Two bottom-mounted ADCPs were deployed in the coastal waters off the National Museum of Marine Biology and Aquarium at southwestern Taiwan coast from June 5, 2004 to December 20, 2005. The long-term observational dataset of wind, currents, water level and drifters were analyzed here to investigate the mechanism and seasonal variations of tidal and subtidal flows. Diurnal tidal constituent of K dominates the tidal energy in this area. The calculated form ratio is 1.53, indicating that the tide is of the diurnal type. Tidal current direction is consistent with the local coastal line, with the principal axis in the NNE-SSW orientation. The tidal waves of two major constituents¡]K an M ¡^are found to exist in the form of propagating waves in this region, rather than the form of standing waves as was found in the east coast of central Taiwan Strait. The subtidal currents flow toward the south with a speed of about 20-50 cm/s during the winter northeastern monsoon. On the other hand, subtidal flow speed is smaller but still toward the south during the summer southwestern monsoon. Current speed in the surface layer is larger than that in the mid and bottom layers. The persistent southward flow in this region is also found to correlate with the wind stress curl. When the wind stress curl reaches a maximum negative value in winter, an anticyclonic eddy develops and the flow in the study area is toward the south. Analysis of Argos drifter data reveal the existence of anticyclone off the southwestern Taiwan coast. Surface drifters were also deployed in this area, and the trajectories indicate that general flow patterns are toward the south. This finding is consistent with the progressive vector diagram from the moored ADCP current data. To summarize, a persistent southward flow exists in the study region all year round. However, the flow intensifies in winter and decays in summer. The southward flow is also associated with the anticyclonic eddy driven by the negative wind stress curl in winter. The intrusion of Kuroshio water from the Luzon Strait into the northern South China Sea is the possible source of water mass for this phenomenon.

This study analyzed historical hydrographic data consist of 95 years of NODC data and 18 years of NCOR data. Variations of upper-layer current in Luzon Strait and its neighboring Northern South China Sea are investigated by the method of dynamical topography. On the other hand, higher salinity characteristic of the North Pacific Tropical Water (NPTW) is used to trace the water mass distribution and its seasonal variations in the studied area. The result shows that the maximum of x-component velocity (along 120.25˚E, relative to 400 m ) in the Luzon Strait occurs in the middle of the strait, the flow direction is westward and the speed increases toward the surface. At the north and south ends of the strait flows are eastward and the maximum speed core is at the depth of 0~50 m. Horizontal distribution of flow fields indicates that intrusion of the north Pacific waters into the South China Sea through the Luzon Strait in the whole year. Westward bifurcation of the intrusion flows occurs at 20~21˚N. A cyclonic eddy exists in the South China Sea all year-round, and its core is located at about 18˚N, 118˚E. This eddy strengthens as the monsoon prevails with its speed reached in January and February. Annual mean upper-layer transport (0~ 400 m) of the Luzon Strait is estimated to be about 3.5 Sv (positive value means westward) with a maximum value of about 6.5 Sv in December and a minimum value of 1.1 Sv in June. The Upper-layer current of the SCS is dominated by the monsoon. When the northeast monsoon prevails, the currents are affected by the Ekman effect to form high sea surface height in the north and low sea surface height in the south to produce a westward current. On the other hand, strong two coexistent wind stress curls with reversing signs during the northeast monsoon produce a westward current along the line of zero curl in the middle of the strait. The distribution of the NPTW(£m£c=23.5¡ã25.5 kg/m3,S >34.5 psu) is mostly at a depth of 120¡ã130 m in the South China Sea. NPTW were traced at 125 m depth, and the result indicates that this water mass enters the South China Sea through the Luzon strait all year-round. The intrusion path is along the continental slope of south China. The extent of intrusion reaches the maximum between December and February, and the water mass can spread into the South China Sea basin.