Observations of a pulsed buoyancy current downstream of Chesapeake Bay

Abstract

A plume of low‐salinity water was observed along the North Carolina coast 100 km south of the mouth of Chesapeake Bay during the Coastal Ocean Processes Pilot field program conducted from August through October 1994. The presence of the plume was episodic, occurring every 3 to 8 days. The timing was related to wind patterns, which influence both the delivery of estuarine discharge to the shelf and the plume's passage down the coast. When not affected by local winds, the low‐salinity water was confined to within 7–9 km of the coast and was about 8 m deep. Downwelling winds narrow and deepen the plume, whereas upwelling winds cause it to thin and spread offshore, eventually detaching from the coast. The low‐salinity plume propagated along the coast at speeds comparable to linear internal wave phase speed, except when strong downwelling wind conditions caused the plume to be in contact with the bottom. The observed propagation speeds are faster than those predicted by previous numerical modeling efforts. The plumes slowed with distance from the source, as mixing with ambient shelf water reduced the density contrast. This buoyancy source was balanced by an alongshore current with a southward velocity of 0.3 to 0.7 m s−1, bounded by a region of high horizontal velocity shear at the offshore salinity front. The currents observed in the nose of the plume are consistent with properties of an internal gravity current under rotation.