中文摘要：

为了更好地开展区域关键期气候监测业务，满足国家级气象服务的需求，本文从区域角度出发，利用1961～2011年西南地区92个气象观测站的逐日雨量资料和1981-2010年美国国家环境预报中心和国家大气研究中心（NCEP／NCAR）逐日的高低层大气环流再分析资料，研究了西南区域雨季开始和结束日期定义的标准问题。结果表明西南区域多年平均雨季开始日期在5月3候（27候），结束日期在10月3候（57候），季节转换期间高低层环流的突变特征进一步表明了该结论的合理性。并通过对多种雨季开始和结束日期判断标准的对比分析，最终提出了西南雨季开始和结束日期的划分标准。同时，对影响西南地区雨季开始和结束日期变化的机理也进行了初步的讨论。

英文摘要：

In order to improve climate monitoring during key periods and meet the needs of meteorological services, a criterion for determination of the onset and end of the rainy season in Southwest China is investigated based on daily rainfall data from 92 stations in Southwest China during 1961-2011 and daily atmospheric circulation reanalysis data from NCEP/NCAR for 1981-2010. The results indicate that, for the mean climate, the rainy season in Southwest China begins in the third pentad in May （pentad 27） and ends in the fourth pentad in October （pentad 57）. Variations in atmospheric circulation confirm the above result. Moreover, a criterion for determination of the annual onset and end of the rainy season in Southwest China is proposed based on the results of comparative analysis of different definitions and the seasonally changing characteristics of atmospheric circulation. Simultaneously, possible mechanisms influencing the onset and end of the rainy season are also discussed preliminarily.