Increased Impacts of ENSO on the Southeast Asian Summer Monsoon under Global Warming and Associated Mechanisms

In this study, the changes in the influence of El Niño–Southern Oscillation (ENSO) on the Southeast Asian summer monsoon (SEASM) during the ENSO decaying summer under global warming are investigated based on 28 coupled general circulation models (CGCMs) from the phase 6 of the Coupled Model Intercomparison Project (CMIP6). The multimodel ensemble mean of CMIP6 models projects an enhanced response of the SEASM to ENSO in a warmer climate, which could be attributed to the weakened warm sea surface temperature (SST) anomaly in the western equatorial Pacific (WEP) during the El Niño decaying summer. Compared with the present climate, in a warmer climate the WEP SST anomaly associated with El Niño decays more rapidly and eventually dissipates from the El Niño mature winter to the following summer, which could intensify the anomalous anticyclonic circulation over the western North Pacific by enhancing the zonal gradient of SST anomaly between the tropical Indian Ocean and the WEP. Further analyses show that the fast decay of WEP SST anomaly is mainly determined by the increased latent heat flux anomaly from the oceans to the atmosphere due to strengthened surface wind speed anomaly. Such a projected change in surface wind speed anomaly in the WEP during the ENSO decaying phase is probably caused by the eastward shifts of both the ENSO-induced anomalous Walker circulation and the associated precipitation anomaly over the equatorial Pacific, resulting from the El Niño-like change in background SST in the equatorial Pacific under global warming.