Intensifying intraseasonal oscillation of South China Sea winter upper-layer circulation in a warming climate

Abstract The upper-layer circulation in the South China Sea (SCS) exhibits significant intraseasonal oscillation (ISO), influencing the regional ocean environment and climate. However, the evolution of this ISO under global warming and its underlying mechanisms remain poorly understood. Using a high-resolution Community Earth System Model, this study identifies a strengthening trend of 7.8% in the ISO intensity of the western boundary current (WBC) speed during 1901–2100. Further analysis reveals a 24% intensification of oceanic stratification, while basin-scale wind stress and its ISO show weakening trends of 10% and 18%, respectively. Quasi-geostrophic model experiments demonstrate that the ISO enhancement is primarily driven by intensified upper-ocean stratification, with the weakening wind field exerting a dampening effect. This amplified upper-layer circulation ISO is closely linked to increased sea surface temperature (SST) variability, suggesting higher intensity and frequency of marine heatwaves in the SCS. Furthermore, enhanced SST variability may amplify intraseasonal ocean heat release, potentially altering regional climate. This study elucidates the intensification mechanism of upper-ocean ISO in the SCS under global warming and its climatic impacts, providing new insights into the response of tropical marginal sea climate variability to a warming climate.