Modulation of quasi-biennial oscillation on the relationship between ENSO and the South China Sea summer monsoon

In this study, the modulation of the equatorial stratospheric quasi-biennial oscillation (QBO) in winter on the impact of the El Niño–Southern Oscillation (ENSO) on the South China Sea summer monsoon intensity (SCSSMI) is investigated during 1952–2020. In the QBO westerly phase (WQBO), ENSO is closely correlated with the SCSSMI, while the relationship is weak in the QBO easterly phase (EQBO). The evolution of ENSO-related sea surface temperature (SST) exhibits distinctive features under different QBO phases. During winter to spring, WQBO suppresses deep convection over the Maritime Continent (MC) during El Niño events, but the suppression effect is weak during La Niña events, which results in the ENSO-related SST evolution in the Indo-Pacific Ocean similar to the canonical ENSO events and a close relationship between SST in the equatorial central and eastern Pacific in ENSO mature winter and the SCSSMI in ENSO decaying summer. While EQBO significantly enhances deep convection over the MC and causes anomalous tropical zonal circulation over the Indo-Pacific during winter to spring. Related to the anomalous zonal circulation over the Indo-Pacific during EQBO, anomalous vertical motion over the MC and anomalous surface wind in the Indo-Pacific modulate the ENSO-related SST evolution in the Indo-Pacific, inducing insignificant warm SST anomalies in the Indian Ocean during the El Niño decaying summer and significant cold SST anomalies in the Indo-Pacific during La Niña decaying summer. These abnormal SST patterns produce negligible influence on the SCSSMI, implying that ENSO exerts weak impact on the SCSSMI during EQBO.