Numerous islands divide the region into different seas of smaller size. These seas are connected to each other by many passages and channels. The SCS has a deep basin in the central and two broad shelves in the north and south. It exchanges water with many different regions: with the ECS through the Taiwan Strait, with the Kuroshio through the Luzon Strait, with the Sulu Sea through the Mindoro Strait, with Java Sea through Karimata Strait, and with the Indian Ocean through the Malacca Strait. Except for the Luzon Strait, the connections between the SCS and its surrounding seas are either narrow or shallow which deprives SCS of its impact directly on its surrounding seas.
The major circulation in the SCS is driven by the monsoon winds [Wyrtki, 1961]. The monsoon dominates most of the year in two distinct seasons. The northeast monsoon starts to intensify in the northern SCS in October, and in early January it reaches its maximum strengthen. It expands southward and covers the entire SCS and then starts to collapse in March. The southwest monsoon first appears in the central basin in April and generally follows the Mei-Yu. In general, the surface current in SCS is also driven by the monsoon. Figure 4 a and b show the surface current patterns when the southwest and northeast monsoon are prevailed, respectively. In the season of southwest monsoon, the water flows out from the SCS through Taiwan Strait and Luzon Strait. On the other hand, this feature is reversed in the
season of northeast monsoon. The Kuroshio Current intrudes into the SCS and keeps flowing westward until it hits the western boundary. A western boundary current is then formed along the coast of Vietnam. The current at the depth is not well known owing to the sparse of data.
Although the water in the SCS does not have significant interact with its adjacent seas, it does have large impact on Taiwan. In the Taiwan Strait, the northward current generally
