The Australian summer monsoon: CSIRO9 GCM simulations for 1 × CO 2 and 2 × CO 2 conditions

Abstract

In this study, the CSIRO9 general circulation model (GCM) simulated temperature, mean sea level pressure (MSLP), circulation features at lower and upper levels of the atmosphere, and interannual variations of monsoon trough and precipitation during the months from December to February are compared with observed data. The study domain includes tropical Australia and the Indonesian region. Changes in monsoon circulation features and in precipitation under 2 × CO 2 conditions are also discussed based on the differences between 2 × CO 2 and 1 × CO 2 experiments. The CSIRO9 GCM simulates many of the observed large-scale and synoptic-scale circulation features of the Australian summer monsoon. It also captures the seasonality of rainfall. However, the model fails to simulate the correct magnitude of the observed wind and the precipitation. It also underestimates the interannual variability of precipitation during the monsoon season. Under a 2 × CO 2 scenario, the simulated Australian monsoon circulation is strengthened and precipitation is increased by about 20%, but the interannual variability of precipitation remains unchanged in north Australia. Similarly in central Australia, the model simulates a greater increase in rainfall under 2 × CO 2 conditions, but does not show a significant change in variability. Results of the CSIRO9 simulations revealed that on average the monsoon shear line moves toward central Australia under 2 × CO 2 conditions, but such a movement is not statistically significant and also shows little variation on interannual time scales. The monsoon shear line does not show any changes under 2 × CO 2 conditions in its location over oceanic regions off the northwest and northeast coasts of Australia.