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  • 1
    Publication Date: 2017-01-04
    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): 1203–1221, doi:10.1175/2007JPO3768.1.
    Description: Analyses of current time series longer than 200 days from 33 sites over the Middle Atlantic Bight continental shelf reveal a consistent mean circulation pattern. The mean depth-averaged flow is equatorward, alongshelf, and increases with increasing water depth from 3 cm s−1 at the 15-m isobath to 10 cm s−1 at the 100-m isobath. The mean cross-shelf circulation exhibits a consistent cross-shelf and vertical structure. The near-surface flow is typically offshore (positive, range −3 to 6 cm s−1). The interior flow is onshore and remarkably constant (−0.2 to −1.4 cm s−1). The near-bottom flow increases linearly with increasing water depth from −1 cm s−1 (onshore) in shallow water to 4 cm s−1 (offshore) at the 250-m isobath over the slope, with the direction reversal near the 50-m isobath. A steady, two-dimensional model (no along-isobath variations in the flow) reproduces the main features of the observed circulation pattern. The depth-averaged alongshelf flow is primarily driven by an alongshelf pressure gradient (sea surface slope of 3.7 × 10−8 increasing to the north) and an opposing mean wind stress that also drives the near-surface offshore flow. The alongshelf pressure gradient accounts for both the increase in the alongshelf flow with water depth and the geostrophic balance onshore flow in the interior. The increase in the near-bottom offshore flow with water depth is due to the change in the relative magnitude of the contributions from the geostrophic onshore flow that dominates in shallow water and the offshore flow driven by the bottom stress that dominates in deeper water.
    Description: This research was funded by Ocean Sciences Division of the National Science Foundation under Grants OCE-820773, OCE-841292, and OCE-848961.
    Keywords: Ocean models ; Ocean circulation ; Continental shelf ; Currents ; In situ observations
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  • 2
    Publication Date: 2017-01-04
    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): 1253-1266, doi:10.1175/2007JPO3786.1.
    Description: Wind stress and tidal dissipation are the most important sources of mechanical energy for maintaining the oceanic general circulation. The contribution of mechanical energy due to tropical cyclones can be a vitally important factor in regulating the oceanic general circulation and its variability. However, previous estimates of wind stress energy input were based on low-resolution wind stress data in which strong nonlinear events, such as tropical cyclones, were smoothed out. Using a hurricane–ocean coupled model constructed from an axisymmetric hurricane model and a three-layer ocean model, the rate of energy input to the world’s oceans induced by tropical cyclones over the period from 1984 to 2003 was estimated. The energy input is estimated as follows: 1.62 TW to the surface waves and 0.10 TW to the surface currents (including 0.03 TW to the near-inertial motions). The rate of gravitational potential energy increase due to tropical cyclones is 0.05 TW. Both the energy input from tropical cyclones and the increase of gravitational potential energy of the ocean show strong interannual and decadal variability with an increasing rate of 16% over the past 20 years. The annual mean diapycnal upwelling induced by tropical cyclones over the past 20 years is estimated as 39 Sv (Sv ≡ 106 m3 s−1). Owing to tropical cyclones, diapycnal mixing in the upper ocean (below the mixed layer) is greatly enhanced. Within the regimes of strong activity of tropical cyclones, the increase of diapycnal diffusivity is on the order of (1 − 6) × 10−4 m2 s−1. The tropical cyclone–related energy input and diapycnal mixing may play an important role in climate variability, ecology, fishery, and environments.
    Description: LLL and WW were supported by the National Basic Research Priorities Programmer of China through Grant 2007CB816004 and National Outstanding Youth Natural Science Foundation of China FIG. 15. Annual-mean vertical diffusivity induced by tropical cyclones from 1984 to 2003 (units: 10 4 m2 s 1): (right) the horizontal distribution and (left) the zonally averaged vertical diffusivity. JUNE 2008 L IU ET AL . 1265 under Grant 40725017. RXH was supported by the W. Alan Clark Chair from Woods Hole Oceanographic Institution.
    Keywords: Tropical cyclones ; Ocean circulation ; Wind stress ; Mixing ; Interannual variability
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  • 3
    Publication Date: 2017-01-04
    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): 1486–1500, doi:10.1175/2007JPO3767.1.
    Description: Fits of an annual harmonic to depth-average along-shelf current time series longer than 200 days from 27 sites over the Middle Atlantic Bight (MAB) continental shelf have amplitudes of a few centimeters per second. These seasonal variations are forced by seasonal variations in the wind stress and the cross-shelf density gradient. The component of wind stress that drives the along-shelf flow over most of the MAB mid- and outer shelf is oriented northeast–southwest, perpendicular to the major axis of the seasonal variation in the wind stress. Consequently, there is not a significant seasonal variation in the wind-driven along-shelf flow, except over the southern MAB shelf and the inner shelf of New England where the wind stress components forcing the along-shelf flow are north–south and east–west, respectively. The seasonal variation in the residual along-shelf flow, after removing the wind-driven component, has an amplitude of a few centimeters per second with maximum southwestward flow in spring onshore of the 60-m isobath and autumn offshore of the 60-m isobath. The spring maximum onshore of the 60-m isobath is consistent with the maximum river discharges in spring enhancing cross-shelf salinity gradients. The autumn maximum offshore of the 60-m isobath and a steady phase increase with water depth offshore of Cape Cod are both consistent with the seasonal variation in the cross-shelf temperature gradient associated with the development and destruction of a near-bottom pool of cold water over the mid and outer shelf (“cold pool”) due to seasonal variations in surface heat flux and wind stress.
    Description: This research was funded by the Ocean Sciences Division of the National Science Foundation under Grants OCE-820773, OCE-841292, and OCE- 848961.
    Keywords: Seasonal variability ; Ocean circulation ; Continental shelf ; Wind stress ; Density currents
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  • 4
    Publication Date: 2017-01-04
    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): 1764-1779, doi:10.1175/2008JPO3921.1.
    Description: Middepth, time-mean circulation in the western North Pacific Ocean (28°–45°N, 140°–165°E) is investigated using drift information from the profiling floats deployed in the Kuroshio Extension System Study (KESS) and the International Argo programs. A well-defined, cyclonic recirculation gyre (RG) is found to exist north of the Kuroshio Extension jet, confined zonally between the Japan Trench (145°E) and the Shatsky Rise (156°E), and bordered to the north by the subarctic boundary along 40°N. This northern RG, which is simulated favorably in the eddy-resolving OGCM for the Earth Simulator (OFES) hindcast run model, has a maximum volume transport at 26.4 Sv across 159°E and its presence persists on the interannual and longer time scales. An examination of the time-mean x-momentum balance from the OFES hindcast run output reveals that horizontal convergence of Reynolds stresses works to accelerate both the eastward-flowing Kuroshio Extension jet and a westward mean flow north of the meandering jet. The fact that the northern RG is eddy driven is further confirmed by examining the turbulent Sverdrup balance, in which convergent eddy potential vorticity fluxes are found to induce the cyclonic RG across the background potential vorticity gradient field. For the strength of the simulated northern RG, the authors find the eddy dissipation effect to be important as well.
    Description: This study was supported by NSF through Grant OCE-0220680 (UH) and OCE-0220161 (WHOI).
    Keywords: Gyres ; Ocean circulation ; Profilers ; Jets ; Transport
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  • 5
    Publication Date: 2017-01-05
    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): 1496-1511, doi:10.1175/jpo3071.1.
    Description: Measurements collected in the York River estuary, Virginia, demonstrate the important impact that tidal and lateral asymmetries in turbulent mixing have on the tidally averaged residual circulation. A reduction in turbulent mixing during the ebb phase of the tide caused by tidal straining of the axial density gradient results in increased vertical velocity shear throughout the water column during the ebb tide. In the absence of significant lateral differences in turbulent mixing, the enhanced ebb-directed transport caused by tidal straining is balanced by a reduction in the net seaward-directed barotropic pressure gradient, resulting in laterally uniform two-layer residual flow. However, the channel–shoal morphology of many drowned river valley estuaries often leads to lateral gradients in turbulent mixing. Tidal straining may then lead to tidal asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well mixed. As a result, the largest lateral asymmetries in turbulent mixing occur at the end of the ebb tide when the channel is significantly more stratified than the shoals. The reduced friction at the end of ebb delays the onset of the flood tide, increasing the duration of ebb in the channel. Conversely, over the shoal regions where stratification is more inhibited by tidal mixing, there is greater friction and the transition from ebb to flood occurs more rapidly. The resulting residual circulation is seaward over the channel and landward over the shoal. The shoal–channel segregation of this barotropically induced estuarine residual flow is opposite to that typically associated with baroclinic estuarine circulation over channel–shoal bathymetry.
    Description: Support for this research was provided by the National Science Foundation Division of Ocean Sciences Grant OCE- 9984941.
    Keywords: Tides ; Ocean circulation ; Estuaries ; Turbulence
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  • 6
    Publication Date: 2017-01-05
    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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  • 7
    Publication Date: 2017-01-07
    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): 291–305, doi:10.1175/JPO-D-11-043.1.
    Description: A number of previous observational studies have found that the waters of the deep Pacific Ocean have an age, or elapsed time since contact with the surface, of 700–1000 yr. Numerical models suggest ages twice as old. Here, the authors present an inverse framework to determine the mean age and its upper and lower bounds given Global Ocean Data Analysis Project (GLODAP) radiocarbon observations, and they show that the potential range of ages increases with the number of constituents or sources that are included in the analysis. The inversion requires decomposing the World Ocean into source waters, which is obtained here using the total matrix intercomparison (TMI) method at up to 2° × 2° horizontal resolution with 11 113 surface sources. The authors find that the North Pacific at 2500-m depth can be no younger than 1100 yr old, which is older than some previous observational estimates. Accounting for the broadness of surface regions where waters originate leads to a reservoir-age correction of almost 100 yr smaller than would be estimated with a two or three water-mass decomposition and explains some of the discrepancy with previous observational studies. A best estimate of mean age is also presented using the mixing history along circulation pathways. Subject to the caveats that inference of the mixing history would benefit from further observations and that radiocarbon cannot rule out the presence of extremely old waters from exotic sources, the deep North Pacific waters are 1200–1500 yr old, which is more in line with existing numerical model results.
    Description: GG is supported by the J. Lamar Worzel Assistant Scientist Fund and the Penzance Endowed Fund in Support of Assistant Scientists. PJH is supported by NSF Award 0960787.
    Description: 2012-08-01
    Keywords: North Pacific Ocean ; Mass fluxes/transport ; Ocean circulation ; Tracers ; Optimization ; Variational analysis
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  • 8
    Publication Date: 2017-01-04
    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 Climate 25 (2012): 1361–1389, doi:10.1175/JCLI-D-11-00091.1.
    Description: The ocean component of the Community Climate System Model version 4 (CCSM4) is described, and its solutions from the twentieth-century (20C) simulations are documented in comparison with observations and those of CCSM3. The improvements to the ocean model physical processes include new parameterizations to represent previously missing physics and modifications of existing parameterizations to incorporate recent new developments. In comparison with CCSM3, the new solutions show some significant improvements that can be attributed to these model changes. These include a better equatorial current structure, a sharper thermocline, and elimination of the cold bias of the equatorial cold tongue all in the Pacific Ocean; reduced sea surface temperature (SST) and salinity biases along the North Atlantic Current path; and much smaller potential temperature and salinity biases in the near-surface Pacific Ocean. Other improvements include a global-mean SST that is more consistent with the present-day observations due to a different spinup procedure from that used in CCSM3. Despite these improvements, many of the biases present in CCSM3 still exist in CCSM4. A major concern continues to be the substantial heat content loss in the ocean during the preindustrial control simulation from which the 20C cases start. This heat loss largely reflects the top of the atmospheric model heat loss rate in the coupled system, and it essentially determines the abyssal ocean potential temperature biases in the 20C simulations. There is also a deep salty bias in all basins. As a result of this latter bias in the deep North Atlantic, the parameterized overflow waters cannot penetrate much deeper than in CCSM3.
    Description: NCAR is sponsored by the National Science Foundation. The CCSM is also sponsored by the Department of Energy. SGY was supported by the NOAA Climate Program Office under Climate Variability and Predictability Program Grant NA09OAR4310163.
    Description: 2012-09-01
    Keywords: Ocean circulation ; Climate models ; General circulation models ; Ocean models
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  • 9
    Publication Date: 2017-01-04
    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): 855–868, doi:10.1175/JPO-D-10-05010.1.
    Description: Data from the Hudson River estuary demonstrate that the tidal variations in vertical salinity stratification are not consistent with the patterns associated with along-channel tidal straining. These observations result from three additional processes not accounted for in the traditional tidal straining model: 1) along-channel and 2) lateral advection of horizontal gradients in the vertical salinity gradient and 3) tidal asymmetries in the strength of vertical mixing. As a result, cross-sectionally averaged values of the vertical salinity gradient are shown to increase during the flood tide and decrease during the ebb. Only over a limited portion of the cross section does the observed stratification increase during the ebb and decrease during the flood. These observations highlight the three-dimensional nature of estuarine flows and demonstrate that lateral circulation provides an alternate mechanism that allows for the exchange of materials between surface and bottom waters, even when direct turbulent mixing through the pycnocline is prohibited by strong stratification.
    Description: The funding for this research was obtained from NSF Grant OCE-08-25226.
    Description: 2012-11-01
    Keywords: Mixing ; Ocean circulation ; Shear structure/flows ; Transport ; Turbulence
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  • 10
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    American Meteorological Society
    Publication Date: 2017-01-05
    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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  • 11
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 27 (2014): 3596–3618, doi:10.1175/JCLI-D-13-00070.1.
    Description: Estimates of the recent mean and time varying water mass transformation rates associated with North Atlantic surface-forced overturning are presented. The estimates are derived from heat and freshwater surface fluxes and sea surface temperature fields from six atmospheric reanalyses—the Japanese 25-yr Reanalysis (JRA), the NCEP–NCAR reanalysis (NCEP1), the NCEP–U.S. Department of Energy (DOE) reanalysis (NCEP2), the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-I), the Climate Forecast System Reanalysis (CFSR), and the Modern-Era Reanalysis for Research and Applications (MERRA)—together with sea surface salinity fields from two globally gridded datasets (World Ocean Atlas and Met Office EN3 datasets). The resulting 12 estimates of the 1979–2007 mean surface-forced streamfunction all depict a subpolar cell, with maxima north of 45°N, near σ = 27.5 kg m−3, and a subtropical cell between 20° and 40°N, near σ = 26.1 kg m−3. The mean magnitude of the subpolar cell varies between 12 and 18 Sv (1 Sv ≡ 106 m3 s−1), consistent with estimates of the overturning circulation from subsurface observations. Analysis of the thermal and haline components of the surface density fluxes indicates that large differences in the inferred low-latitude circulation are largely a result of the biases in reanalysis net heat flux fields, which range in the global mean from −13 to 19 W m−2. The different estimates of temporal variability in the subpolar cell are well correlated with each other. This suggests that the uncertainty associated with the choice of reanalysis product does not critically limit the ability of the method to infer the variability in the subpolar overturning. In contrast, the different estimates of subtropical variability are poorly correlated with each other, and only a subset of them captures a significant fraction of the variability in independently estimated North Atlantic Subtropical Mode Water volume.
    Description: JPG is funded by UK Natural Environment Research Council New Investigator Grant NE/I001654/1. Y-OK was supported by the U.S. National Science Foundation under Grant OCE-0424492. RJB is supported by a fellowship from the UK National Centre for Earth Observation.
    Description: 2014-11-15
    Keywords: Atmosphere-ocean interaction ; Meridional overturning circulation ; Ocean circulation
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  • 12
    Publication Date: 2017-01-04
    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):1189–1204, doi:10.1175/JPO-D-14-0122.1.
    Description: Winter outcropping of the Eighteen Degree Water (EDW) and its subsequent dispersion are studied using a ° eddy-resolving simulation of the Family of Linked Atlantic Modeling Experiments (FLAME). Outcropped EDW columns in the model simulations are detected in each winter from 1990 to 1999, and particles are deployed in the center of each outcropped EDW column. Subsequently, the trajectories of these particles are calculated for the following 5 yr. The particles slowly spread away from the outcropping region into the nonoutcropping/subducted EDW region south of ~30°N and eventually to the non-EDW region in the greater subtropical gyre. Approximately 30% of the particles are found in non-EDW waters 1 yr after deployment; after 5 yr, only 25% of the particles are found within EDW. The reoutcropping time is defined as the number of years between when a particle is originally deployed in an outcropping EDW column and when that particle is next found in an outcropping EDW column. Of the particles, 66% are found to reoutcrop as EDW in 1 yr, and less than 5% of the particles outcrop in each of the subsequent 4 yr. While the individual trajectories exhibit significant eddy-like motions, the time scale of reoutcropping is primarily set by the mean circulation. The dominance of reoutcropping in 1 yr suggests that EDW outcropping contributes considerably to the persistence of surface temperature anomalies from one winter to the next, that is, the reemergence of winter sea surface temperature anomalies.
    Description: We gratefully acknowledge the support from the NSF OCE Physical Oceanography program (NSF OCE-0961090 to Y-OK and J-JP; NSF OCE-0960776 to MSL and SFG; and NSF OCE-1242989 to Y-OK).
    Description: 2015-10-01
    Keywords: Circulation/ Dynamics ; Ocean circulation ; Atm/Ocean Structure/ Phenomena ; Water masses
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  • 13
    Publication Date: 2017-01-05
    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): 1410–1425, doi:10.1175/JPO-D-14-0192.1.
    Description: The west-to-east crossover of boundary currents has been seen in mean circulation schemes from several past models of the Red Sea. This study investigates the mechanisms that produce and control the crossover in an idealized, eddy-resolving numerical model of the Red Sea. The authors also review the observational evidence and derive an analytical estimate for the crossover latitude. The surface buoyancy loss increases northward in the idealized model, and the resultant mean circulation consists of an anticyclonic gyre in the south and a cyclonic gyre in the north. In the midbasin, the northward surface flow crosses from the western boundary to the eastern boundary. Numerical experiments with different parameters indicate that the crossover latitude of the boundary currents changes with f0, β, and the meridional gradient of surface buoyancy forcing. In the analytical estimate, which is based on quasigeostrophic, β-plane dynamics, the crossover is predicted to lie at the latitude where the net potential vorticity advection (including an eddy component) is zero. Various terms in the potential vorticity budget can be estimated using a buoyancy budget, a thermal wind balance, and a parameterization of baroclinic instability.
    Description: This work is supported by Award USA 00002, KSA 00011, and KSA 00011/02 made by King Abdullah University of Science and Technology (KAUST), by National Science Foundation Grants OCE0927017, OCE1154641, and OCE85464100, and by the Woods Hole Oceanographic Institution Academic Program Office.
    Description: 2015-11-01
    Keywords: Circulation/ Dynamics ; Boundary currents ; Buoyancy ; Ocean circulation ; Ocean dynamics
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  • 14
    Publication Date: 2016-05-01
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 28 (2015): 8574–8584, doi:10.1175/JCLI-D-14-00809.1.
    Description: The subsurface ocean response to anthropogenic climate forcing remains poorly characterized. From the Coupled Model Intercomparison Project (CMIP), a robust response of the lower thermocline is identified, where the warming is considerably weaker in the subtropics than in the tropics and high latitudes. The lower thermocline change is inversely proportional to the thermocline depth in the present climatology. Ocean general circulation model (OGCM) experiments show that sea surface warming is the dominant forcing for the subtropical gyre change in contrast to natural variability for which wind dominates, and the ocean response is insensitive to the spatial pattern of surface warming. An analysis based on a ventilated thermocline model shows that the pattern of the lower thermocline change can be interpreted in terms of the dynamic response to the strengthened stratification and downward heat mixing. Consequently, the subtropical gyres become intensified at the surface but weakened in the lower thermcline, consistent with results from CMIP experiments.
    Description: The work was supported by the National Basic Research Program of China (2012CB955600), the National Natural Science Foundation of China (41125019, 41206021), and the U.S. National Science Foundation (AGS 1249145, 1305719).
    Description: 2016-05-01
    Keywords: Circulation/ Dynamics ; Ocean circulation ; Physical Meteorology and Climatology ; Climate change
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  • 15
    Publication Date: 2016-06-01
    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): 2913–2932, doi:10.1175/JPO-D-14-0179.1.
    Description: The oceanic deep circulation is shared between concentrated deep western boundary currents (DWBCs) and broader interior pathways, a process that is sensitive to seafloor topography. This study investigates the spreading and deepening of Denmark Strait overflow water (DSOW) in the western subpolar North Atlantic using two ° eddy-resolving Atlantic simulations, including a passive tracer injected into the DSOW. The deepest layers of DSOW transit from a narrow DWBC in the southern Irminger Sea into widespread westward flow across the central Labrador Sea, which remerges along the Labrador coast. This abyssal circulation, in contrast to the upper levels of overflow water that remain as a boundary current, blankets the deep Labrador Sea with DSOW. Farther downstream after being steered around the abrupt topography of Orphan Knoll, DSOW again leaves the boundary, forming cyclonic recirculation cells in the deep Newfoundland basin. The deep recirculation, mostly driven by the meandering pathway of the upper North Atlantic Current, leads to accumulation of tracer offshore of Orphan Knoll, precisely where a local maximum of chlorofluorocarbon (CFC) inventory is observed. At Flemish Cap, eddy fluxes carry ~20% of the tracer transport from the boundary current into the interior. Potential vorticity is conserved as the flow of DSOW broadens at the transition from steep to less steep continental rise into the Labrador Sea, while around the abrupt topography of Orphan Knoll, potential vorticity is not conserved and the DSOW deepens significantly.
    Description: This work is supported by ONR Award N00014-09-1-0587, the NSF Physical Oceanography Program, and NASA Ocean Surface Topography Science Team Program.
    Description: 2016-06-01
    Keywords: Circulation/ Dynamics ; Abyssal circulation ; Boundary currents ; Ocean circulation ; Ocean dynamics ; Potential vorticity ; Topographic effects
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  • 16
    Publication Date: 2017-04-19
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 633-647, doi:10.1175/JPO-D-16-0089.1.
    Description: Interannual variability in the volumetric water mass distribution within the North Atlantic Subtropical Gyre is described in relation to variability in the Atlantic meridional overturning circulation. The relative roles of diabatic and adiabatic processes in the volume and heat budgets of the subtropical gyre are investigated by projecting data into temperature coordinates as volumes of water using an Argo-based climatology and an ocean state estimate (ECCO version 4). This highlights that variations in the subtropical gyre volume budget are predominantly set by transport divergence in the gyre. A strong correlation between the volume anomaly due to transport divergence and the variability of both thermocline depth and Ekman pumping over the gyre suggests that wind-driven heave drives transport anomalies at the gyre boundaries. This wind-driven heaving contributes significantly to variations in the heat content of the gyre, as do anomalies in the air–sea fluxes. The analysis presented suggests that wind forcing plays an important role in driving interannual variability in the Atlantic meridional overturning circulation and that this variability can be unraveled from spatially distributed hydrographic observations using the framework presented here.
    Description: DGE was supported by a Natural Environment Research Council studentship award at the University of Southampton. JMT’s contribution was supported by the U.S. National Science Foundation (Grant OCE-1332667). GF’s contribution was supported by the U.S. National Science Foundation through Grant OCE-0961713 and by the U.S. National Oceanic and Atmospheric Administration through Grant NA10OAR4310135. The contributions of JDZ and AJGN were supported by the NERC Grant ‘‘Climate scale analysis of air and water masses’’ (NE/ K012932/1). ACNG gratefully acknowledges support from the Leverhulme Trust, the Royal Society, and the Wolfson Foundation. LY was supported by NASA Ocean Vector Wind Science Team (OVWST) activities under Grant NNA10AO86G.
    Keywords: North Atlantic Ocean ; Atmosphere-ocean interaction ; Ekman pumping/transport ; Ocean circulation ; Water masses ; Inverse methods
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  • 17
    Publication Date: 2018-02-11
    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 Atmospheric and Oceanic Technology 34 (2017): 1679-1691, doi:10.1175/JTECH-D-16-0162.1.
    Description: For direction-finding high-frequency (HF) radar systems, the correct separation of backscattered spectral energy due to Bragg resonant waves from that due to more complex double-scattering represents a critical first step toward attaining accurate estimates of surface currents from the range-dependent radar backscatter. Existing methods to identify this “first order” region of the spectra, generally sufficient for lower-frequency radars and low-velocity or low-surface gravity wave conditions, are more likely to fail in higher-frequency systems or locations with more variable current, wave, or noise regimes, leading to elevated velocity errors. An alternative methodology is presented that uses a single and globally relevant smoothing length scale, careful pretreatment of the spectra, and marker-controlled watershed segmentation, an image processing technique, to separate areas of spectral energy due to surface currents from areas of spectral energy due to more complex scattering by the wave field or background noise present. Applied to a number of HF radar datasets with a range of operating frequencies and characteristic issues, the new methodology attains a higher percentage of successful first-order identification, particularly during complex current and wave conditions. As operational radar systems continue to expand to more systematically cover areas of high marine traffic, close approaches to ports and harbors, or offshore energy installations, use of this type of updated methodology will become increasingly important to attain accurate current estimates that serve both research and operational interests.
    Description: This analysis was supported by internal funds from the Woods Hole Oceanographic Institution.
    Description: 2018-02-11
    Keywords: Ocean circulation ; Waves, oceanic ; Data processing ; Radars/Radar observations ; Remote sensing ; Pattern detection
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  • 18
    Publication Date: 2018-10-12
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 739-748, doi:10.1175/JPO-D-17-0089.1.
    Description: McDougall and Ferrari have estimated the global deep upward diapycnal flow in the boundary layer overlying continental slopes that must balance both downward diapycnal flow in the deep interior and the formation of bottom water around Antarctica. The decrease of perimeter of isopycnal surfaces with depth and the observed decay with height above bottom of turbulent dissipation in the deep ocean play a key role in their estimate. They argue that because the perimeter of seamounts increases with depth, the net effect of mixing around seamounts is to produce net downward diapycnal flow. While this is true along much of a seamount, it is shown here that diapycnal flow of the densest water around the seamount is upward, with buoyancy being transferred from water just above. The same is true for midocean ridges, whose perimeter is constant with depth. It is argued that mixing around seamounts and especially midocean ridges contributes positively to the global deep overturning circulation, reducing the amount of turbulence demanded over the continental slopes to balance the buoyancy budget for the bottom and deep water.
    Description: This work was supported by National Science Foundation Grant OCE- 1232962.
    Description: 2018-09-29
    Keywords: Abyssal circulation ; Boundary currents ; Buoyancy ; Diapycnal mixing ; Mass fluxes/transport ; Ocean circulation
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  • 19
    Publication Date: 2017-01-05
    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): 2207-2227, doi:10.1175/jpo3178.1.
    Description: The overturning and horizontal circulations of the Labrador Sea are deduced from a composite vertical section across the basin. The data come from the late-spring/early-summer occupations of the World Ocean Circulation Experiment (WOCE) AR7W line, during the years 1990–97. This time period was chosen because it corresponded to intense wintertime convection—the deepest and densest in the historical record—suggesting that the North Atlantic meridional overturning circulation (MOC) would be maximally impacted. The composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass. The analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux (i.e., the transformation of light water to Labrador Sea Water). The mean overturning cell is calculated to be 1 Sv (1 Sv ≡ 106 m3 s−1), as compared with a horizontal gyre of 18 Sv. The total water mass transformation is 2 Sv. These values are consistent with recent modeling results. The diagnosed heat flux of 37.6 TW is found to result predominantly from the horizontal circulation, both in depth space and density space. These results suggest that the North Atlantic MOC is not largely impacted by deep convection in the Labrador Sea.
    Description: This work was funded by the National Science Foundation through Grants OCE-0450658 (RP) and OCE-024978 (MS).
    Keywords: Convection ; Ocean circulation ; North Atlantic
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  • 20
    Publication Date: 2017-01-05
    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 Climate 25 (2012): 343–349, doi:10.1175/JCLI-D-11-00059.1.
    Description: The Equatorial Undercurrent (EUC) is a major component of the tropical Pacific Ocean circulation. EUC velocity in most global climate models is sluggish relative to observations. Insufficient ocean resolution slows the EUC in the eastern Pacific where nonlinear terms should dominate the zonal momentum balance. A slow EUC in the east creates a bottleneck for the EUC to the west. However, this bottleneck does not impair other major components of the tropical circulation, including upwelling and poleward transport. In most models, upwelling velocity and poleward transport divergence fall within directly estimated uncertainties. Both of these transports play a critical role in a theory for how the tropical Pacific may change under increased radiative forcing, that is, the ocean dynamical thermostat mechanism. These findings suggest that, in the mean, global climate models may not underrepresent the role of equatorial ocean circulation, nor perhaps bias the balance between competing mechanisms for how the tropical Pacific might change in the future. Implications for model improvement under higher resolution are also discussed.
    Description: KBK gratefully acknowledges the J. Lamar Worzel Assistant Scientist Fund. GCJ is supported by NOAA’s Office of Oceanic and Atmospheric Research. RM gratefully acknowledges the generous support and hospitality of the Divecha Centre for Climate Change and CAOS at IISc, Bangalore, and partial support by NASA PO grants.
    Description: 2012-07-01
    Keywords: Tropics ; Ocean circulation ; Ocean dynamics ; Climate models ; Coupled models ; Ocean models
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  • 21
    Publication Date: 2017-01-05
    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): 2283–2296, doi:10.1175/JPO-D-11-0227.1.
    Description: The dynamic influence of thermohaline circulation on wind-driven circulation in the South China Sea (SCS) is studied using a simple reduced gravity model, in which the upwelling driven by mixing in the abyssal ocean is treated in terms of an upward pumping distributed at the base of the upper layer. Because of the strong upwelling of deep water, the cyclonic gyre in the northern SCS is weakened, but the anticyclonic gyre in the southern SCS is intensified in summer, while cyclonic gyres in both the southern and northern SCS are weakened in winter. For all seasons, the dynamic influence of thermohaline circulation on wind-driven circulation is larger in the northern SCS than in the southern SCS. Analysis suggests that the upwelling associated with the thermohaline circulation in the deep ocean plays a crucial role in regulating the wind-driven circulation in the upper ocean.
    Description: G. Wang is supported by the National Science Foundation of China (NSFC Grants 41125019, 40725017, and 40976017).D.Chen is supported by grants from the Ministry of Science and Technology (2010DFA21012), the State Oceanic Administration (201105018), and the NSFC (91128204).
    Description: 2013-06-01
    Keywords: Abyssal circulation ; Dynamics ; Ocean circulation ; Upwelling/downwelling
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  • 22
    Publication Date: 2017-01-05
    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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  • 23
    Publication Date: 2017-01-05
    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): 2593–2616, doi:10.1175/JPO-D-13-0120.1.
    Description: The first direct estimate of the rate at which geostrophic turbulence mixes tracers across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is 710 ± 260 m2 s−1 at 1500-m depth. The estimate is based on an extrapolation of the tracer-based diffusivity using output from numerical tracers released in a one-twentieth of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s−1 and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. These vertical variations are not captured by ocean models presently used for climate studies, but they significantly affect the ventilation of different water masses.
    Description: NSF support through Awards OCE-1233832, OCE-1232962, and OCE-1048926 is gratefully acknowledged.
    Description: 2015-04-01
    Keywords: Geographic location/entity ; Southern Ocean ; Circulation/ Dynamics ; Diffusion ; Eddies ; Ocean circulation ; Turbulence ; Physical Meteorology and Climatology ; Isopycnal mixing
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  • 24
    Publication Date: 2018-10-12
    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): 573-590, doi:10.1175/JPO-D-17-0206.1.
    Description: Motivated by the proximity of the Northern Recirculation Gyre and the deep western boundary current in the North Atlantic, an idealized model is used to investigate how recirculation gyres and a deep flow along a topographic slope interact. In this two-layer quasigeostrophic model, an unstable jet imposed in the upper layer generates barotropic recirculation gyres. These are maintained by an eddy-mean balance of potential vorticity (PV) in steady state. The authors show that the topographic slope can constrain the northern recirculation gyre meridionally and that the gyre’s adjustment to the slope leads to increased eddy PV fluxes at the base of the slope. When a deep current is present along the topographic slope in the lower layer, these eddy PV fluxes stir the deep current and recirculation gyre waters. Increased proximity to the slope dampens the eddy growth rate within the unstable jet, altering the geometry of recirculation gyre forcing and leading to a decrease in overall eddy PV fluxes. These mechanisms may shape the circulation in the western North Atlantic, with potential feedbacks on the climate system.
    Description: We gratefully acknowledge an AMS graduate fellowship (IALB) and U.S. National Science Foundation Grants OCE-1332667 and 1332834 (IALB and JMT).
    Description: 2018-09-06
    Keywords: Boundary currents ; Meridional overturning circulation ; Mesoscale processes ; Ocean circulation ; Potential vorticity ; Quasigeostrophic models
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  • 25
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    American Meteorological Society
    Publication Date: 2019-02-15
    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): 1831-1848, doi:10.1175/JPO-D-18-0068.1.
    Description: We present a simplified theory using reduced-gravity equations for North Atlantic Deep Water (NADW) and its variation driven by high-latitude deep-water formation. The theory approximates layer thickness on the eastern boundary with domain-averaged layer thickness and, in tandem with a mass conservation argument, retains fundamental physics for cross-equatorial flows on interannual and longer forcing time scales. Layer thickness anomalies are driven by a time-dependent northern boundary condition that imposes a southward volume flux representative of a variable source of NADW and damped by diapycnal mixing throughout the basin. Moreover, an outflowing southern boundary condition imposes a southward volume flux that generally differs from the volume flux at the northern boundary, giving rise to temporal storage of NADW within the Atlantic basin. Closed form analytic solutions for the amplitude and phase are provided when the variable source of NADW is sinusoidal. We provide a nondimensional analysis that demonstrates that solution behavior is primarily controlled by two parameters that characterize the meridional extent of the southern basin and the width of the basin relative to the equatorial deformation radius. Similar scaling applied to the time-lagged equations of Johnson and Marshall provides a clear connection to their results. Numerical simulations of reduced-gravity equations agree with analytic predictions in linear, turbulent, and diabatic regimes. The theory introduces a simple analytic framework for studying idealized buoyancy- and wind-driven cross-equatorial flows on interannual and longer time scales.
    Description: This research was supported by the National Science Foundation under Grant OCE- 1634468.
    Description: 2019-02-15
    Keywords: North Atlantic Ocean ; Tropics ; Meridional overturning circulation ; Ocean circulation ; Shallow-water equations
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  • 26
    Publication Date: 2019-03-11
    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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  • 27
    Publication Date: 2017-01-04
    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): 2942-2956, doi:10.1175/2009JPO4041.1.
    Description: Recent work by S. Lentz et al. documents offshore transport in the inner shelf due to a wave-driven return flow associated with the Hasselmann wave stress (the Stokes–Coriolis force). This analysis is extended using observations from the central Oregon coast to identify the wave-driven return flow present and quantify the potential bias of wind-driven across-shelf exchange by unresolved wave-driven circulation. Using acoustic Doppler current profiler (ADCP) measurements at six stations, each in water depths of 13–15 m, observed depth-averaged, across-shelf velocities were generally correlated with theoretical estimates of the proposed return flow. During times of minimal wind forcing, across-shelf velocity profiles were vertically sheared, with stronger velocities near the top of the measured portion of the water column, and increased in magnitude with increasing significant wave height, consistent with circulation due to the Hasselmann wave stress. Yet velocity magnitudes and vertical shears were stronger than that predicted by linear wave theory, and more similar to the stratified “summer” velocity profiles described by S. Lentz et al. Additionally, substantial temporal and spatial variability of the wave-driven return flow was found, potentially due to changing wind and wave conditions as well as local bathymetric variability. Despite the wave-driven circulation found, subtracting estimates of the return flow from the observed across-shelf velocity had no significant effect on estimates of the across-shelf exchange due to along-shelf wind forcing at these water depths along the Oregon coast during summer.
    Description: This work was performed with the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO), funded primarily by the Gordon and Betty Moore Foundation and David and Lucile Packard Foundation. SL acknowledges support from NSF Ocean Science Grant #OCE-0548961. AK acknowledges support from the WHOI Coastal Ocean Institute Fellowship.
    Keywords: Waves, oceanic ; Ekman pumping/transport ; Coastlines ; Ocean circulation ; Gravity waves
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  • 28
    Publication Date: 2017-01-04
    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): 675-690, doi:10.1175/2008JPO3975.1.
    Description: Motivated by observations of surface drifters in the Adriatic Sea, transport in a three-gyre system is studied with the aid of dynamical systems techniques. Particular attention is paid to the issue of intergyre transport. The velocity field is assumed to be two-dimensional and incompressible and composed of a steady three-gyre background flow on which a time-dependent perturbation is superimposed. Two systems of this type are considered: 1) an observationally motivated, analytically prescribed model consisting of a steady background on which a multiperiodic time-dependent perturbation is superimposed, and 2) an observationally based model of the Adriatic Sea consisting of the mean surface circulation derived from surface drifter trajectories on which a time-dependent altimetry-based perturbation velocity field is superimposed. It is shown that for a small perturbation to the steady three-gyre background, two of the gyres exchange no fluid with the third gyre. When the perturbation strength exceeds a certain threshold, transport between all three gyres occurs. This behavior is described theoretically, illustrated using the analytic model and shown to be consistent with the observationally based model of the Adriatic. The relevance of the work presented to more complicated multiple-gyre problems is discussed.
    Description: This work was supported by the National Science Foundation Grants CMG0417425 and CMG0825547.
    Keywords: Transport ; Gyres ; Mediterranean region ; Ocean circulation ; Lagrangian circulation
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  • 29
    Publication Date: 2017-01-05
    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): 968-983, doi:10.1175/2007JPO3807.1.
    Description: Labrador Sea Water (LSW), a dense water mass formed by convection in the subpolar North Atlantic, is an important constituent of the meridional overturning circulation. Understanding how the water mass enters the deep western boundary current (DWBC), one of the primary pathways by which it exits the subpolar gyre, can shed light on the continuity between climate conditions in the formation region and their downstream signal. Using the trajectories of (profiling) autonomous Lagrangian circulation explorer [(P)ALACE] floats, operating between 1996 and 2002, three processes are evaluated for their role in the entry of Labrador Sea Water in the DWBC: 1) LSW is formed directly in the DWBC, 2) eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) a horizontally divergent mean flow advects LSW from the interior to the DWBC. A comparison of the heat flux associated with each of these three mechanisms suggests that all three contribute to the transformation of the boundary current as it transits the Labrador Sea. The formation of LSW directly in the DWBC and the eddy heat flux between the interior Labrador Sea and the DWBC may play leading roles in setting the interannual variability of the exported water mass.
    Description: We are also grateful to the NSF for their support of this research.
    Keywords: Boundary currents ; Water masses ; Ocean circulation ; Lagrangian circulation
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  • 30
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    American Meteorological Society
    Publication Date: 2017-01-04
    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): 2639-2661, doi:10.1175/2008JPO3946.1.
    Description: A theory for the exchange between a rotating, buoyancy-forced marginal sea and an ocean is developed and tested numerically. Cooling over the marginal sea leads to sinking and sets up a two-layer exchange flow, with a warm surface layer entering from the ocean and a cool layer exiting at depth. The connecting strait is sufficiently narrow and shallow to cause the exchange flow to be hydraulically controlled. The incoming surface layer forms a baroclinically unstable boundary current that circles the marginal sea in a cyclonic sense and feeds heat to the interior by way of eddies. Consistent with the overall heat and volume balances for the marginal sea, there is a continuous family of hydraulically controlled states with critical flow at the most constricted section of the strait. Included in this family is a limiting “maximal-exchange” solution with two sections of hydraulic control in the strait and with fixed layer depths at the most constricted section. The state of exchange for a given forcing is predicted using a theory that assumes energy conservation over a certain path connecting the strait to the marginal sea or, in some cases, the ocean. Depending on the configuration of the exchange, long-wave information may be blocked from entering the strait from the marginal sea, from the open ocean, or both. The scenario that holds determines what is predicted and what needs to be input. Numerical tests of the prediction for the temperature difference and the state of exchange are carried out for straits with a pure contraction in width and for a constant width strait with a topographic sill. The comparison is reasonable in most cases, though the numerical model is not able to reproduce cases of multiple states predicted by the theory for certain forcing values. The analytical model is an alternative to the Price and Yang and Siddall et al. models of a marginal sea outflow.
    Description: This work was supported by the National Science Foundation under Grants OCE-0525729 and OCE-0423975.
    Keywords: Ocean circulation ; Coastlines ; Ocean models
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  • 31
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    American Meteorological Society
    Publication Date: 2017-01-05
    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): 1091-1106, doi:10.1175/2007JPO3805.1.
    Description: A model of deep ocean circulation driven by turbulent mixing is produced in a long, rectangular laboratory tank. The salinity difference is substituted for the thermal difference between tropical and polar regions. Freshwater gently flows in at the top of one end, dense water enters at the same rate at the top of the other end, and an overflow in the middle removes the same amount of surface water as is pumped in. Mixing is provided by a rod extending from top to bottom of the tank and traveling back and forth at constant speed with Reynolds numbers 〉500. A stratified upper layer (“thermocline”) deepens from the mixing and spreads across the entire tank. Simultaneously, a turbulent plume (“deep ocean overflow”) from a dense-water source descends through the layer and supplies bottom water, which spreads over the entire tank floor and rises into the upper layer to arrest the upper-layer deepening. Data are taken over a wide range of parameters and compared to scaling theory, energetic considerations, and simple models of turbulently mixed fluid. There is approximate agreement with a simple theory for Reynolds number 〉1000 in experiments with a tank depth less than the thermocline depth. A simple argument shows that mixing and plume potential energy flux rates are equal in magnitude, and it is suggested that the same is approximately true for the ocean.
    Description: The research was supported by the Ocean Climate Change Institute of Woods Hole Oceanographic Institution.
    Keywords: Ocean circulation ; Mixing ; In situ observations ; Vertical motion
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  • 32
    Publication Date: 2017-01-04
    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): 2234–2253, doi:10.1175/JPO-D-12-033.1.
    Description: Meridional velocity, mass, and heat transport in the equatorial oceans are difficult to estimate because of the nonapplicability of the geostrophic balance. For this purpose a steady-state model is utilized in the equatorial Indian Ocean using NCEP wind stress and temperature and salinity data from the World Ocean Atlas 2005 (WOA05) and Argo. The results show a Somali Current flowing to the south during the winter monsoon carrying −11.5 ± 1.3 Sv (1 Sv ≡ 106 m3 s−1) and −12.3 ± 0.3 Sv from WOA05 and Argo, respectively. In the summer monsoon the Somali Current reverses to the north transporting 16.8 ± 1.2 Sv and 19.8 ± 0.6 Sv in the WOA05 and Argo results. Transitional periods are considered together and in consequence, there is not a clear Somali Current present in this period. Model results fit with in situ measurements made around the region, although Argo data results are quite more realistic than WOA05 data results.
    Description: This study has been partly funded by the MOC Project (CTM 2008- 06438) and the Spanish contribution to the Argo network (AC2009 ACI2009-0998), financed by the Spanish Government and Feder.
    Description: 2013-06-01
    Keywords: Indian Ocean ; Subtropics ; Currents ; Ocean circulation ; Transport ; Wind stress
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  • 33
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    American Meteorological Society
    Publication Date: 2017-01-04
    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): 2498–2523, doi:10.1175/JPO-D-13-0183.1.
    Description: This study examines the observability of a stratified ocean in a square flat basin on a midlatitude beta plane. Here, “observability” means the ability to establish, in a finite interval of time, the time-dependent ocean state given density observations over the same interval and with no regard for errors. The dynamics is linearized and hydrostatic, so that the motion can be decomposed into normal modes and the observability analysis is simplified. An observability Gramian (a symmetric matrix) is determined for the flows in an inviscid interior, in frictional boundary layers, and in a closed basin. Its properties are used to establish the condition for complete observability and to identify optimal data locations for each of these flows. It is found that complete observability of an oceanic interior in time-dependent Sverdrup balance requires that the observations originate from the westernmost location at each considered latitude. The degree of observability increases westward due to westward propagation of long baroclinic Rossby waves: data collected in the west are more informative than data collected in the east. Likewise, the best locations for observing variability in the western (eastern) boundary layer are near (far from) the boundary. The observability of a closed basin is influenced by the westward propagation and the boundaries. Optimal data locations that are identified for different resolutions (0.01 to 1 yr) and lengths of data records (0.2 to 20 yr) show a variable influence of the planetary vorticity gradient. Data collected near the meridional boundaries appear always less informative, from the viewpoint of basin observability, than data collected away from these boundaries.
    Description: This work was supported by the U.S. National Science Foundation.
    Description: 2015-03-01
    Keywords: Circulation/ Dynamics ; Ocean circulation ; Rossby waves ; Mathematical and statistical techniques ; Inverse methods ; Variability ; Oceanic variability
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  • 34
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 27 (2014): 9359–9376, doi:10.1175/JCLI-D-14-00228.1.
    Description: Multidecadal variability of the Atlantic meridional overturning circulation (AMOC) is examined based on a comparison of the AMOC streamfunctions in depth and in density space, in a 700-yr present-day control integration of the fully coupled Community Climate System Model, version 3. The commonly used depth-coordinate AMOC primarily exhibits the variability associated with the deep equatorward transport that follows the changes in the Labrador Sea deep water formation. On the other hand, the density-based AMOC emphasizes the variability associated with the subpolar gyre circulation in the upper ocean leading to the changes in the Labrador Sea convection. Combining the two representations indicates that the ~20-yr periodicity of the AMOC variability in the first half of the simulation is primarily due to an ocean-only mode resulting from the coupling of the deep equatorward flow and the upper ocean gyre circulation near the Gulf Stream and North Atlantic Current. In addition, the density-based AMOC reveals a gradual change in the deep ocean associated with cooling and increased density, which is likely responsible for the transition of AMOC variability from strong ~20-yr oscillations to a weaker red noise–like multidecadal variability.
    Description: Support from the NOAA Climate Program Office (Grant NA10OAR4310202 and NA13OAR4310139) and NSF EaSM2 (OCE1242989) is gratefully acknowledged.
    Description: 2015-06-15
    Keywords: North Atlantic Ocean ; Meridional overturning circulation ; Ocean circulation ; Thermocline circulation ; Climate variability ; Multidecadal variability
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  • 35
    Publication Date: 2017-06-02
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 3011-3029, doi:10.1175/JPO-D-15-0248.1.
    Description: Seasonal variability of the tropical Atlantic circulation is dominated by the annual cycle, but semiannual variability is also pronounced, despite weak forcing at that period. This study uses multiyear, full-depth velocity measurements from the central equatorial Atlantic to analyze the vertical structure of annual and semiannual variations of zonal velocity. A baroclinic modal decomposition finds that the annual cycle is dominated by the fourth mode and the semiannual cycle is dominated by the second mode. Similar local behavior is found in a high-resolution general circulation model. This simulation reveals that the annual and semiannual cycles of the respective dominant baroclinic modes are associated with characteristic basinwide structures. Using an idealized, linear, reduced-gravity model to simulate the dynamics of individual baroclinic modes, it is shown that the observed circulation variability can be explained by resonant equatorial basin modes. Corollary simulations of the reduced-gravity model with varying basin geometry (i.e., square basin vs realistic coastlines) or forcing (i.e., spatially uniform vs spatially variable wind) show a structural robustness of the simulated basin modes. A main focus of this study is the seasonal variability of the Equatorial Undercurrent (EUC) as identified in recent observational studies. Main characteristics of the observed EUC including seasonal variability of transport, core depth, and maximum core velocity can be explained by the linear superposition of the dominant equatorial basin modes as obtained from the reduced-gravity model.
    Description: This study was supported by the Deutsche Forschungsgemeinschaft as part of the Sonderforschungsbereich 754 (SFB754) ‘‘Climate–Biogeochemistry Interactions in the Tropical Ocean’’ and through several research cruises with R/V Meteor, R/V Maria S. Merian, andR/VL’Atalante by the German Federal Ministry of Education and Research as part of the cooperative projects RACE (03F0605B) and SACUS (03G0837A) and by European Union 7th Framework Programme (FP7 2007–13) under Grant Agreement 603521 PREFACE project.
    Keywords: Atlantic Ocean ; Ocean circulation ; In situ oceanic observations ; Ocean models ; Seasonal cycle ; Tropical variability
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  • 36
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    American Meteorological Society
    Publication Date: 2018-10-12
    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): 643-646, doi:10.1175/JPO-D-17-0240.1.
    Description: A simple oceanic model is presented for source–sink flow on the β plane to discuss the pathways from source to sink when transport boundary layers have large enough Reynolds numbers to be inertial in their dynamics. A representation of the flow as a Fofonoff gyre, suggested by prior work on inertial boundary layers and eddy-driven circulations in two-dimensional turbulent flows, indicates that even when the source and sink are aligned along the same western boundary the flow must intrude deep into the interior before exiting at the sink. The existence of interior pathways for the flow is thus an intrinsic property of an inertial circulation and is not dependent on particular geographical basin geometry.
    Description: 2018-09-12
    Keywords: Abyssal circulation ; Bottom currents ; Nonlinear dynamics ; Ocean circulation ; Ocean dynamics ; Potential vorticity
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  • 37
    Publication Date: 2017-01-04
    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): 1988–2006, doi:10.1175/2010JPO4336.1.
    Description: Techniques from dynamical systems theory have been applied to study horizontal stirring of fluid in the Philippine Archipelago. The authors’ analysis is based on velocity fields produced by two high-resolution (3 and 6 km) numerical models. Particular attention is paid to identifying robust surface flow patterns and associating them with dominant Lagrangian coherent structures (LCSs). A recurrent wind-driven dipole in the lee of the coastline is considered in detail. The associated LCSs form a template for stirring, exchange, and biological transport in and around the dipole. Chaotic advection is argued to provide a relevant framework for interpreting mesoscale horizontal stirring processes in an archipelago as a whole. Implications for the formation of filaments, the production of tracer variance, and the scale at which stirring leads to mixing are discussed in connection with an observed temperature record.
    Description: This work was supported by the Office of Naval Research: Grant N00014-07-1-0590 to Woods Hole Oceanographic Institution; Grant ONR- 0601153N to Stevens Institute of Technology; Grant N00014-07-1-0417 to Institute of Marine and Coastal Sciences, Rutgers University; and Grant N00014-09-1- 0582 to Lamont-Doherty Earth Observatory of Columbia University.
    Keywords: Advection ; Ocean circulation
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  • 38
    Publication Date: 2017-01-05
    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): 1441–1457, doi:10.1175/2010JPO4293.1.
    Description: Based on the classical Ekman layer theory, a simple analytical solution of the steady flow induced by a stationary hurricane in a homogenous ocean is discussed. The model consists of flow converging in an inward spiral in the deeper layer and diverging in the upper layer. The simple analytical model indicates that both the upwelling flux and the horizontal transport increase linearly with increasing radius of maximum winds. Furthermore, they both have a parabolic relationship with the maximum wind speed. The Coriolis parameter also affects the upwelling flux: the response to a hurricane is stronger at low latitudes than that at middle latitudes. Numerical solutions based on a regional version of an ocean general circulation model are similar to the primary results obtained through the analytical solution. Thus, the simplifications made in formulating the analytical solution are reasonable. Although the analytical solution in this paper is sought for a rather idealized ocean, it can help to make results from the more complicated numerical model understandable. These conceptual models provide a theoretical limit structure of the oceanic response to a moving hurricane over a stratified ocean.
    Description: ZML was supported by CAS (kzcx2-yw-226, SQ200813), NSFC under Grants 40906009 and 40776008, and Dr. Xiaodong Shang through ‘‘100 Talents Program’’ of CAS.
    Keywords: Hurricanes ; Ocean circulation
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  • 39
    Publication Date: 2017-01-04
    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): 429-438, doi:10.1175/2009JPO4187.1.
    Description: Sections of temperature, salinity, dissolved oxygen, and velocity were made crossing the Gulf Stream in late January 2006 to investigate the role of frontal processes in the formation of Eighteen Degree Water (EDW), the Subtropical Mode Water of the North Atlantic. The sections were nominally perpendicular to the stream and measured in a Lagrangian frame by following a floating spar buoy drifting in the Gulf Stream’s warm core. During the survey, EDW was isolated from the mixed layer by the stratified seasonal pycnocline, suggesting that EDW was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted (64°–70°W). However, in two of the sections, the seasonal pycnocline in the core of the Gulf Stream was broken by an intrusion of cold, fresh, weakly stratified water, nearly saturated in oxygen, that appears to have been subducted from the surface mixed layer north of the stream. The intrusion was identified in three of the sections in profiles with a nearly identical temperature–salinity relation. From the western-to-easternmost sections, where the intrusion was observed, the depth of the intrusion’s salinity minimum descended by 90 m in the 71 h it took to complete this part of the survey. This apparent subduction occurred primarily on the upstream side of a meander trough, where the cross-stream velocity was confluent and frontogenetic. Using a variant of the omega equation, the vertical velocity driven by the confluent flow was inferred and yielded downwelling in the vicinity of the intrusion spanning 10–40 m day−1, a range of values consistent with the intrusion’s observed descent, suggesting that frontal subduction was responsible for the formation of the intrusion. In the easternmost section located downstream of the meander trough, the flow was diffluent, driving an inferred vertical circulation that was of the opposite sense to that in the section upstream of the trough. In transiting the two sides of the trough, the intrusion was observed to move toward the center of the stream between the downwelling branches of the opposing vertical circulations, resulting in a downward Lagrangian mean vertical velocity and net subduction. Hydrographic evidence of the subduction of weakly stratified surface waters was seen in the southern flank of the Gulf Stream as well. The solution of the omega equation suggests that this subduction was associated with a relatively shallow vertical circulation confined to the upper 200 m of the water column in the proximity of the front marking the southern edge of the warm core.
    Description: Support came from the National Science Foundation Grant OCE-0424865 (TJ and LT). LT was also supported by NSF Grant OCE- 0549699 and the Worzel Assistant Scientist Endowed Fund at WHOI.
    Keywords: Ocean circulation ; Thermohaline circulation ; Frontogenesis/frontolysis
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  • 40
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    American Meteorological Society
    Publication Date: 2017-01-04
    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): 1231-1243, doi:10.1175/2008JPO4087.1.
    Description: As a driving parameter is slowly altered, thermohaline ocean circulation models show either a smooth evolution of a mode of flow or an abrupt transition of temperature and salinity fields from one mode to another. An abrupt transition might occur at one value or over a range of the driving parameter. The latter has hysteresis because the mode in this range depends on the history of the driving parameter. Although assorted ocean circulation models exhibit abrupt transitions, such transitions have not been directly observed in the ocean. Therefore, laboratory experiments have been conducted to seek and observe actual (physical) abrupt thermohaline transitions. An experiment closely duplicating Stommel’s box model possessed abrupt transitions in temperature and salinity with distinct hysteresis. Two subsequent experiments with more latitude for internal circulation in the containers possessed abrupt transitions over a much smaller range of hysteresis. Therefore, a new experiment with even more latitude for internal circulation was designed and conducted. A large tank of constantly renewed freshwater at room temperature had a smaller cavity in the bottom heated from below with saltwater steadily pumped in. The cavity had either a salt mode, consisting of the cavity filled with heated salty water with an interface at the cavity top, or a temperature mode, in which the heat and saltwater were removed from the cavity by convection. There was no measurable hysteresis between the two modes. Possible reasons for such small hysteresis are discussed.
    Description: Support is gratefully acknowledged from the Woods Hole Oceanographic Institution Climate Change Institute, the National Science Foundation, Physical Oceanography Section under Grant OCE-0081179, and the Paul M. Fye Chair of the Woods Hole Oceanographic Institution.
    Keywords: Thermohaline circulation ; Experimental design ; Ocean circulation ; Temperature ; Salinity
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  • 41
    Publication Date: 2017-01-04
    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): 2563-2569, doi:10.1175/JPO3134.1.
    Description: Along the Taiwan Strait (〈100 m in depth) a northeastward flow persists in all seasons despite the annually averaged wind stress that is strongly southwestward. The forcing mechanism of this countercurrent is examined by using a simple ocean model. The results from a suite of experiments demonstrate that it is the Kuroshio that plays the deciding role for setting the flow direction along the Taiwan Strait. The momentum balance along the strait is mainly between the wind stress, friction, and pressure gradient. Since both wind stress and friction act against the northward flow, it is most likely the pressure gradient that forces the northward flow, as noted in some previous studies. What remains unknown is why there is a considerable pressure difference between the southern and northern strait. The Kuroshio flows along the east coast of Taiwan, and thus the western boundary current layer dynamics applies there. Integrating the momentum equation along Taiwan’s east coast shows that there must be a pressure difference between the southern and the northern tip of Taiwan to counter a considerable friction exerted by the mighty Kuroshio. This same pressure difference is also felt on the other side of the island where it forces the northward flow through Taiwan Strait. The model shows that the local wind stress acts to dampen this northward flow. This mechanism can be illustrated by an integral constraint for flow around an island.
    Description: This study has been supported by National Science Foundation through Grant OCE-0351055.
    Keywords: Ocean circulation ; Wind ; Currents
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  • 42
    Publication Date: 2017-01-07
    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): 241-246, doi:10.1175/2010JPO4557.1.
    Description: The vertical dispersion of a tracer released on a density surface near 1500-m depth in the Antarctic Circumpolar Current west of Drake Passage indicates that the diapycnal diffusivity, averaged over 1 yr and over tens of thousands of square kilometers, is (1.3 ± 0.2) × 10−5 m2 s−1. Diapycnal diffusivity estimated from turbulent kinetic energy dissipation measurements about the area occupied by the tracer in austral summer 2010 was somewhat less, but still within a factor of 2, at (0.75 ± 0.07) × 10−5 m2 s−1. Turbulent diapycnal mixing of this intensity is characteristic of the midlatitude ocean interior, where the energy for mixing is believed to derive from internal wave breaking. Indeed, despite the frequent and intense atmospheric forcing experienced by the Southern Ocean, the amplitude of finescale velocity shear sampled about the tracer was similar to background amplitudes in the midlatitude ocean, with levels elevated to only 20%–50% above the Garrett–Munk reference spectrum. These results add to a long line of evidence that diapycnal mixing in the interior middepth ocean is weak and is likely too small to dictate the middepth meridional overturning circulation of the ocean.
    Description: This material is based upon work supported by the National Science Foundation Grants OCE-0622825,OCE-0622670, OCE-0622630, and OCE-0623177.
    Keywords: Diapycnal mixing ; Currents ; Antarctica ; Ocean circulation ; Meridional overturning circulation
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  • 43
    Publication Date: 2017-01-04
    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): 1874–1893, doi:10.1175/2011JPO4604.1.
    Description: A two-dimensional cross-shelf model of the New England continental shelf and slope is used to investigate the mean cross-shelf and vertical circulation at the shelf break and their seasonal variation. The model temperature and salinity fields are nudged toward climatology. Annual and seasonal mean wind stresses are applied on the surface in separate equilibrium simulations. The along-shelf pressure gradient force associated with the along-shelf sea level tilt is tuned to match the modeled and observed depth-averaged along-shelf velocity. Steady-state model solutions show strong seasonal variation in along-shelf and cross-shelf velocity, with the strongest along-shelf jet and interior onshore flow in winter, consistent with observations. Along-shelf sea level tilt associated with the tuned along-shelf pressure gradient increases shoreward because of decreasing water depth. The along-shelf sea level tilt varies seasonally with the wind and is the strongest in winter and weakest in summer. A persistent upwelling is generated at the shelf break with a maximum strength of 2 m day−1 at 50-m depth in winter. The modeled shelfbreak upwelling differs from the traditional view in that most of the upwelled water is from the upper continental slope instead of from the shelf in the form of a detached bottom boundary layer.
    Description: WGZ was supported by the Woods Hole Oceanographic Institution postdoctoral scholarship program. GGGandDJMwere supported byONRGrant N-00014- 06-1-0739.
    Keywords: Ocean circulation ; North Atlantic Ocean
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  • 44
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 26 (2013): 1669–1684, doi:10.1175/JCLI-D-12-00246.1.
    Description: Climate change west of the Antarctic Peninsula is the most rapid of anywhere in the Southern Hemisphere, with associated changes in the rates and distributions of freshwater inputs to the ocean. Here, results from the first comprehensive survey of oxygen isotopes in seawater in this region are used to quantify spatial patterns of meteoric water (glacial discharge and precipitation) separately from sea ice melt. High levels of meteoric water are found close to the coast, due to orographic effects on precipitation and strong glacial discharge. Concentrations decrease offshore, driving significant southward geostrophic flows (up to ~30 cm s−1). These produce high meteoric water concentrations at the southern end of the sampling grid, where collapse of the Wilkins Ice Shelf may also have contributed. Sea ice melt concentrations are lower than meteoric water and patchier because of the mobile nature of the sea ice itself. Nonetheless, net sea ice production in the northern part of the sampling grid is inferred; combined with net sea ice melt in the south, this indicates an overall southward ice motion. The survey is contextualized temporally using a decade-long series of isotope data from a coastal Antarctic Peninsula site. This shows a temporal decline in meteoric water in the upper ocean, contrary to expectations based on increasing precipitation and accelerating deglaciation. This is driven by the increasing occurrence of deeper winter mixed layers and has potential implications for concentrations of trace metals supplied to the euphotic zone by glacial discharge. As the regional freshwater system evolves, the continuing isotope monitoring described here will elucidate the ongoing impacts on climate and the ecosystem.
    Description: The Palmer LTER participants acknowledge Award 0823101 from the Organisms and Ecosystems program in NSF OPP
    Description: 2013-09-01
    Keywords: Southern Ocean ; Ocean circulation ; Freshwater ; Precipitation ; Snowmelt/icemelt ; Isotopic analysis
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  • 45
    Publication Date: 2017-01-04
    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): 3033–3053, doi:10.1175/JPO-D-13-0227.1.
    Description: The East Greenland Current (EGC) had long been considered the main pathway for the Denmark Strait overflow (DSO). Recent observations, however, indicate that the north Icelandic jet (NIJ), which flows westward along the north coast of Iceland, is a major separate pathway for the DSO. In this study a two-layer numerical model and complementary integral constraints are used to examine various pathways that lead to the DSO and to explore plausible mechanisms for the NIJ’s existence. In these simulations, a westward and NIJ-like current emerges as a robust feature and a main pathway for the Denmark Strait overflow. Its existence can be explained through circulation integrals around advantageous contours. One such constraint spells out the consequences of overflow water as a source of low potential vorticity. A stronger constraint can be added when the outflow occurs through two outlets: it takes the form of a circulation integral around the Iceland–Faroe Ridge. In either case, the direction of overall circulation about the contour can be deduced from the required frictional torques. Some effects of wind stress forcing are also examined. The overall positive curl of the wind forces cyclonic gyres in both layers, enhancing the East Greenland Current. The wind stress forcing weakens but does not eliminate the NIJ. It also modifies the sign of the deep circulation in various subbasins and alters the path by which overflow water is brought to the Faroe Bank Channel, all in ways that bring the idealized model more in line with observations. The sequence of numerical experiments separates the effects of wind and buoyancy forcing and shows how each is important.
    Description: This study has been supported by National Science Foundation (OCE0927017 and ARC1107412).
    Description: 2015-06-01
    Keywords: Circulation/ Dynamics ; Boundary currents ; Channel flows ; Meridional overturning circulation ; Ocean circulation ; Topographic effects
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  • 46
    Publication Date: 2016-12-23
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 966–987, doi:10.1175/JPO-D-14-0110.1.
    Description: A key remaining challenge in oceanography is the understanding and parameterization of small-scale mixing. Evidence suggests that topographic features play a significant role in enhancing mixing in the Southern Ocean. This study uses 914 high-resolution hydrographic profiles from novel EM-APEX profiling floats to investigate turbulent mixing north of the Kerguelen Plateau, a major topographic feature in the Southern Ocean. A shear–strain finescale parameterization is applied to estimate diapycnal diffusivity in the upper 1600 m of the ocean. The indirect estimates of mixing match direct microstructure profiler observations made simultaneously. It is found that mixing intensities have strong spatial and temporal variability, ranging from O(10−6) to O(10−3) m2 s−1. This study identifies topographic roughness, current speed, and wind speed as the main factors controlling mixing intensity. Additionally, the authors find strong regional variability in mixing dynamics and enhanced mixing in the Antarctic Circumpolar Current frontal region. This enhanced mixing is attributed to dissipating internal waves generated by the interaction of the Antarctic Circumpolar Current and the topography of the Kerguelen Plateau. Extending the mixing observations from the Kerguelen region to the entire Southern Ocean, this study infers a large water mass transformation rate of 17 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) across the boundary of Antarctic Intermediate Water and Upper Circumpolar Deep Water in the Antarctic Circumpolar Current. This work suggests that the contribution of mixing to the Southern Ocean overturning circulation budget is particularly significant in fronts.
    Description: AM was supported by the joint CSIRO–University of Tasmania Quantitative Marine Science (QMS) program and the 2009 CSIRO Wealth from Ocean Flagship Collaborative Fund. BMS was supported by the Australian Climate Change Science Program, jointly funded by the Department of the Environment and CSIRO. KLPs salary support was provided by Woods Hole Oceanographic Institution bridge support funds.
    Description: 2015-10-01
    Keywords: Geographic location/entity ; Southern Ocean ; Circulation/ Dynamics ; Diapycnal mixing ; Fronts ; Ocean circulation ; Topographic effects ; Atm/Ocean Structure/ Phenomena ; Mixing
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  • 47
    Publication Date: 2017-01-04
    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): 778–791, doi:10.1175/JPO-D-14-0164.1.
    Description: This study examines anisotropic transport properties of the eddying North Atlantic flow, using an idealized model of the double-gyre oceanic circulation and altimetry-derived velocities. The material transport by the time-dependent flow (quantified by the eddy diffusivity tensor) varies geographically and is anisotropic, that is, it has a well-defined direction of the maximum transport. One component of the time-dependent flow, zonally elongated large-scale transients, is particularly important for the anisotropy, as it corresponds to primarily zonal material transport and long correlation time scales. The importance of these large-scale zonal transients in the material distribution is further confirmed with simulations of idealized color dye tracers, which has implications for parameterizations of the eddy transport in non-eddy-resolving models.
    Description: IK would like to acknowledge support through the NSF Grant OCE-1154923. IR was supported by the NSF OCE-1154641 and NASA Grant NNX14AH29G.
    Description: 2015-09-01
    Keywords: Circulation/ Dynamics ; Eddies ; Lagrangian circulation/transport ; Mesoscale processes ; Ocean circulation ; Models and modeling ; Tracers
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  • 48
    Publication Date: 2017-01-05
    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): 2598–2620, doi:10.1175/JPO-D-14-0249.1.
    Description: Through combining analytical arguments and numerical models, this study investigates the finite-amplitude meanders of shelfbreak fronts characterized by sloping isopycnals outcropping at both the surface and the shelfbreak bottom. The objective is to provide a formula for the meander length scale that can explain observed frontal length scale variability and also be verified with observations. Considering the frontal instability to be a mixture of barotropic and baroclinic instability, the derived along-shelf meander length scale formula is [b1/(1 + a1S1/2)]NH/f, where N is the buoyancy frequency; H is the depth of the front; f is the Coriolis parameter; S is the Burger number measuring the ratio of energy conversion associated with barotropic and baroclinic instability; and a1 and b1 are empirical constants. Initial growth rate of the frontal instability is formulated as [b2(1 + a1S1/2)/(1 + a2αS1/2)]NH/L, where α is the bottom slope at the foot of the front, and a2 and b2 are empirical constants. The formulas are verified using numerical sensitivity simulations, and fitting of the simulated and formulated results gives a1 = 2.69, b1 = 14.65, a2 = 5.1 × 103, and b2 = 6.2 × 10−2. The numerical simulations also show development of fast-growing frontal symmetric instability when the minimum initial potential vorticity is negative. Although frontal symmetric instability leads to faster development of barotropic and baroclinic instability at later times, it does not significantly influence the meander length scale. The derived meander length scale provides a framework for future studies of the influences of external forces on shelfbreak frontal circulation and cross-frontal exchange.
    Description: WGZ and GGG were supported by the National Science Foundation through Grant OCE-1129125.
    Description: 2016-04-01
    Keywords: Circulation/ Dynamics ; Instability ; Ocean circulation ; Topographic effects ; Atm/Ocean Structure/ Phenomena ; Fronts ; Models and modeling ; Numerical analysis/modeling
    Repository Name: Woods Hole Open Access Server
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  • 49
    Publication Date: 2016-05-01
    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): 2806–2819, doi:10.1175/JPO-D-15-0061.1.
    Description: An eastward-flowing current of a homogeneous fluid with velocity U, contained in a channel of width L, impinges on an island of width of O(L), and the resulting interaction and dynamics are studied for values of the supercriticality parameter, b = βL2/U, both larger and smaller than π2. The former case is subcritical with respect to Rossby waves, and the latter is supercritical. The nature of the flow field depends strongly on b, and in particular, the nature of the flow around the island and the proportion of the flow passing to the north or south of the island are sensitive to b and to the position of the island in the channel. The problem is studied analytically in a relatively simple, nonlinear quasigeostrophic and adiabatic framework and numerically with a shallow-water model that allows a qualitative extension of the results to the equator. Although the issues involved are motivated by the interaction of the Equatorial Undercurrent and the Galapagos Islands, the analysis presented here focuses on the fundamental issue of the distinctive nature of the flow as a function of Rossby wave criticality.
    Description: Supported by the National Science Foundation Grant OCE-0959381.
    Description: 2016-05-01
    Keywords: Circulation/ Dynamics ; Ocean circulation ; Ocean dynamics ; Waves, oceanic
    Repository Name: Woods Hole Open Access Server
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  • 50
    Publication Date: 2017-04-20
    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): 3263-3278, doi:10.1175/JPO-D-16-0091.1.
    Description: The halocline of the Beaufort Gyre varies significantly on interannual to decadal time scales, affecting the freshwater content (FWC) of the Arctic Ocean. This study explores the role of eddies in the Ekman-driven gyre variability. Following the transformed Eulerian-mean paradigm, the authors develop a theory that links the FWC variability to the stability of the large-scale gyre, defined as the inverse of its equilibration time. The theory, verified with eddy-resolving numerical simulations, demonstrates that the gyre stability is explicitly controlled by the mesoscale eddy diffusivity. An accurate representation of the halocline dynamics requires the eddy diffusivity of 300 ± 200 m2 s−1, which is lower than what is used in most low-resolution climate models. In particular, on interannual and longer time scales the eddy fluxes and the Ekman pumping provide equally important contributions to the FWC variability. However, only large-scale Ekman pumping patterns can significantly alter the FWC, with spatially localized perturbations being an order of magnitude less efficient. Lastly, the authors introduce a novel FWC tendency diagnostic—the Gyre Index—that can be conveniently calculated using observations located only along the gyre boundaries. Its strong predictive capabilities, assessed in the eddy-resolving model forced by stochastic winds, suggest that the Gyre Index would be of use in interpreting FWC evolution in observations as well as in numerical models.
    Description: GEMacknowledges the support from theHowland Postdoctoral Program Fund at WHOI and the Stanback Fellowship Fund at Caltech.MAS was supported by NSF Grants PLR-1415489 and OCE-1232389. AFT acknowledges support from NASA Award NNN12AA01C.
    Description: 2017-04-20
    Keywords: Arctic ; Eddies ; Ekman pumping/transport ; Large-scale motions ; Ocean circulation ; Stability
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  • 51
    Publication Date: 2017-07-31
    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): 339-351, doi:10.1175/JPO-D-16-0165.1.
    Description: A novel multi-iteration statistical method for studying tracer spreading using drifter data is introduced. The approach allows for the best use of the available drifter data by making use of a simple iterative procedure, which results in the statistically probable map showing the likelihood that a tracer released at some source location would visit different geographical regions, along with the associated arrival travel times. The technique is tested using real drifter data in the North Atlantic. Two examples are considered corresponding to sources in the western and eastern North Atlantic Ocean, that is, Massachusetts Bay–like and Irish Sea–like sources, respectively. In both examples, the method worked well in estimating the statistics of the tracer transport pathways and travel times throughout the entire North Atlantic. The role of eddies versus mean flow is quantified using the same technique, and eddies are shown to significantly broaden the spread of a tracer. The sensitivity of the results to the size of the source domain is investigated and causes for this sensitivity are discussed.
    Description: This work was supported by the Grant OCE-1356630 from the National Science Foundation (NSF). Rypina also acknowledges NSF Grant OCE-1154641 and NASA Grant NNX14AH29G.
    Description: 2017-07-31
    Keywords: Atlantic Ocean ; Mass fluxes/transport ; Ocean circulation ; Trajectories ; Statistics
    Repository Name: Woods Hole Open Access Server
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  • 52
    Publication Date: 2018-06-08
    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): 2927-2947, doi:10.1175/JPO-D-17-0083.1.
    Description: Motivated by observations in Hudson shelf valley showing stronger onshore than offshore flows, this study investigates wind-driven flows in idealized shallow shelf valleys. This first part of a two-part sequence focuses on the mechanism of the asymmetrical flow response in a valley to along-shelf winds of opposite directions. Model simulations show that (i) when the wind is in the opposite direction to coastal-trapped wave (CTW) phase propagation, the shelf flow turns onshore in the valley and generates strong up-valley transport and a standing meander on the upstream side (in the sense of CTW phase propagation) of the valley, and (ii) when the wind is in the same direction as CTW phase propagation, the flow forms a symmetric onshore detour pattern over the valley with negligible down-valley transport. Comparison of the modeled upstream meanders in the first scenario with CTW characteristics confirms that the up-valley flow results from CTWs being arrested by the wind-driven shelf flow establishing lee waves. The valley bathymetry generates an initial excessive onshore pressure gradient force that drives the up-valley flow and induces CTW lee waves that sustain the up-valley flow. When the wind-driven shelf flow aligns with CTW phase propagation, the initial disturbance generated in the valley propagates away, allowing the valley flow to adjust to roughly follow isobaths. Because of the similarity in the physical setup, this mechanism of arrested CTWs generating stronger onshore than offshore flow is expected to be applicable to the flow response in slope canyons to along-isobath background flows of opposite directions.
    Description: WGZ and SJL were supported by the National Science Foundation through GrantOCE1154575.WGZ is also supported by the NSF Grant OCE 1634965 and SJL by NSF Grant OCE 1558874.
    Description: 2018-06-08
    Keywords: Ocean circulation ; Topographic effects ; Transport ; Vertical motion ; Waves, oceanic ; Wind stress
    Repository Name: Woods Hole Open Access Server
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  • 53
    Publication Date: 2018-10-16
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 883-904, doi:10.1175/JPO-D-17-0084.1.
    Description: The dynamics controlling the along-valley (cross shelf) flow in idealized shallow shelf valleys with small to moderate Burger number are investigated, and analytical scales of the along-valley flows are derived. This paper follows Part I, which shows that along-shelf winds in the opposite direction to coastal-trapped wave propagation (upwelling regime) force a strong up-valley flow caused by the formation of a lee wave. In contrast, along-shelf winds in the other direction (downwelling regime) do not generate a lee wave and consequently force a relatively weak net down-valley flow. The valley flows in both regimes are cyclostrophic with 0(1) Rossby number. A major difference between the two regimes is the along-shelf length scales of the along-valley flows L. In the upwelling regime Ls, depends on the valley width W, and the wavelength lambda(1w) of the coastal-trapped lee wave arrested by the along-shelf flow U-s. In the downwelling regime L depends on the inertial length scale U-s|'f and W-c. The along-valley velocity scale in the upwelling regime, given by V-u approximate to root pi H-c/H-s integral W-c lambda(1w)/2 pi L-x (1+L-x(2)/L-c(2))(-1) e(-(pi Wc)/(lambda 1w),) is based on potential vorticity (PV) conservation and lee-wave dynamics (Hs and H, are the shelf and valley depth scales, respectively, and fis the Coriolis parameter). The velocity scale in the downwelling regime, given by |v(d)| approximate to (H-s/H-s)[1 + (L-x(2)/L-x(2))](-1) fL, is based on PV conservation. The velocity scales are validated by the numerical sensitivity simulations and can be useful for observational studies of along -valley transports. The work provides a framework for investigating cross -shelf transport induced by irregular shelf bathymetry and calls for future studies of this type under realistic environmental conditions and over a broader parameter space.
    Description: Both WGZ and SJL were supported by the National Science Foundation (NSF) through Grant OCE 1154575.WGZis also supported by the NSF Grant OCE 1634965 and SJL by NSF Grant OCE 1558874.
    Description: 2018-10-16
    Keywords: Ocean circulation ; Topographic effects ; Upwelling/downwelling ; Waves, oceanic ; Wind stress ; Ocean models
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  • 54
    Publication Date: 2018-12-06
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 9881-9901, doi:10.1175/JCLI-D-17-0889.1.
    Description: The Atlantic meridional overturning circulation and associated poleward heat transport are balanced by northern heat loss to the atmosphere and corresponding water-mass transformation. The circulation of northward-flowing Atlantic Water at the surface and returning overflow water at depth is particularly manifested—and observed—at the Greenland–Scotland Ridge where the water masses are guided through narrow straits. There is, however, a rich variability in the exchange of water masses across the ridge on all time scales. Focusing on seasonal and interannual time scales, and particularly the gateways of the Denmark Strait and between the Faroe Islands and Shetland, we specifically assess to what extent the exchanges of water masses across the Greenland–Scotland Ridge relate to wind forcing. On seasonal time scales, the variance explained of the observed exchanges can largely be related to large-scale wind patterns, and a conceptual model shows how this wind forcing can manifest via a barotropic, cyclonic circulation. On interannual time scales, the wind stress impact is less direct as baroclinic mechanisms gain importance and observations indicate a shift in the overflows from being more barotropically to more baroclinically forced during the observation period. Overall, the observed Greenland–Scotland Ridge exchanges reflect a horizontal (cyclonic) circulation on seasonal time scales, while the interannual variability more represents an overturning circulation.
    Description: This research was supported by the Research Council of Norway project NORTH (Grant 229763). Additional support for M. A. Spall was provided by National Science Foundation Grant OCE- 1558742, for T. Eldevik and S. Østerhus by the European Union’s Horizon 2020 research and innovation program project Blue-Action (Grant 727852), and for S. Østerhus by the European Framework Programs under Grant Agreement 308299 (NACLIM).
    Keywords: Ocean circulation ; Thermocline circulation ; Atmosphere-ocean interaction ; North Atlantic Oscillation ; Statistical techniques ; Time series
    Repository Name: Woods Hole Open Access Server
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