The Characteristics of Atmospheric Circulation Associated with Austral Winter Rainfall in Southwest Western Australia
与西澳州西南部冬季降水相联系的大气环流特征分析

Southwest Western Australia (SWWA) locates in the southwest corner of Western Australia. Its capital, Perth, is not only the political, economic, cultural, educational, and tourism center of Western Australia, but also the wine region and wheat belt of Western Australia. The observed rainy season precipitation over SWWA has significantly declined since the 1950s. The reduction in austral winter rainfall resulted in an even sharper fall in the stream flow in southwestern Australia and heavily impacted the water resources in the state. Based on the observation data and reanalysis datasets in recent 6 decades, the variability and circulation features influencing the SWWA austral winter rainfall (SWR) are investigated. The impacts of the known modes, which may have impacts on the Australian rainfall, El Ni o-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), ENSO Modoki (EM), and Southern Hemisphere Annular Mode (SAM) on the SWR are analyzed. It is found that they all cannot explain the long term rainfall variation of the SWR in neither early (May to July, MJJ) nor late (August to October, ASO) austral winter. Particularly, the reported significant inverse relationship between the SAM and SWR is caused by an extreme year. Based on the investigation on rainfall distribution over SWWA, seasonal march of the circulation features, dynamical and thermal structures of the atmospheric circulation, and thermal contrast between land and sea over wider southwest Australia, the authors find that the circulation over SWWA shows strong seasonality, and exhibits monsoonal characteristics, that is, alternate wet and dry seasons, seasonal reversal of prevailing winds, and an evident land-sea thermal contrast. Taking account of these monsoonal characteristics, the authors propose the concept of monsoon-like southwest Australian circulation (SWAC), and discuss the relationship between the SWAC and the SWR. The result indicates that the variability of SWAC contributes to the variation of SWR, whose weakening is a possible reason for the SWR decreasing.