A numerical study of the South China Sea deep circulation and its relation to the Luzon Strait transport

A fine-resolution MOM code is used to study the South China Sea basin-scale circulation and its relation to the mass transport through the Luzon Strait.The modal domain includes the South China Sea,part of the East China Sea,and part of the Philippine Sea so that the currents in the vicinity of the Luzon Strait are free to evolve.In addition,all channels between the South China,Sea and the Indonesian seas are closed so that the focus is on the Luzon Strait transport.The model is driven by specified Philippine Sea currents and by surface heat and salt flux conditions.For simplicity,no windstress is applied at the surface.
The simulated Luzon Strait transport and the South China Sea circulation feature a sandwich vertical structure from the surface to the bottom.The Philippine Sea water is simulated to enter the South China Sea at the surface and in the deep ocean and is carried to the southern basin by western boundary currents.At the intermediate depth,the net Luzon Strait transport is out of the South China Sea and is fed by a western boundary current flowing to the north at the base of the thermocline.Corresponding to the western boundary currents,the basin circulation of the South China Sea is cyclonic gyres at the surface and in the abyss but an anticyclonic gyre at the intermediate depth.The vorticity balance of the gyre circulation is between the vortex stretching and the meridional change of the planetary vorticity.Based on these facts,it is hypothesized that the Luzon Strait transports are determined by the diapycnal mixing inside the entire South China Sea.The South China Sea plays the role of a "mixing mill" that mixes the surface and deep waters to return them to the Luzon Strait at the intermediate depth.The gyre structures are consistent with the Stommel and Arons theory (1960),which suggests that the mixlng-induced circulation inside the South China Sea should be cyclonic gyres at the surface and at the bottom but an anti-cyclonic gyre at the intermediate depth.The simulated gyre circulation at the intermediate depth has been confirmed by the dynamic height calculation based on the Levitns hydrography data.The sandwich transports in the Luzon Strait are consistent with recent hydrographic,al observations.
Model results suggest that the Kuroshio tends to form a loop current in the northeastern South China Sea.The simulated Kuroshio Loop Current is generated by the pressure head at the Pacific side of the Luzon Strait and is enhanced by the β-plane effects.The β-plane appears to be of paramount importance to the South China Sea circulation and to the Luzon Strait translports.Without the β-plane,the Luzon Strait transports would be greatly reduced and the South China Sea circulation would be complete-ly different.