Simulated Interannual Variations of the Indo-Australian Monsoons

Abstract

Interannual variations of the South Asian and the Australian summer monsoons in a 20-year simulation by an atmospheric general circulation model (GCM) with the observed sea surface temperature (SST) for the period 1970-1989 are investigated. Emphasis is laid on moisture flux variations, as the moisture flux convergence plays a more dominant role in the rainfall variations than the local evaporation, although their relative roles are comparable for the 20-year mean moisture budget.
The anomalous precipitation in the Bay of Bengal in boreal summer is significantly correlated with the SST anomalies in the western Pacific, but not with the in situ SST anomalies. The positive SST anomalies in the western Pacific are favorable to the intensified South Asian summer monsoon circulation with strong surface westerlies over the north Indian Ocean. A contrast in anomalous precipitation in the model between the South Arabian Sea and the India subcontinent is also discussed. The anomalous precipitation over the oceans north of Australia in austral summer is explained mainly by the anomalous moisture flux converging into this region from the west. This is accompanied by an intensified cyclonic circulation northwest of Australia, but cross-equatorial moisture flux from the winter hemisphere is found to be important. Some comparisons with observations are also made and discrepancies between the model and these observation are discussed.