A Modeling Study of the Episodic Cross-Frontal Water Transport over the Inner Shelf of the South Atlantic Bight

Abstract

Physical processes that control the episodic cross-frontal transport over the inner shelf of the South Atlantic Bight were examined using a 3D primitive turbulent closure equation model. The model results showed that significant cross-frontal water exchange occurred under upwelling-favorable wind conditions through the detachment of isolated low-salinity lenses at the outer edge of the front. The formation of isolated lenses was a 3D feature associated with complex nonlinear processes. It depended on 1) the amount of multiple river discharges, 2) direction and magnitude of the wind, and 3) tidal mixing. As the total amount of river discharges increased, stronger winds were required to form the isolated lenses. When the river discharges were larger, tidal mixing tended to delay the occurrence of isolated lenses through 1) increasing the cross-shelf gradients of density and surface elevation and thus enhancing the along-shelf current and reducing the along-shelf variation of salinity, and 2) strengthening the internal stress at the bottom edge of the plume and thus slowing down the offshore water transport. The vertical salt diffusion, along-shelf and cross-shelf salt advections played a critical role in the detachment of the low-salinity water from the front. Upwelling significantly contributed to salinity increases near the coastal region in the early wind stages and the lenses? intensification in the midshelf. After the wind subsided, the eddylike circulation developed over the midshelf through adjustment of currents to surface elevation. The residence time of these eddies depended on the initial strength and location of isolated lenses. In the midshelf, where tidal mixing was relatively weak, these eddies could last about one week. In the inner shelf, where the depth was less than 20 m and tidal mixing was stronger, their lifetimes were less than one day.