中文摘要：

利用1961—2010年NCEP/NCAR逐日再分析资料和云南站点降水资料,通过数理统计和动力诊断方法分析了东亚副热带西风急流（东亚急流）的低频变化特征、形成机理及其与云南雨季开始的关系。研究显示,东亚急流南移增强是云南雨季开始的重要触发因子,即伴随云南雨季开始东亚急流明显南移且强度增强,急流入口区的垂直环流也随之南移增强,而云南正好处于垂直环流上升支的影响范围;与此同时,东亚中纬度冷空气活跃,急流入口区的垂直环流下沉支为干冷气流,十分有利于对流层中低层冷暖空气在云南交汇形成降水。上述过程与欧亚大陆中高纬10~30天低频波列（EU型）的传播密切相关,当波列上的冷性气旋在东亚上空向东南方向移动时即可造成副热带西风急流的增强南移和冷空气的活跃南下。分析还显示EU型波列对4—7月的云南降水也有显著影响,并且波列的年际异常及其与夏季风的相互作用是影响云南雨季开始早晚的重要原因。

英文摘要：

This study examined the low-frequency characteristics of the variation of an East Asian subtropical westerly jet（EASWJ）, analyzed the propagation of the low-frequency circulation in mid-and high-latitudes and its impacts on the onset of Yunnan rainy season by using the daily NCEP/NCAR reanalysis data and observational precipitation in the province from 1961 to 2010. The results indicate that the southward shift of the EASWJ is significant in triggering the Yunnan rainy season onset. With the onset, the EASWJ not only displaces southward but also strengthens, associated with an enhanced secondary circulation cell around the jet entrance. Yunnan is right under the updraft of the vertical circulation. Meanwhile, cold air is active over East Asia and a descending current at the jet entrance is a cold and dry airflow, which is favorable for the interaction of warm and cold air around the Yunnan area. The processes above are closely linked to a 10-to-30-day oscillation in mid-and high-latitudes of the Eurasian continent, generally known as EU teleconnection. It is the propagation of a low-frequency wave train that contributes to the EASWJ meridional variation, and cold cyclonic cells in the wave train are responsible for the cold air activity over East Asia that may also affect the Yunnan rainfall. The findings further suggest that the EU low-frequency oscillation not only influences the rainy season onset in Yunnan significantly but also affects the precipitation during the transitional season from April to July, and the interaction between abnormal EU wave-train and the Asian summer monsoon are both significant factors determining the interannual variation of Yunnan rainy season onset.