Although the Kuroshio and the South China Sea (SCS) waters flow in and out of the Luzon Strait near surface, the SCS water mainly flows out of the SCS at mid depth between 350 and 1350 m in all seasons and years that we studied. The eastward flowing SCS intermediate water is blocked by and joins the northward flowing Kuroshio at about 122 ¿E. Thus the Kuroshio Intermediate Water (KIW) is composed of the West Philippine Sea (WPS) proper water on the east, centered at 600 m depth. Because the ridge east of Taiwan separating the Okinawa Trough from the WPS is mostly shallower than 600 m, the eastern portion of the KIW is largely blocked by the ridge and is forced to turn eastward. This water does not enter the Okinawa Trough freely until it reaches a deep gap in the Ryukyu Ridge farther north, subsequently forming the eastern portion of the intermediate water in the trough. The western portion of the KIW is the mixed water of SCS and WPS. The nutrient-rich SCS Intermediate Water, centered at 500 m, is less influenced by the ridge and is able to flow into the Okinawa Trough. The SCS Intermediate Water thus retains its signature in the southern and western Okinawa Trough, upwells onto the East China Sea shelf, and becomes a major source of nutrients to the shelf water. ¿ 1998 American Geophysical Union