Modulation of Atlantic Multidecadal Oscillation on ENSO-East Asian early summer monsoon connection: role of a tropical pathway

Using observational datasets and model simulation outputs, we investigate the impact of the Atlantic Multidecadal Oscillation (AMO) on the interannual relationship between the El Nino-Southern Oscillation (ENSO) and East Asian early summer monsoon (EAESM). We find that the AMO can modulate the interannual ENSO-EAESM relationship via a tropical pathway by affecting the ENSO amplitude, specifically, the ENSO is stronger during the negative AMO phase, which favors a close ENSO-EAESM relationship, while the ENSO is weaker during the positive AMO phase, which may lead to an insignificant relationship with the EAESM. The AMO-related Atlantic sea-surface temperature anomalies (SSTAs) are conducive to an anomalous Walker circulation, causing trade wind anomalies over the equatorial Pacific, disturbing the thermocline, and finally affecting ENSO amplitude by altering the equatorial Pacific upper-ocean stratification. The amplitude of the ENSO further determines the ENSO-EAESM correlation. The CESM1 large ensemble results of idealized experiment confirm the observational results. The climate models of the Coupled Model Inter-comparison Project Phase 6 (CMIP6) exhibit large inter-model spread in representing the AMO's modulation on the ENSO-EAESM relationship. This is primarily due to their varying performances in simulating the relationship between the AMO and ENSO amplitude through the inter-basin Atlantic-Pacific Ocean coupling capacity, that is, whether the related SSTAs can trigger the Bjerknes feedback over the Pacific Ocean in the model.