中国西南地区5月降水与阿拉伯海季风关系的年代际变化

本文利用1960～2017年中国西南地区115个台站观测降水资料和日本气象厅发布的55年再分析资料集，研究了中国西南地区5月降水变异的主导模态及其与阿拉伯海季风的关系。结果显示，中国西南地区5月降水的第一主导模态主要表现为全区一致的变异特征；该模态与同期5月阿拉伯海季风强度异常关系密切，但两者的关系在20世纪70年代后期发生了显著的年代际变化。在1960～1976年，阿拉伯海季风异常所引起的低层大气环流和水汽输送异常主要集中在阿拉伯海到孟加拉湾一带；阿拉伯海季风异常所引起的大气环流不能到达中国西南地区，因此它对中国西南地区5月降水的影响偏弱。但在1981～2017年，阿拉伯海季风异常可以导致整个北印度洋到南海地区的大气环流异常，进而引起中国西南地区水汽和垂直运动的变化，最终对该地区5月降水产生显著的影响。进一步的研究显示，阿拉伯海季风与中国西南地区5月降水关系的变化可能与季风自身的年代际变率有关。阿拉伯海季风在20世纪70年代末之前变率偏弱，其引起的环流异常也偏弱；相反在20世纪70年代末之后，其变率增强，它引起的大气环流异常也偏强，可以延伸到中国西南地区，进而影响到西南地区的5月降水。因此，季风变率的强弱可能在季风对西南地区5月降水的影响中起着非常重要的作用。

Decadal Variability in the Relationship between May Rainfall over Southwest China and the Arabian Sea Monsoon

The dominant mode of anomalous precipitation in May over Southwest China and its association with the Arabian Sea monsoon are investigated in this study using the 115 observation stations and Japanese 55-year reanalysis for the period of 1960–2017. The result showed approximately consistent variabilities of the leading mode of May precipitation over Southwest China, exhibiting a close relationship with the Arabian Sea monsoon. However, around the late 1970s, the relationship experienced an interdecadal change. In 1960–1976, the anomalous atmospheric circulations and water vapor transportation associated with the Arabian Sea monsoon were mainly over the Arabian Sea to the Bay of Bengal, showing a weak influence on the May precipitation over Southwest China. However, in 1981–2017, the anomalous Arabian Sea monsoon was related to large-scale atmospheric anomalies from the northern Indian Ocean to the South China Sea, leading to anomalous water vapor and vertical motion over Southwest China. Therefore, the Arabian Sea monsoon can significantly influence May precipitation over Southwest China during 1981–2017. Further analysis indicated that the change in the relationship between the Arabian Sea monsoon and May precipitation over Southwest China could be related to the change in decadal variability of the Arabian Sea monsoon. Over the period before the late 1970s, the Arabian Sea monsoon’s variability was relatively weaker, and its related atmospheric circulation anomalies were also weaker. Whereas, after the late 1970s, the Arabian Sea monsoon’ s variability became stronger, and the related atmospheric circulation anomalies extended more eastward, covering Southwest China. Consequently, the Arabian Sea monsoon can significantly influence the May precipitation over Southwest China after the late 1970s. This result indicated the important role of monsoon variability in precipitation.