Vertical Vorticity Balance in Meanders Downstream the Agulhas Retroflection

Abstract

The Agulhas Current flows poleward along the western boundary of the southeastern Indian Ocean where, at the southernmost latitude of the African continent, it executes a dramatic anticyclonic turn, or retroflection, to the east. Since 1978, a large number of drifting buoys have passed through this eastward-flowing Agulhas Return Current (ARC), or the zonal frontal boundary between subtropical and subpolar waters of the south Indian Ocean. The spatial distribution of the ensemble-averaged near-surface velocity along the ARC axis reveals a series of steady-state meanders of 700-km wavelength and amplitudes that decrease from 170 km in the first meander to 50 km in the following four meanders. Here an analysis of vorticity balance of the meandering ARC speed axis is presented that demonstrates a balance between the ? term and advection of curvature vorticity. This balance implies that the ARC axis, or frontal region, is horizontally nondivergent in agreement with the other observations of flow in the surface layers of near-zonal oceanic fronts.