Initial observations of the subsurface structure and short‐term variability of the seaward deflection of the Gulf Stream off Charleston, South Carolina

Abstract

A recurring seaward deflection of the surface layer of the Gulf Stream has been observed near 32°N latitude off the coast of the southeastern United States. It has been suggested that a ridge and trough bottom feature (the so‐called ‘Charleston bump’) on the upper continental slope off the Georgia/South Carolina coast produces the deflection through a bottom steering effect. Present data indicate that the deflection is great enough to direct the Gulf Stream's shoreward surface thermal front to the east, and even south of east, about 70percnt; of the time. Air‐deployed expendable bathythermograph surveys have been made with sufficient coverage to provide several synoptic, three‐dimensional views of the Gulf Stream's thermal frontal zone in the region between Savannah, Georgia, and Cape Hatteras, North Carolina. These views show the subsurface structure of the seaward deflection to exhibit large short‐term variability. During wintertime conditions (February 1979) the greatest deflection (>090° true) of the near‐surface front occurred at a time when the deeper front was more aligned (∼080° true) with local topography. Within the few days following this observation the deflection angle at all depths decreased to near or below 070° true. Two large‐amplitude Gulf Stream meanders progressed northeastward away from the deflection region during this time period. Deflection angles at all levels during late summertime conditions (November 1979) were observed to be near 070° true. A simple, kinematical model which incorporates growing, propagating Gulf Stream meanders is proposed to explain the deflection's short‐term variability. A dome‐shaped volume of cold water was observed to be located over the upper continental slope immediately downstream of the deflection. The existence and persistence of this cold dome suggest that upwelling is important in its maintenance, a hypothesis consistent with its hydrographic properties.