Coupled dynamics of the South China Sea, the Sulu Sea, and the Pacific Ocean

The complex geometry, the seasonally reversing monsoon winds, and the connectivity with the Pacific Ocean all contribute to the coupled dynamics of the circulation in the South China Sea (SCS), the Sulu Sea, and the region around the Philippine Islands. The 1/2 degrees, 1.5-layer global reduced gravity thermodynamic Navy layered ocean model (NLOM) is used to separate these components and to investigate the role of each one. When forced by the Hellerman and Rosenstein [1983] (HR) monthly wind stress climatology, the basic features of the model solution compare well with observations, and with higher-resolution NLOM versions. The dynamics of the flow from the Pacific Ocean into the SCS via the Luzon Strait are emphasized. The effects of Ekman suction/pumping due to wind curl are examined by forming monthly spatial averages of the winds over the SCS/Sulu Sea basins. This maintains a monthly varying stress but with a region of zero curl. Forcing the model with these modified winds leaves the mean Luzon Strait transport unchanged, and the variability actually increases slightly. These results suggest that it is the pressure head created by the pileup of water from the monsoonal wind stress that controls the variability of the Luzon Strait transport. The forcing for wind stress pileup effects could be either internal or external to the SCS/Sulu Sea basin. The effects of internal forcing are studied by applying monthly winds within this basin but annual HR winds outside the region. With this forcing the mean Luzon Strait transport is essentially unchanged, but the variability is only 44% of the standard case value. The external forcing is defined as zero stress in the SCS/Sulu Sea basins and HR monthly winds outside. Again, the mean Luzon Strait transport is unchanged, and here the variability is 60% of the standard case. The mean Luzon Strait transport is largely a function of the model geometry. When the Sulu archipelago is opened, a net cyclonic flow develops around the Philippines, which is essentially an extension of the northern tropical gyre. The bifurcation latitude of the North Equatorial Current (NEC) at the Philippine coast is also affected by the amount of transport through the Sulu archipelago. Opening this archipelago causes the NEC split point to move southward and increases the transport of the Kuroshio east of Luzon while decreasing the Mindanao Current. Opening or closing the Sunda Shelf/Java Sea or the Sulu archipelago does not affect the transport of the Pacific to Indian Ocean throughflow.