Causes of the Diverse Impacts of ENSO on the Southeast Asian Summer Monsoon: Emphasis on Physical Processes

ENSO affects the Southeast Asian summer monsoon (SEASM) in different ways, involved in different physical processes. The response of the SEASM to ENSO shows a large spread among the 47 CMIP6 models analyzed. Good-performance models capture many observed features of the ENSO’s impact on the SEASM, while some bad-performance models (BPMs) even simulate opposite signs of the SEASM anomalies related to ENSO compared to the values in observations and good-performance models. The main problem of the BPMs is that the El Niño-related warm SST anomalies extend too far westward in the western equatorial Pacific (WEP) and they do not dissipate in the El Niño decaying summer as observed, interfering with the effect of ENSO on the SEASM. This feature strongly suggests the importance of the WEP SST anomalies for ENSO-monsoon interaction. A mixed-layer heat budget analysis indicates that the slow decay of WEP SST anomalies from the El Niño mature winter to the decaying summer in BPMs is mainly caused by a weak negative shortwave radiation feedback due to a low sensitivity of precipitation to local SST changes, which is related to the cold bias in climatological SST over this region. On the other hand, from the mature winter to the decaying summer of El Niño, the El Niño-related anomalous eastward current does not reverse to westward current because of the weak El Niño discharge process, which could be attributed to the weak westerly wind anomalies associated with El Niño. The prolonged anomalous eastward current thus also contributes to the slow decay of WEP SST anomalies via inducing excessively persistent warm zonal advection in the WEP.