中国典型夏季风影响过渡区夏季降水异常时空特征及成因分析

基于1961-2016年国家气象信息中心整编的气象台站逐日降水以及NCAR/NCEP再分析等资料，对我国典型夏季风影响过渡区夏季降水的异常时空特征及成因进行分析，结果表明：典型夏季风影响过渡区夏季降水EOF展开第一模态呈全区一致性特征，而且该模态时间系数没有明显的长期变化趋势，第二模态呈西北和东南反位相变化特征.相关分析表明夏季中纬度西风带是影响典型夏季风影响过渡区夏季降水异常的最主要因子，高原夏季风为次要因子，东亚夏季风的影响较弱，而且东亚夏季风主要通过其子系统——西太平洋副热带高压的东西摆动来影响.此外在夏季中纬度西风偏弱年，高空急流位置偏南，急流轴在典型夏季风影响过渡区向东南方向发生了"倾斜"，对应500 hPa呈异常的西北气流控制，同时由于高空急流在过渡区减弱，使得高层发生异常的气流辐合，低层辐散，通过高低层环流之间的质量和动量调整，垂直场表现为异常下沉运动，低层的辐散也减弱了西南暖湿气流的北上，水汽来源少，最终使得典型夏季风影响过渡区夏季降水偏少，反之亦然.这是夏季中纬度西风带影响典型夏季风影响过渡区夏季降水的可能机理.

Spatial-temporal characteristics and causes of summer precipitation anomalies in the transitional zone of typical summer monsoon,China

In this study, station daily precipitation data from 1961 to 2016 provided by the National Meteorological Information Center, and NCAR/NCEP reanalysis data were used to analyze the spatial-temporal characteristics and causes of summer precipitation anomalies in the transitional zone of typical summer monsoon in China. Results showed that the time coefficient of EOF1 of summer precipitation presented regional consistency characteristics in the transitional zone, and the mode did not show significantly long-term trends, the time coefficient of EOF2 presented reversely change in the northwest and the southeast part. Correlation analysis showed that the middle latitude westerlies in summer may be the dominant factor of summer precipitation anomaly in the transitional zone of tropical summer monsoon, while plateau summer monsoon is the secondary factors, and East Asian summer monsoon had less impacts, and east Asian summer affected the precipitation anomaly mainly through the east-west swing of the west edge of the subtropical high in western Pacific. Moreover, during the years with weaker summer middle latitude westerlies, the "sloping" occurred along the southeast direction of the high-altitude jet in the transitional zone of typical summer monsoon, corresponding to the anomalous control of northwest airflow at 500 hPa. At the same time, due to the abnormally weak upper-stream westerly jet, abnormal airflow convergence and divergence occurred at the upper and lower levels, respectively. Vertical field showed anomaly sinking motion as a result of the adjustment of mass and momentum between the high and low circulations, the low-level southwest warm and humid airflow is weakened. Eventually, the precipitation is decreased in the transitional zone of typical summer monsoon, and vice versa. This may be the mechanism that the summer middle latitude westerly affect the summer precipitation in the transitional zone of typical summer monsoon.