Extended Abstract (1.1M)5A.1
Factors contributing to the onset of the Australian summer monsoon
Chih-wen Hung, Univ. of California, Los Angeles, CA; and M. Yanai
The land-sea thermal contrast is believed to be the primary factor for the evolution of monsoons. The Australian summer monsoon is thought to be ideal for examining this concept, because flat land surface of Australia has no complexity of topography. The Australian summer monsoon usually begins in December and ends in March, and its mature stage is characterized by heavy precipitation and low-level westerly wind in northern Australia. Using 15-year (1979-1993) ECMWF reanalyses (ERA) data, daily 3-dimensional Q1 (apparent heat source) and Q2 (apparent moisture sink) are calculated. Significant heating in a layer below 700 hPa over the Australian continent, mainly caused by the sensible heat from the land surface, starts in September prior to the monsoon onset. Reversal of meridional temperature gradient (dT/dy) between Australian continent and the Arafura sea in this layer occurs from September to March. The warming over the continent dominates the meridional temperature gradient, because the temperature over the Arafura sea has very small change. The sensible heating sets up a thermally-induced meridional-vertical circulation, as seen from outflow between 800 hPa and 400 hPa and low-level inflow below 800 hPa over the continent. Stronger monsoon is observed with significantly warmer temperature over Australia in southern spring before the monsoon onset. In this pre-monsoon stage, low-level easterlies begin to weaken and westerlies begin to develop in the northern Australia. After the onset, the core of westerly wind is observed between 925 hPa and 700 hPa. In the post-onset stage (January-March), the air over the northern and middle Australia becomes colder for the strong monsoon years than that for the weak monsoon years. The relationship between the Australian summer monsoon and Asian winter monsoon is investigated. The frequency of the Madden-Julian oscillation (MJO) acting as a trigger for the monsoon onset is also examined.
Supplementary URL: http://www.atmos.ucla.edu/~tropics/AMS/5A1.pdf
Session 5A, Monsoons I (Parallel with Sessions 5B, 5C & 5D)
Tuesday, 30 April 2002, 11:00 AM-12:30 PM
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