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Impact of the Position Abnormalities of East Asian Subtropical Westerly Jet on Summer Precipitation in Middle-Lower Reaches of Yangtze River

KUANG Xue-yuan, ZHANG Yao-cun(Department of Atmospheric Sciences,Nanjing University,Nanjing 210093,China)  
An axis index of the East Asian subtropical westerly jet is defined as the mean location(latitude) of the 200 hPa maximum westerly at various longitudes over East Asia using the NCEP/NCAR monthly reanalysis data.The indexcan reasonably reflect the interannual variation of the meridional migration of the East Asian subtropical westerly jet and its impact on the precipitation in the middle-lower reaches of the Yangtze River.Moreover,the index has good correspondence with the East Asia summer monsoon and its interdecadal and interannual changes are consistent with the variations of the rainfall in the middle-lower reaches of the Yangtze River.Comparisons of the circulation differences of the East Asian westerly jet position anomalousyearsshow that when the jet is abnormally northward,the intensity of South Asian high is weaker and the location is far north-westward;and the intensity of the western Pacific subtropical high is weaker and its position is far east-southward in comparison with normal condition.As a result,the anomalous convergence areas of air flows migrate to the regions over the North China,which results in more precipitation there.While the jet is abnormally southward,the South Asia high is too strong and far south-eastward,and the western Pacific subtropical high is stronger and far west-southward,resulting in stronger convergent and ascending flows over the Yangtze River valley,which can easily lead to heavy precipitation and floods.
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