Observations of a pulsed buoyancy current downstream of Chesapeake Bay

A plume of low-salinity water was observed along the North Carolina coast 1 00 km south of the mouth of Chesapeake Bay during the Coastal Ocean Process es Pilot field program conducted from August through October 1994. The pres ence 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 d ischarge to the shelf and the plume's passage down the coast. When not affe cted by local winds, the low-salinity water was confined to within 7-9 km o f 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, event ually detaching from the coast. The low-salinity plume propagated along the coast at speeds comparable to linear internal wave phase speed, except whe n strong downwelling wind conditions caused the plume to be in contact with the bottom. The observed propagation speeds are faster than those predicte d by previous numerical modeling efforts. The plumes slowed with distance f rom the source, as mixing with ambient shelf water reduced the density cont rast. This buoyancy source was balanced by an alongshore current with a sou thward velocity of 0.3 to 0.7 m s(-1) bounded by a region of high horizonta l velocity shear at the offshore salinity front. currents observed in the n ose of the plume are consistent with properties of an internal gravity curr ent under rotation.