Inter-ocean fluxes south of Africa in an eddy-permitting model

Exchanges of water south of Africa between the South Indian Ocean and the South Atlantic Ocean are an important component of the global thermohaline circulation. Evidence exists that the variability in these exchanges, on both meso- and longer time scales, may significantly influence weather and climate patterns in the southern African region and the significance of these regional ocean–atmosphere interactions is discussed. Observations of the inter-ocean exchange are limited and it is necessary to augment these with estimates derived from models. As a first step in this direction, this study uses an eddy-permitting model to investigate the heat and volume transport in the oceanic region south of Africa and its variability on meso, seasonal and inter-annual time scales. On the annual mean, about 0.84 PW (standard deviation 0.13 PW) of heat flows west into the South Atlantic across 20°E (longitude of Cape Agulhas, the southernmost point of Africa), with just over 1 PW (standard deviation 0.4 PW) flowing north into the South Atlantic across 35°S. The seasonal variations in this transport are about 10% at 35°S in the South Atlantic and around 20% through 20°E; the model value of 0.66 PW for summer (standard deviation ranging from 0.13 PW in January to 0.17 PW in March) appears consistent with respective estimates of 0.51 and 0.60 PW derived from two WOCE summer cruises southwest of Cape Town to 45°S in 1990 and 1993. Volume transports of the Agulhas Current section through 35°S in the SW Indian Ocean range from 58 to 59 Sv in summer/autumn to 64–65 Sv in winter/spring. The model results suggest that the inter-ocean exchange south of Africa is highly variable on seasonal through to interannual scales. If this variability is also the case in the real ocean (and the limited observations suggest that this is so), then there are likely to be significant implications for climate.