[1] Conductivity-temperature-depth and acoustic Doppler current profiler data from six cruises spanning 1989 to 1999 are used to investigate the seasonal structure of the upper layer circulation in the southern South China Sea (SCS). The surveys were made during winter, late spring, summer, and late fall. More detailed structures not presented in previous studies are found. In summer the upper layer circulation of the southern SCS is dominated by an anticyclonic gyre with a strong eastward flow on its northern border, which originates off the southeast coast of Vietnam: the Summer Southeast Vietnam Offshore Current. In winter a stronger cyclonic gyre exists in the western portion of the southern SCS and a weaker anticyclonic circulation in the eastern portion. At the juncture of these two gyres, there is a strong northward upwind flow, called the Winter Natuna Off-Shelf Current. In late spring the anticyclonic gyre begins to form in the northwest, and a trace of the Natuna Off-Shelf Current can still be observed, especially subsurface in the southern part of the study area. Mesoscale eddies are active in this period. In late fall the winter-type pattern begins to appear. The Natuna Off-Shelf Current and the cyclonic gyre in the northwest part of the southern SCS emerge, and the summer anticyclonic gyre vanishes. In the southeast the currents are basically toward the northeast with an anticyclonic trend, revealing a multi-eddy feature. These upper ocean currents appear consistent with the wind forcing.

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.[1] Three across-shelf Acoustic Doppler Current Profiler (ADCP) moorings were deployed on the continental shelf in the northern South China Sea in 2006 and 2007, in order to obtain time series of ocean currents. During each of the three observational periods, in spring, autumn, and winter seasons, the observation was sustained for at least 1 month. Conductivity-temperature-depth data were also collected in the vicinity of these ADCP moorings. The two diurnal baroclinic constituents (O 1 and K 1 ) are found to be more prominent than the semidiurnal baroclinic ones (M 2 and S 2 ) at each mooring site, which are different from the barotropic tides. The highest diurnal (semidiurnal) baroclinic kinetic energy density exceeded 10 (2.7) kJ/m 2 during the observational periods. In each observational period, the vertical structures of baroclinic tidal ellipses indicate mode-1 characteristics for O 1 and K 1 , whereas some of the semidiurnal internal tides show mode-2 characteristics in autumn. Propagation directions of the diurnal and semidiurnal internal tides are basically across-shelf, and their phase speeds estimated from phase changes and buoyancy frequencies indicate deceleration of internal tides, when the internal tidal waves propagate from deeper shelf break to shallower shelf. The increased stratification in autumn is favorable for maintaining internal tidal waveform and preserving internal tidal energy, while the intensity of the internal tidal flow decreases significantly in winter.