Factors contributing to the onset of the Australian summer monsoon

Using the reanalyses from the European Centre for Medium-Range Weather Forecasts, outgoing longwave radiation (OLR) and precipitation data for 1979-93, this paper examines major factors contributing to the onset of the Australian summer monsoon. The low-level (850 hPa) westerly wind and convective activity (OLR, precipitation) over a region in northern Australia (NAU) are used to determine the onset dates. Composite results are then derived, based on these dates. Daily apparent heat source and apparent moisture sink values are obtained as residuals of the large-scale heat and moisture budgets to help clarify the roles of the various heating processes in the onset. Four major factors contributing to the onset are identified: (1) land-sea thermal contrast, (2) barotropic instability, (3) arrival of the Madden-Julian oscillation (MJO) and (4) intrusion of midlatitude troughs. The thermal contrast due to differential heating between land and sea acts as a seasonal preconditioning for the onset. The sensible heating over the continent leads to a reversal of meridional temperature gradient between the Australian continent and the Arafura Sea in a layer below 800 hPa in September-March, and sets up a thermally induced meridional-vertical circulation which helps to transport low-level moist air inland. The barotropic instability criterion is often met at 850 hPa in NAU several days prior to the onset. The sudden onset is then triggered by the arriving MJO and at times by the intrusion of a midlatitude trough.