Circulation in the South China Sea
Su Jilan, Xu Jianping (Second Institute of oceanography, State Oceanic Administration, 9 Xixihexia, Hangzhou, Zhejiang 310012, China)
e-mail: sujil@zgb.com.cn
Cai Shuqun (South China Sea Institute of Oceanology, Chinese Academy of Science, 164 West Xingang Road, Guangzhou, Guangdong 510301, China)
Wang Ou (Department of Oceanography, Texas A&M University, College Station, TX, USA)
ABSTRACT
Circulation in the South China Sea is discussed, based on results from both hydrographic surveys andlayered models. The influence of the Kuroshio seems to be important to areas north of about 17｡?. Advection of vorticity from the Kuroshio front results in a fluctuating cyclonic gyre with migrating cold eddies. The smaller scale anticyclonic gyre next to the northern shelf, often with a warm eddy near the Dongsha Islands, is likely to be induced by the cyclonic gyre. The summer monsoon drives two large anticyclonic gyres with a smaller cyclonic gyre sandwiched in between just off the northeast coast of middle Vietnam. In the winter the monsoon produces a basin-wide cyclonic gyre in the SCS. Inaddition, in the winter an intense anticyclonic eddy may be generated periodically over the Nansha Trough. It then propagates first northwestward and later on southward along the Vietnam coast before being dissipated.
INTRODUTION
The South China Sea (SCS) is the largest coastal ocean in East Asia with an area of about 3.5×106km2. Wide shelves extend from the north, to south along its western boundary, excep t for a break along the middle part of the east Vietnam coast. Its average depth is, howeve, over 1800m because of the presence of an extensive abyssal basin.
The SCS is basically a semi-enclosed sea. Its water mass characteristics suggest that its water masses are derived principally from the Western Philippine Sea (WPS) (Nitani, 1972). It also exports significant arnount of water to other marginal seas through straits with shallow sill dephs. Thus, how the WPS water enters the SCS is of fundamental importance to the understanding of its oceanography.
The SCS is also located as the passage way of the Asian monsoon. Seasonal change of the monsoon winds results in different summer and winter circulation features in the SCS (e. g., Pohlmann, 1987; Shaw and Chao, 1994). Additionally, sea surface dynamic heights derived from the TOPEX/POSEIDON data indicate the presence of active mesoscale eddies in the SCS (e. g., see the Monthly Ocean Report published by the Japan Meteorological Agency).
The countries along the coasts of the SCS have had one of the most bustling economy over the past 30years. However, in spite of its geographical location, the SCS is a relatively under-studied coastal ocean, especially with respect to in situ observations. In this paper, we will present our recent findings on the intrusion of the WPS water (more specifically, the Kuroshio water) into the SCS, as well as on the background gyres and the mesoscale eddies in the SCS, based on observational and modeling results.
