The response of summer monsoon onset/retreat in Sumatra-Java and tropical Australia region to global warming in CMIP3 models

In this study, we assess the potential changes in the onset, retreat and duration of austral summer monsoon covering the domain from south Sumatra and Java region in the tropics to the northern Australian continent. We simply call it the Australian summer monsoon. Daily precipitable water and 850 hPa wind from 13 CMIP3 models are used in the diagnoses. A majority of the models can capture the northwest–southeast evolution of the summer monsoon, which starts from the south Sumatra and Java region around later November and then progressively approaches the Australian continent in late December. Nevertheless, significant biases exist in the modeled onset/retreat dates and the extent of the monsoon inland penetration. Under global warming, the agreement among the model projections varies across the domain. In between the Sumatra-Java archipelago and the top end of the Australian continent, over 80 % of the models simulate delayed monsoon onset and shortened duration by ~10 days, but less model agreement is seen over interior continent where the model ensembles show an approximate 7-day delay of both the onset and retreat with relatively little change in duration. Both El Nino-Southern Oscillation and Indian Ocean SST patterns appear to play important roles in determining the variations of the modeled monsoon onset. Nevertheless, the extent of their influence varies significantly across the models. Under global warming, a large proportion of models show relatively less warming in the eastern Indian Ocean and with a consequent increase in the modeled Indian Ocean Dipole index. Both a weakened and/or eastward shift of the upward branch of Walker circulation and the Indian Ocean contribute to the simulated delayed onset and shortened duration in the tropics under global warming.