Modulation of the Relationship Between ENSO and the Intensity of the South China Sea Summer Monsoon by Quasi‐Biennial Oscillation

ABSTRACT

This study investigates how boreal winter equatorial stratospheric quasi‐biennial oscillation (QBO), defined by 70‐hPa zonal winds, modulates the El Niño–Southern Oscillation (ENSO)–South China Sea summer monsoon intensity (SCSSMI) relationship during 1952–2020 using the ERA5 reanalysis dataset. ENSO exhibits a strong correlation with the SCSSMI during the QBO westerly phase (WQBO), whereas their linkage becomes weak during the QBO easterly phase (EQBO). ENSO‐associated sea surface temperature (SST) evolution exhibits markedly different characteristics across contrasting QBO phases. From boreal winter to spring, WQBO suppresses deep convection over the Maritime Continent (MC) during El Niño events, yet this suppression is weak during La Niña events. This yields ENSO‐related Indo‐Pacific SST evolution analogous to that of canonical ENSO events, establishing a tight linkage between central–eastern equatorial Pacific SST in ENSO mature winter and the SCSSMI in subsequent ENSO decaying summer. However, EQBO substantially enhances MC deep convection and drives anomalous tropical zonal circulation over the Indo‐Pacific from winter to spring. Driven by these EQBO‐induced zonal circulation anomalies, anomalous vertical motion over the MC and anomalous surface winds across the Indo‐Pacific reshape ENSO‐related Indo‐Pacific SST evolution. During El Niño decaying summer, only weak warm SST anomalies emerge in the Indian Ocean; by contrast, significant cold SST anomalies prevail across the Indo‐Pacific during La Niña decaying summer. These anomalous SST patterns exert a negligible modulating effect on the SCSSMI, indicating that ENSO exerts a weak control on the SCSSMI during EQBO.