Mesoscale Currents on the Inner New Jersey Shelf Driven by the Interaction of Buoyancy and Wind Forcing

Abstract

Shipboard hydrographic and acoustic Doppler current profiler surveys conducted in August 1996 on the New Jersey inner shelf revealed a buoyant intrusion advancing southward along the coast. This buoyant intrusion originated from the Hudson estuary more than 100 km upshelf and appeared as a bulge of less saline water with a sharp across-shelf frontal zone at its leading edge. During this time, the study area was also forced by a brief upwelling-favorable wind event opposing the direction of buoyant flow propagation. The interaction of buoyancy and wind forcing generated a spatially variable velocity field. In particular, across-shelf currents were comparable to their alongshelf counterparts. Variability in the alongshelf direction occurred on the scales of the order of the baroclinic Rossby radius. Intensive across-shelf currents reached speeds of 20? 40 cm s?1 and appeared as spatially localized mesoscale flows with a width of O(10 km). They were generated at the leading edge of the buoyant intrusion and persisted over the period of observations, slowly propagating southward along with the buoyant flow. They were essentially baroclinic with strong vertical shear and were further amplified by the wind forcing. The upwelling-favorable wind event also generated cyclonic circulation within the buoyant intrusion, which has not been observed before. Interaction of the opposing wind and buoyancy forcings deformed the pycnocline into a dome. This dome was effectively isolated from wind-induced turbulent mixing by overlying buoyant water. The adjustment of the velocity field to this density disturbance occurred geostrophically, even though the water depth was only 20?30 m and friction was important. Relative vorticity associated with this cyclonic flow was at least 0.3f.