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Characteristics of East Asian Subtropical Monsoon Index and Its Definition

Zhou Bing 1), He Jinhai 2), Wu Guoxiong 1), and Han Guirong 2) 1) (State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029) 2) (Nanjing Institute of Meteorology, Nanjing 210044)  
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.
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