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《Chinese Journal of Atmospheric Sciences》 2004-05
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The Relationship between East Asian Summer Monsoon Activity and Northward Jump of the Upper Westerly Jet Location

Li Chongyin 1), Wang Jough-Tai 2), Lin Shi-Zhei 2), and Cho Hun-Ru 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)(Institute of Atmospheric Physics, Taiwan Central University, Chungli 32054)  
It is studied by using NCEP/NCAR reanalysis data (1980-1999) that the relationship between the northward jumps of the upper westerly jet location and two important events of the East Asian summer monsoon activity the onset of summer monsoon over the South China Sea(SCS) and the beginning of the Meiyu rainfall in the Changjiang River and the Huaihe River basins. The systematic analyses show that the upper westerly jet has northward jump phenomenon for two times during the transforming process from winter to summer and they are closely related to summer monsoon activities. The northward jump of the upper westerly jet in East Asia for the first time (jet location from 25-28°N to the north of 30°N) occurred about on 8 May on average, it is about 7 days earlier than the onset date of the SCS summer monsoon (mean date is 15 May). The northward jump of upper westerly jet in East Asia for the second time (jet location from 32°N to the north of 35°N) occurred about on 7 June on average, it is about 10 days earlier than the beginning date of the Meiyu rainfall in the Changjiang River and the Huaihe River basins and can be the forewarning of Meiyu rainfall beginning. This two-time northward jumps of the upper westerly jet location in East Asia are related to two-time reverses of meridional temperature gradient in the upper-middle troposphere (500-200 hPa). During the seasonal transforming the continent was heated quickly, so that the meridional temperature gradient in the upper-middle troposphere will be reversed in 5-25°N in South Asia. Then through the geostrophic adjustment, the flow field adjusts to the pressure field (temperature field) and it will lead to the northward jumps of the upper westerly jet location. The analyses also show that sometimes the enhancement and northward moving of the upper westerly jet in the southern hemispheric subtropics can also influence the first northward jumps of the upper westerly jet location in East Asia.
【Fund】: 国家自然科学基金资助项目 40 2 3 3 0 3 3;; 中国科学院创新项目ZKCX2 SW 2 1 0共同资助
【CateGory Index】: P425.42
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