El Ni?o对东亚夏季风和夏季降水季节内变化的影响

基于1979~2012年候平均再分析资料,合成分析了El Ni?o对东亚夏季风和夏季降水季节内变化的影响。结果表明,在El Ni?o衰减年夏季,西太平洋副热带高压(副高)明显偏强,位置偏向西南。副高的这种异常特征随夏季的季节进程有明显变化,初夏异常较弱,盛夏期间异常达到最强。此外,根据东亚夏季风降水呈现阶段式北进的特征,将夏季分为华南前汛期、江淮梅雨期、华北和东北雨期以及华南后汛期来分析东亚夏季风和降水的季节内变化。在上述各个时期,大气对流层低层表现为一致的环流异常型,副高及其以南区域为异常反气旋,其北部为异常气旋。这种异常环流型加强了副高南部偏东风及其北部偏北风,增强了热带水汽输送和高纬度地区冷空气的入侵,二者结合造成主汛期地区降水增加。需要强调的是,上述环流异常型随东亚夏季风逐步向北推移,导致东亚各地区的主汛期降水增加,非主汛期降水减少,降水分布更为集中。

The El Niño Influence on Intra-seasonal Variations of East Asian Summer Monsoon and Summer Rainfall

The influence of El Niño on the intra-seasonal variations of East Asian summer monsoon (EASM) and summer rainfall is studied based on pentad mean reanalysis data during 1979-2012. It is shown that, during the El Niño decaying summer, the western Pacific subtropical high (WPSH) tends to extend more southwestward with a stronger intensity. The anomaly enhances with the seasonal march, with a weaker anomaly in the early summer and the anomaly reaches its maximum in the late summer. Based on the feature that summer rainfall in East Asia tends to advance northward in a stepwise way, we divide the major rainy period into four periods, i.e., the pre-flooding period in southern China, the Meiyu period over Yangtze-Huaihe River basin, the rainy period in northern and northeastern China, and the post-flooding period in southern China, and analyze the intraseasonal variations of the EASM and summer rainfall. During the above four periods, there appears a consistent anomalous circulation pattern in the lower troposphere. There is an anomalous anticyclone in the southern flank of the WPSH and a cyclonic anomaly in the northern flank, respectively. As a result, the moisture transport from the tropics and the cold air from high latitudes both intensify due to the enhancement of the easterlies to the south and the northerlies to the north of the WPSH, leading to more rainfall over the major rainy region. It is important to note that this anomalous pattern tends to move northward with the EASM, resulting in more rainfall during the major flooding period and less rainfall in other periods over each individual region of East Asia.