Enhanced impacts of ENSO on the commencement of tropical Asian summer monsoon in May after 1976/77

The commencement of the tropical Asian summer monsoon (TASM) in May is a crucial phase in its seasonal evolution, with critical implications for agriculture and water resources. Based on observational and reanalysis data, this study finds that the relationship between El Niño-Southern Oscillation (ENSO) and monsoon commencement experiences a notable interdecadal strengthening after 1976/77. While the tropical tropospheric temperature response to ENSO remains largely unchanged, ENSO induces a stronger Walker circulation, a more pronounced equatorial Rossby wave, an intensified extratropical Rossby wave train after 1976/77. These enhanced atmospheric processes, which directly reinforce the ENSO–TASM onset relationship, are likely driven by interdecadal shifts in the structure and variance of ENSO. Post-1976/77, ENSO displays increased variance and a more coherent structure, with more pronounced sea surface temperature anomalies in the western North Pacific and subtropical North Pacific. Given the limitations of observational data, a 1000-year piControl experiment further confirms the role of ENSO variance changes in strengthening its influence on monsoon commencement. Our findings underscore the critical influence of evolving ENSO characteristics on climate anomalies such as monsoon commencement, offering potential insights for short-term climate prediction.