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  • Articles  (56)
  • Coastal flows  (33)
  • Baroclinic flows
  • American Meteorological Society  (56)
  • MDPI Publishing
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  • Articles  (56)
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  • American Meteorological Society  (56)
  • MDPI Publishing
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  • 1
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    Interannual and seasonal along-shelf current variability and dynamics: seventeen years of observations from the southern New England inner shelf (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-12-21
    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): 2923–2933, https://doi.org/10.1175/jpo-d-22-0064.1.
    Description: The characteristics and dynamics of depth-average along-shelf currents at monthly and longer time scales are examined using 17 years of observations from the Martha’s Vineyard Coastal Observatory on the southern New England inner shelf. Monthly averages of the depth-averaged along-shelf current are almost always westward, with the largest interannual variability in winter. There is a consistent annual cycle with westward currents of 5 cm s−1 in summer decreasing to 1–2 cm s−1 in winter. Both the annual cycle and interannual variability in the depth-average along-shelf current are predominantly driven by the along-shelf wind stress. In the absence of wind forcing, there is a westward flow of ∼5 cm s−1 throughout the year. At monthly time scales, the depth-average along-shelf momentum balance is primarily between the wind stress, surface gravity wave–enhanced bottom stress, and an opposing pressure gradient that sets up along the southern New England shelf in response to the wind. Surface gravity wave enhancement of bottom stress is substantial over the inner shelf and is essential to accurately estimating the bottom stress variation across the inner shelf.
    Description: The National Science Foundation, Woods Hole Oceanographic Institution, the Massachusetts Technology Collaborative, and the Office of Naval Research have supported the construction and maintenance of MVCO. The analysis presented here was partially funded by the National Science Foundation under Grants OCE 1558874 and OCE 1655686.
    Keywords: Continental shelf/slope ; Coastal flows ; Momentum ; Ocean dynamics ; Wind stress
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Turbulent thermal-wind driven coastal upwelling: current observations and dynamics (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-12-21
    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): 2909-2921, https://doi.org/10.1175/jpo-d-22-0063.1.
    Description: A remarkably consistent Lagrangian upwelling circulation at monthly and longer time scales is observed in a 17-yr time series of current profiles in 12 m of water on the southern New England inner shelf. The upwelling circulation is strongest in summer, with a current magnitude of ∼1 cm s−1, which flushes the inner shelf in ∼2.5 days. The average winter upwelling circulation is about one-half of the average summer upwelling circulation, but with larger month-to-month variations driven, in part, by cross-shelf wind stresses. The persistent upwelling circulation is not wind-driven; it is driven by a cross-shelf buoyancy force associated with less-dense water near the coast. The cross-shelf density gradient is primarily due to temperature in summer, when strong surface heating warms shallower nearshore water more than deeper offshore water, and to salinity in winter, caused by fresher water near the coast. In the absence of turbulent stresses, the cross-shelf density gradient would be in a geostrophic, thermal-wind balance with the vertical shear in the along-shelf current. However, turbulent stresses over the inner shelf attributable to strong tidal currents and wind stress cause a partial breakdown of the thermal-wind balance that releases the buoyancy force, which drives the observed upwelling circulation. The presence of a cross-shelf density gradient has a profound impact on exchange across this inner shelf. Many inner shelves are characterized by turbulent stresses and cross-shelf density gradients with lighter water near the coast, suggesting turbulent thermal-wind-driven coastal upwelling may be a broadly important cross-shelf exchange mechanism.
    Description: The National Science Foundation, Woods Hole Oceanographic Institution, the Massachusetts Technology Collaborative, and the Office of Naval Research have supported the construction and maintenance of MVCO. The analysis presented here was partially funded by the National Science Foundation under Grants OCE 1558874 and OCE 1655686.
    Keywords: Buoyancy ; Coastal flows ; Currents ; Dynamics ; Lagrangian circulation/transport ; Upwelling/downwelling
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  • 3
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    A three-dimensional inertial model for coastal upwelling along western boundaries (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-10-12
    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(10), (2022): 2431-2444, https://doi.org/10.1175/jpo-d-22-0024.1.
    Description: A three-dimensional inertial model that conserves quasigeostrophic potential vorticity is proposed for wind-driven coastal upwelling along western boundaries. The dominant response to upwelling favorable winds is a surface-intensified baroclinic meridional boundary current with a subsurface countercurrent. The width of the current is not the baroclinic deformation radius but instead scales with the inertial boundary layer thickness while the depth scales as the ratio of the inertial boundary layer thickness to the baroclinic deformation radius. Thus, the boundary current scales depend on the stratification, wind stress, Coriolis parameter, and its meridional variation. In contrast to two-dimensional wind-driven coastal upwelling, the source waters that feed the Ekman upwelling are provided over the depth scale of this baroclinic current through a combination of onshore barotropic flow and from alongshore in the narrow boundary current. Topography forces an additional current whose characteristics depend on the topographic slope and width. For topography wider than the inertial boundary layer thickness the current is bottom intensified, while for narrow topography the current is wave-like in the vertical and trapped over the topography within the inertial boundary layer. An idealized primitive equation numerical model produces a similar baroclinic boundary current whose vertical length scale agrees with the theoretical scaling for both upwelling and downwelling favorable winds.
    Description: This research is supported in part by the China Scholarship Council (201906330102). H. G. is financially supported by the China Scholarship Council to study at WHOI for 2 years as a guest student. M.S. is supported by the National Science Foundation Grant OCE-1922538. Z. C. is supported by the ‘Taishan/Aoshan’ Talents program (2017ASTCPES05) the Fundamental Research Funds for the Central Universities (202072001).
    Description: 2023-03-30
    Keywords: Ekman pumping/transport ; Upwelling/downwelling ; Coastal flows
    Repository Name: Woods Hole Open Access Server
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  • 4
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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
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Impact of shelf valleys on the spread of surface-trapped river plumes (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): 247-266, https://doi.org/10.1175/JPO-D-20-0098.1.
    Description: This study focuses on mechanisms of shelf valley bathymetry affecting the spread of riverine freshwater in the nearshore region. In the context of Changjiang River, a numerical model is used with different no-tide idealized configurations to simulate development of unforced river plumes over a sloping bottom, with and without a shelf valley off the estuary mouth. All simulated freshwater plumes are surface-trapped with continuously growing bulges near the estuary mouth and narrow coastal currents downstream. The simulations indicate that a shelf valley tends to compress the bulge along the direction of the valley long axis and modify the incident angle of the bulge flow impinging toward the coast, which then affects the strength of the coastal current. The bulge compression results from geostrophic adjustment and isobath-following tendency of the depth-averaged flow in the bulge region. Generally, the resulting change in the direction of the bulge impinging flow enhances down-shelf momentum advection and freshwater delivery into the coastal current. Sensitivity simulations with altered river discharges Q, Coriolis parameter, shelf bottom slope, valley geometry, and ambient stratification show that enhancement of down-shelf freshwater transport in the coastal current, ΔQc, increases with increasing valley depth within the bulge region and decreasing slope Burger number of the ambient shelf. Assuming potential vorticity conservation, a scaling formula of ΔQc/Q is developed, and it agrees well with results of the sensitivity simulations. Mechanisms of valley influences on unforced river plumes revealed here will help future studies of topographic influence on river plumes under more realistic conditions.
    Description: This work is conducted by Canbo Xiao and Weifeng (Gordon) Zhang during CX’s one-year visit at Woods Hole Oceanographic Institution (WHOI) in 2018–19. CX was supported by China Scholarship Council.
    Keywords: Continental shelf/slope ; Buoyancy ; Coastal flows ; Topographic effects ; Runoff ; Numerical analysis/modeling
    Repository Name: Woods Hole Open Access Server
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  • 6
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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
    Repository Name: Woods Hole Open Access Server
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  • 7
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    Frontogenesis and variability in Denmark Strait and its influence on overflow water (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 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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  • 8
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    Improving surface current resolution using direction finding algorithms for multiantenna high-frequency radars (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 the Atmospheric and Oceanic Technology 36(10), (2019): 1997-2014, doi: 10.1175/JTECH-D-19-0029.1.
    Description: While land-based high-frequency (HF) radars are the only instruments capable of resolving both the temporal and spatial variability of surface currents in the coastal ocean, recent high-resolution views suggest that the coastal ocean is more complex than presently deployed radar systems are able to reveal. This work uses a hybrid system, having elements of both phased arrays and direction finding radars, to improve the azimuthal resolution of HF radars. Data from two radars deployed along the U.S. East Coast and configured as 8-antenna grid arrays were used to evaluate potential direction finding and signal, or emitter, detection methods. Direction finding methods such as maximum likelihood estimation generally performed better than the well-known multiple signal classification (MUSIC) method given identical emitter detection methods. However, accurately estimating the number of emitters present in HF radar observations is a challenge. As MUSIC’s direction-of-arrival (DOA) function permits simple empirical tests that dramatically aid the detection process, MUSIC was found to be the superior method in this study. The 8-antenna arrays were able to provide more accurate estimates of MUSIC’s noise subspace than typical 3-antenna systems, eliminating the need for a series of empirical parameters to control MUSIC’s performance. Code developed for this research has been made available in an online repository.
    Description: This analysis was supported by NSF Grants OCE-1657896 and OCE-1736930 to Kirincich, OCE-1658475 to Emery and Washburn and OCE-1736709 to Flament. Flament is also supported by NOAA’s Integrated Ocean Observing System through Award NA11NOS0120039. The authors thank Lindsey Benjamin, Alma Castillo, Ken Constantine, Benedicte Dousset, Ian Fernandez, Mael Flament, Dave Harris, Garrett Hebert, Ben Hodges, Victoria Futch, Matt Guanci, and Philip Moravcik for assistance in building, deploying, and operating the radars.
    Description: 2020-04-11
    Keywords: Ocean ; Coastal flows ; Algorithms ; Radars/Radar observations ; Remote sensing
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  • 9
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    Reversed lateral circulation in a sharp estuarine bend with weak stratification (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 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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  • 10
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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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  • 11
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    Coral reef drag coefficients—surface gravity wave enhancement (2018)
    American Meteorological Society
    Details
    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): 1555-1566, doi:10.1175/JPO-D-17-0231.1.
    Description: A primary challenge in modeling flow over shallow coral reefs is accurately characterizing the bottom drag. Previous studies over continental shelves and sandy beaches suggest surface gravity waves should enhance the drag on the circulation over coral reefs. The influence of surface gravity waves on drag over four platform reefs in the Red Sea is examined using observations from 6-month deployments of current and pressure sensors burst sampling at 1Hz for 4–5min. Depth-average current fluctuations U0 within each burst are dominated by wave orbital velocities uw that account for 80%–90%of the burst variance and have a magnitude of order 10 cm s21, similar to the lower-frequency depth-average current Uavg. Previous studies have shown that the cross-reef bottom stress balances the pressure gradient over these reefs. A bottom stress estimate that neglects the waves (rCdaUavgjUavgj, where r is water density and Cda is a drag coefficient) balances the observed pressure gradient when uw is smaller than Uavg but underestimates the pressure gradient when uw is larger than Uavg (by a factor of 3–5 when uw 5 2Uavg), indicating the neglected waves enhance the bottom stress. In contrast, a bottom stress estimate that includes the waves [rCda(Uavg 1 U0)jUavg 1 U0j)] balances the observed pressure gradient independent of the relative size of uw and Uavg, indicating that this estimate accounts for the wave enhancement of the bottom stress. A parameterization proposed by Wright and Thompson provides a reasonable estimate of the total bottom stress (including the waves) given the burst-averaged current and the wave orbital velocity.
    Description: The Red Sea field program was supported by Awards USA 00002 and KSA 00011 made by KAUST. S. Lentz was supported for the analysis by NSF Award OCE-1558343.
    Description: 2019-01-13
    Keywords: Coastal flows ; Currents ; Dynamics ; Gravity waves ; Turbulence
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  • 12
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    Evolution of the freshwater coastal current at the southern tip of Greenland (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): 2127-2140, doi:10.1175/JPO-D-18-0035.1.
    Description: Shipboard hydrographic and velocity measurements collected in summer 2014 are used to study the evolution of the freshwater coastal current in southern Greenland as it encounters Cape Farewell. The velocity structure reveals that the coastal current maintains its identity as it flows around the cape and bifurcates such that most of the flow is diverted to the outer west Greenland shelf, while a small portion remains on the inner shelf. Taking into account this inner branch, the volume transport of the coastal current is conserved, but the freshwater transport decreases on the west side of Cape Farewell. A significant amount of freshwater appears to be transported off the shelf where the outer branch flows adjacent to the shelfbreak circulation. It is argued that the offshore transposition of the coastal current is caused by the flow following the isobaths as they bend offshore because of the widening of the shelf on the west side of Cape Farewell. An analysis of the potential vorticity shows that the subsequent seaward flux of freshwater can be enhanced by instabilities of the current. This set of circumstances provides a pathway for the freshest water originating from the Arctic, as well as runoff from the Greenland ice sheet, to be fluxed into the interior Labrador Sea where it could influence convection in the basin.
    Description: Funding for this project was provided by the National Science Foundation under Grant OCE-1259618.
    Description: 2019-03-11
    Keywords: Boundary currents ; Coastal flows ; Instability ; Ocean circulation ; Potential vorticity ; Transport
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  • 13
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    Submesoscale processes at shallow salinity fronts in the Bay of Bengal : observations during the winter monsoon (2018)
    American Meteorological Society
    Details
    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
    Repository Name: Woods Hole Open Access Server
    Type: Article
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    On the dynamics and water mass transformation of a boundary current connecting alpha and beta oceans (2018)
    American Meteorological Society
    Details
    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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  • 15
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    The dynamics of shelf forcing in Greenlandic fjords (2018)
    American Meteorological Society
    Details
    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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  • 16
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    Shelf–open ocean exchange forced by wind jets (2018)
    American Meteorological Society
    Details
    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): 163-174, doi:10.1175/JPO-D-17-0161.1.
    Description: The general problem of exchange from a shallow shelf across sharp topography to the deep ocean forced by narrow, cross-shelf wind jets is studied using quasigeostrophic theory and an idealized primitive equation numerical model. Interest is motivated by katabatic winds that emanate from narrow fjords in southeast Greenland, although similar topographically constrained wind jets are found throughout the world’s oceans. Because there is no net vorticity input by the wind, the circulation is largely confined to the region near the forcing. Circulation over the shelf is limited by bottom friction for weakly stratified flows, but stratification allows for much stronger upper-layer flows that are regulated by weak coupling to the lower layer. Over the sloping topography, the topographic beta effect limits the deep flow, while, for sufficient stratification, the upper-layer flow can cross the topography to connect the shelf to the open ocean. This can be an effective transport mechanism even for short, strong wind events because damping of the upper-layer flow is weak. A variety of transients are generated for an abrupt onset of winds, including short topography Rossby waves, long topographic Rossby waves, and inertial waves. Using parameters representative of southeast Greenland, katabatic wind events will force an offshore transport of O(0.4) Sv (1 Sv ≡ 106 m3 s−1) that, when considered for 2 days, will result in an offshore flux of O(5 × 1010) m3.
    Description: MAS was supported by the National Science Foundation under Grant OCE-1533170.
    Description: 2018-07-18
    Keywords: Coastal flows ; Downslope winds ; Ocean dynamics
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    Estuarine exchange flow is related to mixing through the salinity variance budget (2018)
    American Meteorological Society
    Details
    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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  • 18
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    Surface cooling, winds, and eddies over the continental shelf (2018)
    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 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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  • 19
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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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    Comparison of rip current hazard likelihood forecasts with observed rip current speeds (2017)
    American Meteorological Society
    Details
    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 Weather and Forecasting 32 (2017): 1659-1666, doi:10.1175/WAF-D-17-0076.1.
    Description: Although rip currents are a major hazard for beachgoers, the relationship between the danger to swimmers and the physical properties of rip current circulation is not well understood. Here, the relationship between statistical model estimates of hazardous rip current likelihood and in situ velocity observations is assessed. The statistical model is part of a forecasting system that is being made operational by the National Weather Service to predict rip current hazard likelihood as a function of wave conditions and water level. The temporal variability of rip current speeds (offshore-directed currents) observed on an energetic sandy beach is correlated with the hindcasted hazard likelihood for a wide range of conditions. High likelihoods and rip current speeds occurred for low water levels, nearly shore-normal wave angles, and moderate or larger wave heights. The relationship between modeled hazard likelihood and the frequency with which rip current speeds exceeded a threshold was assessed for a range of threshold speeds. The frequency of occurrence of high (threshold exceeding) rip current speeds is consistent with the modeled probability of hazard, with a maximum Brier skill score of 0.65 for a threshold speed of 0.23 m s−1, and skill scores greater than 0.60 for threshold speeds between 0.15 and 0.30 m s−1. The results suggest that rip current speed may be an effective proxy for hazard level and that speeds greater than ~0.2 m s−1 may be hazardous to swimmers.
    Description: Funding was provided by the National Science Foundation (1232910, 1332705, and 1536365), and by National Security Science and Engineering and Vannevar Bush Faculty Fellowships funded by the assistant secretary of Defense for Research and Engineering.
    Description: 2018-02-28
    Keywords: Coastlines ; Coastal flows ; Waves, oceanic ; Forecast verification/skill ; Probability forecasts/models/distribution ; Statistical forecasting
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    Total exchange flow, entrainment, and diffusive salt flux in estuaries (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 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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    Surface wave effects on the translation of wind stress across the air–sea interface in a fetch-limited, coastal embayment (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 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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    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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    Coupled ocean–atmosphere offshore decay scale of cold SST signals along upwelling eastern boundaries (2017)
    American Meteorological Society
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    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 Climate 29 (2016): 8317-8331, doi:10.1175/JCLI-D-16-0109.1.
    Description: A simple analytic model is developed to represent the offshore decay of cold sea surface temperature (SST) signals that originate from wind-driven upwelling at a coastal boundary. The model couples an oceanic mixed layer to an atmospheric boundary layer through wind stress and air–sea heat exchange. The primary mechanism that controls SST is a balance between Ekman advection and air–sea exchange. The offshore penetration of the cold SST signal decays exponentially with a length scale that is the product of the ocean Ekman velocity and a time scale derived from the air–sea heat flux and the radiative balance in the atmospheric boundary layer. This cold SST signal imprints on the atmosphere in terms of both the boundary layer temperature and surface wind. Nonlinearities due to the feedback between SST and atmospheric wind, baroclinic instability, and thermal wind in the atmospheric boundary layer all slightly modify this linear theory. The decay scales diagnosed from two-dimensional and three-dimensional eddy-resolving numerical ocean models are in close agreement with the theory, demonstrating that the basic physics represented by the theory remain dominant even in these more complete systems. Analysis of climatological SST off the west coast of the United States also shows a decay of the cold SST anomaly with scale roughly in agreement with the theory.
    Description: MASwas supported by the Andrew W. Mellon Foundation Endowed Fund for Innovative Research and the National Science Foundation under Grant OCE-1433170 and PLR-1415489. NS was supported by the National Aeronautics and Space Administration under Grant NNX14AL83G, the Department of Energy, Office of Science Grant DE-SC0006766, and the Japan Agency for Marine-Earth Science and Technology as part of the JAMSTEC-IPRC Joint Investigations.
    Description: 2017-05-03
    Keywords: Coastal flows ; Ekman pumping/transport ; Ocean dynamics
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    Downfront winds over buoyant coastal plumes (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): 3139-3154, doi:10.1175/JPO-D-16-0042.1.
    Description: Downfront, or downwelling favorable, winds are commonly found over buoyant coastal plumes. It is known that these winds can result in mixing of the plume with the ambient water and that the winds influence the transport, spatial extent, and stability of the plumes. In the present study, the interaction of the Ekman velocity in the surface layer and baroclinic instability supported by the strong horizontal density gradient of the plume is explored with the objective of understanding the potential vorticity and buoyancy budgets. The approach makes use of an idealized numerical model and scaling theory. It is shown that when winds are present the weak stratification resulting from vertical mixing and the strong baroclinicity of the front results in near-zero average potential vorticity q. For weak to moderate winds, the reduction of q by diapycnal mixing is balanced by the generation of q through the geostrophic stress term in the regions of strong horizontal density gradients and stable stratification. However, for very strong winds the wind stress overwhelms the geostrophic stress and leads to a reduction in q, which is balanced by the vertical mixing term. In the absence of winds, the geostrophic stress dominates mixing and the flow rapidly restratifies. Nonlinearity, extremes of relative vorticity and vertical velocity, and mixing are all enhanced by the presence of a coast. Scaling estimates developed for the eddy buoyancy flux, the surface potential vorticity flux, and the diapycnal mixing rate compare well with results diagnosed from a series of numerical model calculations.
    Description: This study was supported by NSF Grants OCE-1433170 (MAS) and OCE-1459677 (LNT).
    Description: 2017-04-07
    Keywords: Coastal flows ; Ekman pumping/transport ; Mesoscale processes ; Wind stress
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    Heat, salt, and freshwater budgets for a glacial fjord in Greenland (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): 2735-2768, doi:10.1175/JPO-D-15-0134.1.
    Description: In Greenland’s glacial fjords, heat and freshwater are exchanged between glaciers and the ocean. Submarine melting of glaciers has been implicated as a potential trigger for recent glacier acceleration, and observations of ocean heat transport are increasingly being used to infer the submarine melt rates. The complete heat, salt, and mass budgets that underlie such methods, however, have been largely neglected. Here, a new framework for exploring glacial fjord budgets is developed. Building on estuarine studies of salt budgets, the heat, salt, and mass transports through the fjord are decomposed, and new equations for calculating freshwater fluxes from submarine meltwater and runoff are presented. This method is applied to moored records from Sermilik Fjord, near the terminus of Helheim Glacier, to evaluate the dominant balances in the fjord budgets and to estimate freshwater fluxes. Throughout the year, two different regimes are found. In the nonsummer months, advective transports are balanced by changes in heat/salt storage within their ability to measure; freshwater fluxes cannot be inferred as a residual. In the summer, a mean exchange flow emerges, consisting of inflowing Atlantic water and outflowing glacially modified water. This exchange transports heat toward the glacier and is primarily balanced by changes in storage and latent heat for melting ice. The total freshwater flux increases over the summer, reaching 1200 ± 700 m3 s−1 of runoff and 1500 ± 500 m3 s−1 of submarine meltwater from glaciers and icebergs in August. The methods and results highlight important components of fjord budgets, particularly the storage and barotropic terms, that have been not been appropriately considered in previous estimates of submarine melting.
    Description: The data collection and analysis was funded by NSF Grants ARC-0909373, OCE-113008, and OCE-1434041.
    Keywords: Geographic location/entity ; Estuaries ; Glaciers ; Circulation/ Dynamics ; Coastal flows ; Atm/Ocean Structure/ Phenomena ; Freshwater ; Snowmelt/icemelt ; Observational techniques and algorithms ; In situ oceanic observations
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    Investigating the eddy diffusivity concept in the coastal ocean (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): 2201-2218, doi:10.1175/JPO-D-16-0020.1.
    Description: This paper aims to test the validity, utility, and limitations of the lateral eddy diffusivity concept in a coastal environment through analyzing data from coupled drifter and dye releases within the footprint of a high-resolution (800 m) high-frequency radar south of Martha’s Vineyard, Massachusetts. Specifically, this study investigates how well a combination of radar-based velocities and drifter-derived diffusivities can reproduce observed dye spreading over an 8-h time interval. A drifter-based estimate of an anisotropic diffusivity tensor is used to parameterize small-scale motions that are unresolved and underresolved by the radar system. This leads to a significant improvement in the ability of the radar to reproduce the observed dye spreading.
    Description: IR, AK, and SL were supported by the NSF OCE Grant 1332646. IR was also supported by NASA Grant NNX14AH29G.
    Description: 2016-12-29
    Keywords: Circulation/ Dynamics ; Coastal flows ; Diffusion ; Lagrangian circulation/transport ; Observational techniques and algorithms ; Radars/Radar observations ; Models and modeling ; Tracers
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  • 28
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    Estuarine frontogenesis (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): 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
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    Mechanisms of tidal oscillatory salt transport in a partially stratified estuary (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): 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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    An inertial model of the interaction of Ekman layers and planetary islands (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 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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    Modeling and analysis of internal-tide generation and beamlike onshore propagation in the vicinity of shelfbreak canyons (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 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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    Eulerian and Lagrangian correspondence of high-frequency radar and surface drifter data : effects of radar resolution and flow components (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 Atmospheric and Oceanic Technology 31 (2014): 945–966, doi:10.1175/JTECH-D-13-00146.1.
    Description: This study investigated the correspondence between the near-surface drifters from a mass drifter deployment near Martha’s Vineyard, Massachusetts, and the surface current observations from a network of three high-resolution, high-frequency radars to understand the effects of the radar temporal and spatial resolution on the resulting Eulerian current velocities and Lagrangian trajectories and their predictability. The radar-based surface currents were found to be unbiased in direction but biased in magnitude with respect to drifter velocities. The radar systematically underestimated velocities by approximately 2 cm s−1 due to the smoothing effects of spatial and temporal averaging. The radar accuracy, quantified by the domain-averaged rms difference between instantaneous radar and drifter velocities, was found to be about 3.8 cm s−1. A Lagrangian comparison between the real and simulated drifters resulted in the separation distances of roughly 1 km over the course of 10 h, or an equivalent separation speed of approximately 2.8 cm s−1. The effects of the temporal and spatial radar resolution were examined by degrading the radar fields to coarser resolutions, revealing the existence of critical scales (1.5–2 km and 3 h) beyond which the ability of the radar to reproduce drifter trajectories decreased more rapidly. Finally, the importance of the different flow components present during the experiment—mean, tidal, locally wind-driven currents, and the residual velocities—was analyzed, finding that, during the study period, a combination of tidal, locally wind-driven, and mean currents were insufficient to reliably reproduce, with minimal degradation, the trajectories of real drifters. Instead, a minimum combination of the tidal and residual currents was required.
    Description: I.R. was supported by the WHOI Coastal Ocean Institute Project 27040148 and by the WHOI Access to the Sea Program 27500036. I.R. and A.K. acknowledge support fromthe NSF project 83264600. A.K. acknowledges support from the Massachusetts Clean Energy Center (MassCEC) via the New England Marine Renewable Energy Center (MREC).
    Description: 2014-10-01
    Keywords: Coastal flows ; Currents ; Lagrangian circulation/transport ; Trajectories ; Radars/Radar observations
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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
    Repository Name: Woods Hole Open Access Server
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    Inner-shelf response to cross-chelf wind stress : the importance of the cross-shelf density gradient in an idealized numerical model and field observations (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 Physical Oceanography 44 (2014): 86–103, doi:10.1175/JPO-D-13-075.1.
    Description: This study investigates the effects of horizontal and vertical density gradients on the inner-shelf response to cross-shelf wind stress by using an idealized numerical model and observations from a moored array deployed south of Martha’s Vineyard, Massachusetts. In two-dimensional (no along-shelf variation) numerical model runs of an initially stratified shelf, a cross-shelf wind stress drives vertical mixing that results in a nearly well-mixed inner shelf with a cross-shelf density gradient because of the sloping bottom. The cross-shelf density gradient causes an asymmetric response to on- and offshore wind stresses. For density increasing offshore, an offshore wind stress drives a near-surface offshore flow and near-bottom onshore flow that slightly enhances the vertical stratification and the cross-shelf circulation. An onshore wind stress drives the reverse cross-shelf circulation reducing the vertical stratification and the cross-shelf circulation. A horizontal Richardson number is shown to be the nondimensional parameter that controls the dependence of the wind-driven nondimensional cross-shelf transport on the cross-shelf density gradient. Field observations show the same empirical relationship between the horizontal Richardson number and transport fraction as the model predicts. These results show that it is the cross-shelf rather than vertical density gradient that is critical to predicting the inner-shelf cross-shelf transport driven by a cross-shelf wind stress.
    Description: This work was funded by Ocean Sciences Division of the National Science Foundation Grant OCE-0548961 and by the Woods Hole Oceanographic Institution through the Academic Programs Office and the Coastal Ocean Institute. Data central to this study were provided by the Martha’s Vineyard Coastal Observatory, which is funded by WHOI and the Jewett/EDUC/Harrison Foundation.
    Description: 2014-07-01
    Keywords: Circulation/ Dynamics ; Coastal flows ; Circulation/ Dynamics ; Upwelling/downwelling
    Repository Name: Woods Hole Open Access Server
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    Aerial imaging of fluorescent dye in the near shore (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 Atmospheric and Oceanic Technology 31 (2014): 1410–1421, doi:10.1175/JTECH-D-13-00230.1.
    Description: Aerial images are used to quantify the concentration of fluorescent Rhodamine water tracing (WT) dye in turbid and optically deep water. Tracer releases near the shoreline of an ocean beach and near a tidal inlet were observed with a two-band multispectral camera and a pushbroom hyperspectral imager, respectively. The aerial observations are compared with near-surface in situ measurements. The ratio of upwelling radiance near the Rhodamine WT excitation and emission peaks varies linearly with the in situ dye concentrations for concentrations 〈20 ppb (r2 = 0.70 and r2 = 0.85–0.88 at the beach and inlet, respectively). The linear relationship allows for relative tracer concentration estimates without in situ calibration. The O(1 m) image pixels resolve complex flow structures on the inner shelf that transport and mix tracer.
    Description: We thank ONR and NSF for funding this work.
    Description: 2014-12-01
    Keywords: Coastal flows ; Mixing ; Transport ; Aircraft observations ; Remote sensing ; Tracers
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    Interaction of Ekman layers and islands (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): 1028–1041, doi:10.1175/JPO-D-12-0159.1.
    Description: The circulation induced by the interaction of surface Ekman transport with an island is considered using both numerical models and linear theory. The basic response is similar to that found for the interaction of Ekman layers and an infinite boundary, namely downwelling (upwelling) in narrow boundary layers and deformation-scale baroclinic boundary layers with associated strong geostrophic flows. The presence of the island boundary, however, allows the pressure signal to propagate around the island so that the regions of upwelling and downwelling are dynamically connected. In the absence of stratification the island acts as an effective barrier to the Ekman transport. The presence of stratification supports baroclinic boundary currents that provide an advective pathway from one side of the island to the other. The resulting steady circulation is quite complex. Near the island, both geostrophic and ageostrophic velocity components are typically large. The density anomaly is maximum below the surface and, for positive wind stress, exhibits an anticyclonic phase rotation with depth (direction of Kelvin wave propagation) such that anomalously warm water can lie below regions of Ekman upwelling. The horizontal and vertical velocities exhibit similar phase changes with depth. The addition of a sloping bottom can act to shield the deep return flow from interacting with the island and providing mass transport into/out of the surface Ekman layer. In these cases, the required transport is provided by a pair of recirculation gyres that connect the narrow upwelling/downwelling boundary layers on the eastern and western sides of the island, thus directly connecting the Ekman transport across the island.
    Description: This study was supported by the National Science Foundation under Grants OCE-0826656 and OCE-0959381 (MAS), and OCE-0925061 (JP).
    Description: 2013-11-01
    Keywords: Coastal flows ; Ekman pumping/transport ; Ocean dynamics
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    Residual sediment fluxes in weakly-to-periodically stratified estuaries and tidal inlets (2013)
    American Meteorological Society
    Details
    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): 1841–1861, doi:10.1175/JPO-D-12-0231.1.
    Description: In this idealized numerical modeling study, the composition of residual sediment fluxes in energetic (e.g., weakly or periodically stratified) tidal estuaries is investigated by means of one-dimensional water column models, with some focus on the sediment availability. Scaling of the underlying dynamic equations shows dependence of the results on the Simpson number (relative strength of horizontal density gradient) and the Rouse number (relative settling velocity) as well as impacts of the Unsteadiness number (relative tidal frequency). Here, the parameter space given by the Simpson and Rouse numbers is mainly investigated. A simple analytical model based on the assumption of stationarity shows that for small Simpson and Rouse numbers sediment flux is down estuary and vice versa for large Simpson and Rouse numbers. A fully dynamic water column model coupled to a second-moment turbulence closure model allows to decompose the sediment flux profiles into contributions from the transport flux (product of subtidal velocity and sediment concentration profiles) and the fluctuation flux profiles (tidal covariance between current velocity and sediment concentration). Three different types of bottom sediment pools are distinguished to vary the sediment availability, by defining a time scale for complete sediment erosion. For short erosion times scales, the transport sediment flux may dominate, but for larger erosion time scales the fluctuation sediment flux largely dominates the tidal sediment flux. When quarter-diurnal components are added to the tidal forcing, up-estuary sediment fluxes are strongly increased for stronger and shorter flood tides and vice versa. The theoretical results are compared to field observations in a tidally energetic inlet.
    Description: Project funding was provided by the German Research Foundation (DFG) in the framework of the Project ECOWS (Role of Estuarine Circulation for Transport of Suspended Particulate Matter in the Wadden Sea, BU 1199/11) and by the German Federal Ministry of Research and Education in the framework of the Project PACE [The future of the Wadden Sea sediment fluxes: still keeping pace with sea level rise? (FKZ 03F0634A)].
    Description: 2014-03-01
    Keywords: Channel flows ; Coastal flows ; Mixing ; Transport ; Turbulence ; Single column models
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    The spatial structure of tidal and mean circulation over the inner shelf south of Martha's Vineyard, Massachusetts (2013)
    American Meteorological Society
    Details
    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): 1940–1958, doi:10.1175/JPO-D-13-020.1.
    Description: The spatial structure of the tidal and background circulation over the inner shelf south of Martha's Vineyard, Massachusetts, was investigated using observations from a high-resolution, high-frequency coastal radar system, paired with satellite SSTs and in situ ADCP velocities. Maximum tidal velocities for the dominant semidiurnal constituent increased from 5 to 35 cm s−1 over the 20-km-wide domain with phase variations up to 60°. A northeastward jet along the eastern edge and a recirculation region inshore dominated the annually averaged surface currents, along with a separate along-shelf jet offshore. Owing in part to this variable circulation, the spatial structure of seasonal SST anomalies had implications for the local heat balance. Cooling owing to the advective heat flux divergence was large enough to offset more than half of the seasonal heat gain owing to surface heat flux. Tidal stresses were the largest terms in the mean along- and across-shelf momentum equations in the area of the recirculation, with residual wind stress and the Coriolis term dominating to the west and south, respectively. The recirculation was strongest in summer, with mean winds and tidal stresses accounting for much of the differences between summer and winter mean circulation. Despite the complex bathymetry and short along-shelf spatial scales, a simple model of tidal rectification was able to recreate the features of the northeastward jet and match an estimate of the across-shelf structure of sea surface height inferred from the residual of the momentum analysis.
    Description: 2014-03-01
    Keywords: Coastal flows ; Momentum ; Sea surface temperature ; Tides ; Surface observations
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    A comparison of methods for estimating Reynolds stress from ADCP measurements in wavy environments (2012)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2011. 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 28 (2011): 1539–1553, doi:10.1175/JTECH-D-11-00001.1.
    Description: Turbulent Reynolds stresses are now routinely estimated from acoustic Doppler current profiler (ADCP) measurements in estuaries and tidal channels using the variance method, yet biases due to surface gravity waves limit its use in the coastal ocean. Recent modifications to this method, including spatially filtering velocities to isolate the turbulence from wave velocities and fitting a cospectral model to the below-wave band cospectra, have been used to remove this bias. Individually, each modification performed well for the published test datasets, but a comparative analysis over the range of conditions in the coastal ocean has not yet been performed. This work uses ADCP velocity measurements from five previously published coastal ocean and estuarine datasets, which span a range of wave and current conditions as well as instrument configurations, to directly compare methods for estimating stresses in the presence of waves. The computed stresses from each were compared to bottom stress estimates from a quadratic drag law and, where available, estimates of wind stress. These comparisons, along with an analysis of the cospectra, indicated that spectral fitting performs well when the wave climate is wide-banded and/or multidirectional as well as when instrument noise is high. In contrast, spatial filtering performs better when waves are narrow-banded, low frequency, and when wave orbital velocities are strong relative to currents. However, as spatial filtering uses vertically separated velocity bins to remove the wave bias, spectral fitting is able to resolve stresses over a larger fraction of the water column.
    Description: J. Rosman acknowledges funding from the National Science Foundation (OCE-1061108).
    Keywords: Coastal flows ; Momentum ; Ocean circulation ; Waves, oceanic ; In situ observations ; Instrumentation/sensors
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    Improving HF radar estimates of surface currents using signal quality metrics, with application to the MVCO high-resolution radar system (2012)
    American Meteorological Society
    Details
    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 Atmospheric and Oceanic Technology 29 (2012): 1377–1390, doi:10.1175/JTECH-D-11-00160.1.
    Description: Estimates of surface currents over the continental shelf are now regularly made using high-frequency radar (HFR) systems along much of the U.S. coastline. The recently deployed HFR system at the Martha’s Vineyard Coastal Observatory (MVCO) is a unique addition to these systems, focusing on high spatial resolution over a relatively small coastal ocean domain with high accuracy. However, initial results from the system showed sizable errors and biased estimates of M2 tidal currents, prompting an examination of new methods to improve the quality of radar-based velocity data. The analysis described here utilizes the radial metric output of CODAR Ocean Systems’ version 7 release of the SeaSonde Radial Site Software Suite to examine both the characteristics of the received signal and the output of the direction-finding algorithm to provide data quality controls on the estimated radial currents that are independent of the estimated velocity. Additionally, the effect of weighting spatial averages of radials falling within the same range and azimuthal bin is examined to account for differences in signal quality. Applied to two month-long datasets from the MVCO high-resolution system, these new methods are found to improve the rms difference comparisons with in situ current measurements by up to 2 cm s−1, as well as reduce or eliminate observed biases of tidal ellipses estimated using standard methods.
    Description: 2013-03-01
    Keywords: Coastal flows ; Currents ; Data processing ; Data quality control ; In situ atmospheric observations ; Remote sensing
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    Cross-shelf and out-of-bay transport driven by an open-ocean current (2012)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2011. 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 41 (2011): 2168–2186, doi:10.1175/JPO-D-11-08.1.
    Description: This paper studies the interaction of an Antarctic Circumpolar Current (ACC)–like wind-driven channel flow with a continental slope and a flat-bottomed bay-shaped shelf near the channel’s southern boundary. Interaction between the model ACC and the topography in the second layer induces local changes of the potential vorticity (PV) flux, which further causes the formation of a first-layer PV front near the base of the topography. Located between the ACC and the first-layer slope, the newly formed PV front is constantly perturbed by the ACC and in turn forces the first-layer slope with its own variability in an intermittent but persistent way. The volume transport of the slope water across the first-layer slope edge is mostly directly driven by eddies and meanders of the new front, and its magnitude is similar to the maximum Ekman transport in the channel. Near the bay’s opening, the effect of the topographic waves, excited by offshore variability, dominates the cross-isobath exchange and induces a mean clockwise shelf circulation. The waves’ propagation is only toward the west and tends to be blocked by the bay’s western boundary in the narrow-shelf region. The ensuing wave–coast interaction amplifies the wave amplitude and the cross-shelf transport. Because the interaction only occurs near the western boundary, the shelf water in the west of the bay is more readily carried offshore than that in the east and the mean shelf circulation is also intensified along the bay’s western boundary.
    Description: Y. Zhang acknowledges the support of the MIT-WHOI Joint Program in Physical Oceanography and NSF OCE-9901654 and OCE- 0451086. J. Pedlosky acknowledges the support of NSF OCE-9901654 and OCE-0451086.
    Keywords: Baroclinic flows ; Eddies ; Fronts ; Mass fluxes/transport ; Mesoscale processes ; Topographic effects
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    On the response of a buoyant plume to downwelling-favorable wind stress (2012)
    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): 1083–1098, doi:10.1175/JPO-D-11-015.1.
    Description: Here, the response of a coastally trapped buoyant plume to downwelling-favorable wind forcing is explored using a simplified two-dimensional numerical model and a prognostic theory for the resulting width, depth, and density anomaly and along-shelf transport of the plume. Consistent with the numerical simulations, the analytical model shows that the wind causes mixing of the plume water and that the forced cross-shelf circulation can also generate significant deepening and surface narrowing, as well as increased along-shelf transport. The response is due to a combination of the purely advective process that leads to the steepening of the isopycnals and the entrainment of ambient water into the plume. The advective component depends on the initial plume geometry: plumes that have a large fraction of their total width in contact with the bottom (“bottom trapped”) suffer relatively small depth and width changes compared to plumes that have a large fraction of their total width detached from the bottom (“surface trapped”). Key theoretical parameters are Wγ/Wα, the ratio of the width of the plume detached from the bottom to the width of the plume in contact with it, and the ratio of the wind-generated mixed layer δe to the initial plume depth hp, which determines the amount of water initially entrained into the plume. The model results also show that the cross-shelf circulation can be strongly influenced by the wind-driven response in combination with the geostrophic shear of the plume. The continuous entrainment into the plume, as well as transient events, is also discussed.
    Description: This work has been supported by FONDECYT Grant 1070501. S. Lentz received support by theNational Science Foundation GrantOCE-0751554. C. Moffat had additional support from the National Science Foundation Office of Polar Programs through U.S. Southern Ocean GLOBEC Grants OPP 99-10092 and 06- 23223.
    Description: 2013-01-01
    Keywords: Baroclinic flows ; Boundary currents ; Coastal flows ; Upwelling/downwelling ; Wind ; Ocean models
    Repository Name: Woods Hole Open Access Server
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    Estuarine exchange flow quantified with isohaline coordinates : contrasting long and short estuaries (2012)
    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): 748–763, doi:10.1175/JPO-D-11-086.1.
    Description: Isohaline coordinate analysis is used to compare the exchange flow in two contrasting estuaries, the long (with respect to tidal excursion) Hudson River and the short Merrimack River, using validated numerical models. The isohaline analysis averages fluxes in salinity space rather than in physical space, yielding the isohaline exchange flow that incorporates both subtidal and tidal fluxes and precisely satisfies the Knudsen relation. The isohaline analysis can be consistently applied to both subtidally and tidally dominated estuaries. In the Hudson, the isohaline exchange flow is similar to results from the Eulerian analysis, and the conventional estuarine theory can be used to quantify the salt transport based on scaling with the baroclinic pressure gradient. In the Merrimack, the isohaline exchange flow is much larger than the Eulerian quantity, indicating the dominance of tidal salt flux. The exchange flow does not scale with the baroclinic pressure gradient but rather with tidal volume flux. This tidal exchange is driven by tidal pumping due to the jet–sink flow at the mouth constriction, leading to a linear dependence of exchange flow on tidal volume flux. Finally, a tidal conversion parameter Qin/Qprism, measuring the fraction of tidal inflow Qprism that is converted into net exchange Qin, is proposed to characterize the exchange processes among different systems. It is found that the length scale ratio between tidal excursion and salinity intrusion provides a characteristic to distinguish estuarine regimes.
    Description: SNC is supported by a WHOI postdoctoral scholarship, a NSF Grant OCE-0926427, and a Taiwan National Science Council Grant NSC 100- 2199-M-002-028.WRGis supported byNSFGrantOCE- 0926427. JAL is supported by NSF Grant OCE-0452054.
    Description: 2012-11-01
    Keywords: Coastal flows
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  • 44
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    Buoyancy arrest and shelf–ocean exchange (2012)
    American Meteorological Society
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    Publication Date: 2022-05-26
    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): 644–658, doi:10.1175/JPO-D-11-0143.1.
    Description: When steady flow in a stratified ocean passes between the continental slope and open ocean, its ability to cross isobaths is potentially limited by buoyancy arrest. If the bottom Ekman transport vanishes and there are no interior stresses, then steady linear flow on an f plane must be geostrophic and follow isobaths exactly. The influence of arrest on cross-shelf transport is investigated here to establish 1) whether there are substantial penetration asymmetries between cases with upwelling and downwelling in the bottom boundary layer; 2) over what spatial scales, hence in what parameter regime, buoyancy arrest is important; and 3) the effects of depth-dependent interior flow. The problem is approached using scalings and idealized numerical models. The results show that there is little or no asymmetry introduced by bottom boundary layer behavior. Further, if the stratification is weak or moderate, as measured by a slope Burger number s = αN/f (where α is the bottom slope, N is buoyancy frequency, and f is the Coriolis parameter), buoyancy arrest does not exert a strong constraint on cross-isobath exchange.
    Description: This research was supported by the National Science Foundation Physical Oceanography program through Grant OCE-0849498.
    Description: 2012-10-01
    Keywords: Coastal flows ; Ekman pumping/transport
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  • 45
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    Baroclinic flow around planetary islands in a double gyre : a mechanism for cross-gyre flow (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2010. 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 40 (2010): 1075-1086, doi:10.1175/2009JPO4375.1.
    Description: A quasigeostrophic, two-layer model is used to study the baroclinic circulation around a thin, meridionally elongated island. The flow is driven by either buoyancy forcing or wind stress, each of whose structure would produce an antisymmetric double-gyre flow. The ocean bottom is flat. When the island partially straddles the intergyre boundary, fluid from one gyre is forced to flow into the other. The amount of the intergyre flow depends on the island constant, that is, the value of the geostrophic streamfunction on the island in each layer. That constant is calculated in a manner similar to earlier studies and is determined by the average, along the meridional length of the island, of the interior Sverdrup solution just to the east of the island. Explicit solutions are given for both buoyancy and wind-driven flows. The presence of an island of nonzero width requires the determination of the baroclinic streamfunction on the basin’s eastern boundary. The value of the boundary term is proportional to the island’s area. This adds a generally small additional baroclinic intergyre flow. In all cases, the intergyre flow produced by the island is not related to topographic steering of the flow but rather the pressure anomaly on the island as manifested by the barotropic and baroclinic island constants. The vertical structure of the flow around the island is a function of the parameterization of the vertical mixing in the problem and, in particular, the degree to which long baroclinic Rossby waves can traverse the basin before becoming thermally damped.
    Description: This research was supported in part by NSF Grant OCE 0451086.
    Keywords: Gyres ; Baroclinic flows ; Topographic effects ; Streamfunction ; Orographic effects
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  • 46
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    Response to a steady poleward outflow. Part I : the linear, quasigeostrophic problem (2010)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2009. 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 39 (2009): 1541-1550, doi:10.1175/2008JPO3999.1.
    Description: The response of a zonal channel to a uniform, switched-on but subsequently steady poleward outflow is presented. An eastward coastal current with a Kelvin wave’s cross-shore structure is found to be generated instantly upon initiation of the outflow. The current is essentially in geostrophic balance everywhere except for the vicinity of the outflow channel mouth, where the streamlines must cross planetary vorticity contours to feed the current. The adjustment of this region generates a plume that propagates westward at Rossby wave speeds. The cross-shore structure of the plume varies with longitude, and at any given longitude it evolves with time. The authors show that the plume evolution can be understood both conceptually and quantitatively as the westward propagation of the Kelvin current’s meridional spectrum, with each spectral element propagating at its own Rossby wave group velocity.
    Description: This work was completed at Woods Hole Oceanographic Institution while T.S. Durland was supported by the Ocean and Climate Change Institute. M.A. Spall was supported by NSF Grant OCE-0423975, and J. Pedlosky by NSF Grant OCE-0451086. T.S. Durland acknowledges additional report preparation support from NASA Grant NNG05GN98G.
    Keywords: Coastal flows ; Estuaries ; Currents ; Vorticity ; Plumes
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  • 47
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    Lateral coupling in baroclinically unstable flows (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2008. 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 38 (2008): 1267-1277, doi:10.1175/2007JPO3906.1.
    Description: A two-layer quasigeostrophic model in a channel is used to study the influence of lateral displacements of regions of different sign mean potential vorticity gradient (Πy) on the growth rate and structure of linearly unstable waves. The mean state is very idealized, with a region of positive Πy in the upper layer and a region of negative Πy in the lower layer; elsewhere Πy is zero. The growth rate and structure of the model’s unstable waves are quite sensitive to the amount of overlap between the two regions. For large amounts of overlap (more than several internal deformation radii), the channel modes described by Phillips’ model are recovered. The growth rate decreases abruptly as the amount of overlap decreases below the internal deformation radius. However, unstable modes are also found for cases in which the two nonzero Πy regions are separated far apart. In these cases, the wavenumber of the unstable waves decreases such that the aspect ratio of the wave remains O(1). The waves are characterized by a large-scale barotropic component that has maximum amplitude near one boundary but extends all the way across the channel to the opposite boundary. Near the boundaries, the wave is of mixed barotropic–baroclinic structure with cross-front scales on the order of the internal deformation radius. The perturbation heat flux is concentrated near the nonzero Πy regions, but the perturbation momentum flux extends all the way across the channel. The perturbation fluxes act to reduce the isopycnal slopes near the channel boundaries and to transmit zonal momentum from the region of Πy 〉 0 to the region on the opposite side of the channel where Πy 〈 0. These nonzero perturbation momentum fluxes are found even for a mean state that has no lateral shear in the velocity field.
    Description: This work was supported by NSF Grants OPP-0421904, OCE-0423975 (MAS), and OCE- 85108600 (JP).
    Keywords: Baroclinic flows ; Barotropic flows
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    The influence of lateral advection on the residual estuarine circulation : a numerical modeling study of the Hudson River Estuary (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2009. 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 39 (2009): 107-124, doi:10.1175/2008JPO3952.1.
    Description: In most estuarine systems it is assumed that the dominant along-channel momentum balance is between the integrated pressure gradient and bed stress. Scaling the amplitude of the estuarine circulation based on this balance has been shown to have predictive skill. However, a number of authors recently highlighted important nonlinear processes that contribute to the subtidal dynamics at leading order. In this study, a previously validated numerical model of the Hudson River estuary is used to examine the forces driving the residual estuarine circulation and to test the predictive skill of two linear scaling relationships. Results demonstrate that the nonlinear advective acceleration terms contribute to the subtidal along-channel momentum balance at leading order. The contribution of these nonlinear terms is driven largely by secondary lateral flows. Under a range of forcing conditions in the model runs, the advective acceleration terms nearly always act in concert with the baroclinic pressure gradient, reinforcing the residual circulation. Despite the strong contribution of the nonlinear advective terms to the subtidal dynamical balance, a linear scaling accurately predicts the strength of the observed residual circulation in the model. However, this result is largely fortuitous, as this scaling does not account for two processes that are fundamental to the estuarine circulation. The skill of this scaling results because of the compensatory relationship between the contribution of the advective acceleration terms and the suppression of turbulence due to density stratification. Both of these processes, neither of which is accounted for in the linear scaling, increase the residual estuarine circulation but have an opposite dependence on tidal amplitude and, consequently, strength of stratification.
    Description: This research was supported by the Beacon Institute for Rivers and Estuaries—Woods Hole Oceanographic Institution postdoctoral fellowship program, as well as NSF Grants OCE-0452054 and OCE-0451740.
    Keywords: Advection ; Estuarine circulation ; Friction ; Density currents ; Baroclinic flows
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    Laboratory experiments on the interaction of a buoyant coastal current with a canyon : application to the East Greenland Current (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2009. 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 39 (2009): 1258-1271, doi:10.1175/2008JPO4028.1.
    Description: This paper presents a set of laboratory experiments focused on how a buoyant coastal current flowing over a sloping bottom interacts with a canyon and what controls the separation, if any, of the current from the upstream canyon bend. The results show that the separation of a buoyant coastal current depends on the current width W relative to the radius of curvature of the bathymetry ρc. The flow moved across the mouth of the canyon (i.e., separated) for W/ρc 〉 1, in agreement with previous results. The present study extends previous work by examining both slope-controlled and surface-trapped currents, and using a geometry specific to investigating buoyant current–canyon interaction. The authors find that, although bottom friction is important in setting the position of the buoyant front, the separation process driven by the inertia of the flow could overcome even the strongest bathymetric influence. Application of the laboratory results to the East Greenland Current (EGC), an Arctic-origin buoyant current that is observed to flow in two branches south of Denmark Strait, suggests that the path of the EGC is influenced by the large canyons cutting across the shelf, as the range of W/ρc in the ocean spans those observed in the laboratory. What causes the formation of a two-branched EGC structure downstream of the Kangerdlugssuaq Canyon (68°N, 32°W) is still unclear, but potential mechanisms are discussed.
    Description: This work was partially funded by NSF Grant OCE-0450658. DS also received support from the Academic Programs Office of the Woods Hole Oceanographic Institution, while CC had partial support from NSF OCE-0350891.
    Keywords: Coastal flows ; Buoyancy ; Currents ; Experimental design ; Topographic effects
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    Direct breaking of the internal tide near topography : Kaena Ridge, Hawaii (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2008. 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 38 (2008): 380-399, doi:10.1175/2007JPO3728.1.
    Description: Barotropic to baroclinic conversion and attendant phenomena were recently examined at the Kaena Ridge as an aspect of the Hawaii Ocean Mixing Experiment. Two distinct mixing processes appear to be at work in the waters above the 1100-m-deep ridge crest. At middepths, above 400 m, mixing events resemble their open-ocean counterparts. There is no apparent modulation of mixing rates with the fortnightly cycle, and they are well modeled by standard open-ocean parameterizations. Nearer to the topography, there is quasi-deterministic breaking associated with each baroclinic crest passage. Large-amplitude, small-scale internal waves are triggered by tidal forcing, consistent with lee-wave formation at the ridge break. These waves have vertical wavelengths on the order of 400 m. During spring tides, the waves are nonlinear and exhibit convective instabilities on their leading edge. Dissipation rates exceed those predicted by the open-ocean parameterizations by up to a factor of 100, with the disparity increasing as the seafloor is approached. These observations are based on a set of repeated CTD and microconductivity profiles obtained from the research platform (R/P) Floating Instrument Platform (FLIP), which was trimoored over the southern edge of the ridge crest. Ocean velocity and shear were resolved to a 4-m vertical scale by a suspended Doppler sonar. Dissipation was estimated both by measuring overturn displacements and from microconductivity wavenumber spectra. The methods agreed in water deeper than 200 m, where sensor resolution limitations do not limit the turbulence estimates. At intense mixing sites new phenomena await discovery, and existing parameterizations cannot be expected to apply.
    Description: This work was funded by the National Science Foundation as a component of the Hawaii Ocean Mixing Program. Added support for FLIP was provided by the Office of Naval Research.
    Keywords: Pacific Ocean ; Topographic effects ; Internal waves ; Barotropic flows ; Baroclinic flows
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    Observations and a model of undertow over the inner continental shelf (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2008. 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 38 (2008): 2341-2357, doi:10.1175/2008JPO3986.1.
    Description: Onshore volume transport (Stokes drift) due to surface gravity waves propagating toward the beach can result in a compensating Eulerian offshore flow in the surf zone referred to as undertow. Observed offshore flows indicate that wave-driven undertow extends well offshore of the surf zone, over the inner shelves of Martha’s Vineyard, Massachusetts, and North Carolina. Theoretical estimates of the wave-driven offshore transport from linear wave theory and observed wave characteristics account for 50% or more of the observed offshore transport variance in water depths between 5 and 12 m, and reproduce the observed dependence on wave height and water depth. During weak winds, wave-driven cross-shelf velocity profiles over the inner shelf have maximum offshore flow (1–6 cm s−1) and vertical shear near the surface and weak flow and shear in the lower half of the water column. The observed offshore flow profiles do not resemble the parabolic profiles with maximum flow at middepth observed within the surf zone. Instead, the vertical structure is similar to the Stokes drift velocity profile but with the opposite direction. This vertical structure is consistent with a dynamical balance between the Coriolis force associated with the offshore flow and an along-shelf “Hasselmann wave stress” due to the influence of the earth’s rotation on surface gravity waves. The close agreement between the observed and modeled profiles provides compelling evidence for the importance of the Hasselmann wave stress in forcing oceanic flows. Summer profiles are more vertically sheared than either winter profiles or model profiles, for reasons that remain unclear.
    Description: This research was funded by the Ocean Sciences Division of the National Science Foundation under Grants OCE-0241292 and OCE-0548961.
    Keywords: Continental shelf ; Transport ; Shear structure/flows ; Coastal flows ; Gravity waves
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    The coastal bottom boundary layer : a note on the model of Chapman and Lentz (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2007. 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 37 (2007): 2776-2784, doi:10.1175/2007JPO3710.1.
    Description: The bottom boundary layer of a stratified flow on a coastal continental shelf is examined using the model of Chapman and Lentz. The flow is driven by a surface stress, uniform in the alongshore coordinate, in a downwelling-favorable direction. The stress diminishes in the offshore direction and produces an Ekman pumping, as well as an onshore Ekman flux. The model yields an interior flow, sandwiched between an upper Ekman layer and a bottom boundary layer. The interior has a horizontal density gradient produced by a balance between horizontal diffusion of density and vertical advection of a background vertical density gradient. The interior flow is vertically sheared and in thermal wind balance. Whereas the original model of Chapman and Lentz considered an alongshore flow that is freely evolving, the present note focuses on the equilibrium structure of a flow driven by stress and discusses the vertical and lateral structure of the flow and, in particular, the boundary layer thickness. The vertical diffusivity of density in the bottom boundary layer is considered so strong, locally, as to render the bottom boundary layer’s density a function of only offshore position. Boundary layer budgets of mass, momentum, and buoyancy determine the barotropic component of the interior flow as well as the boundary layer thickness, which is a function of the offshore coordinate. The alongshore flow has enhanced vertical shear in the boundary layer that reduces the alongshore flow in the boundary layer; however, the velocity at the bottom is generally not zero but produces a stress that locally balances the applied surface stress. The offshore transport in the bottom boundary layer therefore balances the onshore surface Ekman flux. The model predicts the thickness of the bottom boundary layer, which is a complicated function of several parameters, including the strength of the forcing stress, the vertical and horizontal diffusion coefficients in the interior, and the horizontal diffusion in the boundary layer. The model yields a boundary layer over only a finite portion of the bottom slope if the interior diffusion coefficients are too large; otherwise, the layer extends over the full lateral extent of the domain.
    Description: This research was supported in part by NSF Grant OCE-851086.00.
    Keywords: Boundary layer ; Continental shelf ; Coastal flows ; Ekman pumping ; Forcing
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    Effect of sea surface temperature-wind stress coupling on baroclinic instability in the ocean (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2007. 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 37 (2007): 1092–1097, doi:10.1175/JPO3045.1.
    Description: The impact of the observed relationship between sea surface temperature and surface wind stress on baroclinic instability in the ocean is explored using linear theory and a nonlinear model. A simple parameterization of the influence of sea surface temperature on wind stress is used to derive a surface boundary condition for the vertical velocity at the base of the oceanic Ekman layer. This boundary condition is applied to the classic linear, quasigeostrophic stability problem for a uniformly sheared flow originally studied by Eady in the 1940s. The results demonstrate that for a wind directed from warm water toward cold water, the coupling acts to enhance the growth rate, and increase the wavelength, of the most unstable wave. Winds in the opposite sense reduce the growth rate and decrease the wavelength of the most unstable wave. For representative coupling strengths, the change in growth rate can be as large as ±O(50%). This effect is largest for shallow, strongly stratified, low-latitude flows.
    Description: This work was supported by the Office of Naval Research Grant N00014-05-1-0300.
    Keywords: Wind stress ; Instability ; Sea surface temperature ; Baroclinic flows ; Ocean dynamics
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    Rossby wave instability and apparent phase speeds in large ocean basins (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2007. 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 37 (2007): 1177-1191, doi:10.1175/jpo3054.1.
    Description: The stability of baroclinic Rossby waves in large ocean basins is examined, and the quasigeostrophic (QG) results of LaCasce and Pedlosky are generalized. First, stability equations are derived for perturbations on large-scale waves, using the two-layer shallow-water system. These equations resemble the QG stability equations, except that they retain the variation of the internal deformation radius with latitude. The equations are solved numerically for different initial conditions through eigenmode calculations and time stepping. The fastest-growing eigenmodes are intensified at high latitudes, and the slower-growing modes are intensified at lower latitudes. All of the modes have meridional scales and growth times that are comparable to the deformation radius in the latitude range where the eigenmode is intensified. This is what one would expect if one had applied QG theory in latitude bands. The evolution of large-scale waves was then simulated using the Regional Ocean Modeling System primitive equation model. The results are consistent with the theoretical predictions, with deformation-scale perturbations growing at rates inversely proportional to the local deformation radius. The waves succumb to the perturbations at the mid- to high latitudes, but are able to cross the basin at low latitudes before doing so. Also, the barotropic waves produced by the instability propagate faster than the baroclinic long-wave speed, which may explain the discrepancy in speeds noted by Chelton and Schlax.
    Description: PEI was supported by a postdoctoral grant from the Norwegian Research Council, JHL was supported under the Norwegian NOCLIM II program, and JP was partly supported by NSF OCE 0451086.
    Keywords: Rossby waves ; Ocean models ; Barotropic flows ; Baroclinic flows
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    Marginal sea overflows and the upper ocean interaction (2010)
    American Meteorological Society
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2009. 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 39 (2009): 387-403, doi:10.1175/2008JPO3934.1.
    Description: Marginal sea overflows and the overlying upper ocean are coupled in the vertical by two distinct mechanisms—by an interfacial mass flux from the upper ocean to the overflow layer that accompanies entrainment and by a divergent eddy flux associated with baroclinic instability. Because both mechanisms tend to be localized in space, the resulting upper ocean circulation can be characterized as a β plume for which the relevant background potential vorticity is set by the slope of the topography, that is, a topographic β plume. The entrainment-driven topographic β plume consists of a single gyre that is aligned along isobaths. The circulation is cyclonic within the upper ocean (water columns are stretched). The transport within one branch of the topographic β plume may exceed the entrainment flux by a factor of 2 or more. Overflows are likely to be baroclinically unstable, especially near the strait. This creates eddy variability in both the upper ocean and overflow layers and a flux of momentum and energy in the vertical. In the time mean, the eddies accompanying baroclinic instability set up a double-gyre circulation in the upper ocean, an eddy-driven topographic β plume. In regions where baroclinic instability is growing, the momentum flux from the overflow into the upper ocean acts as a drag on the overflow and causes the overflow to descend the slope at a steeper angle than what would arise from bottom friction alone. Numerical model experiments suggest that the Faroe Bank Channel overflow should be the most prominent example of an eddy-driven topographic β plume and that the resulting upper-layer transport should be comparable to that of the overflow. The overflow-layer eddies that accompany baroclinic instability are analogous to those observed in moored array data. In contrast, the upper layer of the Mediterranean overflow is likely to be dominated more by an entrainment-driven topographic β plume. The difference arises because entrainment occurs at a much shallower location for the Mediterranean case and the background potential vorticity gradient of the upper ocean is much larger.
    Description: SK’s support during the time of his Ph.D. research in the MIT/WHOI Joint Program was provided by the National Science Foundation through Grant OCE04-24741. JP and JY have also received support from the Climate Process Team on Gravity Current Entrainment, NSF Grant OCE-0611530. JY has also been supported by NSF Grant OCE-0351055.
    Keywords: Baroclinic flows ; Mass fluxes/transport ; Entrainment ; Topographic effects ; Potential vorticity
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    The baroclinic adjustment of time-dependent shear flows (2010)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2010. 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 40 (2010): 1851-1865, doi:10.1175/2010JPO4217.1.
    Description: Motivated by the fact that time-dependent currents are ubiquitous in the ocean, this work studies the two-layer Phillips model on the beta plane with baroclinic shear flows that are steady, periodic, or aperiodic in time to understand their nonlinear evolution better. When a linearly unstable basic state is slightly perturbed, the primary wave grows exponentially until nonlinear advection adjusts the growth. Even though for long time scales these nearly two-dimensional motions predominantly cascade energy to large scales, for relatively short times the wave–mean flow and wave–wave interactions cascade energy to smaller horizontal length scales. The authors demonstrate that the manner through which these mechanisms excite the harmonics depends significantly on the characteristics of the basic state. Time-dependent basic states can excite harmonics very rapidly in comparison to steady basic states. Moreover, in all the simulations of aperiodic baroclinic shear flows, the barotropic component of the primary wave continues to grow after the adjustment by the nonlinearities. Furthermore, the authors find that the correction to the zonal mean flow can be much larger when the basic state is aperiodic compared to the periodic or steady limits. Finally, even though time-dependent baroclinic shear on an f plane is linearly stable, the authors show that perturbations can grow algebraically in the linear regime because of the erratic variations in the aperiodic flow. Subsequently, baroclinicity adjusts the growing wave and creates a final state that is more energetic than the nonlinear adjustment of any of the unstable steady baroclinic shears that are considered.
    Description: FJP was supported by NSERC and JP was supported by NSF OCE 0925061 during the research and writing of this manuscript.
    Keywords: Baroclinic flows ; Shear structure/flows
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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