Causes of Diverse Impacts of ENSO on the Southeast Asian Summer Monsoon among CMIP6 Models

Abstract

Abstract This study examines the fidelity of 47 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) in representing the influence of El Niño–Southern Oscillation (ENSO) on the Southeast Asian summer monsoon (SEASM) during the ENSO decaying summer. The response of the SEASM to ENSO shows a large model spread among the models, some of which even simulate opposite signs of SEASM anomalies compared to the observed values. The bad-performance models (BPMs) are therefore selected to be compared with both the good-performance models (GPMs) and observations to explore the possible causes of the deficiency. Results show that in the BPMs, the ENSO-related warm sea surface temperature (SST) anomalies extend too far westward in the western equatorial Pacific (WEP) and they do not dissipate in the El Niño decaying summer in comparison with those in the GPMs and observations, interfering with the effect of ENSO on the SEASM. The slow decay of WEP SST anomalies from the El Niño mature winter to the decaying summer in the BPMs is mainly caused by a weak negative shortwave radiation feedback due to a low sensitivity of convection to local SST anomalies, 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 a westward current in the BPMs, which also contributes to the slow decay of WEP SST anomalies via inducing excessively persistent warm zonal advection. Significance Statement We investigate the possible causes of the diverse impacts of El Niño–Southern Oscillation (ENSO) on the Southeast Asian summer monsoon (SEASM) among 47 CMIP6 models. We find that a plausible reason for the deficiency of some models in simulating the influence of ENSO on the monsoon is that the sea surface temperature (SST) anomalies associated with ENSO are unrealistic in the western equatorial Pacific (WEP) in these models. Further diagnoses indicate that the unrealistic WEP SST anomalies are related to the cold bias of the climatological SST, which could lead to a weak negative shortwave radiation feedback and excessively persistent warm zonal advection. The information provided in this study is useful for improving the skill of the climate models in representing the ENSO–SEASM relationship.