Sea-surface temperature fluctuations during the Holocene off the south coast of Africa: implications for terrestrial climate and rainfall

Abstract

Models of southern African palaeoclimate implicate surface atmospheric circulation anoma lies as forcing large-scale changes during the Late Quaternary. The available proxy data are insufficient to test the models since they provide information about temperature and rainfall rather 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 data. A Holocene sea-surface temperature (SST) record was constructed by measuring the oxygen isotope composition of marine mollusc shells preserved in an archaeological cave deposit on the coast of the eastern Agulhas Bank, southern Africa. Radiocarbon-dating of individual shells allowed 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-scale trends. During the early Holocene, the sea surface on the eastern Agulhas Bank was colder than it is at present. Maximum summer and winter temperatures obtained 5800 years ago, exceeding by more than 2°C those recorded in the region today during non-El Nino years. On average, temperatures remained high for the following 1500 years but dropped again during the Late Holocene. At 650 BP, at the start of the 'Little Ice Age' in southern Africa, the surface waters on the eastern Agulhas Bank were colder during both winter and summer. Millennial-scale variability was attributed to the same processes which force interannual SST anomalies in the region today, namely surface winds and the Agulhas Current. Negative temperature anomalies were probably associated with higher volume transport of the Agulhas Current, which forces upwelling over the shelf edge, and with increased prevalence of easterly winds, which drive coastal upwelling. In contrast, a northward displacement of the westerly wind belt, weakening of the anticyclonic circulation and thus suppression of both coastal and shelf-edge upwelling may have been responsible for positive temperature anomalies recorded during the mid-Holocene. The conceptual model predicts that negative SST anomalies are associated with wetter conditions over the southern African interior, whereas warm-water anomalies are associated with dry conditions.