Impacts of the long-term change of the summer Asian polar vortex on the circulation system and the water vapor transport in East Asia
夏季亚洲极涡的长期变化对东亚环流和水汽收支的影响

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.