Potential Sources of Decadal Climate Variability over Southern Africa

Abstract

otential sources of decadal climate variability over southern Africa are examined by conducting in-depth analysis of available datasets and coupled general circulation model (CGCM) experiments. The observational data in recent decades show a bidecadal variability noticeable in the southern African rainfall with its positive phase of peak during 1999/2000. It is found that the rainfall variability is related to anomalous moisture advection from the southwestern Indian Ocean, where the anomalous sea level pressure (SLP) develops. The SLP anomaly is accompanied by anomalous sea surface temperature (SST). Both SLP and SST anomalies slowly propagate eastward from the South Atlantic to the southwestern Indian Ocean. The analysis of mixed layer temperature tendency reveals that the SST anomaly in the southwestern Indian Ocean is mainly due to eastward advection of the SST anomaly by the Antarctic Circumpolar Current. The eastward propagation of SLP and SST anomalies are also confirmed in the 270-yr outputs of the CGCM control experiment. However, in a sensitivity experiment where the SST anomalies in the South Atlantic are suppressed by the model climatology, the eastward propagation of the SLP anomaly from the South Atlantic disappears. These results suggest that the local air?sea coupling in the South Atlantic may be important for the eastward propagation of the SLP anomaly from the South Atlantic to the southwestern Indian Ocean. Although remote influences from the tropical Pacific and Antarctica were widely discussed, this study provides new evidence for the potential role of local air?sea coupling in the South Atlantic for the decadal climate variability over southern Africa.