Is there a continuous Subtropical Front south of Africa?

Abstract

A 14.3 year series of weekly absolute sea surface height (SSH) and associated geostrophic velocities is used for a study of the subtropical‐to‐subantarctic frontal system in the region south of Africa. Detecting the fronts from surface velocity maxima confirms a two‐stepped transition in both the southeastern Atlantic Ocean (the Northern and Southern Subtropical fronts (NSTF, SSTF)) and southwestern Indian Ocean (the Agulhas Front and SSTF), as proposed previously from hydrographic data. An additional front associated with westward flow north of the NSTF is indicative of a partial eastern closure of the South Atlantic subtropical between 11°E and 17.5°E. The role of northwestward propagating Agulhas rings in connecting the two fronts explains the varying location. The SSTF, which marks the southern limit of subtropical waters, is found continuous from the Atlantic to the Indian Ocean in the time‐averaged SSH field. In the weekly fields, however, the velocity maximum criterion defining the front is often met at the southern flanks of Agulhas rings or Agulhas eddies at 12°E-23°E, indicating front disruptions. Such discontinuities suggest that no South Atlantic Central Water is directly advected into the Indian Ocean. Instead, the eastward limb of the Indo‐Atlantic super gyre encompassing the subtropical gyres of both oceans would rest, at these longitudes, on diffusive processes at the upper levels, and possibly be enhanced at depth. A schematic diagram of the fronts and their relations to Agulhas rings and eddies is proposed.