西风季风协同作用对云南春末5月降水的影响

基于CN05.1降水资料和ERA5再分析资料，采用多种统计方法对1961~2020年云南春末5月降水的时空变化特征和环流形势进行了分析，并利用西风季风协同指数讨论了西风季风协同作用对云南春末5月降水的影响。结果表明:云南5月降水是春季各月降水量最多月份，其空间上主要表现出自南向北减少的特征。此外，云南5月降水还存在明显的年际变化，但在空间上其气候趋势并不显著。相对于单一西风指数和季风指数，西风季风协同指数对云南5月降水具有更好的指示性，二者之间的相关性以及协同指数与极端降水间的同号率更高。环流场合成分析也表明，当西风季风协同作用增强（减弱）时，受阿拉伯海异常高（低）压影响，云南上空气流以辐散（辐合）为主，有利于上升（下沉）运动，而中低层孟加拉湾主要为异常的气旋（反气旋）控制，促进（抑制）西南水汽向云南输送，造成降水偏多（偏少）。 

Influence of the Synergy between Westerly Wind and Monsoon on Late Spring Precipitation over Yunnan Province

Based on CN05.1 precipitation dataset and ERA5 reanalysis, the spatiotemporal variation characteristics and circulation feature of late spring (May) precipitation in Yunnan during 1961–2020 are analyzed by using various statistical methods, and the influence of the synergy between westerly wind and monsoon on late spring precipitation is discussed by using the synergy index. The results show that: In Yunnan, May is the month with the most precipitation in spring with a spatial distribution of precipitation decreasing from south to north. In addition, the precipitation shows a pronounced interannual variation, while the spatial climate trends are not significant. In comparison with the individual westerly wind and monsoon indexes, the westerly-monsoon synergy index has a better indication for May precipitation in Yunnan, the correlation and the same sign rate between the synergy index and extreme precipitation are higher. Composite analysis of circulation field also implies that when the westerly-monsoon synergy effect enhances (weakens), the air flow over Yunnan is dominated by divergence ( convergence ) due to the influence of the abnormal high ( low ) pressure in the Arabian Sea, which is conducive to ascending (descending) motions. Meanwhile, cyclonic (anticyclonic) anomalies dominate over the Bay of Bengal at the middle and lower troposphere, strengthening (weakening) the water vapor transport towards Yunnan, and eventually causing more (less) precipitation in Yunnan.