Fading influence of El Niño-Southern Oscillation on East Asian Summer Monsoon Northern Boundary after the late-1980s

This study reveals that the linkage between the El Niño-Southern Oscillation (ENSO) and the northern boundary of the East Asian summer monsoon (EASM) has experienced a marked interdecadal weakening since the late 1980s. We further explore the underlying mechanisms of this interdecadal transition, emphasizing the role of Indian Ocean sea surface temperature (SST) anomalies. Before the late-1980s, ENSO-induced warming of the Indian Ocean—driven by atmospheric teleconnections and ocean-atmosphere interaction­­­—­­suppressed Indian summer monsoon rainfall (ISMR) via enhanced convective heating and a strengthened Hadley circulation. The resulting decrease in ISMR triggered a negative-phase Silk Road Pattern (SRP), leading to a southward shift of the EASM northern boundary and a decline in precipitation over the monsoon transition zone. After the late 1980s, concurrent cold SST anomalies in the tropical North Atlantic suppressed the ENSO-driven Indian Ocean warming by enhancing easterly winds, increasing cloud cover, and reducing downward shortwave radiation. This weakened the associated Hadley circulation and SRP response, thereby diminishing the influence of ENSO on the monsoon boundary. The proposed mechanism is further supported by numerical experiments conducted with the atmospheric general circulation model.