A numerical study of the continental shelf circulation of the U . S . South Atlantic Bight during the autumn of 1987

-The autumn circulation on the innerand mid-shelf of the U.S. South Atlantic Bight (SAB) is examined numerically. Using data collected in 1987 during the Fall Experiment (FLEX) the alongshore structure of the currents and the coastal sea level fluctuations were found to be correlated to local winds which were strong and persistently northeasterly. The observed inshore distribution of freshwater during FLEX, characterized by the presence of a coastal front confined to the coast inside the 25 m isobath, reflects the local autumn discharge subjected to strong and persistent downwelling winds. The freshwater signal found outside the 25 m isobath is suggested to be the previous summer's discharge advected northward by the summer winds subsequently returning south forced by the autumn winds. 1. I N T R O D U C T I O N DURING the fall of 1987, the Fall Experiment (FLEX) was conducted on the innerto mid-continental shelf of the U.S. South Atlantic Bight--the SAB (Fig. 1). One of the goals of FLEX was to determine the fate of the fresh, low density water arising from local river discharge during autumn in the SAB. Climatological studies of the SAB winds (WEBER and BLANTON, 1980) and the hydrography of its shelf waters (ATKINSON et al., 1983) suggest a transition of summer to fall regimes much like the 1987 observations indicated. As such, 1987's summer and autumn may be considered typical. Summer winds are southwesterly, and generally upwelling favorable (Fig. 2; also WEBER and BLANTON, 1980). Previous studies (BLANTON and ATKINSON, 1983; BLANTON et al., 1989) have found that these winds spread the freshwater seaward, while at the same time the water column becomes vertically stratified with relatively weak horizontal gradients. October's winds and hydrography are completely different. The winds are downwelling favorable with strong, persistent winds from the northeast (Figs 2 and 3). Climatological hydrographic data, as well as data collected during FLEX, reveal a salinity structure wherein the October freshwater discharge is advected southward hugging the coast, forming a vertically well mixed coastal front with weaker horizontal salinity gradients in the midand outer-shelf regions (Fig. 4). Lastly, since tritiated waters are indicative of *Skidaway Institute of Oceanography, P.O. Box 13687, Savannah, GA 31416, U.S.A. tThayer School of Engineering, Dartmouth College, Hanover, NH 03755, U.S.A. ~Now at: Marine Science Program, University of North Carolina, Chapel Hill, NC 27514, U.S.A. 971 972 F . E . WEg~ER et al.