CLIMATE VARIABILITY IN A COUPLED GCM .2. THE INDIAN-OCEAN AND MONSOON

We have investigated the seasonal cycle and the interannual variabilit y of the tropical Indian Ocean circulation and the Indian summer monso on simulated by a coupled ocean-atmosphere general circulation model i n a 26-year integration. Although the model exhibits significant clima te drift, overall, the coupled GCM simulates realistically the seasona l changes in the tropical Indian Ocean and the onset and evolution of the Indian summer monsoon. The amplitudes of the seasonal changes, how ever, are underestimated. The coupled GCM also simulates considerable interannual variability in the tropical Indian Ocean circulation, whic h is partly related to the El Nino/Southern Oscillation phenomenon and the associated changes in the Walker circulation. Changes in the surf ace wind stress appear to be crucial in forcing interannual variations in the Indian Ocean SST. As in the Pacific Ocean, the net surface hea t flux acts as a negative feedback on the SST anomalies. The interannu al variability in monsoon rainfall, simulated by the coupled GCM, is o nly about half as strong as observed. The reason for this is that the simulated interannual variability in the Indian monsoon appears to be related to internal processes within the atmosphere only. In contrast, an investigation based on observations shows a clear lead-lag relatio nship between interannual variations in the monsoon rainfall and tropi cal Pacific SST anomalies. Furthermore, the atmospheric GCM also fails to reproduce this lead-lag relationship between monsoon rainfall and tropical Pacific SST when run in a stand-alone integration with observ ed SSTs prescribed during the period 1970-1988. These results indicate that important physical processes relating tropical Pacific SST to In dian monsoon rainfall are not adequately modeled in our atmospheric GC M. Monsoon rainfall predictions appear therefore premature.