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Seasonal circulation in the South China Sea ──a TOPEX/POSEIDON satellite altimetry study

Li Li, Wu Risheng, Guo Xiaogang (Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005; Key Laboratory of Marine Environmental Sciences and Numerical Modeling, State Oceanic Administration,Qingdao 266061)  
The mean seasonal circulation in upper layer of the South China Sea (SCS) is studied using TOPEX/ POSEIDON altimetry remote sensing products from 1992 to 1996. The research results reveal that upper layer circulation in the SCS is predominated by seasonal alternating monsoons and its pattern varies seasonally. The winter general circulation appears to be cyclone consisted of two sub-basin cyclonic gyres. The summer circulation is generally anticyclonic, but no systematic current structure is found in the eastern SCS south of 18°N. The results also indicate that circulation in the SCS is notably westen intensified. A jet is observed, in both winter and summer, off the coast of Vietnam and the outer edge of Sunda shelf. It appears to be the strongest current in the SCS, which flows southword in winter, hut northward in summer. Since the dynamic feature of this jet is mainly geostrophic rather than wind drifting, it is suggested that name it as a jet (the SCS Monson Jet) instead of a drift. In winter, the Sunda shelf blocks the southward flowing jet and a northeastward return current is developed along coasts of Kalimantan and Palawan Ialands. In summer, the northward flowing jet splits off the southeast coast of Hainan Island. The northern branch flows along the China shelf, consisting with the traditional path of the SCS Warm Current. The southern branch flows to the east along 18°N across the basin, turns to northeast, and joints the northern branch again off Shantou. The SCS Warm Current is strengthened there by reunification and a zone of weak flow is observed near the shelf break between these branches. In the sense of multiyear mean, the Kuroshio intrudes into the northeastern SCS only in winter and induces a sub-basin cyclonic gyre in the northern SCS. At this period, the SCS Warm Current only exists in the area east of Shantou. In summer, the predominated current in the northern SCS is northeastward. There is no indication of Kuroshio invasion there. Validation using observations from satellite tracking surface drifting buoy shows that these remote sensing results are in good agreement with surface observation.
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