50 research outputs found
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Observations of near-inertial current variability on the New England shelf
Observations from the Coastal Mixing and Optics (CMO) moored array (deployed from August 1996 through June 1997) and supplemental moored observations are used to describe near-inertial current variability over the New England shelf. Near-inertial band current variance comprises 10â20% of the total observed current variance, and has episodic peak speeds exceeding 30 cm/s. Near-inertial current variability during CMO is characterized by a first baroclinic mode vertical structure with one zero-crossing between 15 and 50 m. The zero-crossing is shallower during periods of stronger stratification. Laterally, near-inertial variability is coherent over the extent of the CMO moored array, and cross-shelf decorrelation scales for near-inertial currents are about 100 km, approximately the entire shelf width. The magnitude of near-surface near-inertial variability is stronger in the summer and weaker in the winter, following the seasonal variation in stratification and opposite the seasonal cycle in wind stress variance. During CMO, near-surface near-inertial kinetic energy is inversely related to surface mixed layer depth. Near-inertial variance decreases onshore, matching approximately the cross-shelf decrease in near-inertial energy predicted by a two-dimensional, linear, flat-bottom, twolayer, coastal wall model. In this model, the nullifying effects of a baroclinic wave emanating from the coastal wall play a dominant role in controlling the onshore decrease. Finally, strong persistent anticyclonic relative vorticity shifts near-inertial variability on the New England shelf to subinertial frequencies.Keywords: Inertial waves, Internal waves, Continental shelf processes, Analytical modelin
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 mâ2. 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 N00014â95â1â0339. Analysis was also partially supported
by the National Science Foundation Physical Oceanography program
under grants OCEâ0647050 and OCEâ0548961
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Bottom boundary layer flow and salt injection from the continental shelf to slope
Austral winter oceanographic measurements from the northwest Australian continental shelf reveal salty water forming evaporatively inshore, moving across the wide shelf near the bottom and into the adjacent open ocean when the shelf edge alongshore flow is equatorward. The salt tongue is absent during more normal conditions, when the poleward Leeuwin Current is present. We hypothesize that the flow reversal enables shelf-wide bottom boundary layer (Ekman) transport and thus creates the shelf-edge convergence that accounts for the observed salt tongue. This flow is absent under sustained normal conditions because of buoyancy arrest in the bottom boundary layer
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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 alongâisobath and dominated by episodic bursts of westward flow. The alongâisobath subtidal flow was primarily geostrophic and barotropic, and was correlated with largeâscale alongâcoast wind stress fluctuations oriented 45°T (65° counterclockwise from the local isobath orientation). Subtidal nearâsurface ageostrophic transport matched estimates of windâdriven Ekman transport; however, nearâbottom ageostrophic transport was much larger than estimates of Ekman transport from bottom stress. Lowâfrequency (monthly and longer timescales) flow was generally westward and offâshelf at all sites and depths, with the strongest westward flow during the fall. Lowâfrequency alongâisobath currents were primarily geostrophic with baroclinic and barotropic components of similar magnitude. Depthâaveraged ageostrophic transport was quantitatively consistent with Ekman transport from wind and bottom stress. Measured bottom stress at both subtidal and lowâfrequency timescales was weak, nearly an order of magnitude smaller than the wind stress. Lowâfrequency fluctuations in the predominantly geostrophic alongâisobath flow were attributable to variations in the crossâshelf density field associated with the seasonal cycle in surface heating. During the fall, thermal wind shear was strongest, because the crossâisobath temperature gradient was acting in concert with the persistent crossâisobath salinity gradient to enhance the crossâisobath density gradient (i.e., warmer and fresher water inshore). During the winter, in response to surface cooling, the crossâisobath temperature gradient reversed sign, reducing the crossâisobath density gradient (i.e., cooler and fresher water inshore).Keywords: wind-driven circulation, dynamics, coastal oceanography, shelf-slope front, Middle Atlantic Bigh
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Long-Term Sea Surface Temperature Variability along the U.S. East Coast
Sea surface temperature variations along the entire U.S. East Coast from 1875 to 2007 are characterized using a collection of historical observations from lighthouses and lightships combined with recent buoy and shore-based measurements. Long-term coastal temperature trends are warming in the Gulf of Maine [1.0° ± 0.3°C (100 yr)â»Âč] and Middle Atlantic Bight [0.7° ± 0.3°C (100 yr)â»Âč], whereas trends are weakly cooling or not significant in the South Atlantic Bight [â0.1° ± 0.3°C (100 yr)â»Âč] and off Florida [â0.3° ± 0.2°C (100 yr)â»Âč]. Over the last century, temperatures along the northeastern U.S. coast have warmed at a rate 1.8â2.5 times the regional atmospheric temperature trend but are comparable to warming rates for the Arctic and Labrador, the source of coastal ocean waters north of Cape Hatteras (36°N). South of Cape Hatteras, coastal ocean temperature trends match the regional atmospheric temperature trend. The observations and a simple model show that along-shelf transport, associated with the mean coastal current system running from Labrador to Cape Hatteras, is the mechanism controlling long-term temperature changes for this region and not the local airâsea exchange of heat
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Evaporative dense water formation and crossâshelf exchange over the northwest Australian inner shelf
Highâresolution surveys of oceanographic and atmospheric conditions made during the
winter over the inner shelf off northwest Australia are used to examine the coastal ocean
response to large outgoing heat and freshwater fluxes. Relatively cool, lowâhumidity air
blows off the Australian continent out over the tropical continental shelf, resulting in a large
mean latent heat flux (â177Wmâ2) that overwhelms insolation and, along with the outgoing
longâwave radiation, results in substantial net cooling (â105 W mâ2) and evaporative
freshwater flux (0.6 cm dâ1). The inner shelf is characterized by increasingly cool, salty,
and dense waters onshore, with a strong front near the 25 m isobath. The front is evident
in satellite sea surface temperature (SST) imagery along the majority of the northwest
Australian shelf, exhibiting a complex filamentary and eddy structure. Crossâshelf buoyancy
fluxes estimated from the mean, twoâdimensional heat and salt budgets are comparable
to parameterizations of crossâshelf eddy driven fluxes; however, the same fluxes can be
achieved by crossâshelf transports in the bottom boundary layer of about 0.5 m2 sâ1 (and an
overlying return flow).Keywords: Cross-shelf exchange, Evaporative dense water formatio
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Observations of tidal variablity 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 Mâ 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 sâ»Âč). Estimates of the depth-averaged Mâ 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 Mâ currents decreases in amplitude and phase (phase lead near bottom)
over the bottom 20 m. The Mâ 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 sâ»Âč 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.Keywords: continental shelf, baroclinic tides, barotropic tidesKeywords: continental shelf, baroclinic tides, barotropic tide
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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 Mâ 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 sâ»Âč). Estimates of the depthâaveraged Mâ momentum balance indicate that tidal dynamics are linear, and alongâshelf pressure gradients are as large as crossâshelf 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 Mâ currents decreases in amplitude and phase (phase lead near bottom) over the bottom 20 m. The Mâ 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 sâ»Âč amplitude), with a predominantly mode 1 vertical structure. Highâfrequency (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 lowâmode baroclinic tidal and highâfrequency internal wave energy decrease linearly with bottom depth across the shelf
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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