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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High‐Resolution Observations of Subsurface Fronts and Alongshore Bottom Temperature Variability Over the Inner Shelf
Circulation patterns over the inner continental shelf can be spatially complex and highly variable in time. However, few studies have examined alongshore variability over short scales of kilometers or less. To observe inner‐shelf bottom temperatures with high (5‐m) horizontal resolution, a fiber‐optic distributed temperature sensing system was deployed along a 5‐km‐long portion of the 15‐m isobath within a larger‐scale mooring array south of Martha's Vineyard, MA. Over the span of 4 months, variability at a range of scales was observed along the cable over time periods of less than a day. Notably, rapid cooling events propagated down the cable away from a tidal mixing front, showing that propagating fronts on the inner shelf can be generated locally near shallow bathymetric features in addition to remote offshore locations. Propagation velocities of observed fronts were influenced by background tidal currents in the alongshore component and show a weak correlation with theoretical gravity current speeds in the cross‐shore component. These events provide a source of cold, dense water into the inner shelf. However, differences in the magnitude and frequency of cooling events at sites separated by a few kilometers in the alongshore direction suggest that the characteristics of small‐scale variability can vary dramatically and can result in differential fluxes of water, heat, and other tracers. Thus, under stratified conditions, prolonged subsurface observations with high spatial and temporal resolution are needed to characterize the implications of three‐dimensional circulation patterns on exchange, especially in regions where the coastline and isobaths are not straight.
more » « less- NSF-PAR ID:
- 10459939
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 124
- Issue:
- 1
- ISSN:
- 2169-9275
- Page Range / eLocation ID:
- p. 593-614
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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