Dynamics of mean and subtidal flow on the New England shelf

Abstract

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 near- surface 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).