Water Exchange via the Straits

Several straits connect the SCS with the surrounding waters. However, water exchange at the Taiwan, Kalimantan, and Palawan Straits is mainly concentrated in the surface layer, whose effect on the intermediate circulation can be negligible. In the Luzon Strait, which connects the SCS with the western Pacific at the northern tip of Luzon Island, waters in the NSCS are influenced by the Kuroshio directly. The Strait s water depth and geographical location, the outside western boundary current (WBC), and the western boundary countercurrent (WBCC) mean that the transport at the Luzon Strait has a significant variation in the vertical direction. 

The water exchange between the SCS and Pacific Ocean at the surface layer relates the monsoon to the exchange between the Kuroshio water and the SCS water. The winter monsoon forces the Kuroshio water to enter the Luzon Strait and even to reach the western coast of the NSCS. However, the summer monsoon drives the SCS water into the Pacific. The Philippines seawater enters the SCS perennially above the submarine sill in the Luzon Strait. In a word, the Kuroshio surface water does enter the SCS. 

The Kuroshio enters the SCS in the form of a loop current. After shedding off the Kuroshio, the water mass intrudes into the NSCS in an eddy style all the year round, which is analogous to the Mexico Gulf Stream. The horizontal scale of a cyclonic ring away from the Kuroshio is about 150 km, and the vertical scale is about 1,000 m. The maximum surface velocity is about 1 m/s (Li et al., 1997). 

The backward and forward movements of Kuroshio waters (also named the Philippine Sea waters), which pass through the Luzon Strait at the intermediate layer, can be well confined by the 34.6%o isohaline. The maximum (minimum) intrusion of the salinity tongue arises in June (October). The warm, saltier Kuroshio water enters the SCS through the Luzon Strait from October to March, whose maximum (minimum) intrusion arises in February (September) with a volume transport of about 13.7 Sv (1.4 Sv), and the annual mean volume transport is about 6.5 Sv (Chu and Li, 2000). The saltiest North Pacific water (NPW) enters the SCS all the year round with a strong intrusion present in winter and summer, and a weak intrusion present 

3 Vertical Structure of Water Exchange Through the Luzon Strait 

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