Wind-driven mixed layer and coastal upwelling processes off the south coast of South Africa

Most coastal mixed layer and upwelling investigations have concentrated on the more important fisheries regions situated on the west coasts of the con tinents, while this paper analyzes conditions on the zonally-orientated sou th coast of South Africa. Most of the data originate from a short measureme nt program in the southern summer of 1995, involving both CTD measurements from a small boat and data from a temperature array and coastal sea surface temperature (SST). Limited mixed layer statistics are derived and yield results similar to tho se of Lentz (1992), although a lack of current measurements precludes any a ssessment of the Ekman drift. It is shown that the nature of the summer win d forcing is very different to the situation off the South African west coa st, with wind variability occurring on much shorter time scales from 2 to 6 days: this has important consequences for plankton blooms and is a likely reason why production is not as efficient off this coast. Past results (Til ney et al., 1996) have shown the importance of coastal trapped waves at dep th, but it is apparent that very different conditions occur near the surfac e. It is found that the coastal SST is a very good monitor of wind-forced coas tal upwelling conditions, probably because of the abrupt topography. Substa ntial baroclinic tidal signals are also apparent at times in the temperatur e measurements.