资源环境科技发展态势分析平台

The East Asian summer monsoon (EASM) shows notable change during the summer after El Nino peak. This delayed response, however, is variable and difficult to predict. Here, we revisit this issue by separating El Nino decays into early transition and late transition. In the summer after an early transition, the central-to-eastern Pacific evolves into a La Nina condition, with positive rainfall anomaly occurring over most parts of eastern China. In contrast, in the summer after a late transition, the central-to-eastern Pacific sea surface temperature (SST)anomaly remains neutral or slightly above normal; correspondingly, the East Asian rainfall anomaly shows a tripolar structure with positive anomaly over the Yangtze-Huaihe River valley and negative anomalies over northern and southern China. These different rainfall responses are mainly related to different locations of the anomalous anticyclone (AAC) over the western North Pacific (WNP): it is centered at (165 degrees E, 25 degrees N) for late-transition El Ninos, but at (135 degrees E, 16 degrees N) for early-transition El Ninos. During the late transition, the AAC-SST feedback, identified by the dipole SST mode consisting of WNP cooling and northern Indian Ocean (NIO) warming, mainly works to support the WNP AAC. During the early transition, the AAC-SST feedback is weak and mainly attributed to NIO warming. The strong easterly anomaly over the western equatorial Pacific, which is tied to the central-to-eastern equatorial Pacific cooling and dipole precipitation pattern from western equatorial Pacific to the Maritime Continent, occurs to support the AAC and pulls it equatorward. These distinct responses exist in the last century, and the CMIP5 models can reproduce these distinct responses well except that the models underestimate the AAC-SST feedback for late-transition El Ninos. The findings in this study help predict the EASM rainfall in post-El Nino years, but the key is the accurate prediction of the timing of decay.