【摘要】 利用NCEP/NCAR 200 hPa月平均风场再分析资料,定义东亚大陆对流层上层不同经度上最大西风所在位置的平均纬度为东亚副热带西风急流轴线指数,该指数能准确反映东亚副热带西风急流位置的南北变化及其对长江中下游降水的影响,并能较好地体现东亚夏季风盛行期间对流层低层与高层的纬向风场变化特征。分析表明,该指数的时间变化具有与长江中下游夏季降水较一致的年代际变化及年际振荡特征。对东亚副热带西风急流位置异常年的大气环流差异分析表明,急流异常偏北时,南亚高压偏弱,位置偏北偏西,呈伊朗高压型;西太平洋副热带高压(下称西太副高)偏弱、位置偏东偏北;气流的辐合上升区北移至华北一带,而长江流域低层风场为辐散异常,上升气流较常年偏弱,降水偏少。急流异常偏南时,南亚高压偏强,位置偏南偏东,呈青藏高压型;西太副高偏强、位置偏西偏南;长江流域地区上空低层有较强辐合上升气流,高层有较强的气流辐散,对流旺盛,雨带在此维持,容易引发洪涝。更多还原

【Abstract】 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.更多还原