Local SST Impacts on the Summertime Mascarene High Variability

Abstract The interannual variations in the summertime Mascarene high have great impacts on the southern African climate as well as the sea surface temperature (SST) in the southern Indian Ocean. A set of coupled general circulation model (CGCM) experiments are performed to examine a role of the interannual SST variability in the southern Indian Ocean on the summertime Mascarene high variability. The dominant interannual variability in the summertime Mascarene high shows the strengthening (weakening) in its southern part throughout the austral summer (December–February). However, in the experiment where the interannual SST variability in the southern Indian Ocean is suppressed, the strengthening (weakening) of the Mascarene high in its southern part does not persist until February. Also, the Mascarene high variability and its associated SST anomalies in December and January are found to increase (decrease) the southern African rainfall via more (less) moisture supply from the southern Indian Ocean. The Mascarene high variability is actually associated with a meridional dipole of positive and negative SST anomalies, which in turn produces that of the meridional SST gradient anomaly. This causes a southward (northward) shift of the storm tracks and hence the westerly jet, favoring the strengthening (weakening) of the Mascarene high in its southern part. This local ocean–atmosphere feedback effectively operates in February, when the meridional dipole of the SST anomalies reaches the maximum. These results provide new insight into the important role of the local SST variability in the summertime Mascarene high variability and hence the southern African climate.