与西澳州西南部冬季降水相联系的大气环流特征分析

The Characteristics of Atmospheric Circulation Associated with Austral Winter Rainfall in Southwest Western Australia

西澳大利亚州西南部(SWWA)是西澳大利亚州首府Perth的所在地,也是西澳州政治、经济、文化、教育和旅游的中心。自20世纪中期以来,SWWA地区雨季降水持续减少。本文利用近60年的观测及再分析数据,分析了已知的影响澳大利亚降水的热带海洋模态:厄尔尼诺—南方涛动(ENSO)、印度洋偶极子(IOD)和ENSOModoki(EM)对SWWA地区降水的影响,发现它们均不能解释SWWA地区前冬(5~7月)(全文季节均指南半球)和后冬(8~10月)降水的长期变化趋势。特别地澄清了南半球环状模(SAM)对SWWA地区冬季降水的影响,揭示了前人所指出的两者之间的显著关系是由于极端事件造成的。这表明,已知的影响澳大利亚降水的气候模态并不能解释SWWA地区冬季降水的长期变化趋势。通过对SWWA地区降水、大尺度大气环流的动力、热力结构和季节循环特征的分析,发现其表现出类似季风环流的特征,也即干湿季节的交替出现、风向的季节性反转和明显的海陆热力差异。基于此提出了SWWA地区类季风环流(SWAC)的概念,并分析了其与SWWA地区冬季降水之间的关系,指出SWAC环流的减弱是SWWA地区降水减少的可能原因。

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 Nin^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.