The Australian Summer Monsoon in Current and Future Climate

Abstract

As the counterpart of the Asian monsoon in the Northern Hemisphere, in this chapter the Australian summer monsoon covers a spatial domain encompassing tropical Sumatra and the Java Islands, and the adjacent waters in the west, extending south and eastward into the Timor and Timor Sea region and further penetrating into the tropical Australian continent. This chapter documents its observed features at a range of temporal and spatial scales, evaluates how well the current climate models can reproduce these fundamental features, and finally summarizes current projections of its potential changes in future and primary processes leading to such changes. The monsoon system shows pronounced seasonal variations of rainfall and prevailing wind, with its austral summer season rainfall being supported by the reversal of easterly trade winds into deep and moist westerlies. Its onsets are influenced by a number of factors including the Madden-Julian Oscillation (MJO), land-sea thermal contrast, the influence of middle latitude systems, and the inherent atmospheric instability. The inter-annual variations of the monsoon onset are correlated to El Niño-Southern Oscillation (ENSO), but the total summer monsoon rainfall is not. This is partially attributed to the seasonally varying air-sea interactions over the waters north of the Australian continent. The Asian aerosol and tropical SSTs near the continent have been used in explaining observed rainfall increases northwest of the tropical Australian continent. In fully coupled global climate model simulations, the broad features of the monsoon mean climate, its seasonal and inter-annual variations of rainfall, temperature and circulation, can be reasonably reproduced by a majority of the models, but there are very large variations in individual model skills. The relevant importance of primary large-scale drivers governing the monsoon variations differs significantly across current climate models, with some models having too strong an ENSO influence. There is great uncertainty in model projections of the changes in the monsoon rainfall under global warming. A weak change in mean rainfall from multi-model ensemble averages is largely produced accompanied by large model discrepancies, with the number of the models showing likely increases in rainfall being matched by roughly the same number of the models showing decreased rainfall. There is a lack of consensus regarding the changes in the Australian monsoon onset/retreat, with studies using rainfall in defining the monsoon onset suggesting the onset comes earlier. However, for the studies using circulation as one of the criteria, they showed a possible delay due to the weakening and shifting of the atmospheric circulation. Future progress to improve the modelling skill and increase our confidence of the projection of the Australian monsoon relies on a number of key aspects, including the improved model physics and dynamics with increased model resolutions; improved representations of key drivers of the monsoon system, such as realistic MJO, ENSO and Indian Ocean Dipole (IOD) in climate models; and improved understanding of the model discrepancies.