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  • Atmosphere-ocean interaction  (51)
  • Diapycnal mixing  (33)
  • Ocean models  (33)
  • Baroclinic flows
  • American Meteorological Society  (136)
  • MDPI Publishing
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  • 1
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    Dynamics of eddying abyssal mixing layers over sloping rough topography (2023)
    American Meteorological Society
    Details
    Publication Date: 2023-02-28
    Description: Author Posting. © American Meteorological Society, 2022. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 52(12),(2022): 3199-3219, https://doi.org/10.1175/jpo-d-22-0009.1.
    Description: The abyssal overturning circulation is thought to be primarily driven by small-scale turbulent mixing. Diagnosed water-mass transformations are dominated by rough topography “hotspots,” where the bottom enhancement of mixing causes the diffusive buoyancy flux to diverge, driving widespread downwelling in the interior—only to be overwhelmed by an even stronger upwelling in a thin bottom boundary layer (BBL). These water-mass transformations are significantly underestimated by one-dimensional (1D) sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this 1D boundary layer approach to three-dimensional eddying flows over realistically rough topography. When applied to the Mid-Atlantic Ridge in the Brazil Basin, the idealized simulation results are roughly consistent with available observations. Integral buoyancy budgets isolate the physical processes that contribute to realistically strong BBL upwelling. The downward diffusion of buoyancy is primarily balanced by upwelling along the sloping canyon sidewalls and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies and by the blocking of along-ridge thermal wind within the canyon. Major topographic sills block along-thalweg flows from restratifying the canyon trough, resulting in the continual erosion of the trough’s stratification. We propose simple modifications to the 1D boundary layer model that approximate each of these three-dimensional effects. These results provide local dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the nonlocal coupling to the basin-scale circulation.
    Description: We acknowledge funding support from National Science Foundation Awards 1536515, 1736109, and 2149080. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant 174530.
    Description: 2023-05-18
    Keywords: Abyssal circulation ; Diapycnal mixing ; Meridional overturning circulation ; Topographic effects ; Upwelling/downwelling ; Bottom currents/bottom water
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Diapycnal displacement, diffusion, and distortion of tracers in the ocean (2023)
    American Meteorological Society
    Details
    Publication Date: 2023-03-02
    Description: Author Posting. © American Meteorological Society, 2022. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 52(12), (2022): 3221–3240, https://doi.org/10.1175/jpo-d-22-0010.1.
    Description: Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Indirect microstructure measurements of in situ turbulence suggest that mixing is bottom enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. Tracer release experiments (TREs), in which inert tracers are purposefully released and their dispersion is surveyed over time, have been used to independently infer turbulent diffusivities—but typically provide estimates in excess of microstructure ones. In an attempt to reconcile these differences, Ruan and Ferrari derived exact tracer-weighted buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. A tracer’s diapycnal displacement rate is exactly twice the tracer-averaged buoyancy velocity, itself a convolution of an asymmetric upwelling/downwelling dipole. The tracer’s diapycnal spreading rate, however, involves both the expected positive contribution from the tracer-averaged in situ diffusion as well as an additional nonlinear diapycnal distortion term, which is caused by correlations between buoyancy and the buoyancy velocity, and can be of either sign. Distortion is generally positive (stretching) due to bottom-enhanced mixing in the stratified interior but negative (contraction) near the bottom. Our simulations suggest that these two effects coincidentally cancel for the Brazil Basin Tracer Release Experiment, resulting in negligible net distortion. By contrast, near-bottom tracers experience leading-order distortion that varies in time. Errors in tracer moments due to realistically sparse sampling are generally small (〈20%), especially compared to the O(1) structural errors due to the omission of distortion effects in inverse models. These results suggest that TREs, although indispensable, should not be treated as “unambiguous” constraints on diapycnal mixing.
    Description: We acknowledge funding support from National Science Foundation Awards 1536515 and 1736109. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant 174530. This research is also supported by the NOAA Climate and Global Change Postdoctoral Fellowship Program, administered by UCAR’s Cooperative Programs for the Advancement of Earth System Science (CPAESS) under Award NA18NWS4620043B.
    Description: 2023-05-18
    Keywords: Diapycnal mixing ; Diffusion ; Upwelling/downwelling ; Bottom currents/bottom water ; Tracers
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  • 3
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    Bay of Bengal intraseasonal oscillations and the 2018 monsoon onset (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-05-27
    Description: Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 102(10), (2021): E1936–E1951, https://doi.org/10.1175/BAMS-D-20-0113.1.
    Description: In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST 〉 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.
    Description: This work was supported through the U.S. Office of Naval Research’s Departmental Research Initiative: Monsoon Intraseasonal Oscillations in the Bay of Bengal, the Indian Ministry of Earth Science’s Ocean Mixing and Monsoons Program, and the Sri Lankan National Aquatic Resources Research and Development Agency. We thank the Captain and crew of the R/V Thompson for their help in data collection. Surface atmospheric fields included fluxes were quality controlled and processed by the Boundary Layer Observations and Processes Team within the NOAA Physical Sciences Laboratory. Forecast analysis was completed by India Meteorological Department. Drone image was taken by Shreyas Kamat with annotations by Gualtiero Spiro Jaeger. We also recognize the numerous researchers who supported cruise- and land-based measurements. This work represents Lamont-Doherty Earth Observatory contribution number 8503, and PMEL contribution number 5193.
    Description: 2022-04-01
    Keywords: Atmosphere-ocean interaction ; Monsoons ; In situ atmospheric observations ; In situ oceanic observations
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  • 4
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    Turbulent mixing variability in an energetic standing meander of the Southern Ocean (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-08-29
    Description: Author Posting. © American Meteorological Society, 2022. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 52(8), (2022): 1593-1611, https://doi.org/10.1175/jpo-d-21-0180.1.
    Description: This study presents novel observational estimates of turbulent dissipation and mixing in a standing meander between the Southeast Indian Ridge and the Macquarie Ridge in the Southern Ocean. By applying a finescale parameterization on the temperature, salinity, and velocity profiles collected from Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats in the upper 1600 m, we estimated the intensity and spatial distribution of dissipation rate and diapycnal mixing along the float tracks and investigated the sources. The indirect estimates indicate strong spatial and temporal variability of turbulent mixing varying from O(10−6) to O(10−3) m2 s−1 in the upper 1600 m. Elevated turbulent mixing is mostly associated with the Subantarctic Front (SAF) and mesoscale eddies. In the upper 500 m, enhanced mixing is associated with downward-propagating wind-generated near-inertial waves as well as the interaction between cyclonic eddies and upward-propagating internal waves. In the study region, the local topography does not play a role in turbulent mixing in the upper part of the water column, which has similar values in profiles over rough and smooth topography. However, both remotely generated internal tides and lee waves could contribute to the upward-propagating energy. Our results point strongly to the generation of turbulent mixing through the interaction of internal waves and the intense mesoscale eddy field.
    Description: The observations were funded through grants from the Australian Research Council Discovery Project (DP170102162) and Australia’s Marine National Facility. Surface drifters were provided by Dr. Shaun Dolk of the Global Drifter Program. AC was supported by an Australian Research Council Postdoctoral Fellowship. AC, HEP, and NLB acknowledge support from the Australian Government Department of the Environment and Energy National Environmental Science Program and the ARC Centre of Excellence in Climate Extremes. KP acknowledges the support from the National Science Foundation.
    Keywords: Diapycnal mixing ; Eddies ; Fronts ; Inertia-gravity waves ; Ocean dynamics
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  • 5
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    Impact of SST and surface waves on Hurricane Florence (2018): a coupled modeling investigation (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-06-10
    Description: Author Posting. © American Meteorological Society , 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Zambon, J. B., He, R., Warner, J. C., & Hegermiller, C. A. Impact of SST and surface waves on Hurricane Florence (2018): a coupled modeling investigation. Weather and Forecasting, 36(5), (2021): 1713–1734, https://doi.org/10.1175/WAF-D-20-0171.1.
    Description: Hurricane Florence (2018) devastated the coastal communities of the Carolinas through heavy rainfall that resulted in massive flooding. Florence was characterized by an abrupt reduction in intensity (Saffir–Simpson category 4 to category 1) just prior to landfall and synoptic-scale interactions that stalled the storm over the Carolinas for several days. We conducted a series of numerical modeling experiments in coupled and uncoupled configurations to examine the impact of sea surface temperature (SST) and ocean waves on storm characteristics. In addition to experiments using a fully coupled atmosphere–ocean–wave model, we introduced the capability of the atmospheric model to modulate wind stress and surface fluxes by ocean waves through data from an uncoupled wave model. We examined these experiments by comparing track, intensity, strength, SST, storm structure, wave height, surface roughness, heat fluxes, and precipitation in order to determine the impacts of resolving ocean conditions with varying degrees of coupling. We found differences in the storm’s intensity and strength, with the best correlation coefficient of intensity (r = 0.89) and strength (r = 0.95) coming from the fully coupled simulations. Further analysis into surface roughness parameterizations added to the atmospheric model revealed differences in the spatial distribution and magnitude of the largest roughness lengths. Adding ocean and wave features to the model further modified the fluxes due to more realistic cooling beneath the storm, which in turn modified the precipitation field. Our experiments highlight significant differences in how air–sea processes impact hurricane modeling. The storm characteristics of track, intensity, strength, and precipitation at landfall are crucial to predictability and forecasting of future landfalling hurricanes.
    Description: This work has been supported by the U.S. Geological Survey Coastal/Marine Hazards and Resources Program, and by Congressional appropriations through the Additional Supplemental Appropriations for Disaster Relief Act of 2019 (H.R. 2157). The authors also wish to acknowledge research support through NSF Grant OCE-1559178 and NOAA Grant NA16NOS0120028. We also wish to thank Chris Sherwood from the U.S. Geological Survey for his help in deriving wave length from WAVEWATCH III data.
    Keywords: Hurricanes/typhoons ; Hindcasts ; Numerical weather prediction/forecasting ; Coupled models ; Ocean models
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  • 6
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    Moored turbulence measurements using pulse-coherent doppler sonar (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-06-10
    Description: Author Posting. © American Meteorological Society , 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Zippel, S. F., Farrar, J. T., Zappa, C. J., Miller, U., St Laurent, L., Ijichi, T., Weller, R. A., McRaven, L., Nylund, S., & Le Bel, D. Moored turbulence measurements using pulse-coherent doppler sonar. Journal of Atmospheric and Oceanic Technology, 38(9), (2021): 1621–1639, https://doi.org/10.1175/JTECH-D-21-0005.1.
    Description: Upper-ocean turbulence is central to the exchanges of heat, momentum, and gases across the air–sea interface and therefore plays a large role in weather and climate. Current understanding of upper-ocean mixing is lacking, often leading models to misrepresent mixed layer depths and sea surface temperature. In part, progress has been limited by the difficulty of measuring turbulence from fixed moorings that can simultaneously measure surface fluxes and upper-ocean stratification over long time periods. Here we introduce a direct wavenumber method for measuring turbulent kinetic energy (TKE) dissipation rates ϵ from long-enduring moorings using pulse-coherent ADCPs. We discuss optimal programming of the ADCPs, a robust mechanical design for use on a mooring to maximize data return, and data processing techniques including phase-ambiguity unwrapping, spectral analysis, and a correction for instrument response. The method was used in the Salinity Processes Upper-Ocean Regional Study (SPURS) to collect two year-long datasets. We find that the mooring-derived TKE dissipation rates compare favorably to estimates made nearby from a microstructure shear probe mounted to a glider during its two separate 2-week missions for O(10−8) ≤ ϵ ≤ O(10−5) m2 s−3. Periods of disagreement between turbulence estimates from the two platforms coincide with differences in vertical temperature profiles, which may indicate that barrier layers can substantially modulate upper-ocean turbulence over horizontal scales of 1–10 km. We also find that dissipation estimates from two different moorings at 12.5 and at 7 m are in agreement with the surface buoyancy flux during periods of strong nighttime convection, consistent with classic boundary layer theory.
    Description: This work was funded by NASA as part of the Salinity Processes in the Upper Ocean Regional Study (SPURS), supporting field work for SPURS-1 (NASA Grant NNX11AE84G), for SPURS-2 (NASA Grant NNX15AG20G), and for analysis (NASA Grant 80NSSC18K1494). Funding for early iterations of this project associated with the VOCALS project and Stratus 9 mooring was provided by NSF (Awards 0745508 and 0745442). Additional funding was provided by ONR Grant N000141812431 and NSF Award 1756839. The Stratus Ocean Reference Station is funded by the Global Ocean Monitoring and Observing Program of the National Oceanic and Atmospheric Administration (CPO FundRef Number 100007298), through the Cooperative Institute for the North Atlantic Region (CINAR) under Cooperative Agreement NA14OAR4320158. Microstructure measurements made from the glider were supported by NSF (Award 1129646).
    Keywords: Ocean ; Turbulence ; Atmosphere-ocean interaction ; Boundary layer ; Oceanic mixed layer ; In situ oceanic observations
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  • 7
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    Impacts of ocean currents on the South Indian Ocean extratropical storm track through the relative wind effect (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-06-17
    Description: Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 34(22), (2021): 9093–9113, https://doi.org/10.1175/JCLI-D-21-0142.1.
    Description: This study examines the role of the relative wind (RW) effect (wind relative to ocean current) in the regional ocean circulation and extratropical storm track in the south Indian Ocean. Comparison of two high-resolution regional coupled model simulations with and without the RW effect reveals that the most conspicuous ocean circulation response is the significant weakening of the overly energetic anticyclonic standing eddy off Port Elizabeth, South Africa, a biased feature ascribed to upstream retroflection of the Agulhas Current (AC). This opens a pathway through which the AC transports the warm and salty water mass from the subtropics, yielding marked increases in sea surface temperature (SST), upward turbulent heat flux (THF), and meridional SST gradient in the Agulhas retroflection region. These thermodynamic and dynamic changes are accompanied by the robust strengthening of the local low-tropospheric baroclinicity and the baroclinic wave activity in the atmosphere. Examination of the composite life cycle of synoptic-scale storms subjected to the high-THF events indicates a robust strengthening of the extratropical storms far downstream. Energetics calculations for the atmosphere suggest that the baroclinic energy conversion from the basic flow is the chief source of increased eddy available potential energy, which is subsequently converted to eddy kinetic energy, providing for the growth of transient baroclinic waves. Overall, the results suggest that the mechanical and thermal air–sea interactions are inherently and inextricably linked together to substantially influence the extratropical storm tracks in the south Indian Ocean.
    Description: Seo acknowledges the support from the NSF (OCE-2022846), NOAA (NA19OAR4310376), ONR (N00014-17-12398), and the Andrew W. Mellon Foundation Endowed Fund for Innovative Research at Woods Hole Oceanographic Institution (WHOI). Song is supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2019R1C1C1003663). O’Neill was supported by the NASA Grants 80NSSC19K1117 and 80NSSC19K1011.
    Keywords: Atmosphere-ocean interaction ; Extratropical cyclones ; Wind stress ; Boundary currents ; Storm tracks
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  • 8
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    Near-resonances of superinertial and tidal fluctuations at islands (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-06-03
    Description: Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 51(9), (2021): 2721–2733, https://doi.org/10.1175/JPO-D-20-0298.1.
    Description: A linear numerical model of an island or a tall seamount is used to explore superinertial leaky resonances forced by ambient vertically and horizontally uniform current fluctuations. The model assumes a circularly symmetric topography (including a shallow reef) and allows realistic stratification and bottom friction. As long as there is substantial stratification, a number of leaky resonances are found, and when the island’s flanks are narrow relative to the internal Rossby radius, some of the near-resonant modes resemble leaky internal Kelvin waves. Other “resonances” resemble higher radial mode long gravity waves as explored by Chambers. The near-resonances amplify the cross-reef velocities that help fuel biological activity. Results for cases with the central island replaced by a lagoon do not differ greatly from the island case which has land at the center. As an aside, insight is provided on the question of offshore boundary conditions for superinertial nearly trapped waves along a straight coast.
    Keywords: Baroclinic flows ; Internal waves ; Kelvin waves
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  • 9
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    Time scales of the Greenland freshwater anomaly in the subpolar North Atlantic (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-06-03
    Description: Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 34(22), (2021): 8971–8987, https://doi.org/10.1175/JCLI-D-20-0610.1.
    Description: The impact of increasing Greenland freshwater discharge on the subpolar North Atlantic (SPNA) remains unknown as there are uncertainties associated with the time scales of the Greenland freshwater anomaly (GFWA) in the SPNA. Results from numerical simulations tracking GFWA and an analytical approach are employed to estimate the response time, suggesting that a decadal time scale (13 years) is required for the SPNA to adjust for increasing GFWA. Analytical solutions obtained for a long-lasting increase of freshwater discharge show a non-steady-state response of the SPNA with increasing content of the GFWA. In contrast, solutions for a short-lived pulse of freshwater demonstrate different responses of the SPNA with a rapid increase of freshwater in the domain followed by an exponential decay after the pulse has passed. The derived theoretical relation between time scales shows that residence time scales are time dependent for a non-steady-state case and asymptote the response time scale with time. The residence time of the GFWA deduced from Lagrangian experiments is close to and smaller than the response time, in agreement with the theory. The Lagrangian analysis shows dependence of the residence time on the entrance route of the GFWA and on the depth. The fraction of the GFWA exported through Davis Strait has limited impact on the interior basins, whereas the fraction entering the SPNA from the southwest Greenland shelf spreads into the interior regions. In both cases, the residence time of the GFWA increases with depth demonstrating long persistence of the freshwater anomaly in the subsurface layers.
    Description: D. S. Dukhovskoy and E. P. Chassignet were funded by the DOE (Award DE-SC0014378) and HYCOM NOPP (Award N00014-19-1-2674). The HYCOM-CICE simulations were supported by a grant of computer time from the DoD High-Performance Computing Modernization Program at NRL SSC. G. Platov was funded by the RSF N19-17-00154. P. G. Myers was funded by an NSERC Discovery Grant (Grant RGPIN 04357). A. Proshutinsky was funded by FAMOS project (NSF Grant NSF 14-584).
    Keywords: North Atlantic Ocean ; Lagrangian circulation/transport ; Ocean circulation ; Differential equations ; Ocean models
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  • 10
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    Double diffusion, shear instabilities, and heat impacts of a pacific summer water intrusion in the Beaufort Sea (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-06-13
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Fine, E., MacKinnon, J., Alford, M., Middleton, L., Taylor, J., Mickett, J., Cole, S., Couto, N., Boyer, A., & Peacock, T. Double diffusion, shear instabilities, and heat impacts of a pacific summer water intrusion in the Beaufort Sea. Journal of Physical Oceanography, 52(2), (2022): 189–203, https://doi.org/10.1175/jpo-d-21-0074.1.
    Description: Pacific Summer Water eddies and intrusions transport heat and salt from boundary regions into the western Arctic basin. Here we examine concurrent effects of lateral stirring and vertical mixing using microstructure data collected within a Pacific Summer Water intrusion with a length scale of ∼20 km. This intrusion was characterized by complex thermohaline structure in which warm Pacific Summer Water interleaved in alternating layers of O(1) m thickness with cooler water, due to lateral stirring and intrusive processes. Along interfaces between warm/salty and cold/freshwater masses, the density ratio was favorable to double-diffusive processes. The rate of dissipation of turbulent kinetic energy (ε) was elevated along the interleaving surfaces, with values up to 3 × 10−8 W kg−1 compared to background ε of less than 10−9 W kg−1. Based on the distribution of ε as a function of density ratio Rρ, we conclude that double-diffusive convection is largely responsible for the elevated ε observed over the survey. The lateral processes that created the layered thermohaline structure resulted in vertical thermohaline gradients susceptible to double-diffusive convection, resulting in upward vertical heat fluxes. Bulk vertical heat fluxes above the intrusion are estimated in the range of 0.2–1 W m−2, with the localized flux above the uppermost warm layer elevated to 2–10 W m−2. Lateral fluxes are much larger, estimated between 1000 and 5000 W m−2, and set an overall decay rate for the intrusion of 1–5 years.
    Description: This work was supported by ONR Grant N00014-16-1-2378 and NSF Grants PLR 14-56705 and PLR-1303791, NSF Graduate Research Fellowship Grant DGE-1650112, as well as by the Postdoctoral Scholar Program at Woods Hole Oceanographic Institution, with funding provided by the Weston Howland Jr. Postdoctoral Scholarship.
    Keywords: Arctic ; Diapycnal mixing ; Diffusion ; Fluxes ; Instability ; Mixing ; Turbulence
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  • 11
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    Geometry of the meridional overturning circulation at the Last Glacial Maximum (2022)
    American Meteorological Society
    Details
    Publication Date: 2023-02-01
    Description: Author Posting. © American Meteorological Society, 2022. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 35(17), (2022): 5465-5482, https://doi.org/10.1175/jcli-d-21-0671.1.
    Description: Understanding the contribution of ocean circulation to glacial–interglacial climate change is a major focus of paleoceanography. Specifically, many have tried to determine whether the volumes and depths of Antarctic- and North Atlantic–sourced waters in the deep ocean changed at the Last Glacial Maximum (LGM; ∼22–18 kyr BP) when atmospheric pCO2 concentrations were 100 ppm lower than the preindustrial. Measurements of sedimentary geochemical proxies are the primary way that these deep ocean structural changes have been reconstructed. However, the main proxies used to reconstruct LGM Atlantic water mass geometry provide conflicting results as to whether North Atlantic–sourced waters shoaled during the LGM. Despite this, a number of idealized modeling studies have been advanced to describe the physical processes resulting in shoaled North Atlantic waters. This paper aims to critically assess the approaches used to determine LGM Atlantic circulation geometry and lay out best practices for future work. We first compile existing proxy data and paleoclimate model output to deduce the processes responsible for setting the ocean distributions of geochemical proxies in the LGM Atlantic Ocean. We highlight how small-scale mixing processes in the ocean interior can decouple tracer distributions from the large-scale circulation, complicating the straightforward interpretation of geochemical tracers as proxies for water mass structure. Finally, we outline promising paths toward ascertaining the LGM circulation structure more clearly and deeply.
    Description: S.K.H. was supported by the Investment in Science Fund at WHOI and the John E. and Anne W. Sawyer Endowed Fund in Support of Scientific Staff. F.J.P. was supported by a Stanback Postdoctoral Fellowship at Caltech.
    Description: 2023-02-01
    Keywords: Diapycnal mixing ; Meridional overturning circulation ; Ocean circulation
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  • 12
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    Microstructure mixing observations and finescale parameterizations in the Beaufort Sea (2021)
    American Meteorological Society
    Details
    Publication Date: 2022-05-27
    Description: Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 51(1), (2021): 19-35, https://doi.org/10.1175/JPO-D-19-0233.1.
    Description: In the Beaufort Sea in September of 2015, concurrent mooring and microstructure observations were used to assess dissipation rates in the vicinity of 72°35′N, 145°1′W. Microstructure measurements from a free-falling profiler survey showed very low [O(10−10) W kg−1] turbulent kinetic energy dissipation rates ε. A finescale parameterization based on both shear and strain measurements was applied to estimate the ratio of shear to strain Rω and ε at the mooring location, and a strain-based parameterization was applied to the microstructure survey (which occurred approximately 100 km away from the mooring site) for direct comparison with microstructure results. The finescale parameterization worked well, with discrepancies ranging from a factor of 1–2.5 depending on depth. The largest discrepancies occurred at depths with high shear. Mean Rω was 17, and Rω showed high variability with values ranging from 3 to 50 over 8 days. Observed ε was slightly elevated (factor of 2–3 compared with a later survey of 11 profiles taken over 3 h) from 25 to 125 m following a wind event which occurred at the beginning of the mooring deployment, reaching a maximum of ε= 6 × 10−10 W kg−1 at 30-m depth. Velocity signals associated with near-inertial waves (NIWs) were observed at depths greater than 200 m, where the Atlantic Water mass represents a reservoir of oceanic heat. However, no evidence of elevated ε or heat fluxes was observed in association with NIWs at these depths in either the microstructure survey or the finescale parameterization estimates.
    Description: This work was supported by NSF Grants PLR 14-56705 and PLR-1303791 and by NSF Graduate Research Fellowship Grant DGE-1650112.
    Keywords: Ocean ; Arctic ; Internal waves ; Turbulence ; Diapycnal mixing
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  • 13
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    Mixing and transformation in a deep western boundary current: a case study (2021)
    American Meteorological Society
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    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Spingys, C. P., Garabato, A. C. N., Legg, S., Polzin, K. L., Abrahamsen, E. P., Buckingham, C. E., Forryan, A., & Frajka-Williams, E. E. Mixing and transformation in a deep western boundary current: a case study. Journal of Physical Oceanography, 51(4), (2021): 1205-1222, https://doi.org/10.1175/JPO-D-20-0132.1
    Description: Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this transformation and its underpinning processes in a small Southern Ocean basin: the Orkney Deep. Observations reveal a focusing of the transport in density space as a deep western boundary current (DWBC) flows through the region, associated with lightening and densification of the current’s denser and lighter layers, respectively. These transformations are driven by vigorous turbulent mixing. Comparing this transformation with measurements of the rate of turbulent kinetic energy dissipation indicates that, within the DWBC, turbulence operates with a high mixing efficiency, characterized by a dissipation ratio of 0.6 to 1 that exceeds the common value of 0.2. This result is corroborated by estimates of the dissipation ratio from microstructure observations. The causes of the transformation are unraveled through a decomposition into contributions dependent on the gradients in density space of the: dianeutral mixing rate, isoneutral area, and stratification. The transformation is found to be primarily driven by strong turbulence acting on an abrupt transition from the weakly stratified bottom boundary layer to well-stratified off-boundary waters. The reduced boundary layer stratification is generated by a downslope Ekman flow associated with the DWBC’s flow along sloping topography, and is further regulated by submesoscale instabilities acting to restratify near-boundary waters. Our results provide observational evidence endorsing the importance of near-boundary mixing processes to deep-ocean overturning, and highlight the role of DWBCs as hot spots of dianeutral upwelling.
    Description: CS, ACNG, AF, and EFW were supported by the U.K. Natural Environment Research Council (NERC) Grant NE/K013181/1. ACNG was supported by the Royal Society and Wolfson Foundation. EPA and CEB were supported by NERC Grant NE/K012843/1. CEB was funded by an MSCA grant (No. 798319) from the European Union’s Horizon 2020 program. EPA was supported by NERC Grant NE/N018095/1. SL and KP were supported by U.S. National Science Foundation Grants OCE-1536453 and OCE-1536779. SL acknowledges support of Award NA18OAR4320123 from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the authors, and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration, or the U.S. Department of Commerce.
    Keywords: Bottom currents ; Diapycnal mixing ; Turbulence ; Southern Ocean
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  • 14
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    Uncertainties in ocean latent heat flux variations over recent decades in satellite-based estimates and reduced observation reanalyses (2021)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(19), (2020): 8415-8437, doi:10.1175/JCLI-D-19-0954.1.
    Description: Four state-of-the-art satellite-based estimates of ocean surface latent heat fluxes (LHFs) extending over three decades are analyzed, focusing on the interannual variability and trends of near-global averages and regional patterns. Detailed intercomparisons are made with other datasets including 1) reduced observation reanalyses (RedObs) whose exclusion of satellite data renders them an important independent diagnostic tool; 2) a moisture budget residual LHF estimate using reanalysis moisture transport, atmospheric storage, and satellite precipitation; 3) the ECMWF Reanalysis 5 (ERA5); 4) Remote Sensing Systems (RSS) single-sensor passive microwave and scatterometer wind speed retrievals; and 5) several sea surface temperature (SST) datasets. Large disparities remain in near-global satellite LHF trends and their regional expression over the 1990–2010 period, during which time the interdecadal Pacific oscillation changed sign. The budget residual diagnostics support the smaller RedObs LHF trends. The satellites, ERA5, and RedObs are reasonably consistent in identifying contributions by the 10-m wind speed variations to the LHF trend patterns. However, contributions by the near-surface vertical humidity gradient from satellites and ERA5 trend upward in time with respect to the RedObs ensemble and show less agreement in trend patterns. Problems with wind speed retrievals from Special Sensor Microwave Imager/Sounder satellite sensors, excessive upward trends in trends in Optimal Interpolation Sea Surface Temperature (OISST AVHRR-Only) data used in most satellite LHF estimates, and uncertainties associated with poor satellite coverage before the mid-1990s are noted. Possibly erroneous trends are also identified in ERA5 LHF associated with the onset of scatterometer wind data assimilation in the early 1990s.
    Description: FRR, JBR, and MGB acknowledge support for this investigation through the NASA Energy and Water Cycle Study (NEWS), Dr. Jared Entin, Program Manager. MS acknowledges the financial support by the EUMETSAT member states through CM SAF. The NOAA-CIRES-DOE Twentieth Century Reanalysis Project version 3 used resources of the National Energy Research Scientific Computing Center managed by Lawrence Berkeley National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract DE-AC02-05CH11231 and used resources of NOAA’s remotely deployed high-performance computing systems. Support for the Twentieth Century Reanalysis Project version 3 dataset is provided by the U.S. DOE, Office of Science Biological and Environmental Research (BER), by the NOAA Climate Program Office, and by the NOAA Physical Sciences Laboratory. RSS products are supported by funding from the NASA Earth Science Division. H. Tomita acknowledges support from JSPS Grants JP18H03726, JP18H03737, and JP19H05696 and JAXA Announcement EO-2. We gratefully acknowledge provision and institutional support for the following SST datasets: ESA CCI (http://data.ceda.ac.uk/neodc/esacci/sst/data/CDR_v2/); NOAA Optimum Interpolation 1/4 Degree Daily Sea Surface Temperature (OISST) Analysis, version 2, (https:/doi.org/10.7289/V5SQ8XB5); NOAA ERSST v5 (https:/doi.org/10.7289/V5T72FNM) and access to COBE-SST2 provided by the NOAA/OAR/ESRL PSD (boyin.huang@noaa.gov); 20CRv3 data are available at the NERSC Science Tape Gateway via portal.nersc.gov.
    Description: 2021-03-01
    Keywords: Atmosphere-ocean interaction ; Hydrologic cycle ; Microwave observations ; Satellite observations ; Reanalysis data ; Decadal variability
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  • 15
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    Hydraulics and mixing of the deep overflow in the Lifamatola Passage of the Indonesian Seas (2021)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(9),(2020): 2797-2814, https://doi.org/10.1175/JPO-D-19-0326.1.
    Description: Hydrographic measurements recently acquired along the thalweg of the Lifamatola Passage combined with historical moored velocity measurements immediately downstream of the sill are used to study the hydraulics, transport, mixing, and entrainment in the dense overflow. The observations suggest that the mean overflow is nearly critical at the mooring site, suggesting that a weir formula may be appropriate for estimating the overflow transport. Our assessment suggests that the weir formulas corresponding to a rectangular, triangular, or parabolic cross section all result in transports very close to the observation, suggesting their potential usage in long-term monitoring of the overflow transport or parameterizing the transport in numerical models. Analyses also suggest that deep signals within the overflow layer are blocked by the shear flow from propagating upstream, whereas the shallow wave modes of the full-depth continuously stratified flow are able to propagate upstream from the Banda Sea into the Maluku Sea. Strong mixing is found immediately downstream of the sill crest, with Thorpe-scale-based estimates of the mean dissipation rate within the overflow up to 1.1 × 10−7 W kg−1 and the region-averaged diapycnal diffusivity within the downstream overflow in the range of 2.3 × 10−3 to 10.1 × 10−3 m2 s−1. Mixing in the Lifamatola Passage results in 0.6–1.2-Sv (1 Sv ≡ 106 m3 s−1) entrainment transport added to the overflow, enhancing the deep-water renewal in the Banda Sea. A bulk diffusivity coefficient estimated in the deep Banda Sea yields 1.6 × 10−3 ± 5 × 10−4 m2 s−1, with an associated downward turbulent heat flux of 9 W m−2.
    Description: This study is supported by NSFC (91858204), the CAS Strategic Priority Research Program (XDB42000000), NSFC(41720104008, 41421005, 41876025), QMSNL (2018SDKJ0104-02), and the Shandong Provincial projects (U1606402). L. Pratt was supported by the U.S. NSF Grant OCE-1657870.
    Keywords: Diapycnal mixing ; Entrainment ; Internal waves ; Topographic effects ; In situ oceanic observations
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  • 16
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    Decadal to multidecadal variability of the mixed layer to the south of the Kuroshio Extension region (2021)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(17), (2020): 7697-7714, https://doi.org/10.1175/JCLI-D-20-0115.1.
    Description: The decadal to multidecadal mixed layer variability is investigated in a region south of the Kuroshio Extension (130°E–180°, 25°–35°N), an area where the North Pacific subtropical mode water forms, during 1948–2012. By analyzing the mixed layer heat budget with different observational and reanalysis data, here we show that the decadal to multidecadal variability of the mixed layer temperature and mixed layer depth is covaried with the Atlantic multidecadal oscillation (AMO), instead of the Pacific decadal oscillation (PDO). The mixed layer temperature has strong decadal to multidecadal variability, being warm before 1970 and after 1990 (AMO positive phase) and cold during 1970–90 (AMO negative phase), and so does the mixed layer depth. The dominant process for the mixed layer temperature decadal to multidecadal variability is the Ekman advection, which is controlled by the zonal wind changes related to the AMO. The net heat flux into the ocean surface Qnet acts as a damping term and it is mainly from the effect of latent heat flux and partially from sensible heat flux. While the wind as well as mixed layer temperature decadal changes related to the PDO are weak in the western Pacific Ocean. Our finding proposes the possible influence of the AMO on the northwestern Pacific Ocean mixed layer variability, and could be a potential predictor for the decadal to multidecadal climate variability in the western Pacific Ocean.
    Description: Xiaopei Lin is supported by the China’s national key research and development projects (2016YFA0601803) and the National Natural Science Foundation of China (41925025 and U1606402). Baolan Wu is supported by the China Scholarship Council (201806330010). Lisan Yu thanks NOAA for support for her study on climate change and variability.
    Keywords: Atmosphere-ocean interaction ; Boundary currents ; Hadley circulation ; Ocean dynamics ; Teleconnections
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  • 17
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    Depth structure of Ningaloo Niño/Niña events and associated drivers (2021)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 34(5), (2021): 1767-1788, https://doi.org/10.1175/JCLI-D-19-1020.1.
    Description: Marine heatwaves along the coast of Western Australia, referred to as Ningaloo Niño, have had dramatic impacts on the ecosystem in the recent decade. A number of local and remote forcing mechanisms have been put forward; however, little is known about the depth structure of such temperature extremes. Utilizing an eddy-active global ocean general circulation model, Ningaloo Niño and the corresponding cold Ningaloo Niña events are investigated between 1958 and 2016, with a focus on their depth structure. The relative roles of buoyancy and wind forcing are inferred from sensitivity experiments. Composites reveal a strong symmetry between cold and warm events in their vertical structure and associated large-scale spatial patterns. Temperature anomalies are largest at the surface, where buoyancy forcing is dominant, and extend down to 300-m depth (or deeper), with wind forcing being the main driver. Large-scale subsurface anomalies arise from a vertical modulation of the thermocline, extending from the western Pacific into the tropical eastern Indian Ocean. The strongest Ningaloo Niños in 2000 and 2011 are unprecedented compound events, where long-lasting high temperatures are accompanied by extreme freshening, which emerges in association with La Niñas, that is more common and persistent during the negative phase of the interdecadal Pacific oscillation. It is shown that Ningaloo Niños during La Niña phases have a distinctively deeper reach and are associated with a strengthening of the Leeuwin Current, while events during El Niño are limited to the surface layer temperatures, likely driven by local atmosphere–ocean feedbacks, without a clear imprint on salinity and velocity.
    Description: The following support is gratefully acknowledged: the Feodor-Lynen Fellowship by the Alexander von Humboldt Foundation and the WHOI Postdoctoral Scholar program (to SR), the Office of Naval Research under project number N-00014-19-12646 (to GG), the James E. and Barbara V. Moltz Fellowship for Climate-Related Research (to CCU), and IndoArchipel from the Deutsche Forschungsgemeinschaft (DFG) as part of the Special Priority Program (SPP)-1889 “Regional Sea Level Change and Society” (SeaLevel) (for PW).
    Keywords: Ocean ; Australia ; Indian Ocean ; Extreme events ; General circulation models ; Ocean models
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    Upper-ocean response to precipitation forcing in an ocean model hindcast of Hurricane Gonzalo (2021)
    American Meteorological Society
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    Publication Date: 2022-05-27
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(11),(2020): 3219–3234, https://doi.org/10.1175/JPO-D-19-0277.1.
    Description: Preexisting, oceanic barrier layers have been shown to limit turbulent mixing and suppress mixed layer cooling during the forced stage of a tropical cyclone (TC). Furthermore, an understanding of barrier layer evolution during TC passage is mostly unexplored. High precipitation rates within TCs provide a large freshwater flux to the surface that alters upper-ocean stratification and can act as a potential mechanism to strengthen the barrier layer. Ocean glider observations from the Bermuda Institute of Ocean Sciences (BIOS) indicate that a strong barrier layer developed during the approach and passage of Hurricane Gonzalo (2014), primarily as a result of freshening within the upper 30 m of the ocean. Therefore, an ocean model case study of Hurricane Gonzalo has been designed to investigate how precipitation affects upper-ocean stratification and sea surface temperature (SST) cooling during TC passage. Ocean model hindcasts of Hurricane Gonzalo characterize the upper-ocean response to TC precipitation forcing. Three different vertical mixing parameterizations are tested to determine their sensitivity to precipitation forcing. For all turbulent mixing schemes, TC precipitation produces near-surface freshening of about 0.3 psu, which is consistent with previous studies and in situ ocean observations. The influence of precipitation-induced changes to the SST response is more complicated, but generally modifies SSTs by ±0.3°C. Precipitation forcing creates a dynamical coupling between upper-ocean stratification and current shear that is largely responsible for the heterogeneous response in modeled SSTs.
    Description: This work was supported by the National Aeronautics and Space Administration (NASA; Grant NNX15AD45G) and the National Oceanic and Atmospheric Administration (NOAA; Grant NA11OAR4320199).
    Keywords: Air-sea interaction ; Hurricanes/typhoons ; Salinity ; Ocean models
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  • 19
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    Near-surface salinity reveals the oceanic sources of moisture for Australian precipitation through atmospheric moisture transport (2021)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(15), (2020): 6707-6730, https://doi.org/10.1175/JCLI-D-19-0579.1.
    Description: The long-term trend of sea surface salinity (SSS) reveals an intensification of the global hydrological cycle due to human-induced climate change. This study demonstrates that SSS variability can also be used as a measure of terrestrial precipitation on interseasonal to interannual time scales, and to locate the source of moisture. Seasonal composites during El Niño–Southern Oscillation/Indian Ocean dipole (ENSO/IOD) events are used to understand the variations of moisture transport and precipitation over Australia, and their association with SSS variability. As ENSO/IOD events evolve, patterns of positive or negative SSS anomaly emerge in the Indo-Pacific warm pool region and are accompanied by atmospheric moisture transport anomalies toward Australia. During co-occurring La Niña and negative IOD events, salty anomalies around the Maritime Continent (north of Australia) indicate freshwater export and are associated with a significant moisture transport that converges over Australia to create anomalous wet conditions. In contrast, during co-occurring El Niño and positive IOD events, a moisture transport divergence anomaly over Australia results in anomalous dry conditions. The relationship between SSS and atmospheric moisture transport also holds for pure ENSO/IOD events but varies in magnitude and spatial pattern. The significant pattern correlation between the moisture flux divergence and SSS anomaly during the ENSO/IOD events highlights the associated ocean–atmosphere coupling. A case study of the extreme hydroclimatic events of Australia (e.g., the 2010/11 Brisbane flood) demonstrates that the changes in SSS occur before the peak of ENSO/IOD events. This raises the prospect that tracking of SSS variability could aid the prediction of Australian rainfall.
    Description: This research is funded through the Earth System and Climate Change Hub of the Australian government’s National Environmental Science Programme. The assistance of computing resources from the National Computational Infrastructure supported by the Australian Government is acknowledged. CCU acknowledges support from the U.S. National Science Foundation under Grant OCE-1663704. MF was supported by the by Centre for Southern Hemisphere Oceans Research (CSHOR), which is a joint initiative between the Qingdao National Laboratory for Marine Science and Technology (QNLM), CSIRO, University of New South Wales and University of Tasmania. The authors wish to acknowledge PyFerret (https://ferret.pmel.noaa.gov/Ferret/) and the Cimate Data Operators (https://code.mpimet.mpg.de/projects/cdo/) for the data analysis and graphical representations in this paper.
    Keywords: Atmosphere-ocean interaction ; El Nino ; Extreme events ; La Nina ; Precipitation ; Salinity
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  • 20
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    Impact of multidecadal variability in Atlantic SST on winter atmospheric blocking (2020)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: © The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Kwon, Y., Seo, H., Ummenhofer, C. C., & Joyce, T. M. Impact of multidecadal variability in Atlantic SST on winter atmospheric blocking. Journal of Climate, 33(3), (2020): 867-892, doi: 10.1175/JCLI-D-19-0324.1.
    Description: Recent studies have suggested that coherent multidecadal variability exists between North Atlantic atmospheric blocking frequency and the Atlantic multidecadal variability (AMV). However, the role of AMV in modulating blocking variability on multidecadal times scales is not fully understood. This study examines this issue primarily using the NOAA Twentieth Century Reanalysis for 1901–2010. The second mode of the empirical orthogonal function for winter (December–March) atmospheric blocking variability in the North Atlantic exhibits oppositely signed anomalies of blocking frequency over Greenland and the Azores. Furthermore, its principal component time series shows a dominant multidecadal variability lagging AMV by several years. Composite analyses show that this lag is due to the slow evolution of the AMV sea surface temperature (SST) anomalies, which is likely driven by the ocean circulation. Following the warm phase of AMV, the warm SST anomalies emerge in the western subpolar gyre over 3–7 years. The ocean–atmosphere interaction over these 3–7-yr periods is characterized by the damping of the warm SST anomalies by the surface heat flux anomalies, which in turn reduce the overall meridional gradient of the air temperature and thus weaken the meridional transient eddy heat flux in the lower troposphere. The anomalous transient eddy forcing then shifts the eddy-driven jet equatorward, resulting in enhanced Rossby wave breaking and blocking on the northern flank of the jet over Greenland. The opposite is true with the AMV cold phases but with much shorter lags, as the evolution of SST anomalies differs in the warm and cold phases.
    Description: We gratefully acknowledge support from the NSF Climate and Large-scale Dynamics Program (AGS-1355339) to Y-OK, HS, CCU, and TMJ, the NASA Physical Oceanography Program (NNX13AM59G) to Y-OK, HS, and TMJ, NOAA CPO Climate Variability and Predictability Program (NA13OAR4310139) and DOE CESD Regional and Global Model Analysis Program (DE-SC0019492) to Y-OK, and NSF Physical Oceanography Program (OCE-1419235) to HS. We are very grateful to the three anonymous reviewers and editor Dr. Mingfang Ting, for their thorough and insightful suggestions. The NOAA 20CR dataset was downloaded from the NOAA Earth System Research Laboratory Physical Science Division webpage (https://www.esrl.noaa.gov/psd/data/20thC_Rean/). Support for the 20CR Project version 2c dataset is provided by the U.S. Department of Energy, Office of Science Biological and Environmental Research (BER), and by the National Oceanic and Atmospheric Administration Climate Program Office. The HadISST dataset was downloaded from the U.K. Met Office Hadley Centre webpage (https://www.metoffice.gov.uk/hadobs/hadisst/). The ERA-20C dataset was downloaded from the ECMWF webpage (https://apps.ecmwf.int/datasets/data/era20c-daily/). The ERSST5 dataset was provided by the NOAA Earth System Research Laboratory Physical Science Division (https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.ersst.v5.html).
    Keywords: North Atlantic Ocean ; Atmosphere-ocean interaction ; Blocking ; Climate variability ; Multidecadal variability ; North Atlantic Oscillation
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  • 21
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    Dispersion in the open ocean seasonal pycnocline at scales of 1-10 km and 1-6 days (2020)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(2), (2020): 415-437, doi:10.1175/JPO-D-19-0019.1.
    Description: Results are presented from two dye release experiments conducted in the seasonal thermocline of the Sargasso Sea, one in a region of low horizontal strain rate (~10−6 s−1), the second in a region of intermediate horizontal strain rate (~10−5 s−1). Both experiments lasted ~6 days, covering spatial scales of 1–10 and 1–50 km for the low and intermediate strain rate regimes, respectively. Diapycnal diffusivities estimated from the two experiments were κz = (2–5) × 10−6 m2 s−1, while isopycnal diffusivities were κH = (0.2–3) m2 s−1, with the range in κH being less a reflection of site-to-site variability, and more due to uncertainties in the background strain rate acting on the patch combined with uncertain time dependence. The Site I (low strain) experiment exhibited minimal stretching, elongating to approximately 10 km over 6 days while maintaining a width of ~5 km, and with a notable vertical tilt in the meridional direction. By contrast, the Site II (intermediate strain) experiment exhibited significant stretching, elongating to more than 50 km in length and advecting more than 150 km while still maintaining a width of order 3–5 km. Early surveys from both experiments showed patchy distributions indicative of small-scale stirring at scales of order a few hundred meters. Later surveys show relatively smooth, coherent distributions with only occasional patchiness, suggestive of a diffusive rather than stirring process at the scales of the now larger patches. Together the two experiments provide important clues as to the rates and underlying processes driving diapycnal and isopycnal mixing at these scales.
    Description: Results are presented from two dye release experiments conducted in the seasonal thermocline of the Sargasso Sea, one in a region of low horizontal strain rate (~10−6 s−1), the second in a region of intermediate horizontal strain rate (~10−5 s−1). Both experiments lasted ~6 days, covering spatial scales of 1–10 and 1–50 km for the low and intermediate strain rate regimes, respectively. Diapycnal diffusivities estimated from the two experiments were κz = (2–5) × 10−6 m2 s−1, while isopycnal diffusivities were κH = (0.2–3) m2 s−1, with the range in κH being less a reflection of site-to-site variability, and more due to uncertainties in the background strain rate acting on the patch combined with uncertain time dependence. The Site I (low strain) experiment exhibited minimal stretching, elongating to approximately 10 km over 6 days while maintaining a width of ~5 km, and with a notable vertical tilt in the meridional direction. By contrast, the Site II (intermediate strain) experiment exhibited significant stretching, elongating to more than 50 km in length and advecting more than 150 km while still maintaining a width of order 3–5 km. Early surveys from both experiments showed patchy distributions indicative of small-scale stirring at scales of order a few hundred meters. Later surveys show relatively smooth, coherent distributions with only occasional patchiness, suggestive of a diffusive rather than stirring process at the scales of the now larger patches. Together the two experiments provide important clues as to the rates and underlying processes driving diapycnal and isopycnal mixing at these scales.
    Description: 2020-08-06
    Keywords: Ocean ; Atlantic Ocean ; Diapycnal mixing ; Diffusion ; Dispersion ; Mixing
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  • 22
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    Assessment of numerical simulations of deep circulation and variability in the Gulf of Mexico using recent observations (2020)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(4), (2020): 1045-1064, doi:10.1175/JPO-D-19-0137.1.
    Description: Three simulations of the circulation in the Gulf of Mexico (the “Gulf”) using different numerical general circulation models are compared with results of recent large-scale observational campaigns conducted throughout the deep (〉1500 m) Gulf. Analyses of these observations have provided new understanding of large-scale mean circulation features and variability throughout the deep Gulf. Important features include cyclonic flow along the continental slope, deep cyclonic circulation in the western Gulf, a counterrotating pair of cells under the Loop Current region, and a cyclonic cell to the south of this pair. These dominant circulation features are represented in each of the ocean model simulations, although with some obvious differences. A striking difference between all the models and the observations is that the simulated deep eddy kinetic energy under the Loop Current region is generally less than one-half of that computed from observations. A multidecadal integration of one of these numerical simulations is used to evaluate the uncertainty of estimates of velocity statistics in the deep Gulf computed from limited-length (4 years) observational or model records. This analysis shows that the main deep circulation features identified from the observational studies appear to be robust and are not substantially impacted by variability on time scales longer than the observational records. Differences in strengths and structures of the circulation features are identified, however, and quantified through standard error analysis of the statistical estimates using the model solutions.
    Description: This work was supported by the Gulf Research Program of the National Academy of Sciences under Awards 2000006422 and 2000009966. The content is solely the responsibility of the authors and does not necessarily represent the official views of the Gulf Research Program or the National Academy of Sciences. The authors acknowledge the GLORYS project for providing the ocean reanalysis data used in the ROMS simulation. GLORYS is jointly conducted by MERCATOR OCEAN, CORIOLIS, and CNRS/INSU. Installation, recovery, data acquisition, and processing of the CANEK group current-meter moorings were possible because of CICESE-PetróleosMexicanos Grant PEP-CICESE 428229851 and the dedicated work of the crew of the B/O Justo Sierra and scientists of the CANEK group. The authors thank Dr. Aljaz Maslo, CICESE, for assistance with analysis of model data. The Bureau of Ocean Energy Management (BOEM), U.S. Dept. of the Interior, provided funding for the Lagrangian Study of the Deep Circulation in the Gulf of Mexico and the Observations and Dynamics of the Loop Current study. HYCOM simulation data are available from the HYCOM data server (https://www.hycom.org/data/goml0pt04/expt-02pt2), MITgcm data are available from the ECCO data server (http://ecco.ucsd.edu/gom_results2.html), and the ROMS simulation data are available from GRIIDC (NA.x837.000:0001).
    Keywords: Ocean circulation ; Abyssal circulation ; Bottom currents/bottom water ; Eddies ; Ocean models
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    The modulation of Gulf Stream influence on the troposphere by the eddy-driven jet (2020)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(10), (2020): 4109-4120, doi:10.1175/JCLI-D-19-0294.1.
    Description: This study suggests that the Gulf Stream influence on the wintertime North Atlantic troposphere is most pronounced when the eddy-driven jet (EDJ) is farthest south and better collocated with the Gulf Stream. Using the reanalysis dataset NCEP-CFSR for December–February 1979–2009, the daily EDJ latitude is separated into three regimes (northern, central, and southern). It is found that the average trajectory of atmospheric fronts covaries with EDJ latitude. In the southern EDJ regime (~19% of the time), the frequency of near-surface atmospheric fronts that pass across the Gulf Stream is maximized. Analysis suggests that this leads to significant strengthening in near-surface atmospheric frontal convergence resulting from strong air–sea sensible heat flux gradients (due to strong temperature gradients in the atmosphere and ocean). In recent studies, it was shown that the pronounced band of time-mean near-surface wind convergence across the Gulf Stream is set by atmospheric fronts. Here, it is shown that an even smaller subset of atmospheric fronts—those associated with a southern EDJ—primarily sets the time mean, due to enhanced Gulf Stream air–sea interaction. Furthermore, statistically significant anomalies in vertical velocity extending well above the boundary layer are identified in association with changes in EDJ latitude. These anomalies are particularly strong for a southern EDJ and are spatially consistent with increases in near-surface atmospheric frontal convergence over the Gulf Stream. These results imply that much of the Gulf Stream influence on the time-mean atmosphere is modulated on synoptic time scales, and enhanced when the EDJ is farthest south.
    Description: For part of this study, R. P. was funded by the Weston Howland Jr. postdoctoral scholarship at Woods Hole Oceanographic Institution. We gratefully acknowledge the support to Y.-O. K. from the NOAA CPO Climate Variability and Predictability program (NA13OAR4310139), the DOE Regional and Global Model Analysis program (DE-SC0014433 and DE-SC0019492), and the NSF AGS Climate and Large-scale Dynamics program and OCE Physical Oceanography program (AGS-1355339). We thank NCAR for allowing access to the NCEP-CFSR dataset, accessible at https://rda.ucar.edu. We thank the editor Hisashi Nakamura and the three reviewers whose comments have helped greatly improve the manuscript.
    Description: 2020-10-13
    Keywords: Atmosphere-ocean interaction ; Atmosphere-ocean interaction
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    Energy fluxes in coastal trapped waves (2020)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 49(12), (2019): 3061-3068, doi: 10.1175/JPO-D-18-0172.1.
    Description: The calculation of energy flux in coastal trapped wave modes is reviewed in the context of tidal energy pathways near the coast. The significant barotropic pressures and currents associated with coastal trapped wave modes mean that large errors in estimating the wave flux are incurred if only the baroclinic component is considered. A specific example is given showing that baroclinic flux constitutes only 10% of the flux in a mode-1 wave for a reasonable choice of stratification and bathymetry. The interpretation of baroclinic energy flux and barotropic-to-baroclinic conversion at the coast is discussed: in contrast to the open ocean, estimates of baroclinic energy flux do not represent a wave energy flux; neither does conversion represent the scattering of energy from the tidal Kelvin wave to higher modes.
    Description: This work was supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Weston Howland Jr. Postdoctoral Scholarship, and by NSF under Grant OCE-1756781. I am grateful to K. Brink for the many useful conversations that contributed to this work and to J. Toole for providing detailed comments on an early version of this paper. The comments of three anonymous reviewers were very helpful in improving this paper.
    Description: 2020-06-03
    Keywords: Diapycnal mixing ; Internal waves ; Kelvin waves ; Topographic effects ; Waves, oceanic ; Tides
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    Could the North Pacific Oscillation be modified by the initiation of the East Asian winter monsoon? (2020)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(6), (2020): 2389-2406, doi:10.1175/JCLI-D-19-0112.1.
    Description: This study investigates the modulation of North Pacific Oscillation (NPO) variability upon initiation of the East Asian winter monsoon (EAWM). The data show that the initiation of EAWM in the Philippine Sea strongly connects to the southern lobe variability of the NPO in January followed by a basin-scale oceanic Victoria mode pattern. No apparent connection was found for the northern lobe of the NPO when the ENSO signals are removed. The strengthening of the EAWM in November interacts with the Kuroshio front and generates a low-level heating source in the Philippine Sea. Significant Rossby wave sources are then formed in the lower to midtroposphere. Wave ray tracing analyses confirm the atmospheric teleconnection established by the Rossby wave propagation in the mid- to upper troposphere. Analyses of the origin of wave trajectories from the Philippine Sea show a clear eastward propagating pathway that affects the southern lobe of the NPO from the southern lobe of the western Pacific pattern at 500 hPa and above on the time scale of 20 days. No ray trajectories from the lower troposphere can propagate eastward to influence the central-eastern subtropical Pacific. The wave propagation process is further supported by the coupled model experiments.
    Description: We thank three anonymous reviewers for their constructive comments that have helped to improve the clarity of the presentation. This study was supported by the MOST Grants 107-2611-M-002-013-MY4 and 108-2111-M-002-006 -MY3, Taiwan.
    Description: 2020-08-21
    Keywords: Atmosphere-ocean interaction ; ENSO ; Climate variability ; Interannual variability
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    Transformation and upwelling of bottom water in fracture zone valleys (2020)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(3), (2020): 715-726, doi:10.1175/JPO-D-19-0021.1.
    Description: Closing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading midocean ridges, which cover more than one-half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.
    Description: The data used in this study were collected in the context of several projects funded by the U.S. National Science Foundation (NSF), in particular BBTRE (OCE-9415589 and OCE-9415598) and DoMORE (OCE-1235094). Funding for the analysis was provided as part of the NSF DoMORE and DECIMAL (OCE-1735618) projects. Author Ijichi is a Japan Society for the Promotion of Science (JSPS) Overseas Research Fellow. Comments on an early draft of this paper by Jim Ledwell and Bryan Kaiser, as well as topical discussions with Jörn Callies and Trevor McDougall, are gratefully acknowledged. The paper was greatly improved during the review process, in particular because of the critical comments from one of the two anonymous reviewers.
    Keywords: Diapycnal mixing ; Topographic effects ; Turbulence ; Upwelling/downwelling ; Bottom currents/bottom water
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    Estuarine exchange flow variability in a seasonal, segmented estuary (2020)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(3),(2020): 595-613, doi:10.1175/JPO-D-19-0108.1.
    Description: Small estuaries in Mediterranean climates display pronounced salinity variability at seasonal and event time scales. Here, we use a hydrodynamic model of the Coos Estuary, Oregon, to examine the seasonal variability of the salinity dynamics and estuarine exchange flow. The exchange flow is primarily driven by tidal processes, varying with the spring–neap cycle rather than discharge or the salinity gradient. The salinity distribution is rarely in equilibrium with discharge conditions because during the wet season the response time scale is longer than discharge events, while during low flow it is longer than the entire dry season. Consequently, the salt field is rarely fully adjusted to the forcing and common power-law relations between the salinity intrusion and discharge do not apply. Further complicating the salinity dynamics is the estuarine geometry that consists of multiple branching channel segments with distinct freshwater sources. These channel segments act as subestuaries that import both higher- and lower-salinity water and export intermediate salinities. Throughout the estuary, tidal dispersion scales with tidal velocity squared, and likely includes jet–sink flow at the mouth, lateral shear dispersion, and tidal trapping in branching channel segments inside the estuary. While the estuarine inflow is strongly correlated with tidal amplitude, the outflow, stratification, and total mixing in the estuary are dependent on the seasonal variation in river discharge, which is similar to estuaries that are dominated by subtidal exchange flow.
    Description: We thank two anonymous reviewers for constructive comments, the staff of the South Slough National Estuarine Research Reserve for providing time series data, and Parker MacCready for sharing LiveOcean boundary conditions. This work was partially sponsored by the National Estuarine Research Reserve System Science Collaborative, which supports collaborative research that addresses coastal management problems important to the reserves. The Science Collaborative is funded by the National Oceanic and Atmospheric Administration and managed by the University of Michigan Water Center (NAI4NOS4190145). Computations were performed on the University of Oregon high performance computer Talapas.
    Description: 2020-08-26
    Keywords: Estuaries ; North Pacific Ocean ; Baroclinic flows ; Channel flows ; Dispersion ; Mixing
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    An observational estimate of the direct response of the cold-season atmospheric circulation to the Arctic Sea ice loss (2020)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(9), (2020): 3863-3882, doi:10.1175/JCLI-D-19-0687.1.
    Description: The direct response of the cold-season atmospheric circulation to the Arctic sea ice loss is estimated from observed sea ice concentration (SIC) and an atmospheric reanalysis, assuming that the atmospheric response to the long-term sea ice loss is the same as that to interannual pan-Arctic SIC fluctuations with identical spatial patterns. No large-scale relationship with previous interannual SIC fluctuations is found in October and November, but a negative North Atlantic Oscillation (NAO)/Arctic Oscillation follows the pan-Arctic SIC fluctuations from December to March. The signal is field significant in the stratosphere in December, and in the troposphere and tropopause thereafter. However, multiple regressions indicate that the stratospheric December signal is largely due to concomitant Siberian snow-cover anomalies. On the other hand, the tropospheric January–March NAO signals can be unambiguously attributed to SIC variability, with an Iceland high approaching 45 m at 500 hPa, a 2°C surface air warming in northeastern Canada, and a modulation of blocking activity in the North Atlantic sector. In March, a 1°C northern Europe cooling is also attributed to SIC. An SIC impact on the warm Arctic–cold Eurasia pattern is only found in February in relation to January SIC. Extrapolating the most robust results suggests that, in the absence of other forcings, the SIC loss between 1979 and 2016 would have induced a 2°–3°C decade−1 winter warming in northeastern North America and a 40–60 m decade−1 increase in the height of the Iceland high, if linearity and perpetual winter conditions could be assumed.
    Description: This research was supported by the Blue-Action project (European Union’s Horizon 2020 research and innovation program, Grant 727852) and by the National Science Foundation (OPP 1736738).
    Description: 2020-10-06
    Keywords: Atmosphere-ocean interaction ; Climate change ; Climate variability ; Ice loss/growth
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    Frontogenesis and variability in Denmark Strait and its influence on overflow water (2019)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 49(7), (2019): 1889-1904, doi:10.1175/JPO-D-19-0053.1.
    Description: A high-resolution numerical model, together with in situ and satellite observations, is used to explore the nature and dynamics of the dominant high-frequency (from one day to one week) variability in Denmark Strait. Mooring measurements in the center of the strait reveal that warm water “flooding events” occur, whereby the North Icelandic Irminger Current (NIIC) propagates offshore and advects subtropical-origin water northward through the deepest part of the sill. Two other types of mesoscale processes in Denmark Strait have been described previously in the literature, known as “boluses” and “pulses,” associated with a raising and lowering of the overflow water interface. Our measurements reveal that flooding events occur in conjunction with especially pronounced pulses. The model indicates that the NIIC hydrographic front is maintained by a balance between frontogenesis by the large-scale flow and frontolysis by baroclinic instability. Specifically, the temperature and salinity tendency equations demonstrate that the eddies act to relax the front, while the mean flow acts to sharpen it. Furthermore, the model reveals that the two dense water processes—boluses and pulses (and hence flooding events)—are dynamically related to each other and tied to the meandering of the hydrographic front in the strait. Our study thus provides a general framework for interpreting the short-time-scale variability of Denmark Strait Overflow Water entering the Irminger Sea.
    Description: MAS was supported by the National Science Foundation (NSF) under Grants OCE-1558742 and OCE-1534618. RSP, PL, and DM were supported by NSF under Grants OCE-1558742 and OCE-1259618. WJvA was supported by the Helmholtz Infrastructure Initiative FRAM. TWNH and MA were supported by NSF under Grants OCE-1633124 and OCE-118123.
    Description: 2020-07-01
    Keywords: Baroclinic flows ; Frontogenesis/frontolysis ; Meridional overturning circulation ; Ocean dynamics ; Topographic effects
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    Moored observations of the surface meteorology and air-sea fluxes in the Northern Bay of Bengal in 2015 (2019)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 32(2), (2019): 549-573. doi: 10.1175/JCLI-D-18-0413.1.
    Description: Time series of surface meteorology and air–sea fluxes from the northern Bay of Bengal are analyzed, quantifying annual and seasonal means, variability, and the potential for surface fluxes to contribute significantly to variability in surface temperature and salinity. Strong signals were associated with solar insolation and its modulation by cloud cover, and, in the 5- to 50-day range, with intraseasonal oscillations (ISOs). The northeast (NE) monsoon (DJF) was typically cloud free, with strong latent heat loss and several moderate wind events, and had the only seasonal mean ocean heat loss. The spring intermonsoon (MAM) was cloud free and had light winds and the strongest ocean heating. Strong ISOs and Tropical Cyclone Komen were seen in the southwest (SW) monsoon (JJA), when 65% of the 2.2-m total rain fell, and oceanic mean heating was small. The fall intermonsoon (SON) initially had moderate convective systems and mean ocean heating, with a transition to drier winds and mean ocean heat loss in the last month. Observed surface freshwater flux applied to a layer of the observed thickness produced drops in salinity with timing and magnitude similar to the initial drops in salinity in the summer monsoon, but did not reproduce the salinity variability of the fall intermonsoon. Observed surface heat flux has the potential to cause the temperature trends of the different seasons, but uncertainty in how shortwave radiation is absorbed in the upper ocean limits quantifying the role of surface forcing in the evolution of mixed layer temperature.
    Description: The deployment of the Woods Hole Oceanographic Institution (WHOI) mooring and RW and JTF were supported by the U.S. Office of Naval Research, Grant N00014-13-1-0453. DS acknowledges support from the Ministry of Earth Sciences under India’s National Monsoon Mission. HS acknowledges support from the Office of Naval Research Grants N00014-13-1-0453 and N00014-17-12398. The deployment of the WHOI mooring was done by RV Sagar Nidhi and the recovery by RV Sagar Kanya; the help of the crew and science parties is gratefully acknowledged as is the ongoing support at NIOT in Chennai and by other colleagues in India of this mooring work. The work of the staff of the WHOI Upper Ocean Process Group in the design, building, deployment, and recovery of the mooring and in processing the data is gratefully acknowledged. The software for the wavelet analysis was provided by Torrence and Compo (1998). Feedback on the paper by Dr. Amit Tandon and two anonymous reviewers is gratefully acknowledged. This paper is dedicated to Dr. Frank Bradley.
    Description: 2019-06-28
    Keywords: Atmosphere-ocean interaction ; Monsoons ; Air-sea interaction ; Surface fluxes
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    Influence of the Kuroshio interannual variability on the summertime precipitation over the East China Sea and adjacent area (2019)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 32(8), (2019): 2185-2205. doi:10.1175/JCLI-D-18-0538.1.
    Description: Much attention has been paid to the climatic impacts of changes in the Kuroshio Extension, instead of the Kuroshio in the East China Sea (ECS). This study, however, reveals the prominent influences of the lateral shift of the Kuroshio at interannual time scale in late spring [April–June (AMJ)] on the sea surface temperature (SST) and precipitation in summer around the ECS, based on high-resolution satellite observations and ERA-Interim. A persistent offshore displacement of the Kuroshio during AMJ can result in cold SST anomalies in the northern ECS and the Japan/East Sea until late summer, which correspondingly causes anomalous cooling of the lower troposphere. Consequently, the anomalous cold SST in the northern ECS acts as a key driver to robustly enhance the precipitation from the Yangtze River delta to Kyushu in early summer (May–August) and over the central ECS in late summer (July–September). In view of the moisture budget analysis, two different physical processes modulated by the lateral shift of the Kuroshio are identified to account for the distinct responses of precipitation in early and late summer, respectively. First, the anomalous cold SST in the northern ECS induced by the Kuroshio offshore shift is likely conducive to the earlier arrival of the mei-yu–baiu front at 30°–32°N and its subsequent slower northward movement, which may prolong the local rainy season, leading to the increased rain belt in early summer. Second, the persistent cold SST anomalies in late summer strengthen the near-surface baroclinicity and the associated strong atmospheric fronts embedded in the extratropical cyclones over the central ECS, which in turn enhances the local rainfall.
    Description: We appreciate three anonymous reviewers for their thoughtful and constructive comments. This work is supported by the National Key Research and Development Program of China (2016YFA0601804), the National Natural Science Foundation of China (NSFC) Projects (91858102, 41490643, 41490640, 41506009, U1606402) and the OUC–WHOI joint research program (21366).
    Description: 2019-10-01
    Keywords: Continental shelf/slope ; Atmosphere-ocean interaction ; Boundary currents ; Precipitation ; Interannual variability
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    Southern Ocean seasonal restratification delayed by submesoscale wind-front interactions (2019)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 49(4), (2019): 1035-1053, doi:10.1175/JPO-D-18-0136.1.
    Description: Ocean stratification and the vertical extent of the mixed layer influence the rate at which the ocean and atmosphere exchange properties. This process has direct impacts for anthropogenic heat and carbon uptake in the Southern Ocean. Submesoscale instabilities that evolve over space (1–10 km) and time (from hours to days) scales directly influence mixed layer variability and are ubiquitous in the Southern Ocean. Mixed layer eddies contribute to mixed layer restratification, while down-front winds, enhanced by strong synoptic storms, can erode stratification by a cross-frontal Ekman buoyancy flux. This study investigates the role of these submesoscale processes on the subseasonal and interannual variability of the mixed layer stratification using four years of high-resolution glider data in the Southern Ocean. An increase of stratification from winter to summer occurs due to a seasonal warming of the mixed layer. However, we observe transient decreases in stratification lasting from days to weeks, which can arrest the seasonal restratification by up to two months after surface heat flux becomes positive. This leads to interannual differences in the timing of seasonal restratification by up to 36 days. Parameterizing the Ekman buoyancy flux in a one-dimensional mixed layer model reduces the magnitude of stratification compared to when the model is run using heat and freshwater fluxes alone. Importantly, the reduced stratification occurs during the spring restratification period, thereby holding important implications for mixed layer dynamics in climate models as well as physical–biological coupling in the Southern Ocean.
    Description: MdP acknowledges numerous research visits to the Department of Marine Science, University of Gothenburg, and a visit to Woods Hole Oceanographic Institution, which greatly enhanced this work. We thank SANAP and the captain and crew of the S.A. Agulhas II for their assistance in the deployment and retrieval of the gliders. We acknowledge the work of SAMERC-STS for housing, managing, and piloting the gliders. SS was supported by NRF-SANAP Grant SNA14071475720 and a Wallenberg Academy Fellowship (WAF 2015.0186). Lastly, SS thanks the numerous technical assistance, advice, and IOP hosting provided by Geoff Shilling and Craig Lee of the Applied Physics Laboratory, University of Washington.
    Description: 2020-04-11
    Keywords: Atmosphere-ocean interaction ; Fronts ; Oceanic mixed layer ; In situ oceanic observations ; Interannual variability ; Seasonal cycle
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    Some new aspects of the joint effect of rotation and topography on internal solitary waves. (2019)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 49(6), (2019): 1639-1649, doi: 10.1175/JPO-D-18-0154.1.
    Description: Using a recently developed asymptotic theory of internal solitary wave propagation over a sloping bottom in a rotating ocean, some new qualitative and quantitative features of this process are analyzed for internal waves in a two-layer ocean. The interplay between different singularities—terminal damping due to radiation and disappearing quadratic nonlinearity, and reaching an “internal beach” (e.g., zero lower-layer depth)—is discussed. Examples of the adiabatic evolution of a single solitary wave over a uniformly sloping bottom under realistic conditions are considered in more detail and compared with numerical solutions of the variable-coefficient, rotation-modified Korteweg–de Vries (rKdV) equation.
    Description: LAO is thankful to Yu. Stepanyants for broad discussions of mutual benefit. KRH was supported by Grant N00014-18-1-2542 from the Office of Naval Research.
    Description: 2020-06-13
    Keywords: Internal waves ; Differential equations ; Nonlinear models ; Ocean models
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    Reversed lateral circulation in a sharp estuarine bend with weak stratification (2019)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 49(6), (2019):1619-1637, doi:10.1175/JPO-D-18-0175.1.
    Description: Although the hydrodynamics of river meanders are well studied, the influence of curvature on flow in estuaries, with alternating tidal flow and varying water levels and salinity gradients, is less well understood. This paper describes a field study on curvature effects in a narrow salt-marsh creek with sharp bends. The key observations, obtained during times of negligible stratification, are 1) distinct differences between secondary flow during ebb and flood, with helical circulation as in rivers during ebb and a reversed circulation during flood, and 2) maximum (ebb and flood) streamwise velocities near the inside of the bend, unlike typical river bend flow. The streamwise velocity structure is explained by the lack of a distinct point bar and the relatively deep cross section in the estuary, which means that curvature-induced inward momentum redistribution is not overcome by outward redistribution by frictional and topographic effects. Through differential advection of the along-estuary salinity gradient, the laterally sheared streamwise velocity generates lateral salinity differences, with the saltiest water near the inside during flood. The resulting lateral baroclinic pressure gradient force enhances the standard helical circulation during ebb but counteracts it during flood. This first leads to a reversed secondary circulation during flood in the outer part of the cross section, which triggers a positive feedback mechanism by bringing slower-moving water from the outside inward along the surface. This leads to a reversal of the vertical shear in the streamwise flow, and therefore in the centrifugal force, which further enhances the reversed secondary circulation.
    Description: This project was funded by NSF Grant OCE-1634490. During this work W.M. Kranenburg was supported as USGS Postdoctoral Scholar at Woods Hole Oceanographic Institution. A.M.P. Garcia was supported by the Michael J. Kowalski Fellowship in Ocean Science and Engineering (AMPG), and the Diversity Fellowship of the MIT Office of the Dean of Graduate Education (AMPG). The authors thank Jay Sisson for the technical support and Peter Traykovski for providing the bathymetric data. Also, the suggestions for improvement by Dr. K. Blanckaert and an anonymous reviewer are thankfully acknowledged.
    Keywords: Estuaries ; Advection ; Baroclinic flows ; Barotropic flows
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    Decomposition of vertical velocity for nutrient transport in the upper ocean (2019)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 49(6), (2019): 1561-1575, doi:10.1175/JPO-D-19-0002.1.
    Description: Within the pycnocline, where diapycnal mixing is suppressed, both the vertical movement (uplift) of isopycnal surfaces and upward motion along sloping isopycnals supply nutrients to the euphotic layer, but the relative importance of each of these mechanisms is unknown. We present a method for decomposing vertical velocity w into two components in a Lagrangian frame: vertical velocity along sloping isopycnal surfaces and the adiabatic vertical velocity of isopycnal surfaces . We show that , where is the isopycnal slope and is the geometric aspect ratio of the flow, and that accounts for 10%–25% of the total vertical velocity w for isopycnal slopes representative of the midlatitude pycnocline. We perform the decomposition of w in a process study model of a midlatitude eddying flow field generated with a range of isopycnal slopes. A spectral decomposition of the velocity components shows that while is the largest contributor to vertical velocity, is of comparable magnitude at horizontal scales less than about 10 km, that is, at submesoscales. Increasing the horizontal grid resolution of models is known to increase vertical velocity; this increase is disproportionately due to better resolution of , as is shown here by comparing 1- and 4-km resolution model runs. Along-isopycnal vertical transport can be an important contributor to the vertical flux of tracers, including oxygen, nutrients, and chlorophyll, although we find weak covariance between vertical velocity and nutrient anomaly in our model.
    Description: MAF was supported by a National Defense Science and Engineering Graduate Fellowship and AM by NSF OCE-I434788. The authors thank Glenn Flierl and Ruth Curry for helpful conversations, and three anonymous reviewers for comments that improved the manuscript.
    Description: 2020-06-11
    Keywords: Baroclinic flows ; Mesoscale processes ; Small scale processes ; Subgrid-scale processes ; Vertical motion
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    Comment on “Abyssal upwelling and downwelling driven by near-boundary mixing” (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 739-748, doi:10.1175/JPO-D-17-0089.1.
    Description: McDougall and Ferrari have estimated the global deep upward diapycnal flow in the boundary layer overlying continental slopes that must balance both downward diapycnal flow in the deep interior and the formation of bottom water around Antarctica. The decrease of perimeter of isopycnal surfaces with depth and the observed decay with height above bottom of turbulent dissipation in the deep ocean play a key role in their estimate. They argue that because the perimeter of seamounts increases with depth, the net effect of mixing around seamounts is to produce net downward diapycnal flow. While this is true along much of a seamount, it is shown here that diapycnal flow of the densest water around the seamount is upward, with buoyancy being transferred from water just above. The same is true for midocean ridges, whose perimeter is constant with depth. It is argued that mixing around seamounts and especially midocean ridges contributes positively to the global deep overturning circulation, reducing the amount of turbulence demanded over the continental slopes to balance the buoyancy budget for the bottom and deep water.
    Description: This work was supported by National Science Foundation Grant OCE- 1232962.
    Description: 2018-09-29
    Keywords: Abyssal circulation ; Boundary currents ; Buoyancy ; Diapycnal mixing ; Mass fluxes/transport ; Ocean circulation
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    Submesoscale processes at shallow salinity fronts in the Bay of Bengal : observations during the winter monsoon (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 479-509, doi:10.1175/JPO-D-16-0283.1.
    Description: Lateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a salinity-controlled density front, embedded in a zone of moderate mesoscale strain (0.15 times the Coriolis parameter) and forced by winds with a downfront orientation. Below a thin mixed layer, often ≤10 m, the analysis shows several dynamical signatures indicative of submesoscale processes: (i) negative Ertel potential vorticity (PV); (ii) low-PV anomalies with O(1–10) km lateral extent, where the vorticity estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to yield low PV; (iii) flow conditions susceptible to forced symmetric instability (FSI) or bearing the imprint of earlier FSI events; (iv) negative lateral gradients in the absolute momentum field (inertial instability); and (v) strong contribution from differential sheared advection at O(1) km scales to the growth rate of the depth-averaged stratification. The findings here show one-dimensional vertical processes alone cannot explain the vertical stratification and its lateral variability over O(1–10) km scales at the radiator survey.
    Description: S. Ramachandran acknowledges support from the National Science Foundation through award OCE 1558849 and the U.S. Office of Naval Research, Grants N00014-13-1-0456 and N00014-17- 1-2355. A. Tandon acknowledges support from the U.S. Office of Naval Research, Grants N00014-13-1-0456 and N00014-17-1-2355. J. T. Farrar and R. A. Weller were supported by the U.S. Office of Naval Research, Grant N00014-13-1-0453, to collect the UCTD data and process theUCTD and shipboard meteorological data. J. Nash, J. Mackinnon, and A. F. Waterhouse acknowledge support from the U. S. Office of Naval Research, Grants N00014-13-1-0503 and N00014-14-1-0455. E. Shroyer acknowledges support from the U. S. Office of Naval Research, Grants N00014-14-10236 and N00014-15- 12634. A. Mahadevan acknowledges support fromthe U. S. Office of Naval Research, Grant N00014-13-10451. A. J. Lucas and R. Pinkel acknowledge support from the U. S. Office of Naval Research, Grant N00014-13-1-0489.
    Description: 2018-08-26
    Keywords: Indian Ocean ; Baroclinic flows ; Potential vorticity ; Fronts ; Monsoons ; Oceanic mixed layer
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    A role for the equatorial undercurrent in the ocean dynamical thermostat (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 6245-6261, doi:10.1175/JCLI-D-17-0513.1.
    Description: Reconstructions of sea surface temperature (SST) based on instrumental observations suggest that the equatorial Pacific zonal SST gradient has increased over the twentieth century. While this increase is suggestive of the ocean dynamical thermostat mechanism of Clement et al., observations of a concurrent weakening of the zonal atmospheric (Walker) circulation are not. Here we show, using heat and momentum budget calculations on an ocean reanalysis dataset, that a seasonal weakening of the zonal atmospheric circulation is in fact consistent with cooling in the eastern equatorial Pacific (EEP) and thus an increase in the zonal SST gradient. This cooling is driven by a strengthening Equatorial Undercurrent (EUC) in response to decreased upper-ocean westward momentum associated with weakening equatorial zonal wind stress. This process can help to reconcile the seemingly contradictory twentieth-century trends in the tropical Pacific atmosphere and ocean. Moreover, it is shown that coupled general circulation models (CGCMs) do not correctly simulate this process; we identify a systematic bias in the relationship between changes in equatorial surface zonal wind stress in the EEP and EUC strength that may help to explain why observations and CGCMs have opposing trends in the zonal SST gradient over the twentieth century.
    Description: 2019-01-11
    Keywords: Tropics ; Atmosphere-ocean interaction ; Climate change ; Climate models ; Trends
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    On the dynamics and water mass transformation of a boundary current connecting alpha and beta oceans (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 2457-2475, doi:10.1175/JPO-D-17-0186.1.
    Description: A subpolar marginal sea, like the Nordic seas, is a transition zone between the temperature-stratified subtropics (the alpha ocean) and the salinity-stratified polar regions (the beta ocean). An inflow of Atlantic Water circulates these seas as a boundary current that is cooled and freshened downstream, eventually to outflow as Deep and Polar Water. Stratification in the boundary region is dominated by a thermocline over the continental slope and a halocline over the continental shelves, separating Atlantic Water from Deep and Polar Water, respectively. A conceptual model is introduced for the circulation and water mass transformation in a subpolar marginal sea to explore the potential interaction between the alpha and beta oceans. Freshwater input into the shelf regions has a slight strengthening effect on the Atlantic inflow, but more prominently impacts the water mass composition of the outflow. This impact of freshwater, characterized by enhancing Polar Water outflow and suppressing Deep Water outflow, is strongly determined by the source location of freshwater. Concretely, perturbations in upstream freshwater sources, like the Baltic freshwater outflow into the Nordic seas, have an order of magnitude larger potential to impact water mass transports than perturbations in downstream sources like the Arctic freshwater outflow. These boundary current dynamics are directly related to the qualitative stratification in transition zones and illustrate the interaction between the alpha and beta oceans.
    Description: This research was supported by the Research Council of Norway project NORTH. Support for the publication was provided by the University of Bergen. Ocean Outlook has supported a research visit for EL to Woods Hole Oceanographic Institute where much of the current work has been carried out. Support forMAS was provided by the National Science Foundation Grant OCE-1558742.
    Keywords: Continental shelf/slope ; Baroclinic flows ; Boundary currents ; Buoyancy ; Freshwater ; Thermohaline circulation
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    Wind-driven circulation in a shelf valley. Part II : Dynamics of the along-valley velocity and transport (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 883-904, doi:10.1175/JPO-D-17-0084.1.
    Description: The dynamics controlling the along-valley (cross shelf) flow in idealized shallow shelf valleys with small to moderate Burger number are investigated, and analytical scales of the along-valley flows are derived. This paper follows Part I, which shows that along-shelf winds in the opposite direction to coastal-trapped wave propagation (upwelling regime) force a strong up-valley flow caused by the formation of a lee wave. In contrast, along-shelf winds in the other direction (downwelling regime) do not generate a lee wave and consequently force a relatively weak net down-valley flow. The valley flows in both regimes are cyclostrophic with 0(1) Rossby number. A major difference between the two regimes is the along-shelf length scales of the along-valley flows L. In the upwelling regime Ls, depends on the valley width W, and the wavelength lambda(1w) of the coastal-trapped lee wave arrested by the along-shelf flow U-s. In the downwelling regime L depends on the inertial length scale U-s|'f and W-c. The along-valley velocity scale in the upwelling regime, given by V-u approximate to root pi H-c/H-s integral W-c lambda(1w)/2 pi L-x (1+L-x(2)/L-c(2))(-1) e(-(pi Wc)/(lambda 1w),) is based on potential vorticity (PV) conservation and lee-wave dynamics (Hs and H, are the shelf and valley depth scales, respectively, and fis the Coriolis parameter). The velocity scale in the downwelling regime, given by |v(d)| approximate to (H-s/H-s)[1 + (L-x(2)/L-x(2))](-1) fL, is based on PV conservation. The velocity scales are validated by the numerical sensitivity simulations and can be useful for observational studies of along -valley transports. The work provides a framework for investigating cross -shelf transport induced by irregular shelf bathymetry and calls for future studies of this type under realistic environmental conditions and over a broader parameter space.
    Description: Both WGZ and SJL were supported by the National Science Foundation (NSF) through Grant OCE 1154575.WGZis also supported by the NSF Grant OCE 1634965 and SJL by NSF Grant OCE 1558874.
    Description: 2018-10-16
    Keywords: Ocean circulation ; Topographic effects ; Upwelling/downwelling ; Waves, oceanic ; Wind stress ; Ocean models
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    The dynamics of shelf forcing in Greenlandic fjords (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 2799-2827, doi:10.1175/JPO-D-18-0057.1.
    Description: The fjords that connect Greenland’s glaciers to the ocean are gateways for importing heat to melt ice and for exporting meltwater into the ocean. The transport of heat and meltwater can be modulated by various drivers of fjord circulation, including freshwater, local winds, and shelf variability. Shelf-forced flows (also known as the intermediary circulation) are the dominant mode of variability in two major fjords of east Greenland, but we lack a dynamical understanding of the fjord’s response to shelf forcing. Building on observations from east Greenland, we use numerical simulations and analytical models to explore the dynamics of shelf-driven flows. For the parameter space of Greenlandic fjords, we find that the fjord’s response is primarily a function of three nondimensional parameters: the fjord width over the deformation radius (W/Rd), the forcing time scale over the fjord adjustment time scale, and the forcing amplitude (shelf pycnocline displacements) over the upper-layer thickness. The shelf-forced flows in both the numerical simulations and the observations can largely be explained by a simple analytical model for Kelvin waves propagating around the fjord. For fjords with W/Rd 〉 0.5 (most Greenlandic fjords), 3D dynamics are integral to understanding shelf forcing—the fjord dynamics cannot be approximated with 2D models that neglect cross-fjord structure. The volume flux exchanged between the fjord and shelf increases for narrow fjords and peaks around the resonant forcing frequency, dropping off significantly at higher- and lower-frequency forcing.
    Description: This work was funded by NSF Grant OCE-1536856 and by the NOAA Climate and Global Change Postdoctoral Fellowship.
    Keywords: Estuaries ; Glaciers ; Baroclinic flows ; Coastal flows ; Kelvin waves ; Regional models
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    On estimating the surface wind stress over the sea (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 1533-1541, doi:10.1175/JPO-D-17-0267.1.
    Description: Our study analyzes measurements primarily from two Floating Instrument Platform (FLIP) field programs and from the Air–Sea Interaction Tower (ASIT) site to examine the relationship between the wind and sea surface stress for contrasting conditions. The direct relationship of the surface momentum flux to U2 is found to be better posed than the relationship between and U, where U is the wind speed and is the friction velocity. Our datasets indicate that the stress magnitude often decreases significantly with height near the surface due to thin marine boundary layers and/or enhanced stress divergence close to the sea surface. Our study attempts to correct the surface stress estimated from traditional observational levels by using multiple observational levels near the surface and extrapolating to the surface. The effect of stability on the surface stress appears to be generally smaller than errors due to the stress divergence. Definite conclusions require more extensive measurements close to the sea surface.
    Description: This work was supported by the U.S. Office of Naval Research through Award N00014-16-1-2600. We
    Description: 2019-01-10
    Keywords: Atmosphere-ocean interaction ; Marine boundary layer
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    Influences of Pacific climate variability on decadal subsurface ocean heat content variations in the Indian Ocean (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 4157-4174, doi:10.1175/JCLI-D-17-0654.1.
    Description: Decadal variabilities in Indian Ocean subsurface ocean heat content (OHC; 50–300 m) since the 1950s are examined using ocean reanalyses. This study elaborates on how Pacific variability modulates the Indian Ocean on decadal time scales through both oceanic and atmospheric pathways. High correlations between OHC and thermocline depth variations across the entire Indian Ocean Basin suggest that OHC variability is primarily driven by thermocline fluctuations. The spatial pattern of the leading mode of decadal Indian Ocean OHC variability closely matches the regression pattern of OHC on the interdecadal Pacific oscillation (IPO), emphasizing the role of the Pacific Ocean in determining Indian Ocean OHC decadal variability. Further analyses identify different mechanisms by which the Pacific influences the eastern and western Indian Ocean. IPO-related anomalies from the Pacific propagate mainly through oceanic pathways in the Maritime Continent to impact the eastern Indian Ocean. By contrast, in the western Indian Ocean, the IPO induces wind-driven Ekman pumping in the central Indian Ocean via the atmospheric bridge, which in turn modifies conditions in the southwestern Indian Ocean via westward-propagating Rossby waves. To confirm this, a linear Rossby wave model is forced with wind stresses and eastern boundary conditions based on reanalyses. This linear model skillfully reproduces observed sea surface height anomalies and highlights both the oceanic connection in the eastern Indian Ocean and the role of wind-driven Ekman pumping in the west. These findings are also reproduced by OGCM hindcast experiments forced by interannual atmospheric boundary conditions applied only over the Pacific and Indian Oceans, respectively.
    Description: This research was supported by a scholarship from the China Scholarship Council (CSC) to X. J., a research fellowship by the Alexander von Humboldt Foundation to C. C. U., an NSF OCE PO Grant (OCE- 1242989) to Y.-O. K., the ONR Young Investigator Award (N00014-15-1-2588) to H. S., and a research grant from the Ministry of Science and Technology of the People’s Republic of China to Tsinghua University (2017YFA0603902).
    Description: 2018-10-30
    Keywords: Atmosphere-ocean interaction
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    A multivariate estimate of the cold season atmospheric response to North Pacific SST variability (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 2771-2796, doi:10.1175/JCLI-D-17-0061.1.
    Description: The Generalized Equilibrium Feedback Analysis (GEFA) is used to distinguish the influence of the Oyashio Extension (OE) and the Kuroshio Extension (KE) variability on the atmosphere from 1979 to 2014 from that of the main SST variability modes, using seasonal mean anomalies. Remote SST anomalies are associated with each single oceanic regressor, but the multivariate approach efficiently confines their SST footprints. In autumn [October–December (OND)], the OE meridional shifts are followed by a North Pacific Oscillation (NPO)-like signal. The OE influence is not investigated in winter [December–February (DJF)] because of multicollinearity, but a robust response with a strong signal over the Bering Sea is found in late winter/early spring [February–April (FMA)], a northeastward strengthening of the Aleutian low following a northward OE shift. A robust response to the KE variability is found in autumn, but not in winter and late winter when the KE SST footprint becomes increasingly small and noisy as regressors are added in GEFA. In autumn, a positive PDO is followed by a northward strengthening of the Aleutian low and a southward shift of the storm track in the central Pacific, reflecting the surface heat flux footprint in the central Pacific. In winter, the PDO shifts the maximum baroclinicity and storm track southward, the response strongly tilts westward with height in the North Pacific, and there is a negative NAO-like teleconnection. In late winter, the North Pacific NPO-like response to the PDO interferes negatively with the response to the OE and is only detected when the OE is represented in GEFA. A different PDO influence on the atmospheric circulation is found from 1958 to 1977.
    Description: This research has received funding from the European Union 7th Framework Program (FP7 2007-2013) under Grant Agreement 308299 (NACLIM) and from NSF Grants AGS CLD 1035423 and OCE PO 1242989.
    Keywords: Atmosphere-ocean interaction ; Boundary currents ; Pacific decadal oscillation ; Atmosphere-ocean interaction ; Empirical orthogonal functions ; Regression analysis
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    A global glacial ocean state estimate constrained by upper-ocean temperature proxies (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 8059-8079, doi:10.1175/JCLI-D-17-0769.1.
    Description: We use the method of least squares with Lagrange multipliers to fit an ocean general circulation model to the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) estimate of near sea surface temperature (NSST) at the Last Glacial Maximum (LGM; circa 23–19 thousand years ago). Compared to a modern simulation, the resulting global, last-glacial ocean state estimate, which fits the MARGO data within uncertainties in a free-running coupled ocean–sea ice simulation, has global-mean NSSTs that are 2°C lower and greater sea ice extent in all seasons in both the Northern and Southern Hemispheres. Increased brine rejection by sea ice formation in the Southern Ocean contributes to a stronger abyssal stratification set principally by salinity, qualitatively consistent with pore fluid measurements. The upper cell of the glacial Atlantic overturning circulation is deeper and stronger. Dye release experiments show similar distributions of Southern Ocean source waters in the glacial and modern western Atlantic, suggesting that LGM NSST data do not require a major reorganization of abyssal water masses. Outstanding challenges in reconstructing LGM ocean conditions include reducing effects from model biases and finding computationally efficient ways to incorporate abyssal tracers in global circulation inversions. Progress will be aided by the development of coupled ocean–atmosphere–ice inverse models, by improving high-latitude model processes that connect the upper and abyssal oceans, and by the collection of additional paleoclimate observations.
    Description: DEA was supported by a NSF Graduate Research Fellowship and NSF Grant OCE-1060735. OM acknowledges support from the NSF. GF was supported by NASA Award 1553749 and Simons Foundation Award 549931.
    Keywords: Ocean ; Abyssal circulation ; Sea surface temperature ; Paleoclimate ; Inverse methods ; Ocean models
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    Estuarine exchange flow is related to mixing through the salinity variance budget (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 1375-1384, doi:10.1175/JPO-D-17-0266.1.
    Description: The relationship between net mixing and the estuarine exchange flow may be quantified using a salinity variance budget. Here “mixing” is defined as the rate of destruction of volume-integrated salinity variance, and the exchange flow is quantified using the total exchange flow. These concepts are explored using an idealized 3D model estuary. It is shown that in steady state (e.g., averaging over the spring–neap cycle) the volume-integrated mixing is approximately given by Mixing ≅ SinSoutQr, where Sin and Sout are the representative salinities of in- and outflowing layers at the mouth and Qr is the river volume flux. This relationship provides an extension of the familiar Knudsen relation, in which the exchange flow is diagnosed based on knowledge of these same three quantities, quantitatively linking mixing to the exchange flow.
    Description: The work was supported by the National Science Foundation through Grants OCE-1736242 to PM and OCE-1736539 to WRG and by the German Research Foundation through Grants TRR 181 and GRK 2000 to HB.
    Keywords: Coastal flows ; Diapycnal mixing ; Ocean dynamics ; Streamflow ; Diagnostics ; Isopycnal coordinates
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    Structure and forcing of observed exchanges across the Greenland–Scotland Ridge (2018)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 9881-9901, doi:10.1175/JCLI-D-17-0889.1.
    Description: The Atlantic meridional overturning circulation and associated poleward heat transport are balanced by northern heat loss to the atmosphere and corresponding water-mass transformation. The circulation of northward-flowing Atlantic Water at the surface and returning overflow water at depth is particularly manifested—and observed—at the Greenland–Scotland Ridge where the water masses are guided through narrow straits. There is, however, a rich variability in the exchange of water masses across the ridge on all time scales. Focusing on seasonal and interannual time scales, and particularly the gateways of the Denmark Strait and between the Faroe Islands and Shetland, we specifically assess to what extent the exchanges of water masses across the Greenland–Scotland Ridge relate to wind forcing. On seasonal time scales, the variance explained of the observed exchanges can largely be related to large-scale wind patterns, and a conceptual model shows how this wind forcing can manifest via a barotropic, cyclonic circulation. On interannual time scales, the wind stress impact is less direct as baroclinic mechanisms gain importance and observations indicate a shift in the overflows from being more barotropically to more baroclinically forced during the observation period. Overall, the observed Greenland–Scotland Ridge exchanges reflect a horizontal (cyclonic) circulation on seasonal time scales, while the interannual variability more represents an overturning circulation.
    Description: This research was supported by the Research Council of Norway project NORTH (Grant 229763). Additional support for M. A. Spall was provided by National Science Foundation Grant OCE- 1558742, for T. Eldevik and S. Østerhus by the European Union’s Horizon 2020 research and innovation program project Blue-Action (Grant 727852), and for S. Østerhus by the European Framework Programs under Grant Agreement 308299 (NACLIM).
    Keywords: Ocean circulation ; Thermocline circulation ; Atmosphere-ocean interaction ; North Atlantic Oscillation ; Statistical techniques ; Time series
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    Surface cooling, winds, and eddies over the continental shelf (2018)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 879-894, doi:10.1175/JPO-D-16-0196.1.
    Description: Models show that surface cooling over a sloping continental shelf should give rise to baroclinic instability and thus tend toward gravitationally stable density stratification. Less is known about how alongshore winds affect this process, so the role of surface momentum input is treated here by means of a sequence of idealized, primitive equation numerical model calculations. The effects of cooling rate, wind amplitude and direction, bottom slope, bottom friction, and rotation rate are all considered. All model runs lead to instability and an eddy field. While instability is not strongly affected by upwelling-favorable alongshore winds, wind-driven downwelling substantially reduces eddy kinetic energy, largely because the downwelling circulation plays a similar role to baroclinic instability by flattening isotherms and so reducing available potential energy. Not surprisingly, cross-shelf winds appear to have little effect. Analysis of the model runs leads to quantitative relations for the wind effect on eddy kinetic energy for the equilibrium density stratification (which increases as the cooling rate increases) and for eddy length scale.
    Description: This research was supported by the National Science Foundation Physical Oceanography Program through Grant OCE-1433953.
    Keywords: Continental shelf/slope ; Baroclinic flows ; Eddies ; Instability
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    Rossby waves with continuous stratification and bottom friction (2018)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 2209-2219, doi:10.1175/JPO-D-18-0070.1.
    Description: Published observations of subinertial ocean current variability show that the vertical structure is often well described by a vertical mode that has a node of horizontal velocity at the bottom rather than the traditional node of vertical velocity. The theory of forced and free linear Rossby waves in a continuously stratified ocean with a sloping bottom and bottom friction is treated here to see if frictional effects can plausibly contribute to this phenomenon. For parameter values representative of the mesoscale, bottom dissipation by itself appears to be too weak to be an explanation, although caution is required because the present approach uses a linear model to address a nonlinear phenomenon. One novel outcome is the emergence of a short-wave, bottom-trapped, strongly damped mode that is present even with a flat bottom.
    Description: Partial funding for this article is provided by the National Science Foundation Physical Oceanography section through Award OCE-1433953.
    Description: 2019-03-17
    Keywords: Baroclinic flows ; Ekman pumping/transport ; Rossby waves
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    The continuum of drought in southwestern North America (2018)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 8627-8643, doi:10.1175/JCLI-D-18-0010.1.
    Description: Drought has severe consequences for humans and their environment, yet we have a limited understanding of the drivers of drought across the full range of time scales on which it occurs. Here, the atmosphere and ocean conditions that drive this continuum of drought variability in southwestern North America (SWNA) are studied using the latest observationally based products, paleoclimate reconstructions, and state-of-the-art Earth system model simulations of the last millennium. A novel application of the self-organizing maps (SOM) methodology allows for a visualization of the continuum of climate states coinciding with thousands of droughts of varying lengths in last millennium simulations from the Community Earth System Model (CESM), the Goddard Institute for Space Studies Model E2-R (GISS E2-R), and eight other members from phase 5 of the Coupled Model Intercomparison Project (CMIP5). It is found that most droughts are associated with a cool Pacific decadal oscillation (PDO) pattern, but persistent droughts can coincide with a variety of ocean–atmosphere states, including time periods showing a warm PDO or weak ocean–atmosphere anomalies. Many CMIP5 models simulate similar SWNA teleconnection patterns, but the SOM analysis demonstrates that models simulate different continuums of ocean–atmosphere states coinciding with droughts of different lengths, suggesting fundamental differences in their drought dynamics. These findings have important implications for our understanding and simulation of the drivers of persistent drought, and for their potential predictability.
    Description: The National Science Foundation EaSM2 Grant (AGS1243125) supported this work.
    Keywords: Atmosphere-ocean interaction ; Drought ; Paleoclimate ; Ensembles ; General circulation models ; Climate variability
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    High-frequency variability in the circulation and hydrography of the Denmark Strait Overflow from a high-resolution numerical model (2018)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 2999-3013, doi:10.1175/JPO-D-17-0129.1.
    Description: Initial results are presented from a yearlong, high-resolution (~2 km) numerical simulation covering the east Greenland shelf and the Iceland and Irminger Seas. The model hydrography and circulation in the vicinity of Denmark Strait show good agreement with available observational datasets. This study focuses on the variability of the Denmark Strait overflow (DSO) by detecting and characterizing boluses and pulses, which are the two dominant mesoscale features in the strait. The authors estimate that the yearly mean southward volume flux of the DSO is about 30% greater in the presence of boluses and pulses. On average, boluses (pulses) are 57.1 (27.5) h long, occur every 3.2 (5.5) days, and are more frequent during the summer (winter). Boluses (pulses) increase (decrease) the overflow cross-sectional area, and temperatures around the overflow interface are colder (warmer) by about 2.6°C (1.8°C). The lateral extent of the boluses is much greater than that of the pulses. In both cases the along-strait equatorward flow of dense water is enhanced but more so for pulses. The sea surface height (SSH) rises by 4–10 cm during boluses and by up to 5 cm during pulses. The SSH anomaly contours form a bowl (dome) during boluses (pulses), and the two features cross the strait with a slightly different orientation. The cross streamflow changes direction; boluses (pulses) are associated with veering (backing) of the horizontal current. The model indicates that boluses and pulses play a major role in controlling the variability of the DSO transport into the Irminger Sea.
    Description: This work was supported by the NSF Grants OCE-1433448, OCE-1633124, and OCE- 1259618 and the Institute for Data Intensive Engineering and Science (IDIES) seed grant funding.
    Description: 2018-06-13
    Keywords: North Atlantic Ocean ; Mesoscale processes ; Ocean models ; Regional models
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    Recent wind-driven variability in Atlantic water mass distribution and meridional overturning circulation (2017)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 633-647, doi:10.1175/JPO-D-16-0089.1.
    Description: Interannual variability in the volumetric water mass distribution within the North Atlantic Subtropical Gyre is described in relation to variability in the Atlantic meridional overturning circulation. The relative roles of diabatic and adiabatic processes in the volume and heat budgets of the subtropical gyre are investigated by projecting data into temperature coordinates as volumes of water using an Argo-based climatology and an ocean state estimate (ECCO version 4). This highlights that variations in the subtropical gyre volume budget are predominantly set by transport divergence in the gyre. A strong correlation between the volume anomaly due to transport divergence and the variability of both thermocline depth and Ekman pumping over the gyre suggests that wind-driven heave drives transport anomalies at the gyre boundaries. This wind-driven heaving contributes significantly to variations in the heat content of the gyre, as do anomalies in the air–sea fluxes. The analysis presented suggests that wind forcing plays an important role in driving interannual variability in the Atlantic meridional overturning circulation and that this variability can be unraveled from spatially distributed hydrographic observations using the framework presented here.
    Description: DGE was supported by a Natural Environment Research Council studentship award at the University of Southampton. JMT’s contribution was supported by the U.S. National Science Foundation (Grant OCE-1332667). GF’s contribution was supported by the U.S. National Science Foundation through Grant OCE-0961713 and by the U.S. National Oceanic and Atmospheric Administration through Grant NA10OAR4310135. The contributions of JDZ and AJGN were supported by the NERC Grant ‘‘Climate scale analysis of air and water masses’’ (NE/ K012932/1). ACNG gratefully acknowledges support from the Leverhulme Trust, the Royal Society, and the Wolfson Foundation. LY was supported by NASA Ocean Vector Wind Science Team (OVWST) activities under Grant NNA10AO86G.
    Keywords: North Atlantic Ocean ; Atmosphere-ocean interaction ; Ekman pumping/transport ; Ocean circulation ; Water masses ; Inverse methods
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    Total exchange flow, entrainment, and diffusive salt flux in estuaries (2017)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 1205-1220, doi:10.1175/JPO-D-16-0258.1.
    Description: The linkage among total exchange flow, entrainment, and diffusive salt flux in estuaries is derived analytically using salinity coordinates, revealing the simple but important relationship between total exchange flow and mixing. Mixing is defined and quantified in this paper as the dissipation of salinity variance. The method uses the conservation of volume and salt to quantify and distinguish the diahaline transport of volume (i.e., entrainment) and diahaline diffusive salt flux. A numerical model of the Hudson estuary is used as an example of the application of the method in a realistic estuary with a persistent but temporally variable exchange flow. A notable finding of this analysis is that the total exchange flow and diahaline salt flux are out of phase with respect to the spring–neap cycle. Total exchange flow reaches its maximum near minimum neap tide, but diahaline salt transport reaches its maximum during the maximum spring tide. This phase shift explains the strong temporal variation of stratification and estuarine salt content through the spring–neap cycle. In addition to quantifying temporal variation, the method reveals the spatial variation of total exchange flow, entrainment, and diffusive salt flux through the estuary. For instance, the analysis of the Hudson estuary indicates that diffusive salt flux is intensified in the wider cross sections. The method also provides a simple means of quantifying numerical mixing in ocean models because it provides an estimate of the total dissipation of salinity variance, which is the sum of mixing due to the turbulence closure and numerical mixing.
    Description: T. Wang was supported by the Open Research Fund of State Key Laboratory of Estuarine and Coastal Research (Grant SKLEC-KF201509), the Fundamental Research Funds for the Central Universities (Grant 2017B03514), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDA11010203). W. R. Geyer was supported by NSF Grant OCE 0926427 and ONR Grant N00014-16-1-2948. P. MacCready was supported by NSF Grant OCE-1634148.
    Description: 2017-09-14
    Keywords: Baroclinic flows ; Conservation equations ; Diapycnal mixing ; Diffusion ; Entrainment ; Mixing
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    Spatial patterns and intensity of the surface storm tracks in CMIP5 models (2017)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 4965-4981, doi:10.1175/JCLI-D-16-0228.1.
    Description: To improve the understanding of storm tracks and western boundary current (WBC) interactions, surface storm tracks in 12 CMIP5 models are examined against ERA-Interim. All models capture an equatorward displacement toward the WBCs in the locations of the surface storm tracks’ maxima relative to those at 850 hPa. An estimated storm-track metric is developed to analyze the location of the surface storm track. It shows that the equatorward shift is influenced by both the lower-tropospheric instability and the baroclinicity. Basin-scale spatial correlations between models and ERA-Interim for the storm tracks, near-surface stability, SST gradient, and baroclinicity are calculated to test the ability of the GCMs’ match reanalysis. An intermodel comparison of the spatial correlations suggests that differences (relative to ERA-Interim) in the position of the storm track aloft have the strongest influence on differences in the surface storm-track position. However, in the North Atlantic, biases in the surface storm track north of the Gulf Stream are related to biases in the SST. An analysis of the strength of the storm tracks shows that most models generate a weaker storm track at the surface than 850 hPa, consistent with observations, although some outliers are found. A linear relationship exists among the models between storm-track amplitudes at 500 and 850 hPa, but not between 850 hPa and the surface. In total, the work reveals a dual role in forcing the surface storm track from aloft and from the ocean surface in CMIP5 models, with the atmosphere having the larger relative influence.
    Description: JFB was partially supported by the NOAA Climate Program Office’s Modeling, Analysis, Predictions, and Projections program (Grant NA15OAR4310094). Y-OK was supported by NSF Division of Atmospheric and Geospace Science Climate and Large-scale Dynamics Program (AGS-1355339), NASA Physical Oceanography Program (NNX13AM59G), and DOE Office of Biological and Environmental Research Regional and Global Climate Modeling Program (DE-SC0014433). RJS was supported by DOE Office of Biological and Environmental Research (DE-SC0006743) and NSF Directorate for Geosciences Division of Ocean Sciences (1419584),
    Description: 2017-10-03
    Keywords: Atmosphere-ocean interaction ; Storm tracks
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    Surface wave effects on the translation of wind stress across the air–sea interface in a fetch-limited, coastal embayment (2017)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 1921-1939, doi:10.1175/JPO-D-16-0146.1.
    Description: The role of surface gravity waves in structuring the air–sea momentum flux is examined in the middle reaches of Chesapeake Bay. Observed wave spectra showed that wave direction in Chesapeake Bay is strongly correlated with basin geometry. Waves preferentially developed in the direction of maximum fetch, suggesting that dominant wave frequencies may be commonly and persistently misaligned with local wind forcing. Direct observations from an ultrasonic anemometer and vertical array of ADVs show that the magnitude and direction of stress changed across the air–sea interface, suggesting that a stress divergence occurred at or near the water surface. Using a numerical wave model in combination with direct flux measurements, the air–sea momentum flux was partitioned between the surface wave field and the mean flow. Results indicate that the surface wave field can store or release a significant fraction of the total momentum flux depending on the direction of the wind. When wind blew across dominant fetch axes, the generation of short gravity waves stored as much as 40% of the total wind stress. Accounting for the storage of momentum in the surface wave field closed the air–sea momentum budget. Agreement between the direction of Lagrangian shear and the direction of the stress vector in the mixed surface layer suggests that the observed directional difference was due to the combined effect of breaking waves producing downward sweeps of momentum in the direction of wave propagation and the straining of that vorticity field in a manner similar to Langmuir turbulence.
    Description: This work was supported by National Science Foundation Grants OCE-1061609 and OCE-1339032.
    Description: 2018-01-13
    Keywords: Atmosphere-ocean interaction ; Coastal flows ; Mixing ; Momentum ; Wind stress ; Wind waves
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    Dynamics and energetics of trapped diurnal internal Kelvin waves around a midlatitude lsland (2017)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 2479-2498, doi:10.1175/JPO-D-16-0167.1.
    Description: The generation of trapped and radiating internal tides around Izu‐Oshima Island located off Sagami Bay, Japan, is investigated using the three-dimensional Stanford Unstructured Nonhydrostatic Terrain-following Adaptive Navier–Stokes Simulator (SUNTANS) that is validated with observations of isotherm displacements in shallow water. The model is forced by barotropic tides, which generate strong baroclinic internal tides in the study region. Model results showed that when diurnal K1 barotropic tides dominate, resonance of a trapped internal Kelvin wave leads to large-amplitude internal tides in shallow waters on the coast. This resonance produces diurnal motions that are much stronger than the semidiurnal motions. The weaker, freely propagating, semidiurnal internal tides are generated on the western side of the island, where the M2 internal tide beam angle matches the topographic slope. The internal wave energy flux due to the diurnal internal tides is much higher than that of the semidiurnal tides in the study region. Although the diurnal internal tide energy is trapped, this study shows that steepening of the Kelvin waves produces high-frequency internal tides that radiate from the island, thus acting as a mechanism to extract energy from the diurnal motions.
    Description: This study was supported by JST CREST Grant Number JPRMJCR12A6.
    Description: 2018-04-12
    Keywords: Pacific Ocean ; Internal waves ; Kelvin waves ; In situ oceanic observations ; Baroclinic models ; Ocean models
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    Turbulent mixing in a deep fracture zone on the Mid-Atlantic Ridge (2017)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 1873-1896, doi:10.1175/JPO-D-16-0264.1.
    Description: Midocean ridge fracture zones channel bottom waters in the eastern Brazil Basin in regions of intensified deep mixing. The mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones, whether affected by the local or the nonlocal canyon topography, are still subject to debate. To discriminate those mechanisms and to discern the canyon mean flow, two moorings sampled a deep canyon over and away from a sill/contraction. A 2-layer exchange flow, accelerated at the sill, transports 0.04–0.10-Sv (1 Sv ≡ 106 m3 s−1) up canyon in the deep layer. At the sill, the dissipation rate of turbulent kinetic energy ε increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy. Cross-sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides. During spring to neap tides, ε varies from O(10−9) to O(10−10) W kg−1 below 3500 m around the 2-layer interface. The detection of temperature overturns during tidal flow reversal, which almost fully opposes the deep up-canyon mean flow, confirms the canyon middepth enhancement of ε. The internal tide energy flux, particularly enhanced at the sill, compares with the lower-layer energy loss across the sill. Throughout the canyon away from the sill, near-inertial waves with downward-propagating energy dominate the internal wave field. The present study underlines the intricate pattern of the deep turbulent mixing affected by the mean flow, internal tides, and near-inertial waves.
    Description: The DoMORE project was supported by NSF under the Grant OCE-1235094.
    Description: 2018-01-13
    Keywords: Abyssal circulation ; Bottom currents/bottom water ; Diapycnal mixing
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    Oblique internal hydraulic jumps at a stratified estuary mouth (2017)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 85-100, doi:10.1175/JPO-D-15-0234.1.
    Description: Observations and analyses of two tidally recurring, oblique, internal hydraulic jumps at a stratified estuary mouth (Columbia River, Oregon/Washington) are presented. These hydraulic features have not previously been studied due to the challenges of both horizontally resolving the sharp gradients and temporally resolving their evolution in numerical models and traditional observation platforms. The jumps, both of which recurred during ebb, formed adjacent to two engineered lateral channel constrictions and were identified in marine radar image time series. Jump occurrence was corroborated by (i) a collocated sharp gradient in the surface currents measured via airborne along-track interferometric synthetic aperture radar and (ii) the transition from supercritical to subcritical flow in the cross-jump direction via shipborne velocity and density measurements. Using a two-layer approximation, observed jump angles at both lateral constrictions are shown to lie within the theoretical bounds given by the critical internal long-wave (Froude) angle and the arrested maximum-amplitude internal bore angle, respectively. Also, intratidal and intertidal variability of the jump angles are shown to be consistent with that expected from the two-layer model, applied to varying stratification and current speed over a range of tidal and river discharge conditions. Intratidal variability of the upchannel jump angle is similar under all observed conditions, whereas the downchannel jump angle shows an additional association with stratification and ebb velocity during the low discharge periods. The observations additionally indicate that the upchannel jump achieves a stable position that is collocated with a similarly oblique bathymetric slope.
    Description: We acknowledge the financial support of the Office of Naval Research under Awards N00014-10-1-0932 and N00014-13-1-0364.
    Description: 2017-07-04
    Keywords: Estuaries ; Baroclinic flows ; Internal waves ; Microwave observations ; Remote sensing
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    On the predominant nonlinear response of the extratropical atmosphere to meridional shifts of the Gulf Stream (2017)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 9679-9702, doi:10.1175/JCLI-D-16-0707.1.
    Description: The North Atlantic atmospheric circulation response to the meridional shifts of the Gulf Stream (GS) path is examined using a large ensemble of high-resolution hemispheric-scale Weather Research and Forecasting Model simulations. The model is forced with a broad range of wintertime sea surface temperature (SST) anomalies derived from a lag regression on a GS index. The primary result of the model experiments, supported in part by an independent analysis of a reanalysis dataset, is that the large-scale quasi-steady North Atlantic circulation response is remarkably nonlinear about the sign and amplitude of the SST anomaly chosen over a wide range of GS shift scenarios. The nonlinear response prevails over the weak linear response and resembles the negative North Atlantic Oscillation (NAO), the leading intrinsic mode of variability in the model and the observations. Further analysis of the associated dynamics reveals that the nonlinear responses are accompanied by the shift of the North Atlantic eddy-driven jet, which is reinforced, with nearly equal importance, by the high-frequency transient eddy feedback and the low-frequency wave-breaking events. Additional sensitivity simulations confirm that the nonlinearity of the circulation response is a robust feature found over the broad parameter space encompassing not only the varied SST but also the absence/presence of tropical influence, the varying lateral boundary conditions, and the initialization scheme. The result highlights the fundamental importance of the intrinsically nonlinear transient eddy dynamics and the eddy–mean flow interactions in generating the nonlinear downstream response to the meridional shifts in the Gulf Stream.
    Description: The authors are grateful for the support from NASA (NNX13AM59G) and the NSF (AGS-1355339, OCE-1419235).
    Description: 2018-05-07
    Keywords: North Atlantic Ocean ; Blocking ; North Atlantic Oscillation ; Atmosphere-ocean interaction ; Regional models ; Climate variability
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    Annual and semiannual cycle of equatorial Atlantic circulation associated with basin-mode resonance (2016)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 3011-3029, doi:10.1175/JPO-D-15-0248.1.
    Description: Seasonal variability of the tropical Atlantic circulation is dominated by the annual cycle, but semiannual variability is also pronounced, despite weak forcing at that period. This study uses multiyear, full-depth velocity measurements from the central equatorial Atlantic to analyze the vertical structure of annual and semiannual variations of zonal velocity. A baroclinic modal decomposition finds that the annual cycle is dominated by the fourth mode and the semiannual cycle is dominated by the second mode. Similar local behavior is found in a high-resolution general circulation model. This simulation reveals that the annual and semiannual cycles of the respective dominant baroclinic modes are associated with characteristic basinwide structures. Using an idealized, linear, reduced-gravity model to simulate the dynamics of individual baroclinic modes, it is shown that the observed circulation variability can be explained by resonant equatorial basin modes. Corollary simulations of the reduced-gravity model with varying basin geometry (i.e., square basin vs realistic coastlines) or forcing (i.e., spatially uniform vs spatially variable wind) show a structural robustness of the simulated basin modes. A main focus of this study is the seasonal variability of the Equatorial Undercurrent (EUC) as identified in recent observational studies. Main characteristics of the observed EUC including seasonal variability of transport, core depth, and maximum core velocity can be explained by the linear superposition of the dominant equatorial basin modes as obtained from the reduced-gravity model.
    Description: This study was supported by the Deutsche Forschungsgemeinschaft as part of the Sonderforschungsbereich 754 (SFB754) ‘‘Climate–Biogeochemistry Interactions in the Tropical Ocean’’ and through several research cruises with R/V Meteor, R/V Maria S. Merian, andR/VL’Atalante by the German Federal Ministry of Education and Research as part of the cooperative projects RACE (03F0605B) and SACUS (03G0837A) and by European Union 7th Framework Programme (FP7 2007–13) under Grant Agreement 603521 PREFACE project.
    Keywords: Atlantic Ocean ; Ocean circulation ; In situ oceanic observations ; Ocean models ; Seasonal cycle ; Tropical variability
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    Wind sea behind a cold front and deep ocean acoustics (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 1705-1716, doi:10.1175/JPO-D-15-0221.1.
    Description: A rapid and broadband (1 h, 1 〈 f 〈 400 Hz) increase in pressure and vertical velocity on the deep ocean floor was observed on seven instruments comprising a 20-km array in the northeastern subtropical Pacific. The authors associate the jump with the passage of a cold front and focus on the 4- and 400-Hz spectra. At every station, the time of the jump is consistent with the front coming from the northwest. The apparent rate of progress, 10–20 km h−1 (2.8–5.6 m s−1), agrees with meteorological observations. The acoustic radiation below the front is modeled as arising from a moving half-plane of uncorrelated acoustic dipoles. The half-plane is preceded by a 10-km transition zone, over which the radiator strength increases linearly from zero. With this model, the time derivative of the jump at a station yields a second and independent estimate of the front’s speed, 8.5 km h−1 (2.4 m s−1). For the 4-Hz spectra, the source physics is taken to be Longuet-Higgins radiation. Its strength depends on the quantity , where Fζ is the wave amplitude power spectrum and I the overlap integral. Thus, the 1-h time constant observed in the bottom data implies a similar time constant for the growth of the wave field quantity behind the front. The spectra at 400 Hz have a similar time constant, but the jump occurs 25 min later. The implications of this difference for the source physics are uncertain.
    Description: The OBSANP cruise was funded by the Office of Naval Research under Grants N00014-10-1-0987, N00014-14-1-0324, N00014-10-1-0510, and N00014-10-1-0990.
    Keywords: Atm/Ocean Structure/ Phenomena ; Atmosphere-ocean interaction ; Cold fronts ; Marine boundary layer ; Sea state
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    Can Australian multiyear droughts and wet spells be generated in the absence of oceanic variability? (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 29 (2016): 6201-6221, doi:10.1175/JCLI-D-15-0694.1.
    Description: Anomalous conditions in the tropical oceans, such as those related to El Niño–Southern Oscillation and the Indian Ocean dipole, have been previously blamed for extended droughts and wet periods in Australia. Yet the extent to which Australian wet and dry spells can be driven by internal atmospheric variability remains unclear. Natural variability experiments are examined to determine whether prolonged extreme wet and dry periods can arise from internal atmospheric and land variability alone. Results reveal that this is indeed the case; however, these dry and wet events are found to be less severe than in simulations incorporating coupled oceanic variability. Overall, ocean feedback processes increase the magnitude of Australian rainfall variability by about 30% and give rise to more spatially coherent rainfall impacts. Over mainland Australia, ocean interactions lead to more frequent extreme events, particularly during the rainy season. Over Tasmania, in contrast, ocean–atmosphere coupling increases mean rainfall throughout the year. While ocean variability makes Australian rainfall anomalies more severe, droughts and wet spells of duration longer than three years are equally likely to occur in both atmospheric- and ocean-driven simulations. Moreover, they are essentially indistinguishable from what one expects from a Gaussian white noise distribution. Internal atmosphere–land-driven megadroughts and megapluvials that last as long as ocean-driven events are also identified in the simulations. This suggests that oceanic variability may be less important than previously assumed for the long-term persistence of Australian rainfall anomalies. This poses a challenge to accurate prediction of long-term dry and wet spells for Australia.
    Description: This study was supported by the Australian Research Council (ARC) under ARC-DP1094784, ARC-DP-150101331, ARC-FL100100214, and funding for C.C.U. from the National Science Foundation under AGS-1602455 and the ARC Centre of Excellence for Climate System Science.
    Description: 2017-02-19
    Keywords: Circulation/ Dynamics ; Atmosphere-ocean interaction ; Atm/Ocean Structure/ Phenomena ; Drought ; Precipitation ; Physical Meteorology and Climatology ; Climate variability ; Forecasting ; Climate prediction ; Variability
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    Enhanced diapycnal diffusivity in intrusive regions of the Drake Passage (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 1309-1321, doi:10.1175/JPO-D-15-0068.1.
    Description: Direct measurements of oceanic turbulent parameters were taken upstream of and across Drake Passage, in the region of the Subantarctic and Polar Fronts. Values of turbulent kinetic energy dissipation rate ε estimated by microstructure are up to two orders of magnitude lower than previously published estimates in the upper 1000 m. Turbulence levels in Drake Passage are systematically higher than values upstream, regardless of season. The dissipation of thermal variance χ is enhanced at middepth throughout the surveys, with the highest values found in northern Drake Passage, where water mass variability is the most pronounced. Using the density ratio, evidence for double-diffusive instability is presented. Subject to double-diffusive physics, the estimates of diffusivity using the Osborn–Cox method are larger than ensemble statistics based on ε and the buoyancy frequency.
    Description: This work was supported by grants from the U.S. National Science Foundation.
    Description: 2016-10-05
    Keywords: Geographic location/entity ; Southern Ocean ; Circulation/ Dynamics ; Diapycnal mixing ; Mixing ; Turbulence ; Atm/Ocean Structure/ Phenomena ; Fronts ; Observational techniques and algorithms ; Profilers, oceanic
    Repository Name: Woods Hole Open Access Server
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    Eddy–wind interaction in the California Current System : dynamics and impacts (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 439-459, doi:10.1175/JPO-D-15-0086.1.
    Description: The summertime California Current System (CCS) is characterized by energetic mesoscale eddies, whose sea surface temperature (SST) and surface current can significantly modify the wind stress and Ekman pumping. Relative importance of the eddy–wind interactions via SST and surface current in the CCS is examined using a high-resolution (7 km) regional coupled model with a novel coupling approach to isolate the small-scale air–sea coupling by SST and surface current. Results show that when the eddy-induced surface current is allowed to modify the wind stress, the spatially averaged surface eddy kinetic energy (EKE) is reduced by 42%, and this is primarily due to enhanced surface eddy drag and reduced wind energy transfer. In contrast, the eddy-induced SST–wind coupling has no significant impact on the EKE. Furthermore, eddy-induced SST and surface current modify the Ekman pumping via their crosswind SST gradient and surface vorticity gradient, respectively. The resultant magnitudes of the Ekman pumping velocity are comparable, but the implied feedback effects on the eddy statistics are different. The surface current-induced Ekman pumping mainly attenuates the amplitude of cyclonic and anticyclonic eddies, acting to reduce the eddy activity, while the SST-induced Ekman pumping primarily affects the propagation. Time mean–rectified change in SST is determined by the altered offshore temperature advection by the mean and eddy currents, but the magnitude of the mean SST change is greater with the eddy-induced current effect. The demonstrated remarkably strong dynamical response in the CCS system to the eddy-induced current–wind coupling indicates that eddy-induced current should play an important role in the regional coupled ocean–atmosphere system.
    Description: We thank NSF for support under GrantsOCE-0960770,OCE-1419235, andOCE-1419306. HS is grateful for the WHOI internal support from the Andrew W. Mellon Foundation Awards for Innovative Research and the additional support from the ONR We thank NSF for support under GrantsOCE-0960770,OCE-1419235, andOCE-1419306. HS is grateful for the WHOI internal support from the Andrew W. Mellon Foundation Awards for Innovative Research and the additional support from the ONR
    Description: 2016-05-30
    Keywords: Atm/Ocean Structure/ Phenomena ; Atmosphere-ocean interaction ; Ekman pumping ; Models and modeling ; Ocean models ; Regional models
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    Satellite-derived ocean thermal structure for the North Atlantic hurricane season (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Monthly Weather Review 144 (2016): 877-896, doi:10.1175/MWR-D-15-0275.1.
    Description: This paper describes a new model (method) called Satellite-derived North Atlantic Profiles (SNAP) that seeks to provide a high-resolution, near-real-time ocean thermal field to aid tropical cyclone (TC) forecasting. Using about 139 000 observed temperature profiles, a spatially dependent regression model is developed for the North Atlantic Ocean during hurricane season. A new step introduced in this work is that the daily mixed layer depth is derived from the output of a one-dimensional Price–Weller–Pinkel ocean mixed layer model with time-dependent surface forcing. The accuracy of SNAP is assessed by comparison to 19 076 independent Argo profiles from the hurricane seasons of 2011 and 2013. The rms differences of the SNAP-estimated isotherm depths are found to be 10–25 m for upper thermocline isotherms (29°–19°C), 35–55 m for middle isotherms (18°–7°C), and 60–100 m for lower isotherms (6°–4°C). The primary error sources include uncertainty of sea surface height anomaly (SSHA), high-frequency fluctuations of isotherm depths, salinity effects, and the barotropic component of SSHA. These account for roughly 29%, 25%, 19%, and 10% of the estimation error, respectively. The rms differences of TC-related ocean parameters, upper-ocean heat content, and averaged temperature of the upper 100 m, are ~10 kJ cm−2 and ~0.8°C, respectively, over the North Atlantic basin. These errors are typical also of the open ocean underlying the majority of TC tracks. Errors are somewhat larger over regions of greatest mesoscale variability (i.e., the Gulf Stream and the Loop Current within the Gulf of Mexico).
    Description: IFP is supported by Grants NSC 101-2628-M-002-001-MY4 and MOST 103-2111-M-002 -002 -MY3. JFP and SRJ were supported by the U.S. Office of Naval Research under the project “Impact of Typhoons on the North Pacific, ITOP.”
    Description: 2016-06-08
    Keywords: Atm/Ocean Structure/ Phenomena ; Atmosphere-ocean interaction ; Oceanic mixed layer ; Tropical cyclones ; Observational techniques and algorithms ; Satellite observations
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    Resource allocation for Lagrangian tracking (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Atmospheric and Oceanic Technology 33 (2016): 1225-1235, doi:10.1175/JTECH-D-15-0115.1.
    Description: Accurate estimation of the transport probabilities among regions in the ocean provides valuable information for understanding plankton transport, the spread of pollutants, and the movement of water masses. Individual-based particle-tracking models simulate a large ensemble of Lagrangian particles and are a common method to estimate these transport probabilities. Simulating a large ensemble of Lagrangian particles is computationally expensive, and appropriately allocating resources can reduce the cost of this method. Two universal questions in the design of studies that use Lagrangian particle tracking are how many particles to release and how to distribute particle releases. A method is presented for tailoring the number and the release location of particles to most effectively achieve the objectives of a study. The method detailed here is a sequential analysis procedure that seeks to minimize the number of particles that are required to satisfy a predefined metric of result quality. The study assesses the result quality as the precision of the estimates for the elements of a transport matrix and also describes how the method may be extended for use with other metrics. Applying this methodology to both a theoretical system and a particle transport model of the Gulf of Maine results in more precise estimates of the transport probabilities with fewer particles than from uniformly or randomly distributing particle releases. The application of this method can help reduce the cost of and increase the robustness of results from studies that use Lagrangian particles.
    Description: This research was supported by the Department of Defense (DoD) through the National Defense Science and Engineering Graduate Fellowship (NDSEG) program and the National Science Foundation through Grant OCE-1459133 and Grant OCE-1031256.
    Description: 2016-12-02
    Keywords: Circulation/ Dynamics ; Lagrangian circulation/transport ; Models and modeling ; Model evaluation/performance ; Ocean models ; Tracers
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    Estuarine frontogenesis (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 546–561, doi:10.1175/JPO-D-14-0082.1.
    Description: Model studies and observations in the Hudson River estuary indicate that frontogenesis occurs as a result of topographic forcing. Bottom fronts form just downstream of lateral constrictions, where the width of the estuary increases in the down-estuary (i.e., seaward) direction. The front forms during the last several hours of the ebb, when the combination of adverse pressure gradient in the expansion and baroclinicity cause a stagnation of near-bottom velocity. Frontogenesis is observed in two dynamical regimes: one in which the front develops at a transition from subcritical to supercritical flow and the other in which the flow is everywhere supercritical. The supercritical front formation appears to be associated with lateral flow separation. Both types of fronts are three-dimensional, with strong lateral gradients along the flanks of the channel. During spring tide conditions, the fronts dissipate during the flood, whereas during neap tides the fronts are advected landward during the flood. The zone of enhanced density gradient initiates frontogenesis at multiple constrictions along the estuary as it propagates landward more than 60 km during several days of neap tides. Frontogenesis and frontal propagation may thus be essential elements of the spring-to-neap transition to stratified conditions in partially mixed estuaries.
    Description: Support for this research was provided by NSF Grant OCE 0926427.
    Description: 2015-08-01
    Keywords: Circulation/ Dynamics ; Baroclinic flows ; Coastal flows ; Frontogenesis/frontolysis ; Fronts
    Repository Name: Woods Hole Open Access Server
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    Formation of intrathermocline lenses by eddy–wind interaction (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 606–612, doi:10.1175/JPO-D-14-0221.1.
    Description: Mesoscale intrathermocline lenses are observed throughout the World Ocean and are commonly attributed to water mass anomalies advected from a distant origin. An alternative mechanism of local generation is offered herein, in which eddy–wind interaction can create lens-shaped disturbances in the thermocline. Numerical simulations illustrate how eddy–wind-driven upwelling in anticyclones can yield a convex lens reminiscent of a mode water eddy, whereas eddy–wind-driven downwelling in cyclones produces a concave lens that thins the mode water layer (a cyclonic “thinny”). Such transformations should be observable with long-term time series in the interiors of mesoscale eddies.
    Description: Support of this research by the National Science Foundation and National Aeronautics and Space Administration is gratefully acknowledged.
    Description: 2015-08-01
    Keywords: Circulation/ Dynamics ; Eddies ; Ekman pumping/transport ; Mesoscale processes ; Models and modeling ; Ocean models ; Primitive equations model
    Repository Name: Woods Hole Open Access Server
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    Investigating the local atmospheric response to a realistic shift in the Oyashio Sea surface Temperature Front (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 28 (2015): 1126–1147, doi:10.1175/JCLI-D-14-00285.1.
    Description: The local atmospheric response to a realistic shift of the Oyashio Extension SST front in the western North Pacific is analyzed using a high-resolution (HR; 0.25°) version of the global Community Atmosphere Model, version 5 (CAM5). A northward shift in the SST front causes an atmospheric response consisting of a weak surface wind anomaly but a strong vertical circulation extending throughout the troposphere. In the lower troposphere, most of the SST anomaly–induced diabatic heating is balanced by poleward transient eddy heat and moisture fluxes. Collectively, this response differs from the circulation suggested by linear dynamics, where extratropical SST forcing produces shallow anomalous heating balanced by strong equatorward cold air advection driven by an anomalous, stationary surface low to the east. This latter response, however, is obtained by repeating the same experiment except using a relatively low-resolution (LR; 1°) version of CAM5. Comparison to observations suggests that the HR response is closer to nature than the LR response. Strikingly, HR and LR experiments have almost identical vertical profiles of . However, diagnosis of the diabatic quasigeostrophic vertical pressure velocity (ω) budget reveals that HR has a substantially stronger response, which together with upper-level mean differential thermal advection balances stronger vertical motion. The results herein suggest that changes in transient eddy heat and moisture fluxes are critical to the overall local atmospheric response to Oyashio Front anomalies, which may consequently yield a stronger downstream response. These changes may require the high resolution to be fully reproduced, warranting further experiments of this type with other high-resolution atmosphere-only and fully coupled GCMs.
    Description: We gratefully acknowledge funding provided by NSF to DS and MN (AGS CLD 1035325) and Y-OK and CF (AGS CLD 1035423) and by DOE to Y-OK (DE-SC0007052).
    Description: 2015-08-01
    Keywords: Atmosphere-ocean interaction ; Atmospheric circulation ; Boundary layer ; Cyclogenesis/cyclolysis ; Diabatic heating ; Extratropical cyclones
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    Mixing variability in the Southern Ocean (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 966–987, doi:10.1175/JPO-D-14-0110.1.
    Description: A key remaining challenge in oceanography is the understanding and parameterization of small-scale mixing. Evidence suggests that topographic features play a significant role in enhancing mixing in the Southern Ocean. This study uses 914 high-resolution hydrographic profiles from novel EM-APEX profiling floats to investigate turbulent mixing north of the Kerguelen Plateau, a major topographic feature in the Southern Ocean. A shear–strain finescale parameterization is applied to estimate diapycnal diffusivity in the upper 1600 m of the ocean. The indirect estimates of mixing match direct microstructure profiler observations made simultaneously. It is found that mixing intensities have strong spatial and temporal variability, ranging from O(10−6) to O(10−3) m2 s−1. This study identifies topographic roughness, current speed, and wind speed as the main factors controlling mixing intensity. Additionally, the authors find strong regional variability in mixing dynamics and enhanced mixing in the Antarctic Circumpolar Current frontal region. This enhanced mixing is attributed to dissipating internal waves generated by the interaction of the Antarctic Circumpolar Current and the topography of the Kerguelen Plateau. Extending the mixing observations from the Kerguelen region to the entire Southern Ocean, this study infers a large water mass transformation rate of 17 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) across the boundary of Antarctic Intermediate Water and Upper Circumpolar Deep Water in the Antarctic Circumpolar Current. This work suggests that the contribution of mixing to the Southern Ocean overturning circulation budget is particularly significant in fronts.
    Description: AM was supported by the joint CSIRO–University of Tasmania Quantitative Marine Science (QMS) program and the 2009 CSIRO Wealth from Ocean Flagship Collaborative Fund. BMS was supported by the Australian Climate Change Science Program, jointly funded by the Department of the Environment and CSIRO. KLPs salary support was provided by Woods Hole Oceanographic Institution bridge support funds.
    Description: 2015-10-01
    Keywords: Geographic location/entity ; Southern Ocean ; Circulation/ Dynamics ; Diapycnal mixing ; Fronts ; Ocean circulation ; Topographic effects ; Atm/Ocean Structure/ Phenomena ; Mixing
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    The Tokar Gap jet : regional circulation, diurnal variability, and moisture transport based on numerical simulations (2015)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 28 (2015): 5885–5907, doi:10.1175/JCLI-D-14-00635.1.
    Description: The structure, variability, and regional connectivity of the Tokar Gap jet (TGJ) are described using WRF Model analyses and supporting atmospheric datasets from the East African–Red Sea–Arabian Peninsula (EARSAP) region during summer 2008. Sources of the TGJ’s unique quasi-diurnal nature and association with atypically high atmospheric moisture transport are traced back to larger-scale atmospheric dynamics influencing its forcing. These include seasonal shifts in the intertropical convergence zone (ITCZ), variability of the monsoon and North African wind regimes, and ties to other orographic flow patterns. Strong modulation of the TGJ by regional processes such as the desert heating cycle, wind convergence at the ITCZ surface front, and the local land–sea breeze cycle are described. Two case studies present the interplay of these influences in detail. The first of these was an “extreme” gap wind event on 12 July, in which horizontal velocities in the Tokar Gap exceeded 26 m s−1 and the flow from the jet extended the full width of the Red Sea basin. This event coincided with development of a large mesoscale convective complex (MCC) and precipitation at the entrance of the Tokar Gap as well as smaller gaps downstream along the Arabian Peninsula. More typical behavior of the TGJ during the 2008 summer is discussed using a second case study on 19 July. Downwind impact of the TGJ is evaluated using Lagrangian model trajectories and analysis of the lateral moisture fluxes (LMFs) during jet events. These results suggest means by which TGJ contributes to large LMFs and has potential bearing upon Sahelian rainfall and MCC development.
    Description: This work was supported by a grant from the King Abdullah University of Science and Technology (KAUST) as well as National Science Foundation Grant OCE0927017 and from DOD (MURI) Grant N000141110087, administered by the Office of Naval Research.
    Description: 2016-02-01
    Keywords: Africa ; Orographic effects ; Monsoons ; Atmosphere-land interaction ; Atmosphere-ocean interaction ; Hydrometeorology
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    Thermally forced transients in the thermohaline circulation (2015)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 2820–2835, doi:10.1175/JPO-D-15-0101.1.
    Description: The response of a convective ocean basin to variations in atmospheric temperature is explored using numerical models and theory. The results indicate that the general behavior depends strongly on the frequency at which the atmosphere changes relative to the local response time to air–sea heat flux. For high-frequency forcing, the convective region in the basin interior is essentially one-dimensional and responds to the integrated local surface heat flux anomalies. For low-frequency forcing, eddy fluxes from the boundary current into the basin interior become important and act to suppress variability forced by the atmosphere. A theory is developed to quantify this time-dependent response and its influence on various oceanic quantities. The amplitude and phase of the temperature and salinity of the convective water mass, the meridional overturning circulation, the meridional heat flux, and the air–sea heat flux predicted by the theory compare well with that diagnosed from a series of numerical model calculations in both strongly eddying and weakly eddying regimes. Linearized analytic solutions provide direct estimates of each of these quantities and demonstrate their dependence on the nondimensional numbers that characterize the domain and atmospheric forcing. These results highlight the importance of mesoscale eddies in modulating the mean and time-dependent ocean response to atmospheric variability and provide a dynamical framework with which to connect ocean observations with changes in the atmosphere and surface heat flux.
    Description: This study was supported by the National Science Foundation under Grant OCE-1232389.
    Description: 2016-05-01
    Keywords: Circulation/ Dynamics ; Atmosphere-ocean interaction ; Deep convection ; Eddies ; Meridional overturning circulation
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    Mechanisms of tidal oscillatory salt transport in a partially stratified estuary (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 2773–2789, doi:10.1175/JPO-D-15-0031.1.
    Description: Tidal oscillatory salt transport, induced by the correlation between tidal variations in salinity and velocity, is an important term for the subtidal salt balance under the commonly used Eulerian method of salt transport decomposition. In this paper, its mechanisms in a partially stratified estuary are investigated with a numerical model of the Hudson estuary. During neap tides, when the estuary is strongly stratified, the tidal oscillatory salt transport is mainly due to the hydraulic response of the halocline to the longitudinal variation of topography. This mechanism does not involve vertical mixing, so it should not be regarded as oscillatory shear dispersion, but instead it should be regarded as advective transport of salt, which results from the vertical distortion of exchange flow obtained in the Eulerian decomposition by vertical fluctuations of the halocline. During spring tides, the estuary is weakly stratified, and vertical mixing plays a significant role in the tidal variation of salinity. In the spring tide regime, the tidal oscillatory salt transport is mainly due to oscillatory shear dispersion. In addition, the transient lateral circulation near large channel curvature causes the transverse tilt of the halocline. This mechanism has little effect on the cross-sectionally integrated tidal oscillatory salt transport, but it results in an apparent left–right cross-channel asymmetry of tidal oscillatory salt transport. With the isohaline framework, tidal oscillatory salt transport can be regarded as a part of the net estuarine salt transport, and the Lagrangian advective mechanism and dispersive mechanism can be distinguished.
    Description: Tao Wang was supported by the Open Research Fund of State Key Laboratory of Estuarine and Coastal Research (Grant SKLEC-KF201509) and Chinese Scholarship Council. Geyer was supported by by NSF Grant OCE 0926427. Wensheng Jiang was supported by NSFC-Shandong Joint Fund for Marine Science Research Centers (Grant U1406401).
    Description: 2016-05-01
    Keywords: Geographic location/entity ; Estuaries ; Circulation/ Dynamics ; Baroclinic flows ; Dispersion ; Shear structure/flows ; Atm/Ocean Structure/ Phenomena ; Diapycnal mixing ; Models and modeling ; Regional models
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    Observations of water mass transformation and eddies in the Lofoten basin of the Nordic Seas (2015)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 1735–1756, doi:10.1175/JPO-D-14-0238.1.
    Description: The Lofoten basin of the Nordic Seas is recognized as a crucial component of the meridional overturning circulation in the North Atlantic because of the large horizontal extent of Atlantic Water and winter surface buoyancy loss. In this study, hydrographic and current measurements collected from a mooring deployed in the Lofoten basin from July 2010 to September 2012 are used to describe water mass transformation and the mesoscale eddy field. Winter mixed layer depths (MLDs) are observed to reach approximately 400 m, with larger MLDs and denser properties resulting from the colder 2010 winter. A heat budget of the upper water column requires lateral input, which balances the net annual heat loss of ~80 W m−2. The lateral flux is a result of mesoscale eddies, which dominate the velocity variability. Eddy velocities are enhanced in the upper 1000 m, with a barotropic component that reaches the bottom. Detailed examination of two eddies, from April and August 2012, highlights the variability of the eddy field and eddy properties. Temperature and salinity properties of the April eddy suggest that it originated from the slope current but was ventilated by surface fluxes. The properties within the eddy were similar to those of the mode water, indicating that convection within the eddies may make an important contribution to water mass transformation. A rough estimate of eddy flux per unit boundary current length suggests that fluxes in the Lofoten basin are larger than in the Labrador Sea because of the enhanced boundary current–interior density difference.
    Description: The work was supported by NSF OCE 0850416.
    Description: 2015-12-01
    Keywords: Circulation/ Dynamics ; Atmosphere-ocean interaction ; Boundary currents ; Eddies ; Fluxes ; Mesoscale processes ; Atm/Ocean Structure/ Phenomena ; Thermohaline circulation
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    Biases in Thorpe-scale estimates of turbulence dissipation. Part I : Assessments from large-scale overturns in oceanographic data (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 2497–2521, doi:10.1175/JPO-D-14-0128.1.
    Description: Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale LT and the Ozmidov length scale LO. Historic evidence supporting LT ~ LO has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of LT and LO, using vertical profiles of temperature and microstructure shear collected in the Luzon Strait—a site characterized by topographically influenced overturns up to O(100) m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes. A key result is that LT/LO increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin–Helmholtz (K–H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting LT ~ LO are revisited, and conceptual regimes explaining the relationship between LT/LO and a nondimensional overturn size are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.
    Description: B.D.M. and S.K.V. gratefully acknowledge the support of the Office of Naval Research under Grants N00014-12-1-0279, N00014-12-1-0282, and N00014-12-1-0938 (Program Manager: Dr. Terri Paluszkiewicz). S.K.V. also acknowledges support of the National Science Foundation under Grant OCE-1151838. L.S.L. acknowledges support for BBTRE by the National Science Foundation by Contract OCE94-15589 and NATRE and IWISE by the Office of Naval Research by Contracts N00014-92-1323 and N00014-10-10315. J.N.M. was supported through Grant 1256620 from the National Science Foundation and the Office of Naval Research (IWISE Project).
    Description: 2016-04-01
    Keywords: Circulation/ Dynamics ; Diapycnal mixing ; Small scale processes ; Turbulence ; Atm/Ocean Structure/ Phenomena ; Mixing ; Observational techniques and algorithms ; Profilers, oceanic ; Models and modeling ; Parameterization
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    Rapid generation of upwelling at a shelf break caused by buoyancy shutdown (2015)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 294–312, doi:10.1175/JPO-D-14-0104.1.
    Description: Model analyses of an alongshelf flow over a continental shelf and slope reveal upwelling near the shelf break. A stratified, initially uniform, alongshelf flow undergoes a rapid adjustment with notable differences onshore and offshore of the shelf break. Over the shelf, a bottom boundary layer and an offshore bottom Ekman transport develop within an inertial period. Over the slope, the bottom offshore transport is reduced from the shelf’s bottom transport by two processes. First, advection of buoyancy downslope induces vertical mixing, destratifying, and thickening the bottom boundary layer. The downward-tilting isopycnals reduce the geostrophic speed near the bottom. The reduced bottom stress weakens the offshore Ekman transport, a process known as buoyancy shutdown of the Ekman transport. Second, the thickening bottom boundary layer and weakening near-bottom speeds are balanced by an upslope ageostrophic transport. The convergence in the bottom transport induces adiabatic upwelling offshore of the shelf break. For a time period after the initial adjustment, scalings are identified for the upwelling speed and the length scale over which it occurs. Numerical experiments are used to test the scalings for a range of initial speeds and stratifications. Upwelling occurs within an inertial period, reaching values of up to 10 m day−1 within 2 to 7 km offshore of the shelf break. Upwelling drives an interior secondary circulation that accelerates the alongshelf flow over the slope, forming a shelfbreak jet. The model results are compared with upwelling estimates from other models and observations near the Middle Atlantic Bight shelf break.
    Description: J. Benthuysen acknowledges support from the ARC Centre of Excellence for Climate System Science (CE110001028) and the MIT/WHOI Joint Program, where this work was initiated.
    Description: 2015-07-01
    Keywords: Circulation/ Dynamics ; Boundary currents ; Diapycnal mixing ; Ekman pumping/transport ; Mixing ; Topographic effects ; Upwelling/downwelling
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    Satellite observations of mesoscale eddy-induced Ekman pumping (2015)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 104–132, doi:10.1175/JPO-D-14-0032.1.
    Description: Three mechanisms for self-induced Ekman pumping in the interiors of mesoscale ocean eddies are investigated. The first arises from the surface stress that occurs because of differences between surface wind and ocean velocities, resulting in Ekman upwelling and downwelling in the cores of anticyclones and cyclones, respectively. The second mechanism arises from the interaction of the surface stress with the surface current vorticity gradient, resulting in dipoles of Ekman upwelling and downwelling. The third mechanism arises from eddy-induced spatial variability of sea surface temperature (SST), which generates a curl of the stress and therefore Ekman pumping in regions of crosswind SST gradients. The spatial structures and relative magnitudes of the three contributions to eddy-induced Ekman pumping are investigated by collocating satellite-based measurements of SST, geostrophic velocity, and surface winds to the interiors of eddies identified from their sea surface height signatures. On average, eddy-induced Ekman pumping velocities approach O(10) cm day−1. SST-induced Ekman pumping is usually secondary to the two current-induced mechanisms for Ekman pumping. Notable exceptions are the midlatitude extensions of western boundary currents and the Antarctic Circumpolar Current, where SST gradients are strong and all three mechanisms for eddy-induced Ekman pumping are comparable in magnitude. Because the polarity of current-induced curl of the surface stress opposes that of the eddy, the associated Ekman pumping attenuates the eddies. The decay time scale of this attenuation is proportional to the vertical scale of the eddy and inversely proportional to the wind speed. For typical values of these parameters, the decay time scale is about 1.3 yr.
    Description: This work was funded by NASA Grants NNX08AI80G, NNX08AR37G, NNX13AD78G, NNX10AE91G, NNX13AE47G, and NNX10AO98G.
    Description: 2015-07-01
    Keywords: Circulation/ Dynamics ; Atmosphere-ocean interaction ; Eddies ; Ekman pumping/transport ; Atm/Ocean Structure/ Phenomena ; Eddies ; Ekman pumping ; Observational techniques and algorithms ; Satellite observations
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    Strengthening of the Pacific Equatorial Undercurrent in the SODA reanalysis : mechanisms, ocean dynamics, and implications (2014)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 27 (2014): 2405–2416, doi:10.1175/JCLI-D-13-00359.1.
    Description: Several recent studies utilizing global climate models predict that the Pacific Equatorial Undercurrent (EUC) will strengthen over the twenty-first century. Here, historical changes in the tropical Pacific are investigated using the Simple Ocean Data Assimilation (SODA) reanalysis toward understanding the dynamics and mechanisms that may dictate such a change. Although SODA does not assimilate velocity observations, the seasonal-to-interannual variability of the EUC estimated by SODA corresponds well with moored observations over a ~20-yr common period. Long-term trends in SODA indicate that the EUC core velocity has increased by 16% century−1 and as much as 47% century−1 at fixed locations since the mid-1800s. Diagnosis of the zonal momentum budget in the equatorial Pacific reveals two distinct seasonal mechanisms that explain the EUC strengthening. The first is characterized by strengthening of the western Pacific trade winds and hence oceanic zonal pressure gradient during boreal spring. The second entails weakening of eastern Pacific trade winds during boreal summer, which weakens the surface current and reduces EUC deceleration through vertical friction. EUC strengthening has important ecological implications as upwelling affects the thermal and biogeochemical environment. Furthermore, given the potential large-scale influence of EUC strength and depth on the heat budget in the eastern Pacific, the seasonal strengthening of the EUC may help reconcile paradoxical observations of Walker circulation slowdown and zonal SST gradient strengthening. Such a process would represent a new dynamical “thermostat” on CO2-forced warming of the tropical Pacific Ocean, emphasizing the importance of ocean dynamics and seasonality in understanding climate change projections.
    Description: EJDis supported by NSFGrantsOCE-1031971 and OCE-1233282. KBK is supported by NSF Grant OCE-1233282.
    Description: 2014-09-15
    Keywords: Tropics ; Currents ; Ocean dynamics ; Atmosphere-ocean interaction ; Climate variability ; Reanalysis data
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    Northern hemisphere winter atmospheric transient eddy heat fluxes and the Gulf Stream and Kuroshio–Oyashio Extension variability (2014)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 26 (2013): 9839–9859, doi:10.1175/JCLI-D-12-00647.1.
    Description: Spatial and temporal covariability between the atmospheric transient eddy heat fluxes (i.e., υ′T′ and υ′q′) in the Northern Hemisphere winter (January–March) and the paths of the Gulf Stream (GS), Kuroshio Extension (KE), and Oyashio Extension (OE) are examined based on an atmospheric reanalyses and ocean observations for 1979–2009. For the climatological winter mean, the northward heat fluxes by the synoptic (2–8 days) transient eddies exhibit canonical storm tracks with their maxima collocated with the GS and KE/OE. The intraseasonal (8 days–3 months) counterpart, while having overall similar amplitude, shows a spatial pattern with more localized maxima near the major orography and blocking regions. Lateral heat flux divergence by transient eddies as the sum of the two frequency bands exhibits very close coupling with the exact locations of the ocean fronts. Linear regression is used to examine the lead–lag relationship between interannual changes in the northward heat fluxes by the transient eddies and the meridional changes in the paths of the GS, KE, and OE, respectively. One to three years prior to the northward shifts of each ocean front, the atmospheric storm tracks shift northward and intensify, which is consistent with wind-driven changes of the ocean. Following the northward shifts of the ocean fronts, the synoptic storm tracks weaken in all three cases. The zonally integrated northward heat transport by the synoptic transient eddies increases by ~5% of its maximum mean value prior to the northward shift of each ocean front and decreases to a similar amplitude afterward.
    Description: Support from the National Aeronautics and Space Administration (NASA) Physical Oceanography Program (NNX09AF35G to TJ and Y-OK) and the Department of Energy (DOE) Climate and Environmental Sciences Division (DE-SC0007052 to Y-OK) is gratefully acknowledged.
    Description: 2014-06-15
    Keywords: Atmosphere-ocean interaction ; Eddies ; Energy transport ; Storm tracks ; Heat budgets/fluxes
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    The influence of the AMOC variability on the atmosphere in CCSM3 (2014)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 26 (2013): 9774–9790, doi:10.1175/JCLI-D-12-00862.1.
    Description: The influence of the Atlantic meridional overturning circulation (AMOC) variability on the atmospheric circulation is investigated in a control simulation of the NCAR Community Climate System Model, version 3 (CCSM3), where the AMOC evolves from an oscillatory regime into a red noise regime. In the latter, an AMOC intensification is followed during winter by a positive North Atlantic Oscillation (NAO). The atmospheric response is robust and controlled by AMOC-driven SST anomalies, which shift the heat release to the atmosphere northward near the Gulf Stream/North Atlantic Current. This alters the low-level atmospheric baroclinicity and shifts the maximum eddy growth northward, affecting the storm track and favoring a positive NAO. The AMOC influence is detected in the relation between seasonal upper-ocean heat content or SST anomalies and winter sea level pressure. In the oscillatory regime, no direct AMOC influence is detected in winter. However, an upper-ocean heat content anomaly resembling the AMOC footprint precedes a negative NAO. This opposite NAO polarity seems due to the southward shift of the Gulf Stream during AMOC intensification, displacing the maximum baroclinicity southward near the jet exit. As the mode has somewhat different patterns when using SST, the wintertime impact of the AMOC lacks robustness in this regime. However, none of the signals compares well with the observed influence of North Atlantic SST anomalies on the NAO because SST is dominated in CCSM3 by the meridional shifts of the Gulf Stream/North Atlantic Current that covary with the AMOC. Hence, although there is some potential climate predictability in CCSM3, it is not realistic.
    Description: Support from the NOAA Climate Program Office (Grant Number NA10OAR4310202) and the European Community 7th Framework Programme (FP7 2007-2013) under Grant Agreements GA212643 (THOR) and n.308299 (NACLIM) is gratefully acknowledged.
    Description: 2014-06-15
    Keywords: Atmosphere-ocean interaction ; North Atlantic Oscillation ; Thermohaline circulation ; Decadal variability
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    An inertial model of the interaction of Ekman layers and planetary islands (2014)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 1398–1406, doi:10.1175/JPO-D-13-028.1.
    Description: An adiabatic, inertial, and quasigeostrophic model is used to discuss the interaction of surface Ekman transport with an island. The theory extends the recent work of Spall and Pedlosky to include an analytical and nonlinear model for the interaction. The presence of an island that interrupts a uniform Ekman layer transport raises interesting questions about the resulting circulation. The consequential upwelling around the island can lead to a local intake of fluid from the geostrophic region beneath the Ekman layer or to a more complex flow around the island in which the fluid entering the Ekman layer on one portion of the island's perimeter is replaced by a flow along the island's boundary from a downwelling region located elsewhere on the island. This becomes especially pertinent when the flow is quasigeostrophic and adiabatic. The oncoming geostrophic flow that balances the offshore Ekman flux is largely diverted around the island, and the Ekman flux is fed by a transfer of fluid from the western to the eastern side of the island. As opposed to the linear, dissipative model described earlier, this transfer takes place even in the absence of a topographic skirt around the island. The principal effect of topography in the inertial model is to introduce an asymmetry between the circulation on the northern and southern sides of the island. The quasigeostrophic model allows a simple solution to the model problem with topography and yet the resulting three-dimensional circulation is surprisingly complex with streamlines connecting each side of the island.
    Description: This research was supported in part by NSF Grant OCE Grant 0925061.
    Keywords: Baroclinic flows ; Large-scale motions ; Nonlinear dynamics ; Ocean circulation ; Ocean dynamics ; Topographic effects
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    Impact of surface forcing on Southern Hemisphere atmospheric blocking in the Australia–New Zealand sector (2014)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 26 (2013): 8476–8494, doi:10.1175/JCLI-D-12-00860.1.
    Description: Characteristics of atmospheric blocking in the Southern Hemisphere (SH) are explored in atmospheric general circulation model (AGCM) simulations with the Community Atmosphere Model, version 3, with a particular focus on the Australia–New Zealand sector. Preferred locations of blocking in SH observations and the associated seasonal cycle are well represented in the AGCM simulations, but the observed magnitude of blocking is underestimated throughout the year, particularly in late winter and spring. This is related to overly zonal flow due to an enhanced meridional pressure gradient in the model, which results in a decreased amplitude of the longwave trough/ridge pattern. A range of AGCM sensitivity experiments explores the effect on SH blocking of tropical heating, midlatitude sea surface temperatures, and land–sea temperature gradients created over the Australian continent during austral winter. The combined effects of tropical heating and extratropical temperature gradients are further explored in a configuration that is favorable for blocking in the Australia–New Zealand sector with warm SST anomalies to the north of Australia, cold to the southwest of Australia, warm to the southeast, and cool Australian land temperatures. The blocking-favorable configuration indicates a significant strengthening of the subtropical jet and a reduction in midlatitude flow, which results from changes in the thermal wind. While these overall changes in mean climate, predominantly forced by the tropical heating, enhance blocking activity, the magnitude of atmospheric blocking compared to observations is still underestimated. The blocking-unfavorable configuration with surface forcing anomalies of opposite sign results in a weakening subtropical jet, enhanced midlatitude flow, and significantly reduced blocking.
    Description: C.C.U. received support from the Australian Research Council through funding awarded to the Centre of Excellence for Climate System Science and the Penzance Endowed Fund at WHOI. P.C.M., M.J.P., and J.S.R. were funded by the CSIRO Climate Adaptation Flagship and the Managing Climate Variability R&D Program.
    Description: 2014-05-01
    Keywords: Australia ; Southern Hemisphere ; Atmosphere-ocean interaction ; Atmospheric circulation ; Blocking ; General circulation models
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    The impact of finite-amplitude bottom topography on internal wave generation in the Southern Ocean (2014)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 2938–2950, doi:10.1175/JPO-D-13-0201.1.
    Description: Direct observations in the Southern Ocean report enhanced internal wave activity and turbulence in a kilometer-thick layer above rough bottom topography collocated with the deep-reaching fronts of the Antarctic Circumpolar Current. Linear theory, corrected for finite-amplitude topography based on idealized, two-dimensional numerical simulations, has been recently used to estimate the global distribution of internal wave generation by oceanic currents and eddies. The global estimate shows that the topographic wave generation is a significant sink of energy for geostrophic flows and a source of energy for turbulent mixing in the deep ocean. However, comparison with recent observations from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean shows that the linear theory predictions and idealized two-dimensional simulations grossly overestimate the observed levels of turbulent energy dissipation. This study presents two- and three-dimensional, realistic topography simulations of internal lee-wave generation from a steady flow interacting with topography with parameters typical of Drake Passage. The results demonstrate that internal wave generation at three-dimensional, finite bottom topography is reduced compared to the two-dimensional case. The reduction is primarily associated with finite-amplitude bottom topography effects that suppress vertical motions and thus reduce the amplitude of the internal waves radiated from topography. The implication of these results for the global lee-wave generation is discussed.
    Description: This research was supported by the National Science Foundation under Award CMG-1024198.
    Description: 2015-05-01
    Keywords: Circulation/ Dynamics ; Diapycnal mixing ; Internal waves ; Mixing ; Mountain waves ; Topographic effects ; Waves, oceanic
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    Tropical Pacific influence on the source and transport of marine aerosols to West Antarctica (2014)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 27 (2014): 1343–1363, doi:10.1175/JCLI-D-13-00148.1.
    Description: The climate of West Antarctica is strongly influenced by remote forcing from the tropical Pacific. For example, recent surface warming over West Antarctica reflects atmospheric circulation changes over the Amundsen Sea, driven by an atmospheric Rossby wave response to tropical sea surface temperature (SST) anomalies. Here, it is demonstrated that tropical Pacific SST anomalies also influence the source and transport of marine-derived aerosols to the West Antarctic Ice Sheet. Using records from four firn cores collected along the Amundsen coast of West Antarctica, the relationship between sea ice–modulated chemical species and large-scale atmospheric variability in the tropical Pacific from 1979 to 2010 is investigated. Significant correlations are found between marine biogenic aerosols and sea salts, and SST and sea level pressure in the tropical Pacific. In particular, La Niña–like conditions generate an atmospheric Rossby wave response that influences atmospheric circulation over Pine Island Bay. Seasonal regression of atmospheric fields on methanesulfonic acid (MSA) reveals a reduction in onshore wind velocities in summer at Pine Island Bay, consistent with enhanced katabatic flow, polynya opening, and coastal dimethyl sulfide production. Seasonal regression of atmospheric fields on chloride (Cl−) reveals an intensification in onshore wind velocities in winter, consistent with sea salt transport from offshore source regions. Both the source and transport of marine aerosols to West Antarctica are found to be modulated by similar atmospheric dynamics in response to remote forcing. Finally, the regional ice-core array suggests that there is both a temporally and a spatially varying response to remote tropical forcing.
    Description: This research was supported by an award from the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF) to ASC, a James E. and Barbara V. Moltz Research Fellowship to SBD, and grants from NSF-OPP (ANT- 0632031 and ANT-0631973), NSF-MRI (EAR- 1126217), and the NASA Cryosphere Program (NNX10AP09G), and a WHOI Andrew W. Mellon Foundation Award for Innovative Research.
    Description: 2014-08-01
    Keywords: Antarctica ; Sea ice ; Teleconnections ; Atmosphere-ocean interaction ; Climate records ; Interannual variability
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    Modeling and analysis of internal-tide generation and beamlike onshore propagation in the vicinity of shelfbreak canyons (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 834-849, doi:10.1175/JPO-D-13-0179.1.
    Description: A hydrostatic numerical model with alongshore-uniform barotropic M2 tidal boundary forcing and idealized shelfbreak canyon bathymetries is used to study internal-tide generation and onshore propagation. A control simulation with Mid-Atlantic Bight representative bathymetry is supported by other simulations that serve to identify specific processes. The canyons and adjacent slopes are transcritical in steepness with respect to M2 internal wave characteristics. Although the various canyons are symmetrical in structure, barotropic-to-baroclinic energy conversion rates Cυ are typically asymmetrical within them. The resulting onshore-propagating internal waves are the strongest along beams in the horizontal plane, with the stronger beam in the control simulation lying on the side with higher Cυ. Analysis of the simulation results suggests that the cross-canyon asymmetrical Cυ distributions are caused by multiple-scattering effects on one canyon side slope, because the phase variation in the spatially distributed internal-tide sources, governed by variations in the orientation of the bathymetry gradient vector, allows resonant internal-tide generation. A less complex, semianalytical, modal internal wave propagation model with sources placed along the critical-slope locus (where the M2 internal wave characteristic is tangent to the seabed) and variable source phasing is used to diagnose the physics of the horizontal beams of onshore internal wave radiation. Model analysis explains how the cross-canyon phase and amplitude variations in the locally generated internal tides affect parameters of the internal-tide beams. Under the assumption that strong internal tides on continental shelves evolve to include nonlinear wave trains, the asymmetrical internal-tide generation and beam radiation effects may lead to nonlinear internal waves and enhanced mixing occurring preferentially on one side of shelfbreak canyons, in the absence of other influencing factors.
    Description: All three authors were supported by Office of Naval Research (ONR) Grant N00014-11-1-0701. WGZ was additionally supported by the National Science Foundation (NSF) Grant OCE-1154575, and TFD was additionally supported by NSF Grant OCE-1060430.
    Description: 2014-09-01
    Keywords: Circulation/ Dynamics ; Baroclinic flows ; Internal waves ; Ocean circulation ; Topographic effects ; Waves, oceanic ; Models and modeling ; Numerical analysis/modeling
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    Suppression of internal wave breaking in the Antarctic Circumpolar Current near topography (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 1466–1492, doi:10.1175/JPO-D-12-0154.1.
    Description: Simultaneous full-depth microstructure measurements of turbulence and finestructure measurements of velocity and density are analyzed to investigate the relationship between turbulence and the internal wave field in the Antarctic Circumpolar Current. These data reveal a systematic near-bottom overprediction of the turbulent kinetic energy dissipation rate by finescale parameterization methods in select locations. Sites of near-bottom overprediction are typically characterized by large near-bottom flow speeds and elevated topographic roughness. Further, lower-than-average shear-to-strain ratios indicative of a less near-inertial wave field, rotary spectra suggesting a predominance of upward internal wave energy propagation, and enhanced narrowband variance at vertical wavelengths on the order of 100 m are found at these locations. Finally, finescale overprediction is typically associated with elevated Froude numbers based on the near-bottom shear of the background flow, and a background flow with a systematic backing tendency. Agreement of microstructure- and finestructure-based estimates within the expected uncertainty of the parameterization away from these special sites, the reproducibility of the overprediction signal across various parameterization implementations, and an absence of indications of atypical instrument noise at sites of parameterization overprediction, all suggest that physics not encapsulated by the parameterization play a role in the fate of bottom-generated waves at these locations. Several plausible underpinning mechanisms based on the limited available evidence are discussed that offer guidance for future studies.
    Description: The SOFine project is funded by the United Kingdom’s Natural Environmental Research Council (NERC) (Grant NE/G001510/1). SW acknowledges the support of anARCDiscovery Early CareerResearchAward (Grant DE120102927), as well as the Grantham Institute for Climate Change, Imperial College London, and the ARC Centre of Excellence for Climate System Science (Grant CE110001028). ACNG acknowledges the support of a NERC Advanced Research Fellowship (Grant NE/C517633/1).KLP acknowledges support fromWoods Hole Oceanographic Institution bridge support funds.
    Description: 2014-11-01
    Keywords: Circulation/ Dynamics ; Diapycnal mixing ; Internal waves ; Small scale processes ; Turbulence ; Observational techniques and algorithms ; In situ oceanic observations ; Profilers, oceanic
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    Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 1854–1872, doi:10.1175/JPO-D-13-0104.1.
    Description: The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10−4) m2 s−1 and above 1000-m depth is O(10−5) m2 s−1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.
    Description: This research was funded by the Climate Process Team (CPT) on internal wave–driven mixing throughNSF GrantOCE-0968721. GSC acknowledges support from NSF Grants OCE-0825266 (EXITS), OCE-1029483 (SPAM), and OCE-1029722 (MIXET). LDT and CBW acknowledge support from NSF Grant OCE-0927650. SWand ACNG acknowledge support from NERC Grant NE/G001510/1 (SOFine).
    Description: 2015-01-01
    Keywords: Circulation/ Dynamics ; Diapycnal mixing ; Internal waves
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  • 88
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    Variations of the North Pacific Subtropical Mode Water from direct observations (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 27 (2014): 2842–2860, doi:10.1175/JCLI-D-13-00227.1.
    Description: Mooring measurements from the Kuroshio Extension System Study (June 2004–June 2006) and from the ongoing Kuroshio Extension Observatory (June 2004–present) are combined with float measurements of the Argo network to study the variability of the North Pacific Subtropical Mode Water (STMW) across the entire gyre, on time scales from days, to seasons, to a decade. The top of the STMW follows a seasonal cycle, although observations reveal that it primarily varies in discrete steps associated with episodic wind events. The variations of the STMW bottom depth are tightly related to the sea surface height (SSH), reflecting mesoscale eddies and large-scale variations of the Kuroshio Extension and recirculation gyre systems. Using the observed relationship between SSH and STMW, gridded SSH products and in situ estimates from floats are used to construct weekly maps of STMW thickness, providing nonbiased estimates of STMW total volume, annual formation and erosion volumes, and seasonal and interannual variability for the past decade. Year-to-year variations are detected, particularly a significant decrease of STMW volume in 2007–10 primarily attributable to a smaller volume formed. Variability of the heat content in the mode water region is dominated by the seasonal cycle and mesoscale eddies; there is only a weak link to STMW on interannual time scales, and no long-term trends in heat content and STMW thickness between 2002 and 2011 are detected. Weak lagged correlations among air–sea fluxes, oceanic heat content, and STMW thickness are found when averaged over the northwestern Pacific recirculation gyre region.
    Description: This work was sponsored by the National Science Foundation (Grants OCE-0220161, OCE-0825152, and OCE-0827125).
    Description: 2014-10-15
    Keywords: Atmosphere-ocean interaction ; Mesoscale processes ; Mesoscale systems ; Ocean dynamics ; Eddies ; Water masses
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  • 89
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    CMIP5 model intercomparison of freshwater budget and circulation in the North Atlantic (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 27 (2014): 3298–3317, doi:JCLI-D-12-00700.1.
    Description: The subpolar North Atlantic is a center of variability of ocean properties, wind stress curl, and air–sea exchanges. Observations and hindcast simulations suggest that from the early 1970s to the mid-1990s the subpolar gyre became fresher while the gyre and meridional circulations intensified. This is opposite to the relationship of freshening causing a weakened circulation, most often reproduced by climate models. The authors hypothesize that both these configurations exist but dominate on different time scales: a fresher subpolar gyre when the circulation is more intense, at interannual frequencies (configuration A), and a saltier subpolar gyre when the circulation is more intense, at longer periods (configuration B). Rather than going into the detail of the mechanisms sustaining each configuration, the authors’ objective is to identify which configuration dominates and to test whether this depends on frequency, in preindustrial control runs of five climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). To this end, the authors have developed a novel intercomparison method that enables analysis of freshwater budget and circulation changes in a physical perspective that overcomes model specificities. Lag correlations and a cross-spectral analysis between freshwater content changes and circulation indices validate the authors’ hypothesis, as configuration A is only visible at interannual frequencies while configuration B is mostly visible at decadal and longer periods, suggesting that the driving role of salinity on the circulation depends on frequency. Overall, this analysis underscores the large differences among state-of-the-art climate models in their representations of the North Atlantic freshwater budget.
    Description: JD and RC were funded by NSF through Project 0751896. JD was also funded by IFREMER through project RICCO.
    Description: 2014-11-01
    Keywords: Atmosphere-ocean interaction ; Freshwater ; Climate models ; Model comparison ; Climate variability ; North Atlantic Oscillation
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  • 90
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    Scaling turbulent dissipation in the transition layer (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 2475–2489, doi:10.1175/JPO-D-13-057.1.
    Description: Data from three midlatitude, month-long surveys are examined for evidence of enhanced vertical mixing associated with the transition layer (TL), here defined as the strongly stratified layer that exists between the well mixed layer and the thermocline below. In each survey, microstructure estimates of turbulent dissipation were collected concurrently with fine-structure stratification and shear. Survey-wide averages are formed in a “TL coordinate” zTL, which is referenced around the depth of maximum stratification for each profile. Averaged profiles show characteristic TL structures such as peaks in stratification N2 and shear variance S2, which fall off steeply above zTL = 0 and more gradually below. Turbulent dissipation rates ɛ are 5–10 times larger than those found in the upper thermocline (TC). The gradient Richardson number Ri = N2/S2 becomes unstable (Ri 〈 0.25) within ~10 m of the TL upper boundary, suggesting that shear instability is active in the TL for zTL 〉 0. Ri is stable for zTL ≤ 0. Turbulent dissipation is found to scale exponentially with depth for zTL ≤ 0, but the decay scales are different for the TL and upper TC: ɛ scales well with either N2 or S2. Owing to the strong correlation between S2 and N2, existing TC scalings of the form ɛ ~ |S|p|N|q overpredict variations in ɛ. The scale dependence of shear variance is not found to significantly affect the scalings of ɛ versus N2 and S2 for zTL ≤ 0. However, the onset of unstable Ri at the top of the TL is sensitively dependent to the resolution of the shears.
    Description: This work was funded by NSF Grant OCE-0968787 as part of a Climate Process Team for internal wave-driven mixing.
    Keywords: Atm/Ocean Structure/ Phenomena ; Diapycnal mixing ; Mixed layer ; Thermocline ; Physical Meteorology and Climatology ; Heat budgets/fluxes ; Observational techniques and algorithms ; In situ oceanic observations ; Profilers, oceanic
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    Diurnal restratification events in the southeast Pacific trade wind regime (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 2569–2587, doi:10.1175/JPO-D-14-0026.1.
    Description: This paper describes the occurrence of diurnal restratification events found in the southeast trade wind regime off northern Chile. This is a region where persistent marine stratus clouds are found and where there is a less than complete understanding of the dynamics that govern the maintenance of the sea surface temperature. A surface mooring deployed in the region provides surface meteorological, air–sea flux, and upper-ocean temperature, salinity, and velocity data. In the presence of steady southeast trade winds and strong evaporation, a warm, salty surface mixed layer is found in the upper ocean. During the year, these trade winds, at times, drop dramatically and surface heating leads to the formation of shallow, warm diurnal mixed layers over one to several days. At the end of such a low wind period, mean sea surface temperature is warmer. Though magnitudes of the individual diurnal warming events are consistent with local forcing, as judged by running a one-dimensional model, the net warming at the end of a low wind event is more difficult to predict. This is found to stem from differences between the observed and predicted near-inertial shear and the depths over which the warmed water is distributed. As a result, the evolution of SST has a dependency on these diurnal restratification events and on near-surface processes that govern the depth over which the heat gained during such events is distributed.
    Description: RAW was supported by the NOAA Climate Program Office. SM and AT were supported by NASA Grant NNX12AD47G,ONR Grant N000140910196, and NSF-OCE 0928138 RAW.
    Description: 2015-03-01
    Keywords: Atm/Ocean Structure/ Phenomena ; Atmosphere-ocean interaction ; Boundary layer ; Diurnal effects ; Mixed layer
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  • 92
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    The surface-forced overturning of the North Atlantic : estimates from modern era atmospheric reanalysis datasets (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 27 (2014): 3596–3618, doi:10.1175/JCLI-D-13-00070.1.
    Description: Estimates of the recent mean and time varying water mass transformation rates associated with North Atlantic surface-forced overturning are presented. The estimates are derived from heat and freshwater surface fluxes and sea surface temperature fields from six atmospheric reanalyses—the Japanese 25-yr Reanalysis (JRA), the NCEP–NCAR reanalysis (NCEP1), the NCEP–U.S. Department of Energy (DOE) reanalysis (NCEP2), the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-I), the Climate Forecast System Reanalysis (CFSR), and the Modern-Era Reanalysis for Research and Applications (MERRA)—together with sea surface salinity fields from two globally gridded datasets (World Ocean Atlas and Met Office EN3 datasets). The resulting 12 estimates of the 1979–2007 mean surface-forced streamfunction all depict a subpolar cell, with maxima north of 45°N, near σ = 27.5 kg m−3, and a subtropical cell between 20° and 40°N, near σ = 26.1 kg m−3. The mean magnitude of the subpolar cell varies between 12 and 18 Sv (1 Sv ≡ 106 m3 s−1), consistent with estimates of the overturning circulation from subsurface observations. Analysis of the thermal and haline components of the surface density fluxes indicates that large differences in the inferred low-latitude circulation are largely a result of the biases in reanalysis net heat flux fields, which range in the global mean from −13 to 19 W m−2. The different estimates of temporal variability in the subpolar cell are well correlated with each other. This suggests that the uncertainty associated with the choice of reanalysis product does not critically limit the ability of the method to infer the variability in the subpolar overturning. In contrast, the different estimates of subtropical variability are poorly correlated with each other, and only a subset of them captures a significant fraction of the variability in independently estimated North Atlantic Subtropical Mode Water volume.
    Description: JPG is funded by UK Natural Environment Research Council New Investigator Grant NE/I001654/1. Y-OK was supported by the U.S. National Science Foundation under Grant OCE-0424492. RJB is supported by a fellowship from the UK National Centre for Earth Observation.
    Description: 2014-11-15
    Keywords: Atmosphere-ocean interaction ; Meridional overturning circulation ; Ocean circulation
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  • 93
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    Cold tongue and warm pool ENSO events in CMIP5 : mean state and future projections (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 27 (2014): 2861–2885, doi:10.1175/JCLI-D-13-00437.1.
    Description: The representation of the El Niño–Southern Oscillation (ENSO) under historical forcing and future projections is analyzed in 34 models from the Coupled Model Intercomparison Project phase 5 (CMIP5). Most models realistically simulate the observed intensity and location of maximum sea surface temperature (SST) anomalies during ENSO events. However, there exist systematic biases in the westward extent of ENSO-related SST anomalies, driven by unrealistic westward displacement and enhancement of the equatorial wind stress in the western Pacific. Almost all CMIP5 models capture the observed asymmetry in magnitude between the warm and cold events (i.e., El Niños are stronger than La Niñas) and between the two types of El Niños: that is, cold tongue (CT) El Niños are stronger than warm pool (WP) El Niños. However, most models fail to reproduce the asymmetry between the two types of La Niñas, with CT stronger than WP events, which is opposite to observations. Most models capture the observed peak in ENSO amplitude around December; however, the seasonal evolution of ENSO has a large range of behavior across the models. The CMIP5 models generally reproduce the duration of CT El Niños but have biases in the evolution of the other types of events. The evolution of WP El Niños suggests that the decay of this event occurs through heat content discharge in the models rather than the advection of SST via anomalous zonal currents, as seems to occur in observations. No consistent changes are seen across the models in the location and magnitude of maximum SST anomalies, frequency, or temporal evolution of these events in a warmer world.
    Description: 2014-10-15
    Keywords: Atmosphere-ocean interaction ; Climate change ; Climate variability ; ENSO ; Climate models ; Model evaluation/performance
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    Coastal trapped waves, alongshore pressure gradients, and the California Undercurrent (2014)
    American Meteorological Society
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    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 319–342, doi:10.1175/JPO-D-13-095.1.
    Description: The California Undercurrent (CUC), a poleward-flowing feature over the continental slope, is a key transport pathway along the west coast of North America and an important component of regional upwelling dynamics. This study examines the poleward undercurrent and alongshore pressure gradients in the northern California Current System (CCS), where local wind stress forcing is relatively weak. The dynamics of the undercurrent are compared in the primitive equation Navy Coastal Ocean Model and a linear coastal trapped wave model. Both models are validated using hydrographic data and current-meter observations in the core of the undercurrent in the northern CCS. In the linear model, variability in the predominantly equatorward wind stress along the U.S. West Coast produces episodic reversals to poleward flow over the northern CCS slope during summer. However, reproducing the persistence of the undercurrent during late summer requires additional incoming energy from sea level variability applied south of the region of the strongest wind forcing. The relative importance of the barotropic and baroclinic components of the modeled alongshore pressure gradient changes with latitude. In contrast to the southern and central portions of the CCS, the baroclinic component of the alongshore pressure gradient provides the primary poleward force at CUC depths over the northern CCS slope. At time scales from weeks to months, the alongshore pressure gradient force is primarily balanced by the Coriolis force associated with onshore flow.
    Description: This work was supported by grants to B. Hickey from the Coastal Ocean Program of the National Oceanic and Atmospheric Administration (NOAA) (NA17OP2789 and NA09NOS4780180) and the National Science Foundation (NSF) (OCE0234587 and OCE0942675) as part of the Ecology of Harmful Algal Blooms Pacific Northwest (ECOHAB PNW) and Pacific Northwest Toxin (PNWTOX) projects. I. Shulman was supported by the Naval Research Laboratory.
    Description: 2014-07-01
    Keywords: Geographic location/entity ; Continental shelf/slope ; Circulation/ Dynamics ; Baroclinic flows ; Coastal flows ; Models and modeling ; Model evaluation/performance ; Variability ; Intraseasonal variability ; Seasonal variability
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    Parametric subharmonic instability of the internal tide at 29°N (2013)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 17–28, doi:10.1175/JPO-D-11-0108.1.
    Description: Observational evidence is presented for transfer of energy from the internal tide to near-inertial motions near 29°N in the Pacific Ocean. The transfer is accomplished via parametric subharmonic instability (PSI), which involves interaction between a primary wave (the internal tide in this case) and two smaller-scale waves of nearly half the frequency. The internal tide at this location is a complex superposition of a low-mode waves propagating north from Hawaii and higher-mode waves generated at local seamounts, making application of PSI theory challenging. Nevertheless, a statistically significant phase locking is documented between the internal tide and upward- and downward-propagating near-inertial waves. The phase between those three waves is consistent with that expected from PSI theory. Calculated energy transfer rates from the tide to near-inertial motions are modest, consistent with local dissipation rate estimates. The conclusion is that while PSI does befall the tide near a critical latitude of 29°N, it does not do so catastrophically.
    Description: This work was sponsored by NSF OCE 04-25283.
    Description: 2013-07-01
    Keywords: Diapycnal mixing ; Internal waves ; Nonlinear dynamics
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    On eddy viscosity, energy cascades, and the horizontal resolution of gridded satellite altimeter products (2013)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 283–300, doi:10.1175/JPO-D-11-0240.1.
    Description: Motivated by the recent interest in ocean energetics, the widespread use of horizontal eddy viscosity in models, and the promise of high horizontal resolution data from the planned wide-swath satellite altimeter, this paper explores the impacts of horizontal eddy viscosity and horizontal grid resolution on geostrophic turbulence, with a particular focus on spectral kinetic energy fluxes Π(K) computed in the isotropic wavenumber (K) domain. The paper utilizes idealized two-layer quasigeostrophic (QG) models, realistic high-resolution ocean general circulation models, and present-generation gridded satellite altimeter data. Adding horizontal eddy viscosity to the QG model results in a forward cascade at smaller scales, in apparent agreement with results from present-generation altimetry. Eddy viscosity is taken to roughly represent coupling of mesoscale eddies to internal waves or to submesoscale eddies. Filtering the output of either the QG or realistic models before computing Π(K) also greatly increases the forward cascade. Such filtering mimics the smoothing inherent in the construction of present-generation gridded altimeter data. It is therefore difficult to say whether the forward cascades seen in present-generation altimeter data are due to real physics (represented here by eddy viscosity) or to insufficient horizontal resolution. The inverse cascade at larger scales remains in the models even after filtering, suggesting that its existence in the models and in altimeter data is robust. However, the magnitude of the inverse cascade is affected by filtering, suggesting that the wide-swath altimeter will allow a more accurate determination of the inverse cascade at larger scales as well as providing important constraints on smaller-scale dynamics.
    Description: BKA received support from Office of Naval Research Grant N00014-11-1-0487, National Science Foundation (NSF) Grants OCE-0924481 and OCE- 09607820, and University of Michigan startup funds. KLP acknowledges support from Woods Hole Oceanographic Institution bridge support funds. RBS acknowledges support from NSF grants OCE-0960834 and OCE-0851457, a contract with the National Oceanography Centre, Southampton, and a NASA subcontract to Boston University. JFS and JGR were supported by the projects ‘‘Global and remote littoral forcing in global ocean models’’ and ‘‘Agesotrophic vorticity dynamics of the ocean,’’ respectively, both sponsored by the Office of Naval Research under program element 601153N.
    Description: 2013-08-01
    Keywords: Eddies ; Nonlinear dynamics ; Ocean dynamics ; Satellite observations ; Ocean models
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  • 97
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    On the exchange of momentum over the open ocean (2013)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 1589–1610, doi:10.1175/JPO-D-12-0173.1.
    Description: This study investigates the exchange of momentum between the atmosphere and ocean using data collected from four oceanic field experiments. Direct covariance estimates of momentum fluxes were collected in all four experiments and wind profiles were collected during three of them. The objective of the investigation is to improve parameterizations of the surface roughness and drag coefficient used to estimate the surface stress from bulk formulas. Specifically, the Coupled Ocean–Atmosphere Response Experiment (COARE) 3.0 bulk flux algorithm is refined to create COARE 3.5. Oversea measurements of dimensionless shear are used to investigate the stability function under stable and convective conditions. The behavior of surface roughness is then investigated over a wider range of wind speeds (up to 25 m s−1) and wave conditions than have been available from previous oversea field studies. The wind speed dependence of the Charnock coefficient α in the COARE algorithm is modified to , where m = 0.017 m−1 s and b = −0.005. When combined with a parameterization for smooth flow, this formulation gives better agreement with the stress estimates from all of the field programs at all winds speeds with significant improvement for wind speeds over 13 m s−1. Wave age– and wave slope–dependent parameterizations of the surface roughness are also investigated, but the COARE 3.5 wind speed–dependent formulation matches the observations well without any wave information. The available data provide a simple reason for why wind speed–, wave age–, and wave slope–dependent formulations give similar results—the inverse wave age varies nearly linearly with wind speed in long-fetch conditions for wind speeds up to 25 m s−1.
    Description: This work was funded by the National Science Foundation Grant OCE04-24536 as part of the CLIVAR Mode Water Dynamics Experiment (CLIMODE) and the Office of Naval Research Grant N00014-05-1-0139 as part of the CBLAST-LOW program.
    Description: 2014-02-01
    Keywords: Wind shear ; Wind stress ; Atmosphere-ocean interaction ; Fluxes ; Momentum ; Algorithms
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    Turbulence and diapycnal mixing in Drake Passage (2013)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2012. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 42 (2012): 2143–2152, doi:10.1175/JPO-D-12-027.1.
    Description: Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) is constricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10−8 W kg−1 at abyssal and middepths. The mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 × 10−4 m2 s−1. This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smoother bathymetry. Outside of the frontal regions, dissipation rates are O(10−10) W kg−1, comparable to the background levels of turbulence found throughout most mid- and low-latitude regions of the global ocean.
    Description: This work was supported by the U.S. National Science Foundation and by the Natural Environment Research Council of the United Kingdom.
    Description: 2013-06-01
    Keywords: Southern Ocean ; Turbulence ; Diapycnal mixing
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    Estimates of cabbeling in the global ocean (2013)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 698–705, doi:10.1175/JPO-D-12-0119.1.
    Description: Owing to the larger thermal expansion coefficient at higher temperatures, more buoyancy is put into the ocean by heating than is removed by cooling at low temperatures. The authors show that, even with globally balanced thermal and haline surface forcing at the ocean surface, there is a negative density flux and hence a positive buoyancy flux. As shown by McDougall and Garrett, this must be compensated by interior densification on mixing due to the nonlinearity of the equation of state (cabbeling). Three issues that arise from this are addressed: the estimation of the annual input of density forcing, the effects of the seasonal cycle, and the total cabbeling potential of the ocean upon complete mixing. The annual expansion through surface density forcing in a steady-state ocean driven by balanced evaporation–precipitation–runoff (E–P–R) and net radiative budget at the surface Qnet is estimated as 74 000 m3 s−1 (0.07 Sv; 1 Sv ≡ 106 m3 s−1), which would be equivalent to a sea level rise of 6.3 mm yr−1. This is equivalent to approximately 3 times the estimated rate of sea level rise or 450% of the average Mississippi River discharge. When seasonal variations are included, this density forcing increases by 35% relative to the time-mean case to 101 000 m3 s−1 (0.1 Sv). Likely bounds are established on these numbers by using different Qnet and E–P–R datasets and the estimates are found to be robust to a factor of ~2. These values compare well with the cabbeling-induced contraction inferred from independent thermal dissipation rate estimates. The potential sea level decrease upon complete vertical mixing of the ocean is estimated as 230 mm. When horizontal mixing is included, the sea level drop is estimated as 300 mm.
    Description: The authors would like to acknowledge support from the National Aeronautics and Space Administration, Grant NNX12AF59G and the National Science Foundation, Grant OCE-0647949.
    Description: 2013-10-01
    Keywords: Buoyancy ; Conservation equations ; Diapycnal mixing ; Heating ; Mixing ; Heat budgets/fluxes
    Repository Name: Woods Hole Open Access Server
    Type: Article
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    Internal waves and turbulence in the Antarctic Circumpolar Current (2013)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 259–282, doi:10.1175/JPO-D-11-0194.1.
    Description: This study reports on observations of turbulent dissipation and internal wave-scale flow properties in a standing meander of the Antarctic Circumpolar Current (ACC) north of the Kerguelen Plateau. The authors characterize the intensity and spatial distribution of the observed turbulent dissipation and the derived turbulent mixing, and consider underpinning mechanisms in the context of the internal wave field and the processes governing the waves’ generation and evolution. The turbulent dissipation rate and the derived diapycnal diffusivity are highly variable with systematic depth dependence. The dissipation rate is generally enhanced in the upper 1000–1500 m of the water column, and both the dissipation rate and diapycnal diffusivity are enhanced in some places near the seafloor, commonly in regions of rough topography and in the vicinity of strong bottom flows associated with the ACC jets. Turbulent dissipation is high in regions where internal wave energy is high, consistent with the idea that interior dissipation is related to a breaking internal wave field. Elevated turbulence occurs in association with downward-propagating near-inertial waves within 1–2 km of the surface, as well as with upward-propagating, relatively high-frequency waves within 1–2 km of the seafloor. While an interpretation of these near-bottom waves as lee waves generated by ACC jets flowing over small-scale topographic roughness is supported by the qualitative match between the spatial patterns in predicted lee wave radiation and observed near-bottom dissipation, the observed dissipation is found to be only a small percentage of the energy flux predicted by theory. The mismatch suggests an alternative fate to local dissipation for a significant fraction of the radiated energy.
    Description: SW acknowledges the support of the Grantham Institute for Climate Change, Imperial College London. ACNG acknowledges the support of a NERC Advanced Research Fellowship (Grant NE/C517633/1). KLP acknowledges support from Woods Hole Oceanographic Institution bridge support funds.
    Description: 2013-08-01
    Keywords: Diapycnal mixing ; Internal waves ; Turbulence
    Repository Name: Woods Hole Open Access Server
    Type: Article
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