Plausible impact of ENSO on the enhanced correlation of the onset date between the Bay of Bengal and South China sea summer monsoon

Evidence shows that the correlation between the Bay of Bengal summer monsoon (BOBSM) and the South China Sea summer monsoon (SCSSM) onset experiences an interdecadal enhancement around the late 1970s. The potential impacts of the El Nino-Southern Oscillation (ENSO) on this interdecadal change in BOBSM-SCSSM correlation is investigated in this study. In 1980-2018, the ENSO-associated large-scale anomalous circulations, which include the dipole pattern of anomalous geopotential height over the South Asia accompanied by anomalous zonal wind at upper level, as well as the local vertical movement anomaly at middle level and the anomalous geopotential height at lower level, can widely cover the Bay of Bengal (BOB) and South China Sea (SCS) and improve the consistency of BOBSM and SCSSM onset. The observational analysis and numerical experiments both suggest that the stronger anomalous Walker circulation induced by ENSO in 1980-2018 has a more effective modulation on the onset dates of both BOBSM and SCSSM than that in 1958-1979. Moreover, the sea surface temperature (SST) anomalies over the South Indian Ocean (SIO) triggered by ENSO drive a large-scale anomalous cross-equational meridional circulation and thus influence the onset of BOBSM and SCSSM more synchronously in 1980-2018. This study focuses on the impacts of ENSO on the correlation between BOBSM and SCSSM onset, helping improve our understanding of the Asian summer monsoon onset and the relationship among its subsystems.