Observed summertime slope current in northern South China Sea

• Persistent southwestward current extends 750 km along northern South China Sea continental slope in summer. • The southwestward slope current appears locally generated and maintained in geostrophic balance. • Two underlying dynamic mechanisms contribute to this current: mesoscale Kuroshio eddies and wind stress curl. The summertime slope current in the northern South China Sea (SCS) constitutes a dynamically distinctive yet inadequately understood component of the regional circulation system. This study synthesizes extensive in-situ hydrographic measurements, multi-year mooring current records, satellite altimetry products, surface drifter trajectories, and numerical model outputs to provide a comprehensive physical description of the slope current and to investigate its underlying dynamical forcing mechanisms. Observations reveal a persistent southwestward flow along the continental slope isobaths at depths between approximately 200 and 2000 m, spanning approximately 750 km from the Dongsha Plateau to the southeastern waters off Hainan Island. The current is roughly 100 km wide, reaches depths of at least 500 m, and exhibits maximum mean along-slope velocities exceeding 10 cm/s, with substantial synoptic to intraseasonal variability. Dynamically, the current appears locally generated and sustained, governed by a cross-slope pressure gradient in geostrophic balance. This pressure structure is associated with a pronounced, elongated ridge in absolute dynamic topography aligned with the outer shelf and upper slope. The ridge is likely maintained by warm, saline water mass, sourced from mesoscale Kuroshio eddies. Although positive wind stress curl over the northern SCS favors a basin-scale cyclonic circulation, with the southwestward slope current forming its northern limb. Yet mooring observations show the current emerges prior to the onset of dominant wind forcing. This temporal mismatch suggests that wind stress curl alone cannot sustain its persistence. We therefore identify two underlying dynamical drivers contributing to this flow: mesoscale Kuroshio eddies and wind stress curl. Given its documented geographic extent and distinct dynamics, we propose redesignating this historically localized “Dongsha Current” as the “Yue-Qiong Slope Current”, a name that accurately reflects its full pathway along the continental slope regions off Guangdong (Yue) and Hainan (Qiong).