Observations of a pulsed buoyancy current downstream of Chesapeake Bay

Abstract

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.

Larval distributions in inner‐shelf waters: The roles of wind‐driven cross‐shelf currents and diel vertical migrations

“…For example, if the synchronous peaks that occurred at all pumps on 24 August (gastropods) and 25 August (bivalves) contain plume fauna, then the signature of plume water should have been evident at all depths. Plume water rarely exceeds a salinity of 32 psu (Rennie et al 1999), but this salinity was recorded only briefly at the middle and upper pumps and never at the bottom pump (minimum of 32.2 psu). Otherwise, salinities were much higher.…”

“…2, 3) (Austin 1999;Austin and Lentz 1999;Lentz et al 2001). Along-shore winds controlled stratification/vertical mixing, initiated cross-shelf flows that drove upwelling and downwelling (Austin 1999;Lentz in press), generated swift along-shore currents , and influenced transport of Chesapeake Bay plume water to the site (Rennie et al 1999). The inner-shelf water column was thermally stratified, with the thermocline at 5-10 m (22-25ЊC above, 16-20ЊC below) ( Fig.…”

“…Salinity variability in the Duck region resulted primarily from wind-forced cross-shelf flows and along-shore transport of the Chesapeake Bay plume (Ͻ32 psu) (Boicourt et al 1987;Rennie et al 1999). Typically, near-surface water is fresher than near the bottom.…”

Algorithm development and validation for satellite‐derived distributions of DOC and CDOM in the U.S. Middle Atlantic Bight

“…The general circulation pattern of the MAB is an along-shore southward flow of shelf water from George's Bank (northeast of the MAB) to Cape Hatteras, North Carolina. During winter and early spring northerly winds and the along-shore current force estuarine plumes, including Chesapeake Bay (Rennie et al 1999;Verity et al 2002) and Delaware Bay (Sanders and Garvine 2001) to flow southward along the coast. As winds reverse later in spring the southerly along-shore flow weakens, and the Chesapeake Bay plume broadens and flows offshore, primarily to the south and east.…”

Gravity currents from a dam-break in a rotating channel

“…Examples include the penetration of coastally trapped disturbances in the marine atmospheric boundary layer (Beardsley et al 1987;Dorman 1987) and the discharge of relatively fresh river water into the coastal ocean (e.g. Munchow & Garvine 1993;Rennie, Largier & Lentz 1999). Studies of rotating gravity currents have typically focused on the local dynamics of the gravity current head produced by a dam-break in a rectangular channel.…”