中文摘要：

主要分析了1951～2004年夏季亚洲极涡强度和面积的长期变化趋势及其对东亚夏季环流,水汽输送和降水量的影响,发现1951～2004年,夏季亚洲极涡表现出了明显的强度减弱,面积缩小的变化趋势,并以面积缩小更为显著,这正对应于北极涛动（AO）指数在该时段的显著升高.在这种北半球中高纬大尺度环流变化的影响下,东亚夏季高空西风急流在近54年显著南移,冷空气活动的南侵程度明显增强,从而造成低空偏北风显著增强而偏南风减弱.与此相应,近54年整个中国区域内低空纬向风速呈明显的减小趋势.总的来看,东亚夏季风环流发生了明显减弱.同时,流经中国的中纬度西风水汽输送在近54年也表现出一致减弱的趋势,而南风水汽输送大致以110°E为界,以东的夏季风区呈显著的减弱趋势而以西则有明显的增加趋势.这种水汽输送的变化影响了中国不同区域内水汽输送通量散度的改变,进而使得夏季降水量发生变化.分析表明,夏季亚洲极涡的面积和强度与东北、华北和西北东部的水汽输送通量散度和夏季降水呈正相关,而与长江中下游、华南、西南、青藏高原和西北西部呈显著负相关,夏季亚洲极涡在近几十年的面积缩小和强度减弱是中国夏季降水长期变化的一个可能原因.

英文摘要：

The long-term trend of the intensity and the extension of the summer Asian polar vortex during 1951 to 2004 is analyzed and its relationships with the circulation system,the water vapor transport and rainfall amounts of the East Asian summer monsoon are discussed.The main results are shown as follows：both the intensity and the extension of the summer Asian polar vortex show significant decreasing trend from 1951 to 2004,with the change in the extension being more obvious.And the trend is consistent with the significant increase of the AO index.By the impacts of the change of the large-scale circulation in the mid-high latitude,the location of the upper-level westerly jet shifted significantly southward,the southward invasion of the cold airs intensified greatly,the low-level northerly strengthened while the southerly weakened obviously and the zonal wind speed decreased to some degree in recent 54 years,presenting the obvious weakening of the East Asian summer monsoon.At the same time,the westward water vapor transport weakened consistently over China.And the southward water vapor transport showed opposite trends in the west part and the east part of China,taking about 110°E as the boundary,with the increasing trend in the west while the deceasing trend in the east.Changes in the water vapor transport affected the convergence and divergence of the water vapor in different regions of China,and then resulted in the change of the summer rainfalls.Further analyses present that these changes in the water vapor transport and its convergence and divergence are related closely to the weakening and area decrease of the summer Asian polar vortex.The summer Asian polar vortex has an important impact on the summer rainfalls in China through the circulation and the water vapor transport of the East Asian summer monsoon and its long-term change is a possible cause of the change of summer rainfalls in China.