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

Abstract

Most coastal mixed layer and upwelling investigations have concentrated on the more important fisheries regions situated on the west coasts of the continents, while this paper analyzes conditions on the zonally-orientated south coast of South Africa. Most of the data originate from a short measurement 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 those of Lentz (1992), although a lack of current measurements precludes any assessment of the Ekman drift. It is shown that the nature of the summer wind forcing is very different to the situation off the South African west coast, 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 productions is not as efficient off this coast. Past results (Tilney et al., 1996) have shown the importance of coastal trapped waves at depth, but it is apparent that very different conditions occur near the surface. It is found that the coastal SST is a very good monitor of wind-forced coastal upwelling conditions, probably because of the abrupt topography. Substantial baroclinic tidal signals are also apparent at times in the temperature measurements.