Evidence for the Influence of the Agulhas Current on Regional Atmospheric Circulation Patterns

Abstract

The Agulhas Current is much the strongest western boundary current in the Southern Hemisphere transporting about 65 Sv of warm water along the southeast coast of Africa. South of Africa, it retroflects back into the South Indian Ocean and flows east as the Agulhas Return Current. Large amounts of heat are lost to the atmosphere from the warm waters of the southern Agulhas Current, its retroflection, and the Return Current particularly during winter and the transition seasons. It is thought that this large exchange of heat may have a significant influence on the regional atmospheric circulation, the tracks and intensity of transient weather disturbances such as extratropical cyclones, and on rainfall received over large parts of neighboring southern Africa. In an attempt to investigate the potential influence of the Agulhas Current on the regional atmosphere, two ensembles of year-long integrations of an atmospheric general circulation model are analyzed. In the first ensemble, the sea surface temperature (SST) forcing is a monthly climatology and therefore contains the signature of the greater Agulhas Current system at the model resolution. In the second ensemble, the pronounced signature of this current is smoothed out so that the waters of the region only show a typical latitudinal variation in SST similar to that found in the central ocean away from boundary currents. An objective vortex tracking scheme is used to assess the differences in the number, track, and intensity of extratropical cyclones in the Southern Hemisphere between the two integrations. It is found that a near-surface cold anticyclonic anomaly is generated over the greater Agulhas Current region in the ensemble with the smoothed SST. Cyclonic systems are weaker and shifted farther south in this ensemble compared to climatology with significant reductions in rainfall over neighboring South Africa. Downstream, over the southeastern Australian?Tasman Sea region, there is an increase in cyclonic activity and rainfall. In general, the differences between the two ensembles are more coherent and pronounced during winter than in summer consistent with the surface temperature contrast between the Agulhas Current waters and the overlying atmosphere being larger in winter. With the caveats that the atmosphere?ocean system is nonlinear and that the model resolution does not represent the turbulent processes of air?sea heat/moisture exchange as well as one would like, the results provide some evidence for the potential importance of the greater Agulhas Current system for regional climate and weather patterns.