ENSO-driven Variability in Winter Shelf Circulation of the Northern South China Sea

In this study, we integrate long-term observational data from 1999 to 2021 with numerical simulations to investigate the interannual variability of shelf circulation in the Northern South China Sea (NSCS) during the winter season and its connection to the El Niño-Southern Oscillation (ENSO). Our findings suggest that the eastward shift of the warm pool in the tropical Pacific Ocean, characteristic of El Niño events, not only results in colder sea surface temperatures and enhanced Kuroshio intrusion in the eastern South China Sea but also significantly influences the formation of a distinct subtropical high over the NSCS. During El Niño years, an anomalous anticyclonic wind stress curl in the South China Sea (SCS) is observed, which contributes to the weakening of the northeasterly winter monsoon. The southwesterly local wind stress anomaly over the shelf then exerts a collective influence on the temperature and salinity variations in the NSCS. Our analysis, based on the along-isobath depth-integrated vorticity equation, indicates that the cross-isobath water exchange over the shelf is primarily regulated by the nonlinear advection of relative vorticity, while wind stress curl and bottom stress curl play a secondary role in controlling the structure of the cross-isobath exchanges. The joint effect of baroclinic and relief is likely to dominate the dynamics over the slope.