Interannual variability of summertime cross-isobath exchanges in the northern South China Sea: ENSO and riverine influences

Abstract. This study investigates the interannual variability of summer shelf circulation in the Northern South China Sea (NSCS) from 2000 to 2022, combining long-term observations and high-resolution simulations. We elucidate the responses of NSCS shelf circulation to ENSO and Pearl River Estuary (PRE) freshwater runoff, revealing distinct spatial and mechanistic signatures. During El Niño years, a pronounced sea level anomaly dipole forms between the central and southern South China Sea, intensifying northward geostrophic currents in the southern basin and modulating Kuroshio intrusion. Simultaneously, an amplified PRE plume extends eastward to the 100 m isobath, markedly reducing nearshore salinity. Analysis of depth-integrated vorticity equations indicates that the pressure gradient force—driven by the joint effect of baroclinicity and bottom relief (JEBAR) and bottom pressure gradients—governs NSCS circulation variability. In coastal regions, cross-isobath velocity anomalies are primarily controlled by bottom stress curl and nonlinear vorticity advection, whereas JEBAR dominates offshore dynamics beyond the 100 m isobath. During El Niño summers, bottom density anomalies generate positive cross-isobath velocity anomalies through JEBAR, partially offset by negative anomalies from altered vertical stratification, sustaining a meandering shelf current. These results highlight the interplay of regional and remote forcings, advancing understanding of NSCS hydrographic dynamics.