Dynamical Mechanisms for the South China Sea Seasonal Circulation and Thermohaline Variabilities

Abstract

The seasonal ocean circulation and the seasonal thermal structure in the South China Sea (SCS) were studied numerically using the Princeton Ocean Model (POM) with 20-km horizontal resolution and 23 sigma levels conforming to a realistic bottom topography. A 16-month control run was performed using climatological monthly mean wind stresses, restoring-type surface salt and heat, and observational oceanic inflow/outflow at the open boundaries. The seasonally averaged effects of isolated forcing terms are presented and analyzed from the following experiments: 1) nonlinear dynamic effects removed, 2) wind effects removed, and 3) open boundary inflow/outflow set to zero. This procedure allowed analysis of the contribution of individual parameters to the general hydrology and specific features of the SCS: for example, coastal jets, mesoscale topographic gyres, and countercurrents. The results show that the POM model has the capability of simulating seasonal variations of the SCS circulation and thermohaline structure. The simulated SCS surface circulation is generally anticyclonic (cyclonic) during the summer (winter) monsoon period with a strong western boundary current, a mean maximum speed of 0.5 m s?1 (0.95 m s?1), a mean volume transport of 5.5 Sv (10.6 Sv) (Sv ≡ 106 m3 s?1), and extending to a depth of around 200 m (500 m). During summer, the western boundary current splits and partially leaves the coast; the bifurcation point is at 14°N in May and shifts south to 10°N in July. A mesoscale eddy on the Sunda shelf (Natuna Island eddy) was also simulated. This eddy is cyclonic (anticyclonic) with maximum swirl velocity of 0.6 m s?1 at the peak of the winter (summer) monsoon. The simulated thermohaline structure for summer and winter are nearly horizontal from east to west except at the coastal regions. Coastal upwelling and downwelling are also simulated: localized lifting (descending) of the isotherms and isohalines during summer (winter) at the west boundary. The simulation is reasonable when compared to the observations. Sensitivity experiments were designed to investigate the driving mechanisms. Nonlinearity is shown to be important to the transport of baroclinic eddy features, but otherwise insignificant. Transport from lateral boundaries is of considerable importance to summer circulation and thermal structure, with lesser effect on winter monsoon hydrology. In general, seasonal circulation patterns and upwelling phenomena are determined and forced by the wind, while the lateral boundary forcing plays a secondary role in determining the magnitude of the circulation velocities.