The rapid development process of western boundary undercurrent in the South China Sea

The western boundary undercurrent (WBUC) is crucial for driving deep water and meridional heat transport, yet its presence has not been verified in monsoon-driven semi-closed marginal seas. Based on mooring observations and reanalysis data, we identified a WBUC below 100 m in the South China Sea (SCS) during spring and summer. The SCS WBUC emerges in April, following the seasonal transition of the SCS surface western boundary current (WBC). It rapidly strengthens and gradually rises in July, dissipating in August. Momentum diagnostics reveal that the SCS WBUC is primarily governed by quasi-geostrophic balance. In the upper layer, the northward SCS WBC is driven by a westward barotropic pressure gradient force (BTPGF). Below 100 m, the eastward baroclinic pressure gradient force (BCPGF) dominates and counteracts the barotropic component, generating the southward WBUC. The rapid development of SCS WBUC is attributed to a faster decline in BTPGF compared with BCPGF. Continuous westward transport of water encountering western boundary topography provides conditions for the zonal baroclinic pressure gradient in the subsurface layer.