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Abstract Along‐shelf wind stresses drive substantial along‐coast variations in sea level that result in significant along‐coast pressure gradients in the Middle Atlantic Bight (MAB) at time scales from days to years. Forty years of sea‐level data and reanalysis wind stresses are examined to determine the characteristics and dynamics of pressure gradients along the New England and Central MAB coasts. Along‐coast dynamic sea level (pressure) gradients often exceed 5 cm/100 km at daily time scales, 2 cm/100 km at monthly time scales and 0.2 cm/100 km at yearly time scales. Along‐shelf wind stresses account for more than 50% of the along‐coast pressure gradient variance at daily and monthly time scales and more than 25% at yearly time scales. Pressure gradients along the New England coast are primarily driven by local wind stresses along the New England shelf, while pressure gradients along the Central MAB shelf are driven by both local wind stresses along the Central MAB shelf and remote wind stresses along the New England shelf. A steady depth‐average model (Csanady, 1978,https://doi.org/10.1175/1520‐0485(1978)008<0047:tatw>2.0.co;2) accurately reproduces the wind‐driven along‐coast pressure gradients in both regions. The along‐coast pressure gradients typically oppose the local wind stress and, in the along‐shelf momentum balance, are 50%–80% of the along‐shelf wind stress over the inner shelf (water depth 15 m).
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Turbulent Thermal-Wind-Driven Coastal Upwelling: Current Observations and Dynamics
Abstract A remarkably consistent Lagrangian upwelling circulation at monthly and longer time scales is observed in a 17-yr time series of current profiles in 12 m of water on the southern New England inner shelf. The upwelling circulation is strongest in summer, with a current magnitude of ∼1 cm s −1 , which flushes the inner shelf in ∼2.5 days. The average winter upwelling circulation is about one-half of the average summer upwelling circulation, but with larger month-to-month variations driven, in part, by cross-shelf wind stresses. The persistent upwelling circulation is not wind-driven; it is driven by a cross-shelf buoyancy force associated with less-dense water near the coast. The cross-shelf density gradient is primarily due to temperature in summer, when strong surface heating warms shallower nearshore water more than deeper offshore water, and to salinity in winter, caused by fresher water near the coast. In the absence of turbulent stresses, the cross-shelf density gradient would be in a geostrophic, thermal-wind balance with the vertical shear in the along-shelf current. However, turbulent stresses over the inner shelf attributable to strong tidal currents and wind stress cause a partial breakdown of the thermal-wind balance that releases the buoyancy force, which drives the observed upwelling circulation. The presence of a cross-shelf density gradient has a profound impact on exchange across this inner shelf. Many inner shelves are characterized by turbulent stresses and cross-shelf density gradients with lighter water near the coast, suggesting turbulent thermal-wind-driven coastal upwelling may be a broadly important cross-shelf exchange mechanism. Significance Statement A remarkably consistent upwelling circulation at monthly time scales is observed in a 17-yr time series of current profiles in shallow water off southern New England. This is not the traditional wind-driven coastal upwelling; instead, it is forced by cross-shelf buoyancy (density) gradients, released by turbulent stresses in shallow water. The persistent upwelling circulation is strongest in summer, when wind and wave forcing are weak, and flushes the inner portion of the continental shelf in a few days. Consequently, this buoyancy-driven coastal upwelling is important for cooling the inner shelf and provides a reliable mechanism for cross-shelf exchange. Many inner shelves are characterized by cross-shelf density gradients and turbulent stresses, suggesting this may be a broadly important cross-shelf exchange mechanism.
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- Award ID(s):
- 1655686
- PAR ID:
- 10441261
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 52
- Issue:
- 12
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- 2909 to 2921
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Analysis of 40 years of tide gauge data and reanalysis wind stresses from the Middle Atlantic Bight (MAB) indicate that along‐shelf wind stresses are a dominant driver of coastal dynamic sea level (sea level plus atmospheric pressure) variability at daily to yearly time scales. The sea‐level response to along‐shelf wind stress varies substantially along the coast and is accurately reproduced by a steady, barotropic, depth‐averaged model (Csanady, 1978,https://doi.org/10.1175/1520‐0485(1978)008<0047:tatw>2.0.co;2, Arrested Topographic Wave). The model indicates that the sea‐level response in the MAB depends primarily on the along‐shelf distribution of the along‐shelf wind stress, the Coriolis frequency, the bottom drag coefficient, and the cross‐shelf bottom slope. The along‐shelf wind stress varies along the MAB shelf due primarily to changes in the shelf orientation. The sea‐level response depends on both the local and upstream (in the sense of Kelvin wave propagation) along‐shelf wind stresses. Consequently, sea‐level variability at daily, monthly and yearly time scales along much of the central MAB coast is more strongly driven by upstream winds along the southern New England shelf than by local winds along the central MAB shelf. The residual coastal sea‐level variability, after removing the wind‐driven response and the trend, is roughly uniform along the MAB coast. The along‐coast average of the residual sea level at monthly and yearly time scales is caused by variations in shelf water densities primarily associated with the large annual cycle in water temperature and interannual variations in salinity.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JPO-D-22-0063.1
