Oceanic variability in the western sector of Algoa Bay, South Africa

Abstract

The city of Port Elizabeth lies in the western sector of Algoa Bay, and a number of projects have investigated the oceanic temperature, salinity and current structures in the vicinity. This paper analyses past results, and then incorporates measurements of salinity, sea temperatures and currents from projects over the years 1989 to 1998 in order to gain a better understanding of the oceanic variability in this region. Because of the limited inflow of freshwater from rivers, salinity variations from oceanic values were minimal, and it is surmised that the effects of freshwater influxes during floods are likely to be short-lived. The seasonal variation in temperature structures shows that intense thermoclines are established in summer in the deeper sections of the bay, with more isothermal conditions in winter. Marked variability occurred over spatial scales of kilometres, and over time scales of days. Average current speeds close to the coast were generally low, increasing farther offshore, though there was substantial variability. Moreover, in the lee of the harbour and close to the coast spatial scales were also small, and spectral analyses show that tidal and inertial currents were important at times. Driving forces were investigated, and it was found that wind is important in upwelling processes and in bringing colder water onshore from deeper regions of Algoa Bay and around Cape Recife. On the other hand, coastal-trapped waves do not appear to play a significant role. Sporadic intrusions of both warm and colder water were recorded at sites near the harbour, and are attributed to inputs of warm surface water from the Agulhas Current and the above-mentioned colder upwelled water. Such intrusions are probably important in flushing out accumulations of pollutants discharged to sea from Port Elizabeth. Dominant south-westward flow farther offshore at 40m depth also indicates that the Agulhas Current could be important in entraining water from the central areas of Algoa Bay.

African Journal of Marine Science 2005, 27(1): 65–80