Seasonal characteristics of internal tides on the continental shelf in the northern South China Sea

Abstract

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 (O1 and K1) are found to be more prominent than the semidiurnal baroclinic ones (M2 and S2) at each mooring site, which are different from the barotropic tides. The highest diurnal (semidiurnal) baroclinic kinetic energy density exceeded 10 (2.7) kJ/m2 during the observational periods. In each observational period, the vertical structures of baroclinic tidal ellipses indicate mode‐1 characteristics for O1 and K1, 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.