Thermal structure of Agulhas Bank water

Cyclonic frontal eddies accompanying the sinuous meanders of the Agulhas Current along the south-eastern edge of the Agulhas Bank force both warm Subtropical Surface water and cold, less-saline Indian Ocean Central water onto the continental shelf. The advective juxtaposition of these water masses establishes an arealy extensive vertical thermocline of remarkable intensity over the shelf. Temperature differentials of 8–10°C over depth intervals of as little as 10 m are not uncommon. Several manifestations of upwelling, viz. coastal, midshelf and shelf-edge, are evident in the results of three CTD-surveys made since 1982. Advective maintenance of the thermocline appears to compete with turbulent breakdown, a seasonal change of dominance occurring in parts remote from Agulhas Current influence. The gravitationally unstable salinity distribution could facilitate double-diffusive diapycnal mixing as well as ease turbulent mixing with winter cooling of the surface mixed layer.