Latitudinal dependence of circulation seasonality in the South China Sea and its response to ENSO

Abstract

Surface circulation in the South China Sea (SCS), primarily driven by water exchange through the Luzon Strait and regional wind forcing, exhibits a strong seasonal cycle, typically intensifying in winter and weakening in summer. However, this seasonality varies significantly across the basin, reflecting complex interactions between local dynamics and external forcing. Using satellite altimetry and numerical simulations, this study identifies a latitudinal dependence in the timing of surface circulation transitions. From south to north, the decay phase, when mean kinetic energy declines from its seasonal peak, becomes progressively longer, while the growth phase shortens. Energy budget analysis reveals that in the northern SCS, mean kinetic energy is sustained longer due to joint contributions from local wind power and external kinematic energy (KE) input. In contrast, the southern SCS experiences a rapid drop in KE, driven primarily by a sharp decline in wind power. This spatial pattern also varies interannually, modulated by the El Niño–Southern Oscillation (ENSO). In the south, decay phase duration is positively correlated with ENSO strength, largely due to ENSO-driven variations in wind stress. In the north, ENSO influences wind stress and Kuroshio intrusion in opposite ways, resulting in a negative correlation between ENSO and decay time. These findings enhance our understanding of how large-scale climate variability modulates marginal sea circulation and offer new insights for improving regional ocean modeling.