Observations and a model of the mean circulation over the Middle Atlantic Bight continental shelf

Analyses of current time series longer than 200 days from 33 sites over the Middle Atlantic Bight continental shelf reveal a consistent mean circulation pattern. The mean depth-averaged flow is equator-ward, alongshelf, and increases with increasing water depth from 3 cm s(-1) at the 15-m isobath to 10 cm s(-1) at the 100-m isobath. The mean cross-shelf circulation exhibits a consistent cross-shelf and vertical structure. The near-surface flow is typically offshore (positive, range -3 to 6 cm s(-1)). The interior flow is onshore and remarkably constant (-0.2 to -1.4 cm s(-1)). The near-bottom flow increases linearly with increasing water depth from -1cm s (-1) (onshore) in shallow water to 4 cm s(-1) (offshore) at the 250-m isobath over the slope, with the direction reversal near the 50-m isobath. A steady, two-dimensional model (no along-isobath variations in the flow) reproduces the main features of the observed circulation pattern. The depth-averaged alongshelf flow is primarily driven by an alongshelf pressure gradient (sea surface slope of 3.7 x 10(-8) increasing to the north) and an opposing mean wind stress that also drives the near-surface offshore flow. The alongshelf pressure gradient accounts for both the increase in the alongshelf flow with water depth and the geostrophic balance onshore flow in the interior. The increase in the near-bottom offshore flow with water depth is due to the change in the relative magnitude of the contributions from the geostrophic onshore flow that dominates in shallow water and the offshore flow driven by the bottom stress that dominates in deeper water.