A three-dimensional ocean model has been utilized to study circulation and its seasonal variation in the South China Sea (SCS) in response to the forcing of the Asian monsoon and the Kuroshio intrusion. The SCS ocean model has a resolution of approximately 10 km horizontal spacing and 30 vertical levels with a realistic bottom topography. The model is forced with time-dependent wind stress and heat flux from National Center for Environmental Prediction Reanalysis data as well as with lateral fluxes from a Pacific Ocean model of 40 km horizontal resolution. This study reports on the analysis of the mean seasonal circulation and dynamic processes in response to monsoonal wind stress, the Kuroshio intrusion, and other intrinsic forcing processes. It is found that the seasonal circulation in the SCS is mainly driven by the monsoonal wind stress and greatly influenced by the inflow from the Kuroshio intrusion. Strong currents along the continental margin of the SCS form mean basin-wide cyclonic and anticyclonic circulations in the winter and summer, respectively. Multiscale eddies are embedded in the general circulation across the basin. While mainstream of the Kuroshio passes through the Luzon Strait without intruding into the SCS, partial intrusion occurs in the upper 200 m near the shelf margin southwest of Taiwan at times when winter dynamic conditions prevail in the north SCS. The intrusion of the Kuroshio into the SCS also occurs at depths in all seasons, mainly along the continental slope. The coastal current separation to the east off southern Vietnam and the associated eddy formations characterize the circulation in the south SCS. The simulated results compare well with the corresponding observed fields. Dynamical processes involved in the forced flow fields are investigated by examination of the momentum balances. The analyses reveal that the circulation in the SCS is generally dominated by the geostrophic currents. North of the Luzon Strait, positive nonlinearity in the zonal direction is locally intensified, which leads to the formation of centripetal acceleration for the mainstream of the Kuroshio to turn eastward. The Kuroshio intrusion at depths is governed by the ageostrophic flows and highly associated with the net westward pressure gradient force. Coastal jet separation to the east off Vietnam is mainly associated with the local wind stress field and with the shelf topography in the summer and winter, respectively. Sensitivity study reveals that the weakening of the Kuroshio markedly enhances Kuroshio's intrusion and forms an anticyclonic eddy west of the Luzon Strait.
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