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

Using a collection of high{resolution shipboard acoustic Doppler current pro ̄ler (ADCP) velocity sections which cross the Middle Atlantic Bight shelfbreak jet near 70±W, we describe the mean structure of the frontal jet, and examine the dominant modes of variability of the jet. 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 °ow exceeding .10 m/s. The corresponding cross{stream °ow reveals a clear pattern of convergence which 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 o®shore of, the jet core. The collection of ADCP sections also suggests a previously undetected mode of variability, whereby the jet systematically °uctuates between a convergent, bottom{reaching state and a surface{trapped state with weaker cross{stream velocities. This variability is associated with signi ̄cant variations in the southwestward transport of the jet, and does not seem to be related to simple meandering of the current.