The Influence of Stratification on the Wind-Driven Cross-Shelf Circulation over the North Carolina Shelf

Abstract

Wind-driven, cross-shelf circulation is studied using current observations spanning the 90 km wide North Carolina shelf. Most of the shelf is less than 40 m deep. Current measurements were made at five sites within 16 km of the coast from August through October or early December 1994 and at mid- and outer-shelf sites from February 1992 through February 1994. In both studies the water column was stratified in summer and often unstratified during fall and winter. The presence or absence of stratification had a profound influence on the wind-driven, cross-shelf circulation over this shallow shelf. When the water column was stratified, the wind-driven cross-shelf circulation was consistent with a two-dimensional upwelling/downwelling response. Over the mid and outer shelf, near-surface and near-bottom cross-shelf transports had similar magnitudes but opposite directions and were approximately equal to the Ekman transports associated with the alongshelf wind stress and bottom stress, respectively. Wind-driven cross-shelf transports decreased toward the coast over a cross-shelf scale of ≈10 km, suggesting that upwelling and downwelling were confined near the coast during stratified conditions. Stratification may be maintained in the region of transport divergence near the coast by a balance between vertical mixing and buoyancy forcing. When the water column was unstratified the wind-driven cross-shelf circulation at all mooring sites was substantially reduced relative to the stratified response for moderate to strong wind stresses (|τs| > 0.1 N m?2); consistent with an Ekman depth greater than the water depth. The dependence of the cross-shelf transport on wind stress and water depth is roughly consistent with an unstratified, two-dimensional model where the eddy-viscosity profile depends on the stress and distance from the boundaries. Both observations and model results suggest that during unstratified conditions much of the divergence or convergence in the wind-driven cross-shelf transport, and hence the associated upwelling and downwelling, occurs near the shelfbreak on this shallow shelf.