热带印度洋—西太平洋水汽输送异常对中国东部夏季降水的影响

Effects of Anomalous Water Vapor Transport from Tropical Indian Ocean–Western Pacific on Summer Rainfall in Eastern China

利用1979~2015年NCEP/NCAR发布的月平均全球再分析资料,分析了热带印度洋—西太平洋水汽输送异常对中国东部夏季降水的影响及其形成机理。研究结果表明:热带印度洋—西太平洋地区(10°S^30°N,60°~140°E)夏季异常水汽输送主要包括两个模态,他们可以解释总的水汽输送异常34%的方差。其中,第一模态(EOF1)表现为异常水汽沿反气旋从热带西太平洋经过南海及孟加拉湾输送到中国东部上空,对应南海、孟加拉湾水汽路径输送均偏多,此时西太平洋副热带高压显著偏强,异常水汽在长江中下游地区辐合并伴随显著上升运动,有利于长江中下游降水偏多;第二模态(EOF2)表现为异常水汽从热带印度洋沿阿拉伯海、印度半岛、中南半岛等呈反气旋式输送,华南上空相应出现气旋式水汽输送异常,并对应异常水汽辐合和上升运动,有利于华南降水偏多。就可能的外部成因而言,EOF1与ENSO关系密切,表现为前冬热带中东太平洋显著偏暖,夏季同期热带北印度洋、南海上空显著偏暖,造成西太平洋副热带高压显著偏强,异常水汽主要来源于热带西太平洋和南海;EOF2与同期热带印度洋偶极子(TIOD)异常有关,TIOD为正位相时热带印度洋上空出现异常东风,华南上空出现异常气旋并伴随水汽异常辐合,异常水汽主要来源于热带南印度洋。

Using monthly reanalysis datasets of NCEP/NCAR for 1979–2015, the impact of anomalous water vapor transport from the tropical Indian Ocean–western Pacific on summer precipitation in eastern China was analyzed in this paper. The results show that the summertime anomalous water vapor transport from the tropical India Ocean–western Pacific（10°S–30°N, 60°–140°E） to eastern China mainly includes two modes, which can explain 34% of the total water vapor transport variance. The first mode（EOF1） shows anomalous water vapor transport from the tropical western Pacific across the South China Sea and the Bay of Bengal to eastern China along the western flank of the anticyclone. The water vapor transport from the South China Sea and the Bay of Bengal both are abnormally high. Meanwhile, the WPSH（western Pacific subtropical high） is significantly stronger than normal, and anomalous water vapor convergence occurs over the middle and lower reaches of the Yangtze River valley accompanied with significant upward movements, which are favorable for precipitation over this region. The second mode（EOF2） shows anomalies of anticyclonic water vapor transport from the tropical India Ocean across the Arabia sea, India Peninsula, and Indo-China Peninsula and anomalies of cyclonic water vapor transport over southern China, which lead to anomalous water vapor convergence and ascending motions over southern China. As a result, more precipitation occurs in southern China. Furthermore, possible external causes for the above two modes were analyzed. Results indicate that the EOF1 is closely related to ENSO. SST（sea surface temperature） over the tropical eastern Pacific in the preceding winter was significantly warmer than normal, while SSTs over the tropical North India Ocean and the South China Sea in the subsequent summer were significantly warmer than normal. These SST anomalies resulted in a significantly strong WPSH and anomalously high water vapor mainly originated from the tropical western Pacific and the South China Sea. The EOF2 is related to the simultaneous positive phase of the tropical Indian Ocean Dipole（IOD）. The anomalous easterly winds appeared over the tropical India Ocean when the IOD was positive phase, which was linked to anomalous cyclonic circulation and moisture convergence over South China. The anomalous water vapor mainly originated from the tropical southern India Ocean.