Uncertainty in CMIP5 model-projected changes in the onset/retreat of the Australian summer monsoon

This study addresses several significant drawbacks in our previous analyses of how Australian summer monsoon onset/retreat may respond to global warming in CMIP3 model simulations. We have analysed daily 850 hPa wind, volumetric precipitable water, precipitation and temperature data from 26 CMIP5 models over a pair of 55-year simulations. Firstly, the CMIP5 models do not show significant improvement in capturing observed features of the monsoon onset/retreat in the region, despite of a slightly reduced bias in multi-model ensemble results. We show that wind–rainfall relationship varies with models and rainfall-based wet season onsets may not adequately represent the monsoon development. Under global warming, although 26-model averages show delayed onset and shortened duration, significant uncertainty exists: 10 models simulated delayed onset but it became earlier in another group of 7 models. Similar model discrepancies are seen in the modelled changes in retreat dates. The range of uncertainty in the projected changes is similar in CMIP3 and CMIP5 models and further analysis re-affirms previously proposed reasons: one is the different influence of a number of drivers in these models and the other is the different changes in these drivers themselves in future climate. Overall, most of the models showed impacts of ENSO and the Indian Ocean on the Australian summer monsoon onset/retreat, but the models differed quite significantly in the magnitude of such impacts. Another factor is different warming patterns and magnitudes in the tropical Pacific and Indian Oceans. When combined, the two provide a better explanation of the scatter among the 26 CMIP5 model results.