A new study of El Niño impacts on summertime water vapor transport and rainfall in China

Based on observed precipitation data and multiple reanalysis datasets from 1961 to 2016, different impacts of eastern Pacific (EP) El Niño and central Pacific (CP) El Niño events on summertime moisture transport and rainfall in China have been investigated. Results suggest that:(1) the impacts of El Niño events on summertime rainfall in China are significantly different in the El Niño developing summer and decaying summer. The main impacts occur in the El Niño-decaying summer when obvious rainfall signals are found in most areas; (2) in the summer of EP El Niño developing, the western North Pacific (WNP) subtropical high (SH) tends to retreat eastward and becomes weak. The weak moisture transport conditions are not conductive to the occurrence of rainfall in most areas. In the summer of CP El Niño developing, evaporation over the western Pacific Ocean and low latitudes of the Indian Ocean becomes stronger. Abundant moisture originating from the Arabian Sea, the Bay of Bengal and the western Pacific Ocean is transported to South China and the net moisture income increase, tending to induce positive rainfall anomalies in South China; (3) in the El Niño decaying summer, extra tropical Pacific evaporation increases significantly. The westward expansion of the WNPSH and the establishment of East Asia/Pacific (EAP) teleconnection are favorable for the southwesterly winds along the western side of the WNPSH to carry abundant water vapor from the Pacific to central-eastern China. Meanwhile, the double-blocking set up in Eurasia high-mid latitudes and the northerly flow between the low trough and upstream blocking high tend to carry more water vapor from the Arctic Ocean to Northwest China and northern part of North China. The positive net moisture income anomalies are found in most areas in China, providing favorable moisture conditions for rainfall. Two rain belts form in southern and northern China, respectively. In the CP El Niño decaying summer, the WNPSH tends to extend westward and northward, which is favorable for the transport of water vapor originating from the strong Pacific evaporation to Jianghuai region, leading to increases in net moisture income in Jianghuai region. Hence, El Niño events not only influence rainfall anomalies in Yangtze River valley and Huai River valley, but also influence rainfall anomalies in North China, Northeast China and Northwest China.