Satellite-Tracked Drifters between Africa and Antarctica

Abstract

Trajectories and velocities are reported for six drifters launched in February 1976 in the South Atlantic Ocean mostly along the 10°W meridian between 35° and 57°S. These drifters had drogues centered on 7 or 57 m and the tracking durations were 33–302 days. In the Circumpolar Current the tracks were remarkably zonal, remaining generally within ±1° of latitude, except for one drifter deployed at ~42°S, which when it was south of the Cape curved equatorward in the Agulhas Current terminal region, and one at 57°S, which slanted equatorward along the Atlantic-Indian Ridge. The expected deflections occurred at the mid-ocean ridges, and generally the course of the drifters was consistent with the published streamlines of geostrophic transport. The trajectory of a drifter launched equatorward of the subtropical convergence, at 35°S, was one of complex eddying motion, accompanied by a slow drift to the northeast. In the Circumpolar Current, the latitudinal variation of zonal current velocity was qualitatively similar to the mean annual wind shear and (in the region of 0° meridian) to the geostrophic zonal surface velocities calculated by Ostapoff (1962). Drifter velocities were, however, considerably larger than the geostrophic velocities. Maximum mean velocities (~0.3 m/s) occurred along 49°50′.