2016年夏季我国东部降水异常特征及成因简析

摘要

提2016年夏季（6—8月），我国东部降水呈南、北两条多雨带，长江中下游和华北大部降水均较常年同期明显偏多。其中，6—7月的降水主要发生在长江流域，而8月发生显著转折，除了华南地区降水偏多外，我国东部大部地区降水都较常年同期明显偏少。6—7月长江流域的降水偏多主要是受到偏强、偏西的西太平洋副热带高压（以下简称副高）的影响，副高脊线位置总体接近常年，但南北摆动较大，阶段性偏南对应了长江流域降水明显偏多的时段。同时，菲律宾附近低层异常反气旋环流导致来自副高西侧的水汽通量异常辐合区主要位于长江中下游。热带印度洋全区一致暖海温在超强El Ni珘no衰减年的持续发展是导致上述环流异常的重要外强迫因子。8月，副高发生断裂，西北太平洋对流层低层转为异常气旋性环流控制，水汽输送异常辐散区控制我国东部大部地区，长江流域持续高温少雨。8月的热带大气季节内振荡（Madden-Julian Oscillation，MJO）活动偏强，MJO东传至西太平洋并持续长达25 d，为历史少见。异常的MJO活动是导致8月热带和副热带大气发生转折的重要原因。%Anstract:Two rainfall belts were observed over eastern China during the summer of 2016,with the south-ern one in the Yangtze River Valley (YRV)and the northern one over North China.In June and July,pre-cipitation occurred mainly in the YRV.In August,however,precipitation was below normal in most part of eastern China,except for more precipitation over South China.The more intensified and more west-ward-extending west Pacific subtropical high (WPSH )was one of the most important circulation factors for the southern rainfall belt over the YRV in June and July.The mean position of the high ridge was near normal.However,the high ridge retreated southward sometimes,causing more precipitation over the YRV during that time.The associated anomalous low-level Philippine Sea anticyclone resulted in anoma-lous convergence of moisture flux over the middle and lower reaches of the YRV.Moreover,the persistent warming in the entire basin of the tropical Indian Ocean during the decaying year of extremely strong El Niñno event acted as the important external-forcing factor for the above tropical and subtropical circulation anomalies.In August,however,the WPSH fractured.Anomalous low-level cyclonic circulation controlled the northwestern Pacific,which caused anomalous divergence of moisture flux over most part of eastern China.Therefore,high temperature and little precipitation occurred in the middle and lower reaches of the YRV.Further analysis indicated that the unusually active Madden-Julian Oscillation (MJO)in August was the important cause for the significant turning of the tropical and subtropical circulations.MJO transmitted eastward to the western Pacific and stayed there for 25 days,which is seldom seen in history.