Buoyant gravity currents along a sloping bottom in a rotating fluid

Author Posting. © Cambridge University Press, 2002. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 464 (2002): 251-278, doi:10.1017/S0022112002008868.The dynamics of buoyant gravity currents in a rotating reference frame is a classical problem relevant to geophysical applications such as river water entering the ocean. However, existing scaling theories are limited to currents propagating along a vertical wall, a situation almost never realized in the ocean. A scaling theory is proposed for the structure (width and depth), nose speed and flow field characteristics of buoyant gravity currents over a sloping bottom as functions of the gravity current transport Q, density anomaly g[prime prime or minute], Coriolis frequency f, and bottom slope [alpha]. The nose propagation speed is cp [similar] cw/ (1 + cw/c[alpha]) and the width of the buoyant gravity current is Wp [similar] cw/ f(1 + cw/c[alpha]), where cw = (2Qg[prime prime or minute] f)1/4 is the nose propagation speed in the vertical wall limit (steep bottom slope) and c[alpha] = [alpha]g/f is the nose propagation speed in the slope-controlled limit (small bottom slope). The key non-dimensional parameter is cw/c[alpha], which indicates whether the bottom slope is steep enough to be considered a vertical wall (cw/c[alpha] [rightward arrow] 0) or approaches the slope-controlled limit (cw/c[alpha] [rightward arrow] [infty infinity]). The scaling theory compares well against a new set of laboratory experiments which span steep to gentle bottom slopes (cw/c[alpha] = 0.11–13.1). Additionally, previous laboratory and numerical model results are reanalysed and shown to support the proposed scaling theory.This research was supported by NSF grant OCE-0095059

The summer heat balance of the Oregon inner shelf over two decades: mean and interannual variability

Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 125(2), (2020): e2019JC015856, doi:10.1029/2019JC015856.Summer temperature and velocity measurements from 14 years in 15 m of water over the inner shelf off Oregon were used to investigate interannual temperature variability and the capacity of the across‐shelf heat flux to buffer net surface warming. There was no observable trend in summer mean temperatures, and the standard deviation of interannual variability (0.5°C) was less than the standard deviation in daily temperatures each summer (1.6°C, on average). Yet net surface heat flux provided a nearly constant source of heat each year, with a standard deviation less than 15 urn:x-wiley:jgrc:media:jgrc23812:jgrc23812-math-0001 of the interannual mean. The summer mean across‐shelf upwelling circulation advected warmer water offshore near the surface, cooling the inner shelf and buffering the surface warming. In most years (11 out of 14), this two‐dimensional heat budget roughly closed with a residual less than 20 urn:x-wiley:jgrc:media:jgrc23812:jgrc23812-math-0002 of the leading term. Even in years when the heat budget did not balance, the observed temperature change was negligible, indicating that an additional source of cooling was needed to close the budget. A comparison of the residual to the interannual variability in fields such as along‐shelf wind stress, stratification, and along‐shelf currents found no significant correlation, and further investigation into the intraseasonal dynamics is recommended to explain the results. An improved understanding of the processes that contribute to warming or cooling of the coastal ocean has the potential to improve predictions of the impact of year‐to‐year changes in local winds and circulation, such as from marine heat waves or climate change, on coastal temperatures.The authors would like to acknowledge the David and Lucile Packard Foundation and The Gordon and Betty Moore Foundation for their support of the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) mooring program. This paper is PISCO contribution 504. The contributions of A. Kirincich and S. Lentz were supported by National Science Foundation (NSF) Grant OCE‐1558874). E. Lemagie was partially supported by NSF Grant OCE‐1558874 as well as the Woods Hole Oceanographic Institution Postdoctoral Scholars program. Temperature and velocity data were collected and made available by PISCO (www.piscoweb.org). The NDBC and NWPO3 buoy data are freely available from NOAA (www.ndbc.noaa.gov). Surface heat flux reanalyses were download online: ERA5 was accessed through www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5, and NCEP and OAFlux data were downloaded from www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html and http://oaflux.whoi.edu/, respectively.2020-07-2

Observations of cross-shelf flow driven by cross-shelf winds on the inner continental shelf

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): 2358-2378, doi:10.1175/2008JPO3990.1.Six-yr-long time series of winds, waves, and water velocity from a cabled coastal observatory in 12 m of water reveal the separate dependence of the cross-shelf velocity profile on cross-shelf and along-shelf winds, waves, and tides. During small waves, cross-shelf wind is the dominant mechanism driving the cross-shelf circulation after tides and tidal residual motions are removed. The along-shelf wind does not drive a substantial cross-shelf circulation. During offshore winds, the cross-shelf circulation is offshore in the upper water column and onshore in the lower water column, with roughly equal and opposite volume transports in the surface and bottom layers. During onshore winds, the circulation is nearly the reverse. The observed profiles and cross-shelf transport in the surface layer during winter agree with a simple two-dimensional unstratified model of cross-shelf wind stress forcing. The cross-shelf velocity profile is more vertically sheared and the surface layer transport is stronger in summer than in winter for a given offshore wind stress. During large waves, the cross-shelf circulation is no longer roughly symmetric in the wind direction. For onshore winds, the cross-shelf velocity profile is nearly vertically uniform, because the wind- and wave-driven shears cancel; for offshore winds, the profile is strongly vertically sheared because the wind- and wave-driven shears have the same sign. The Lagrangian velocity profile in winter is similar to the part of the Eulerian velocity profile due to cross-shelf wind stress alone, because the contribution of Stokes drift to the Lagrangian velocity approximately cancels the contribution of waves to the Eulerian velocity.This research was funded by the Ocean Sciences Division of the National Science Foundation under Grants OCE-0241292 and OCE-0548961 and by National Aeronautics and Space Administration Headquarters under Grant NNG04GL03G and the Earth System Science Fellowship Grant NNG04GQ14H. MVCO is partly funded by the Woods Hole Oceanographic Institution and the Jewett/EDUC/Harrison Foundation

Heat and salt balances over the New England continental shelf, August 1996 to June 1997

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): C07017, doi:10.1029/2009JC006073.Heat and salt balances over the New England shelf are examined using 10 month time series of currents, temperature, and salinity from a four element moored array and surface heat and freshwater fluxes from a meteorological buoy. A principal result is closure of the heat budget to 10 W m−2. The seasonal variation in depth-average temperature, from 14°C in September to 5°C in March, was primarily due to the seasonal variation in surface heat flux and a heat loss in winter caused by along-shelf advection of colder water from the northeast. Conductivity sensor drifts precluded closing the salt balance on time scales of months or longer. For time scales of days to weeks, depth-average temperature and salinity variability were primarily due to advection. Advective heat and salt flux divergences were strongest and most complex in winter, when there were large cross-shelf temperature and salinity gradients at the site due to the shelf-slope front that separates cooler, fresher shelf water from warmer, saltier slope water. Onshore flow of warm, salty slope water near the bottom and offshore flow of cooler, fresher shelf water due to persistent eastward (upwelling-favorable) winds caused a temperature increase of nearly 3°C and a salinity increase of 0.8 in winter. Along-shelf barotropic tidal currents caused a temperature decrease of 1.5°C and a salinity decrease of 0.7. Wave-driven Stokes drift caused a temperature increase of 0.5°C and a salinity increase of 0.4 from mid December to January when there were large waves and large near-surface cross-shelf temperature and salinity gradients.The field program was funded by the Office of Naval Research, Code 322, under grant N00014‐95‐1‐0339. Analysis was also partially supported by the National Science Foundation Physical Oceanography program under grants OCE‐0647050 and OCE‐0548961

A note on the depth of sidelobe contamination in acoustic doppler current profiles

Author Posting. © American Meteorological Society, 2022. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of the Atmospheric and Oceanic Technology 39(1), (2022): 31–35, https://doi.org/10.1175/JTECH-D-21-0075.1.Acoustic Doppler current profilers (ADCP) do not provide reliable water velocity measurements near the sea surface or bottom because acoustic sidelobe reflections from the boundary contaminate the Doppler velocity measurements. The apparent depth of the center of the sidelobe reflection is zsl = ha[1 − cos(θ)], where ha is the distance from the ADCP acoustic head to the sea surface and θ is the ADCP beam angle. However, sidelobe contamination extends one and a half ADCP bins below zsl as the range gating of the acoustic return causes overlap between adjacent ADCP bins. Consequently, the contaminated region z < zsl + 3Δz/2 is deeper than traditionally suggested, with a dependence on bin size Δz. Direct observations confirming both the center depth of the sidelobe reflection and the depth of contamination are presented for six bottom-mounted, upward-looking ADCPs. The sidelobe reflection is isolated by considering periods of weak wind stresses when the sea surface is smooth and there is nearly perfect reflection of the main beams away from the ADCP and hence little acoustic return from the main beams to the ADCP.This analysis was supported by NSF OCE 1558874 for Kirincich and Lentz. Plueddemann was supported by the Global Ocean Monitoring and Observing Program of the National Oceanic and Atmospheric Administration (CPO Fund Reference Number 100007298), through the Cooperative Institute for the North Atlantic Region (CINAR) under Cooperative Agreement NA14OAR4320158

The importance of lateral variability on exchange across the inner shelf south of Martha's Vineyard, MA

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 2360–2381, doi:10.1002/2016JC012491.This study examines the spatial variability of transport within the inner-shelf south of Martha's Vineyard Massachusetts, its time and space dependence, and its importance to the total volume exchanged between the nearshore and the coastal ocean. The exchange of water across the inner shelf is often considered to be driven primarily by wind forcing, yet the effects of small-scale O(1–10 km) variability on the total exchange have not been well quantified. Using a combination of high-resolution HF radar-based surface currents and a dense array of moorings to document the lateral variability of across-shelf exchange, the cumulative wind-driven across-shelf transport over the summer stratified period was less than the volume of the inner-shelf onshore of the 25 m isobath. Along-shelf variations in the wind-driven exchange were as large as the spatial mean of the wind-driven exchange. A spatially varying time-averaged circulation caused by tidal rectification resulted in across-shelf exchange larger in magnitude than, and independent of, the integrated wind-forced exchange. Coherent submesoscale eddies also occurred frequently within the domain due to flow-topography effects onshore and horizontal density gradients offshore, generally with lifespans shorter than 10 h, diameters smaller than 6 km, and vertical depths shallower than 10 m. The across-shelf volume transport due to eddies, estimated by seeding particles within the surface current fields, was more than half the wind-driven depth-dependent exchange. Thus, accounting for the potential coherent along-shelf variability present over the inner shelf can significantly increase estimates of the across-shelf transfer of water masses and particles.The 2014 ISLE field study as well as this analysis was supported by NSF OCE grant 1332646 and the Woods Hole Oceanographic Institution.2017-09-2

The spatial structure of tidal and mean circulation over the inner shelf south of Martha's Vineyard, Massachusetts

Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 1940–1958, doi:10.1175/JPO-D-13-020.1.The spatial structure of the tidal and background circulation over the inner shelf south of Martha's Vineyard, Massachusetts, was investigated using observations from a high-resolution, high-frequency coastal radar system, paired with satellite SSTs and in situ ADCP velocities. Maximum tidal velocities for the dominant semidiurnal constituent increased from 5 to 35 cm s−1 over the 20-km-wide domain with phase variations up to 60°. A northeastward jet along the eastern edge and a recirculation region inshore dominated the annually averaged surface currents, along with a separate along-shelf jet offshore. Owing in part to this variable circulation, the spatial structure of seasonal SST anomalies had implications for the local heat balance. Cooling owing to the advective heat flux divergence was large enough to offset more than half of the seasonal heat gain owing to surface heat flux. Tidal stresses were the largest terms in the mean along- and across-shelf momentum equations in the area of the recirculation, with residual wind stress and the Coriolis term dominating to the west and south, respectively. The recirculation was strongest in summer, with mean winds and tidal stresses accounting for much of the differences between summer and winter mean circulation. Despite the complex bathymetry and short along-shelf spatial scales, a simple model of tidal rectification was able to recreate the features of the northeastward jet and match an estimate of the across-shelf structure of sea surface height inferred from the residual of the momentum analysis.2014-03-0

Investigating the eddy diffusivity concept in the coastal ocean

Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 2201-2218, doi:10.1175/JPO-D-16-0020.1.This paper aims to test the validity, utility, and limitations of the lateral eddy diffusivity concept in a coastal environment through analyzing data from coupled drifter and dye releases within the footprint of a high-resolution (800 m) high-frequency radar south of Martha’s Vineyard, Massachusetts. Specifically, this study investigates how well a combination of radar-based velocities and drifter-derived diffusivities can reproduce observed dye spreading over an 8-h time interval. A drifter-based estimate of an anisotropic diffusivity tensor is used to parameterize small-scale motions that are unresolved and underresolved by the radar system. This leads to a significant improvement in the ability of the radar to reproduce the observed dye spreading.IR, AK, and SL were supported by the NSF OCE Grant 1332646. IR was also supported by NASA Grant NNX14AH29G.2016-12-2

Observations of tidal variability on the New England shelf

Observations from the Coastal Mixing and Optics experiment moored array, deployed from August 1996 through June 1997, are used to describe barotropic and baroclinic tidal variability over the New England shelf. The dominant M₂ tidal elevations decrease toward the northeast to a minimum over the Nantucket shoals (about 34 cm), and barotropic tidal current amplitudes increase strongly toward the northeast to a maximum over the shoals (about 35 cm s⁻¹). Estimates of the depth‐averaged M₂ momentum balance indicate that tidal dynamics are linear, and along‐shelf pressure gradients are as large as cross‐shelf pressure gradients. In addition, tidal current ellipses are weakly polarized, confirming that the dynamics are more complex than simple plane waves. The vertical structure of the M₂ currents decreases in amplitude and phase (phase lead near bottom) over the bottom 20 m. The M₂ momentum deficit near the bottom approximately matches direct covariance estimates of stress, confirming the effects of stress on current structure in the tidally driven bottom boundary layer. Baroclinic current variability at tidal frequencies is small (2 cm s⁻¹ amplitude), with a predominantly mode 1 vertical structure. High‐frequency (approaching the buoyancy frequency) internal solitons are observed following the pycnocline. The internal solitons switch from waves of depression to waves of elevation when the depth of maximum stratification is deeper than half the water column depth. Both low‐mode baroclinic tidal and high‐frequency internal wave energy decrease linearly with bottom depth across the shelf

Dynamics of mean and subtidal flow on the New England shelf

Current and hydrographic observations from the Coastal Mixing and Optics experiment moored array, deployed from August 1996 through June 1997, are used to describe the velocity variability and evaluate the dynamics of circulation over the New England shelf on timescales ranging from a few days to several months. Subtidal (days to weeks) current variability was polarized along-isobath and dominated by episodic bursts of westward flow. The along-isobath subtidal flow was primarily geostrophic and barotropic, and was correlated with large-scale along-coast wind stress fluctuations oriented 45°T (65° counterclockwise from the local isobath orientation). Subtidal nearsurface ageostrophic transport matched estimates of wind-driven Ekman transport; however, near-bottom ageostrophic transport was much larger than estimates of Ekman transport from bottom stress. Low-frequency (monthly and longer timescales) flow was generally westward and off-shelf at all sites and depths, with the strongest westward flow during the fall. Low-frequency along-isobath currents were primarily geostrophic with baroclinic and barotropic components of similar magnitude. Depth-averaged ageostrophic transport was quantitatively consistent with Ekman transport from wind and bottom stress. Measured bottom stress at both subtidal and low-frequency timescales was weak, nearly an order of magnitude smaller than the wind stress. Low-frequency fluctuations in the predominantly geostrophic along-isobath flow were attributable to variations in the cross-shelf density field associated with the seasonal cycle in surface heating. During the fall, thermal wind shear was strongest, because the cross-isobath temperature gradient was acting in concert with the persistent cross-isobath salinity gradient to enhance the cross-isobath density gradient (i.e., warmer and fresher water inshore). During the winter, in response to surface cooling, the cross-isobath temperature gradient reversed sign, reducing the cross-isobath density gradient (i.e., cooler and fresher water inshore).Keywords: Middle Atlantic Bight, Shelf-slope front, Wind-driven circulation, Dynamics, Coastal oceanographyKeywords: Middle Atlantic Bight, Shelf-slope front, Wind-driven circulation, Dynamics, Coastal oceanograph