THE INFLUENCE OF THE INDIAN-OCEAN INDONESIAN SST GRADIENT ON THE AUSTRALIAN WINTER RAINFALL AND CIRCULATION IN AN ATMOSPHERIC GCM

A number of atmospheric general circulation experiments have been conducted with the aim of studying the impact of an enhanced sea surface temperature (SST) gradient, between the Indonesian archipelago and the central Indian Ocean, on the Australian winter circulation and rainfall. Particular emphasis is on modelled synoptic events and the sensitivity of mean winter climate anomalies to changes in the strength of the anomalous SST gradient. It is shown that an atmospheric general circulation model, forced by an anomalous SST gradient to produce mean winter circulation and precipitation anomalies qualitatively similar to the observed, simulates quite realistic synoptic events over the Australian continent. In particular, the model simulates an increased frequency of major Australian north-west cloud bands. These events are associated with a triad of tropical systems in the Bay of Bengal, the Cocos Islands and the Indonesian archipelago, strong cross-equatorial flow into a region north-west of Australia, and interactions with short-wave mid-latitude disturbances. The mean climate anomalies, over Australia, are shown to be largely independent of whether a fixed or evolving anomalous gradient is used, but very sensitive to the magnitude of the SST anomaly. This is seen in the greater variability of the results over Australia. It is also suggested that the presence and strength of the warm anomaly in the Indonesian region has a greater bearing on the results than the cold anomaly, in the Indian Ocean. Global tropospheric circulation and precipitation anomaly patterns, associated with an enhanced gradient, are also shown to be consistent with some observed patterns, and to be robust with respect to changes in the magnitude and distribution of the SST anomalies. This suggests that an enhanced SST gradient may also be associated with rainfall variability in a number of geographical locations.