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  • Gulf Stream  (6)
  • Baroclinic flows
  • Eddies
  • Ocean dynamics
  • American Geophysical Union  (10)
  • MDPI Publishing
  • 2020-2023  (10)
  • 1
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    Revisiting the seasonal cycle of the Timor throughflow: impacts of winds, waves and eddies (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-10-26
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Peña‐Molino, B., Sloyan, B., Nikurashin, M., Richet, O., & Wijffels, S. Revisiting the seasonal cycle of the Timor throughflow: impacts of winds, waves and eddies. Journal of Geophysical Research: Oceans, 127, (2022): e2021JC018133, https://doi.org/10.1029/2021jc018133.
    Description: The tropical Pacific and Indian Oceans are connected via a complex system of currents known as the Indonesian Throughflow (ITF). More than 30% of the variability in the ITF is linked to the seasonal cycle, influenced by the Monsoon winds. Despite previous efforts, a detailed knowledge of the ITF response to the components of the seasonal forcing is still lacking. Here, we describe the seasonal cycle of the ITF based on new observations of velocity and properties in Timor Passage, satellite altimetry and a high-resolution regional model. These new observations reveal a complex mean and seasonally varying flow field. The amplitude of the seasonal cycle in volume transport is approximately 6 Sv. The timing of the seasonal cycle, with semi-annual maxima (minima) in May and December (February and September), is controlled by the flow below 600 m associated with semi-annual Kelvin waves. The transport of thermocline waters (〈300 m) is less variable than the deep flow but larger in magnitude. This top layer is modulated remotely by cycles of divergence in the Banda Sea, and locally through Ekman transport, coastal upwelling, and non-linearities of the flow. The latter manifests through the formation of eddies that reduce the throughflow during the Southeast Monsoon, when is expected to be maximum. While the reduction in transport associated with the eddies is small, its impact on heat transport is large. These non-linear dynamics develop over small scales (〈10 km), and without high enough resolution, both observations and models will fail to capture them adequately.
    Description: B. Peña-Molino, B. M. Sloyan, M. Nikurashin, and O. Richet were supported by the Centre for Southern Hemisphere Oceans Research (CSHOR). CSHOR is a joint research Centre for Southern Hemisphere Ocean Research between QNLM and CSIRO. S. E. Wijffels was supported by the US National Science Foundation Grant No. OCE-1851333.
    Keywords: Indonesian Throughflow ; Timor Passage ; Seasonal cycle ; Moorings ; Transport ; Eddies
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Iron depletion in the deep chlorophyll maximum: mesoscale eddies as natural iron fertilization experiments (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-10-26
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hawco, N. J., Barone, B., Church, M. J., Babcock-Adams, L., Repeta, D. J., Wear, E. K., Foreman, R. K., Bjorkman, K. M., Bent, S., Van Mooy, B. A. S., Sheyn, U., DeLong, E. F., Acker, M., Kelly, R. L., Nelson, A., Ranieri, J., Clemente, T. M., Karl, D. M., & John, S. G. Iron depletion in the deep chlorophyll maximum: mesoscale eddies as natural iron fertilization experiments. Global Biogeochemical Cycles, 35(12), (2021): e2021GB007112, https://doi.org/10.1029/2021GB007112.
    Description: In stratified oligotrophic waters, phytoplankton communities forming the deep chlorophyll maximum (DCM) are isolated from atmospheric iron sources above and remineralized iron sources below. Reduced supply leads to a minimum in dissolved iron (dFe) near 100 m, but it is unclear if iron limits growth at the DCM. Here, we propose that natural iron addition events occur regularly with the passage of mesoscale eddies, which alter the supply of dFe and other nutrients relative to the availability of light, and can be used to test for iron limitation at the DCM. This framework is applied to two eddies sampled in the North Pacific Subtropical Gyre. Observations in an anticyclonic eddy center indicated downwelling of iron-rich surface waters, leading to increased dFe at the DCM but no increase in productivity. In contrast, uplift of isopycnals within a cyclonic eddy center increased supply of both nitrate and dFe to the DCM, and led to dominance of picoeukaryotic phytoplankton. Iron addition experiments did not increase productivity in either eddy, but significant enhancement of leucine incorporation in the light was observed in the cyclonic eddy, a potential indicator of iron stress among Prochlorococcus. Rapid cycling of siderophores and low dFe:nitrate uptake ratios also indicate that a portion of the microbial community was stressed by low iron. However, near-complete nitrate drawdown in this eddy, which represents an extreme case in nutrient supply compared to nearby Hawaii Ocean Time-series observations, suggests that recycling of dFe in oligotrophic ecosystems is sufficient to avoid iron limitation in the DCM under typical conditions.
    Description: The expedition and analyses were supported by the Simons Foundation SCOPE Grant 329108 to S. G. John, M. J. Church, D. J. Repeta, B. Van Mooy, E. F. DeLong, and D. M. Karl. N. J. Hawco was supported by a Simons Foundation Marine Microbial Ecology and Evolution postdoctoral fellowship (602538) and Simons Foundation grant 823167.
    Keywords: Chlorophyll ; Photosynthesis ; Iron limitation ; Oligotrophic ; Prochlorococcus ; Eddies
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  • 3
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    A survival analysis of the gulf stream warm core rings (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-10-26
    Description: Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 125(10),(2020): e2020JC016507, https://doi.org/10.1029/2020JC016507.
    Description: Survival of Gulf Stream (GS) warm core rings (WCRs) was investigated using a census consisting of a total of 961 rings formed during the period 1980–2017. Kaplan‐Meier survival probability and Cox hazard proportional models were used for the analysis. The survival analysis was performed for rings formed in four 5° zones between 75° W and 55° W. The radius, latitude, and distance from the shelf‐break of a WCR at formation all had a significant effect on the survival of WCRs. A pattern of higher survival was observed in WCRs formed in Zone 2 (70°–65° W) or Zone 3 (65°–60° W) and then demised in Zone 1 (75°–70° W). Survival probability of the WCRs increased to more than 70% for those formed within a latitude band from 39.5° to 41.5° N. Survival probability is reduced when the WCRs are formed near the New England Seamounts.
    Description: We are grateful for financial supports from NOAA (NA11NOS0120038), NSF (OCE‐1851242), SMAST, and UMass Dartmouth. G. G. was supported by NSF under grant OCE‐1851261.
    Description: 2021-04-14
    Keywords: Gulf Stream ; Warm core rings ; Survival analysis
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  • 4
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    On assessing ERA5 and MERRA2 representations of cold-air outbreaks across the Gulf Stream (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-10-20
    Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 48(19), (2021): e2021GL094364, https://doi.org/10.1029/2021GL094364.
    Description: The warm Gulf Stream sea surface temperatures strongly impact the evolution of winter clouds behind atmospheric cold fronts. Such cloud evolution remains challenging to model. The Gulf Stream is too wide within the ERA5 and MERRA2 reanalyses, affecting the turbulent surface fluxes. Known problems within the ERA5 boundary layer (too-dry and too-cool with too strong westerlies), ascertained primarily from ACTIVATE 2020 campaign aircraft dropsondes and secondarily from older buoy measurements, reinforce surface flux biases. In contrast, MERRA2 winter surface winds and air-sea temperature/humidity differences are slightly too weak, producing surface fluxes that are too low. Reanalyses boundary layer heights in the strongly forced winter cold-air-outbreak regime are realistic, whereas late-summer quiescent stable boundary layers are too shallow. Nevertheless, the reanalysis biases are small, and reanalyses adequately support their use for initializing higher-resolution cloud process modeling studies of cold-air outbreaks.
    Description: This work was supported by NASA grant 80NSSC19K0390 to ACTIVATE, a NASA Earth Venture Suborbital-3 (EVS-3) investigation funded by NASA's Earth Science Division and managed through the Earth System Science Pathfinder Program Office. The Pacific Northwest National Laboratory (PNNL) is operated for the US Department of Energy (DOE) by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.
    Description: 2022-03-08
    Keywords: Cold-air outbreaks ; Surface fluxes ; Gulf Stream ; ACTIVATE ; ERA5 ; MERRA2
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  • 5
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    Modeled three-dimensional currents and eddies on an alongshore-variable barred beach (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-10-20
    Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(7), (2021): e2020JC016899, https://doi.org/10.1029/2020JC016899.
    Description: Circulation in the nearshore region, which is critical for material transport along the coast and between the surf zone and the inner shelf, includes strong vortical motions. The horizontal length scales and vertical structure associated with vortical motions are not well documented on alongshore-variable beaches. Here, a three-dimensional phase-resolving numerical model, Simulating WAves till SHore (SWASH), is compared with surfzone waves and flows on a barred beach, and is used to investigate surfzone eddies. Model simulations with measured bathymetry reproduce trends in the mean surfzone circulation patterns, including alongshore currents and rip current circulation cells observed for offshore wave heights from 0.5 to 2.0 m and incident wave directions from 0 to 15° relative to shore normal. The length scales of simulated eddies, quantified using the alongshore wavenumber spectra of vertical vorticity, suggest that increasing wave directional spread intensifies small-scale eddies ( (10) m). Simulations with bathymetric variability ranging from alongshore uniform to highly alongshore variable indicate that large-scale eddies ( (100) m) may be enhanced by surfzone bathymetric variability, whereas small-scale eddies ( (10) m) are less dependent on bathymetric variability. The simulated vertical dependence of the magnitude and mean length scale (centroid) of the alongshore wavenumber spectra of vertical vorticity and very low-frequency (f ≈ 0.005 Hz) currents is weak in the outer surf zone, and decreases toward the shoreline. The vertical dependence in the simulations may be affected by the vertical structure of turbulence, mean shear, and bottom boundary layer dynamics.
    Description: Support was provided by the University of Washington Royalty Research Fund, the National Science Foundation, the Office of Naval Research, a National Defense Science and Engineering Graduate Fellowship, a Vannevar Bush Faculty Fellowship, the United States Army Corps of Engineers, the United States Coastal Research Program, Sea Grant, and the WHOI Investment in Science Program.
    Description: 2021-12-26
    Keywords: Surf zone ; Eddies ; Circulation ; Vorticity ; Wave breaking ; Modeling
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  • 6
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    Spatial and temporal variability of the Gulf Stream near Cape Hatteras (2021)
    American Geophysical Union
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    Publication Date: 2022-10-26
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Andres, M. Spatil and temporal variability of the Gulf Stream near Cape Hatteras. Journal of Geophysical Research: Oceans, 126(9), (2021): e2021JC017579, https://doi.org/10.1029/2021JC017579.
    Description: In situ observations from a 19-month deployment of current- and pressure-sensor equipped inverted echo sounders (CPIESs) along and across the Gulf Stream near Cape Hatteras capture spatial and temporal variability where this western boundary current separates from the continental margin. Regional hydrographic casts and two temperature cross-sections spanning the Gulf Stream southeast of Cape Hatteras are used with the CPIESs' records of acoustic travel time to infer changes in thermocline depth DT and Gulf Stream position. Wave-like Gulf Stream meanders are observed where the Stream approaches the separation location with periods less than 15 days, wavelengths less than 500-km, and phase speeds between 40 and 70 km d−1. Though meander amplitudes typically decrease by ∼30% on the final approach to Cape Hatteras, some signals are still coherent across the Gulf Stream separation location. Temporal variability in meander intensity may be related to the Loop Current ∼1,400 km upstream. Mesoscale variability is strongest downstream of the separation location where Gulf Stream position is no longer constrained by the steep continental slope. Low frequency transport changes in the Florida Straits are correlated with sea-surface height gradients along the entire South Atlantic Bight (SAB) and with DT inferred at the CPIES sites. The correlations with DT are likely due to coherent transport anomalies in the Gulf Stream approaching the separation location, which then drive Gulf Stream position changes downstream of the separation location. The patterns of coherent transport anomalies may reflect large-scale atmospheric forcing patterns or rapid equatorward propagation of barotropic signals along the SAB.
    Description: This research was supported by the National Science Foundation through grant OCE-1558521.
    Keywords: Altimetry ; Cape Hatteras ; CPIES ; Gulf Stream ; Meanders
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  • 7
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    Forecasting the Gulf Stream path using buoyancy and wind forcing over the North Atlantic (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-05-27
    Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(8), (2021): e2021JC017614, https://doi.org/10.1029/2021JC017614.
    Description: Fluctuations in the path of the Gulf Stream (GS) have been previously studied by primarily connecting to either the wind-driven subtropical gyre circulation or buoyancy forcing via the subpolar gyre. Here we present a statistical model for 1 year predictions of the GS path (represented by the GS northern wall—GSNW) between 75°W and 65°W incorporating both mechanisms in a combined framework. An existing model with multiple parameters including the previous year's GSNW index, center location, and amplitude of the Icelandic Low and the Southern Oscillation Index was augmented with basin-wide Ekman drift over the Azores High. The addition of the wind is supported by a validation of the simpler two-layer Parsons-Veronis model of GS separation over the last 40 years. A multivariate analysis was carried out to compare 1-year-in-advance forecast correlations from four different models. The optimal predictors of the best performing model include: (a) the GSNW index from the previous year, (b) gyre-scale integrated Ekman Drift over the past 2 years, and (c) longitude of the Icelandic Low center lagged by 3 years. The forecast correlation over the 27 years (1994–2020) is 0.65, an improvement from the previous multi-parameter model's forecast correlation of 0.52. The improvement is attributed to the addition of the wind-drift component. The sensitivity of forecasting the GS path after extreme atmospheric years is quantified. Results indicate the possibility of better understanding and enhanced predictability of the dominant wind-driven variability of the Atlantic Meridional Overturning Circulation and of fisheries management models that use the GS path as a metric.
    Description: The authors are grateful for financial supports from NSF (OCE-1851242), SMAST, and UMass Dartmouth. GG was supported by NSF under grants OCE-1657853 and OCE-1558521.
    Description: 2022-01-28
    Keywords: Gulf Stream ; Azores high ; Icelandic low ; forecasting ; AMOC ; North Atlantic
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  • 8
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    Variability of deep water in Jordan Basin of the Gulf of Maine: influence of Gulf Stream warm core rings and the Nova Scotia Current (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-05-27
    Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(5), (2021): e2020JC017136, https://doi.org/10.1029/2020JC017136.
    Description: As the nutrient-rich subsurface slope water intruding into the deep basin of the Gulf of Maine (GoM) supports the high biological productivity in the semi-enclosed gulf, it is important to understand the process and time scale of such slope water intrusion. This study focuses on variations of the GoM deep water on seasonal to interannual time scales and the influences of open ocean processes on the temporal variation of the deep water properties. Based on long-term monitoring data, it is found that the deep water at Jordan Basin (one of three major basins in the GoM) is persistently warmer in winter than in summer, which is distinctly different from the seasonality of surface water in the basin and the deep water on neighboring shelf seas. The unique seasonality in the deep GoM reflects a time-lagged response to shoreward intrusion of the subsurface slope water off the GoM. Both observation-based lag-correlation analyses and numerical simulations confirm a timescale of approximately 3 months for the intruding subsurface slope water to flow from Northeast Channel to Jordan Basin. Properties of the intruding slope water at the Northeast Channel were significantly correlated with the Gulf Stream position and dramatically impacted by episodic warm-core rings shed from Gulf Stream. Inside the deep GoM, the intruding slope water was also indirectly affected by the fresher water input from Nova Scotia Current. Spreading of the fresher water inside the gulf strengthens near-surface stratification, suppresses deep convection, and preserves heat and salt in the deep GoM during the wintertime.
    Description: This study is supported by the National Science Foundation through grant OCE 1634965.
    Description: 2021-11-06
    Keywords: Gulf of Maine deep water ; Gulf Stream ; mooring data ; numerical simulation ; remote sensing data ; warm-core ring
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  • 9
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    The role of wind stress in driving the along-shelf flow in the Northwest Atlantic Ocean (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-12-16
    Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(4), (2021): e2020JC016757, https://doi.org/10.1029/2020JC016757.
    Description: The along-shelf circulation in the Northwest Atlantic (NWA) Ocean is characterized by an equatorward flow from Greenland's south coast to Cape Hatters. The mean flow is considered to be primarily forced by freshwater discharges from rivers and glaciers while its variability is driven by both freshwater fluxes and wind stress. In this study, we hypothesize and test that the wind stress is important for the mean along-shelf flow. A two-layer model with realistic topography when forced by wind stress alone simulates a circulation system on the NWA shelves that is broadly consistent with that derived from observations, including an equatorward flow from Greenland coast to the Mid-Atlantic Bight (MAB). The along-shelf sea-level gradient is close to a previous estimate based on observations. The along-shelf flows exhibit strong seasonal variations with along-shelf transports being strong in fall/winter and weak in spring/summer, consistent with available observations. It is found that the NWA shelf circulation is affected by both wind-driven gyres through their western boundary currents and wind-stress forcing on the shelf especially along the coasts of Newfoundland and Labrador. The local wind stress forcing has more direct impacts on flows in shallower waters along the coast while the open-ocean gyres tend to affect the circulations along the outer shelf. Our conclusion is that wind stress is an important forcing of the main along-shelf flows in the NWA. One objective of this study is to motivate further examination of whether wind stress is as important as freshwater forcing for the mean flow.
    Description: Both Yang and Chen are also supported by NOAA Climate Program Office's Climate Variability and Prediction Program under grant NA20OAR4310398. JY is supported by Woods Hole Oceanographic Institution (WHOI) W. V. A. Clark Chair for Excellence in Oceanography and NSF Ocean Science Division under grant OCE1634886. Chen is supported by WHOI Independent Research and Development award.
    Description: 2021-09-30
    Keywords: Cross-shelf interactions ; Northwest Atlantic Ocean ; Numerical models ; Ocean dynamics ; Shelf flows ; Wind stress forcing
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  • 10
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    A census of the warm-core rings of the Gulf Stream: 1980-2017 (2020)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 125(8), (2020): e2019JC016033, doi:10.1029/2019JC016033.
    Description: A census of Gulf Stream (GS) warm‐core rings (WCRs) is presented based on 38 years (1980–2017) of data. The census documents formation and demise times and locations, and formation size for all 961 WCRs formed in the study period that live for a week or more. A clear regime shift was observed around the Year 2000 and was reported by a subset of authors (Gangopadhyay et al., 2019, https://doi.org/10.1038/s41598-019-48661-9). The WCR formation over the whole region (75–55°W) increased from an average of 18 per year during Regime 1 (1980–1999) to 33 per year during Regime 2 (2000–2017). For geographic analysis formation locations were grouped in four 5° zones between 75°W and 55°W. Seasonally, WCR formations show a significant summer maxima and winter minima, a pattern that is consistent through all zones and both temporal regimes. The lifespan and size distribution show progressively more rings with higher longevity and greater size when formed to the east of 70°W. The average lifespan of the WCRs in all four zones decreased by 20–40% depending on zones and/or seasons from Regime 1 to Regime 2, while the size distribution remained unchanged across regimes. The ring footprint index, a first‐order signature of impact of the WCRs on the slope, increased significantly (26–90%) for all zones from Regime 1 to Regime 2, with the highest percent increase in Zone 2 (70–65°W). This observational study establishes critical statistical and dynamical benchmarks for validating numerical models and highlights the need for further dynamical understanding of the GS‐ring formation processes.
    Description: The authors acknowledge financial support from NOAA (NA11NOS0120038), NSF (OCE‐0815679 and OCE‐1851242), and SMAST and UMass Dartmouth. G. G. was supported by NSF under Grant OCE‐1657853 as well as a Senior Scientist Chair from WHOI. We have benefitted from many discussions on Gulf Stream and WCR with Magdalena Andres, Andre Schmidt, Paula Fratantoni, Jon Hare, Wendell Brown, Kathy Donohue, Tom Rossby, Peter Cornillon, and Randy Watts.
    Description: 2020-12-29
    Keywords: Gulf Stream ; Warm Core Rings ; Regime‐Shift ; Lifespan of rings ; size of rings ; Ring Census
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