东亚夏季经向水汽输送的变异及其对极端降水的影响

东亚夏季降水的异常与水汽输送的变异密切相关。基于1958—2016年资料，研究了夏季东亚季风区经向水汽输送的主要变异特征及其对东亚夏季极端降水的影响。经向水汽输送的第一主变异模态表现出中国东部和西北太平洋上的水汽经向输送呈现反向异常，以年际变化为主。当中国东部向北输送的水汽增强(减弱)而西北太平洋向北输送减弱(增强)，则中国东部大范围的极端降水量及频次增加(减少)。该模态与西太(西太平洋)副高西伸(东撤)有关，并主要受到热带中东印度洋海温的影响。第二变异模态以年代际变化为主兼有年际变化，表现在1980年后中国东部及邻近海域上空的经向水汽输送减弱，使得环渤海地区和华南沿海的极端降水量及频次减少而长江上、下游和贵州的极端降水量及频次增加。该模态与西太副高的减弱有关，并受到热带西太海温年代际增温的影响。第三变异模态以年际变化为主兼有年代际变化，反映中国长江以北地区和日本南部及附近区域的经向水汽输送的反相变化结构。长江以北水汽输送减弱(增强)，可导致华北、东北的极端降水量及频次减少(增加)和长江下游及江南地区的极端降水量及频次的减少(增加)。该模态主要受欧亚大陆上空中高纬度纬向遥相关波列和热带印太（印度洋太平洋）海温异常的影响。 

VARIATION OF SUMMER MERIDIONAL WATER VAPOR TRANSPORT OVER EAST ASIA AND ITS IMPACT ON EXTREME PRECIPITATION

The abnormality of summer precipitation over East Asia is closely associated with anomalous water vapor transport. Based on the datasets from 1958 to 2016, this study investigates the variation features of meridional water vapor transport (MWVT) over East Asian monsoon region and its influence on the extreme precipitation over east China. Three leading anomalous patterns are identified by using the empirical orthogonal function (EOF) method. The first leading EOF mode reveals the inverse variation of MWVT over east China and over the western North Pacific, and the variation is mainly interannual time-scale. When MWVT is enhanced or weakened over east China and weakened or enhanced over the western Northern Pacific, the extreme precipitation rate, and the frequency over a large portion of east China increases or lessens. This mode is associated with the zonal shift of the western North Pacific subtropical high, which is attributed to the sea surface temperature over the tropical central-eastern Indian Ocean. The second EOF mode displays a significant interdecadal change around 1979/1980, and shows that MWVT is suppressed over east China and its adjacent marine area after 1980, leading to a decrease of extreme precipitation and its frequency over south China and around the Bohai Sea, but an increase over the upper and lower reaches of the Yangtze River and Guizhou Province. It is closely related to the intensity weakening of the western North Pacific subtropical high, associated with an interdecadal warming over the tropical western North Pacific. The third anomalous mode presents dominant interannual and minor interdecadal changes and shows an inverse change of MWVT over regions north of the Yangtze River and south Japan. The weakened MWVT over north China may give rise to less extreme precipitation and its frequency over north China and northeast China, and more over the lower reaches and the south of the Yangtze River. This mode is attributed to the impacts of a zonal Eurasian teleconnection over the mid-high latitudes and the sea surface temperature over the tropical Indo-Pacific Oceans.