Topographic influences on recirculation in the deep western boundary current: Results from RAFOS float trajectories between the Blake-Bahama outer ridge and the San Salvador "gate"

Abstract

Over a period of several years, RAFOS floats were launched into three levels of the deep western boundary current (DWBC) east of the northern Bahamas in order to identify and study any local recirculations that might be present in addition to the thermohaline-driven component of the current. These float trajectories reveal the presence of recirculations that are clearly caused by features of the lateral and bottom topography. In particular, the San Salvador Spur exerts a major influence on the paths of these floats. Although the floats exhibit a complicated set of motions, some order is imposed by relating periods when floats move directly along the boundary versus periods when they leave the launch site "anomalously" (i.e., to the east or northeast) due to motions of the DWBC core. Comparison to current meter records along 26°30′ N near the launch site shows that floats in the latter group were deployed when the DWBC core was located offshore. The "eruption" of floats into the interior recirculation at the San Salvador Spur causes a reduction (by a process similar to what elsewhere has been termed "arrested dispersion" ) in the mean rate at which the floats, and presumably other tracers, move southward along the boundary. The "effective southward spreading rate" of these floats is estimated as 1.97 cm s-1, in reasonable agreement with analogous results from tracer studies in the same region.