Subtropical SST dipole events in the southern Indian Ocean

Abstract

Sea surface temperature (SST) anomalies in the subtropical southern Indian Ocean show interannual dipole events that are seasonally phase-locked to the austral summer. A positive phase of the event is characterized by cold SST anomalies in the eastern part i.e. off Australia and warm SST anomalies in the southwestern part, south of Madagascar. Such an event is found to produce above normal rainfall over many regions in south-central Africa. The cooling of SST in the eastern part is mainly caused by the enhanced evaporation. This is associated with stronger winds along the eastern edge of the subtropical high, which is strengthened and shifted slightly to the south during the event. On the other hand, relative decrease in the seasonal latent heat loss due to reduced evaporation dominates the warming in the southwestern part. Evolution of such subtropical dipole events shows quite a contrast to that of the tropical dipole events discovered recently in the Indian Ocean.