Dynamical mechanisms for the South China Sea seasonal circulation and thermohaline variabilities

The seasonal ocean circulation and the seasonal thermal structure in the So uth China Sea (SCS) were studied numerically using the Princeton Ocean Mode l (POM) with 20-km horizontal resolution and 23 sigma levels conforming to a realistic bottom topography. A 16-month control run was performed using c limatological monthly mean wind stresses, restoring-type surface salt and h eat, and observational oceanic inflow/outflow at the open boundaries. The s easonally averaged effects of isolated forcing terms are presented and anal yzed from the following experiments: 1) nonlinear dynamic effects removed, 2) wind effects removed, and 3) open boundary inflow/outflow set to zero. T his procedure allowed analysis of the contribution of individual parameters to the general hydrology and specific features of the SCS: for example, co astal jets, mesoscale topographic gyres, and countercurrents. The results s how that the POM model has the capability of simulating seasonal variations of the SCS circulation and thermohaline structure. The simulated SCS surfa ce circulation is generally anticyclonic (cyclonic) during the summer (wint er) monsoon period with a strong western boundary current, a mean maximum s peed of 0.5 m s(-1) (0.95 m s(-1)), a mean volume transport of 5.5 Sv (10.6 Sv) (Sv = 10(6) m(3) s(-1)), and extending to a depth of around 200 m (500 m). During summer, the western boundary current splits and partially leave s the coast; the bifurcation point is at 14 degrees N in May and shifts sou th to 10 degrees N in July. A mesoscale eddy on the Sunda shelf (Natuna Isl and eddy) was also simulated. This eddy is cyclonic (anticyclonic) with max imum swirl velocity of 0.6 m s(-1) at the peak of the winter (summer) monso on. The simulated thermohaline structure for summer and winter are nearly h orizontal from east to west except at the coastal regions. Coastal upwellin g 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 exp eriments 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 considerab le importance to summer circulation and thermal structure, with lesser effe ct on winter monsoon hydrology. In general, seasonal circulation patterns a nd upwelling phenomena are determined and forced by the wind, while the lat eral boundary forcing plays a secondary role in determining the magnitude o f the circulation velocities.