Kinematics of Inner Shelf Motions during the Summer Stratified Season off New Jersey*

Abstract

Subinertial currents on a wide (∼100 km), shallow (∼20 m), but nevertheless vertically stratified shelf off the Atlantic seaboard of the United States are investigated at spatial scales of about 20 km in the alongshore and 10 km in the across-shore direction. During the summer of 1996 the inner shelf off New Jersey was stratified due to both temperature and salinity that varied vertically by more than 12°C and 4 psu, respectively. Upwelling favorable winds and an intermittent buoyancy-driven Hudson coastal current impact this stratification inshore of the 15-m isobath. Waters offshore were always stratified except during the passage of Tropical Storm Bertha. Mean currents are weak because wind-forced upwelling and buoyancy-forced downwelling events occurred about evenly during the observational study period. At monthly to daily timescales currents always veered counterclockwise with depth in a bottom Ekman-layer sense by more than 30° inshore and 50° offshore. Complex empirical orthogonal function (CEOF) analyses revealed that these veering angles are contained in the first mode that explains 80%–85% of the total variance at individual locations. It also explains 72% of the variance of 44 current time series of an across-shore section. The veering constitutes a robust feature that cannot be rationalized by Ekman dynamics in shallow water alone. The authors hypothesize that the veering represents a frictional response common to both upwelling and downwelling events. The CEOF analysis does not separate wind from buoyancy forcing. The two forcing mechanisms thus appear to be dynamically coupled. Nevertheless, the first two CEOFs suggest distinctly different circulation regimes: For positive and negative temporal amplitudes mode 1 represents a wind-forced upwelling and a buoyancy-forced downwelling circulation while mode 2 represents the lateral shear of the flow field. Synoptic maps of surface currents from ocean surface current radar reveal realizations of these event types.