Abstract: Using the data from the 3th version of U.S. Navy Generalized Digital Environment Model(GDEM), we diagnosed the intermediate circulation and deep circulation in the west of Luzon Island with the P-vector method. The diagnosed result shows that it is an anticyclonic circulation in the west of Luzon Island in the intermediate layer (1 000~2 000 m vertical-averaged), different from the deep layer circulation (2 400 m to bottom vertical-averaged) in the same region, which has been reported as cyclone in the previous studies. These two eddies both correspond with the low salinity water in geographical location. The results further show that the velocity of anticyclonic circulation reduces as the depth increases, and the velocity of cyclonic circulation is stronger in the deeper layer. The current at the depletion layer (about 2 200 m) is weak and disorder. Further research demonstrates that low salinity water and anticyclonic circulation in the intermediate layer have distinct seasonal variation, they both become stronger in May to July, which may suggests that the anticyclonic circulation is influenced by the upper layer circulation to some degree.

Key words: South China Sea, Luzon Island, intermediate circulation, deep circulation, diagnose, P-vector method