Overview of the multi-layer circulation in the South China Sea

Abstract The South China Sea (SCS) is a semi-enclosed ocean basin, with its sole deepwater renewal channel in the northeast, the Luzon Strait, connected to the northwestern Pacific. The circulation in the SCS is endowed with a distinctive multi-layer structure, which is influenced by the seasonally reversing Southeast Asian monsoon at the surface, modulated Kuroshio intrusion in the upper layer, and enhanced diapycnal mixing and persistent Luzon Strait deepwater overflow in the deep layer. After being diapycnally mixed by energetic internal tides, internal waves, and mesoscale eddies, waters entering the SCS in the upper and deep layers through the Luzon Strait are returned to the northwestern Pacific in the intermediate layer. This inflow-outflow-inflow structure in vertical induces net inflows of potential vorticity in the upper and deep layers and a net outflow of potential vorticity in the intermediate layer of the Luzon Strait, which are believed to be closely related with the circulation structure in the SCS. A sandwiched circulation in the SCS, namely, anticyclonic in the intermediate layer and cyclonic in the upper and deeper layers, was recently reported, but controversial issues still remain on its spatial pattern and temporal variability. To ascertain this sandwiched circulation and reveal its driving mechanism is of much scientific interest to the on-going discussion on the SCS circulation and its role in regional climate. Different from previous review articles focusing on eddies or circulation in a specific layer or area, we overview the full-depth circulation of the SCS layer-by-layer and outline its possible forcing mechanisms, and furthermore, provide perspectives for further investigation.