Mean Structure and Dynamics of the Shelfbreak Jet in the Middle Atlantic Bight during Fall and Winter*

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Abstract
Using a collection of high-resolution shipboard acoustic Doppler current profiler (ADCP) velocity sections that cross the Middle Atlantic Bight shelfbreak jet near 70°W, the mean structure of the frontal jet is described and the dominant modes of variability of the jet are examined. A mean section is constructed in a translating coordinate frame whose origin tracks the instantaneous position of the core of the jet, thereby minimizing variability associated with the lateral meandering of the current. The mean jet so constructed extends to the bottom, tilting onshore with depth, with near-bottom flow exceeding 0.10 m s−1. The corresponding cross-stream flow reveals a clear pattern of convergence that extends along the tilted axis of the jet, with enhanced convergence both near the surface and near the bottom. This convergence is largely attributed to the locally convergent topography and is shown to drive an ageostrophic circulation dominated by downwelling at, and offshore of, the jet core. The collection of ADCP sections also suggests a previously undetected mode of variability, whereby the jet systematically fluctuates between a convergent, bottom-reaching state and a surface-trapped state with weaker cross-stream velocities. This variability is associated with significant variations in the southwestward transport of the jet and does not seem to be related to simple meandering of the current.