Structure of the Gulf Stream and Its Recirculations at 55°W

Abstract

Two years of direct current and temperature observations from an army of 13 current meter moorings deployed near 55°W as part of the SYNOP (Synoptic Ocean Prediction) Experiment have been used to explore the spatial and temporal variability of the Gulf Stream from three points of view. In the geographic reference frame, mean eastward velocities were observed from the surface to 4000 m. There was no evidence of westward flow south of the eastward jet, suggesting that the Worthington recirculation gyre was located south of the array during this time period. Westward flow was observed north of the jet only at 4000 m, where it had a magnitude comparable to the mean Gulf Stream (5?10 em s?1). These data also indicate that the mean eastward jet is much more vertically aligned than was depicted in an earlier picture constructed from noncontemporaneous observations. In the Lagrangian, or streamwise, reference frame, it was found that, at the thermocline level, the width of the ?average synoptic? Gulf Stream and the velocity structure remain virtually unchanged between Cape Hatteras (73°W) and 55°W, in spite of large amplitude meandering. The barotropic velocity component of the average synoptic stream increases fivefold over this distance, and the baroclinic component weakens. The northern recirculation appears more clearly in the stream coordinate frame as a 130-km wide barotropic flow with peak westward velocities of about 8 cm s?1. South of the stream, there was no evidence of westward flow, even in the stream coordinate system. Finally, a consideration of eddy-mean flow interactions in terms of the eddy energy equations shows that at the thermocline level, there were no significant cross-gradient fluxes of heat or momentum, supporting the notion that 55°W is at a maximum in eddy energy. At 4000 m, there was some indication of upgradient heat and momentum fluxes in the Gulf Stream, consistent with decreasing eddy energy following the moan flow to the east. These results point to the region between 55°W and the Tail of the Grand Banks (50°W) as the site of eddy energy decay in the Gulf Stream system.