SEA-SURFACE TEMPERATURE-FLUCTUATIONS DURING THE HOLOCENE OFF THE SOUTH COAST OF AFRICA - IMPLICATIONS FOR TERRESTRIAL CLIMATE AND RAINFALL

Models of southern African palaeoclimate implicate surface atmospheric circulation anomalies as forcing large-scale changes during the Late Quaternary. The available proxy data are insufficient to test the mode ls since they provide information about temperature and rainfall rathe r than surface circulation. A conceptual model is proposed which links coastal ocean temperatures with atmospheric circulation and allows a history of surface circulation to be inferred from sea-temperature dat a. A Holocene sea-surface temperature (SST) record was constructed by measuring the oxygen isotope composition of marine mollusc shells pres erved in an archaeological cave deposit on the coast of the eastern Ag ulhas Bank, southern Africa. Radiocarbon-dating of individual shells a llowed definition of the timing and timespan of events in the record. By serially sampling along the growth axis of each shell, information was obtained about intra-annual variability as well as millennial-scal e trends. During the early Holocene, the sea surface on the eastern Ag ulhas Bank was colder than it is at present. Maximum summer and winter temperatures obtained 5800 years ago, exceeding by more than 2 degree s C those recorded in the region today during non-El Nino years. On av erage, temperatures remained high for the following 1500 years but dro pped again during the Late Holocene. At 650 BP, at the start of the 'L ittle Ice Age' in southern Africa, the surface waters on the eastern A gulhas Bank were colder during both winter and summer. Millennial-scal e variability was attributed to the same processes which force interan nual SST anomalies in the region today, namely surface winds and the A gulhas Current. Negative temperature anomalies were probably associate d with higher volume transport of the Agulhas Current, which forces up welling over the shelf edge, and with increased prevalence of easterly winds, which drive coastal upwelling. In contrast, a northward displa cement of the westerly wind belt, weakening of the anticyclonic circul ation and thus suppression of both coastal and shelf-edge upwelling ma y have been responsible for positive temperature anomalies recorded du ring the mid-Holocene. The conceptual model predicts that negative SST anomalies are associated with wetter conditions over the southern Afr ican interior, whereas warm-water anomalies are associated with dry co nditions.