Factors contributing to the onset of the Australian summer monsoon
Chih-wen Hung and Michio Yanai
Submitted to Q. J. R. Meteorol. Soc.
Updated on 6 August 2003
Summary
Using the ECMWF re-analysis, OLR and precipitation data during 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 made based on these dates chosen in 1979-93. Daily Q1 (apparent heat source) and Q2 (apparent moisture sink) are obtained as residuals of the large-scale heat and moisture budgets for clarifying the roles of 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 criterion of the barotropic instability 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.
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