On the dynamics of the South China Sea deep circulation

In the South China Sea (SCS), the deep water is confined to a bowel-type trench, and the maximum depth is approximately 4700 m. The Luzon Strait is the only deep connection between the SCS and the Pacific Ocean, with the deepest sill at about 2400 m in the Bashi Channel. Using the Hybrid Coordinate Ocean Model, this study gives a good description of the SCS deep circulation. The most obvious features are basin-scale cyclonic gyre and western intensification. The gyre is elliptical shaped, and its major axis is northeast-southwest. A numerical experiment is designed to investigate what could be possibly responsible for the SCS deep circulation. The results reveal that the deepwater overflow through the Luzon Strait controls over the basin-scale circulation structure in the deep SCS basin. The driving mechanism is elucidated based on the potential vorticity (PV) integral constraint, which means the PV inflow across the boundary is balanced by the net PV dissipation along the boundary.