The Principal Feature of QinghaiXizang Plateau Monsoon Variation in 40 Years

The daily monsoon index of QinghaiXizang Plateau during 1961—1995 is calculated, the start and end time of summer monsoon of QinghaiXizang Plateau are preliminarily determined. Based upon that, we investigate the interannual variation features of the monsoon index and its relationship with the 500 hPa geopotential height fields and monsoon of East Asia. The results are as follows: The anticorrelation exists in the start and the end time of summer monsoon; The interannual and interdecadal variation of Plateau monsoon is obvious; The strong(weak) winter Plateau monsoon is contemporarily related to the high (low) 500 hPa geopotential height fields over QinghaiXizang Plateau and the Ural mountains and to the strong(weak) winter monsoon of East Asia; The strong(weak) summer Plateau monsoon is contemporarily related to the low(high) 500 hPa geopotential height fields over the Baykal lake and south of QinghaiXizang Plateau and to the high(low) 500 hPa potential height fields over West Asia and East China and also to the strong(weak) summer monsoon of East Asia.