Diverse Impacts Of ENSO On Southeast Asia Rainfall

In this study, the diverse influences of the El Nino Southern Oscillation (ENSO) on wintertime precipitation over Southeast Asian and China are investigated using observational data. The ENSO events are divided into four categories:strong El Nino, strong La Nina, moderate El Nino and moderate La Nina events according to their amplitudes. The main results are summarized as follows.Significant negative Southeast Asia precipitation anomalies are observed during strong El Nino events whereas wet conditions occur during strong La Nina events. In the case of moderate La Nina events, the Southeast Asia rainfall anomalies are similar in spatial structure and comparable in amplitude to strong La Nina events whereas almost no significant rainfall anomalies are observed in moderate El Nino years. Significant precipitation anomalies can be observed during strong El Nino over Southeast China, while no significant rainfall anomalies are noticed during the other three categories of ENSO events.Further analysis shows that strong El Nino (strong/moderate La Nina) events are associated with significant anticyclone (cyclone) surface response over the western tropical Pacific and related to significant descending (ascending) branch of the Walker circulation around the Philippines. In contrast, the local negative sea surface temperature anomalies (SSTA) over the western tropical Pacific and the descending branch of the walker circulation around the Philippines during moderate El Nino events are quite weak which can not cause significant reduced convection over there and therefore the anomalous precipitation response is quite weak. Besides,the pronounced asymmetric Southeast Asian rainfall response to moderate El Nino and moderate La Nina events appears only after 1980s. The tropical Pacific SSTA and the related changes of the Walker circulation during moderate El Nino events is the weakest among the four categories of ENSO events and it becomes even weaker after 1980s. In contrast the tropical Pacific SSTA and the related changes of the Walker circulation during both strong and moderate La Nina events is obviously increased after 1980s causing a signicant positive anomalous precipitation response over Southeast Asia after 1980s.In addition, there is large case-to-case variance of the tropical Pacific SSTA associated with moderate El Nino events which also contributes to weak Maritime Continent rainfall response in the composite analysis.There is a significant asymmetry in the time evolution of the Southeast Asia precipitation of four categories of ENSO events. During strong La Nina events, the Southeast Asia rainfall anomalies persist through the following year and often reintensify in the subsequent winter while the related Southeast Asia rainfall anomalies decay rapidly by next summer during the other three categories of ENSO events. Besides, positive rainfall anomalies are observed over the Southeast Asia after the decaying phase of moderate El Nino events while no significant negative Southeast Asia rainfall anomalies can be observed after the moderate La Nina events decay. Comparing with the strong El Nino events, the westward displacement of atmospheric deep convection anomalies during strong La Nina enables surface winds in the western equatorial Pacific to be more affected by remote forcing from the central and eastern equatorial Pacific Ocean, which acts to the persistence of the negative Southeast Aisa Pacific rainfall anomalies. In contrast, no obvious westward displacement of atmospheric deep convection anomalies can be observed during moderate La Nina events, leading to the terminate of the positive Southeast Asia rainfall anomalies during moderate La Nina events.