东亚副热带季风特征及其指数的建立

Characteristics of East Asian Subtropical Monsoon Index and Its Definition

基于大气对流活动和非绝热加热的分析 ,给出东亚夏季风区域变化特征 ,客观地确定东亚副热带季风及热带季风对流强度 ,并由此建立其与大尺度季风环流的内在联系 ,提出分别用所对应区域经向风垂直切变来构造季风指数。对比分析显示 ,上述季风指数在反映夏季环流和降水等方面效果显著 ,能很好地刻划东亚夏季风强度。同时指出 ,东亚副热带季风指数与西太平洋副热带高压及长江中下游降水密切相关 ,在空间场上表现出东亚 /太平洋型 (EAP型 )遥相关特征。高指数年副高偏南 ,长江中下游为涝 ;低指数年则相反。

East Asian subtropical monsoon has close relation with rainfall anomaly over the Yangtze River valley and much researches have been done for the exponents about the estimation of its strength. It is known that monsoon area in East Asian, crossing zonally 40～50 latitudes, involves subtropical and tropical systems, therefore, further analysis about the choice of such indices and their features are necessary. This paper, in the first place, has defined convective indices for subtropical and tropical monsoon and set up certain relation between convection and circulation on which the meridional wind vertical shear indices (I es/I ss) in the two regions are based. It is shown that I es is closely related to precipitation in the subtropics while I ss more related to that in the tropics. In addition, I es possesses remarkable interannual change features: more rain corresponding to more intensity, and vise versa. The results indicate that the mean rainfall coefficient of I es and seventeen stations in the middle and lower reaches of the Yangtze River is 0.44, α=0.01 validity check is passable. When I es is higher (lower), subtropical high at 500 hPa is southward (northward) of its normal position and flood (dry) year occurs over the middle and lower reaches of the Yangtze River. Besides, the teleconnection of subtropical monsoon at 500 hPa presents EAP pattern.