Current observations of internal tides and parametric subharmonic instability in Luzon Strait

Acoustic Doppler Current Profiler (ADCP) measurements of the velocity structure in the thermocline in Luzon Strait are presented. The statistics for current, vertical variation of the inertia-gravity waveband, parametric subharmonic instability (PSI), and current shear are analyzed. It was found that 1) barotropic flow primarily consists of a nearly circular mixed tide. Diurnal tides are strongest and show smooth variance with a fortnightly spring-neap cycle, indicative of the astronomical tide-generating force. However, the semi-diurnal band power exhibits a high-frequency oscillation as a result of non-linear interactions. The high-frequency band power with high values during the spring tide oscillates with the tidal cycle. Near-inertial wave motions showing random variance may be caused by changes in the wind forcing at the sea surface or by random forcing. 2) Baroclinic velocities exhibit strong shear structure. The observed large changes in the amplitude of the baroclinic velocity and the limited vertical extent of the high-velocity cores may be interpreted as internal wave beams that pass through the observed water column. Semi-diurnal tides are dominant in the baroclinic velocity. Kinetic energy spectra also revealed that additional peaks were centred at sum-tidal-inertial interaction frequencies (such as M-2 + f) and difference-interaction frequencies (such as M-2 - f). The spectral exponent of the baroclinic velocity is omega(-alpha) (1 < alpha < 3). 3) Strong non-linear interactions among internal waves exist, and the semi-diurnal (M-2) component plays a key role in these interactions. Bicoherence analysis showed that M-2/2 waves were non-linearly coupled with the dominant M-2 internal tide. 4) The polarization relations were used to diagnose observational internal tidal motions. Diurnal waves propagate to the east-northeast, and the semi-diurnal-diurnal waves propagate westward. In the case of diurnal tides, the minor to major axis ratio is different from the expected value of f/omega(K1) because of the deviation of inclinations, whereas, for semi-diurnal tides, it is close to the expected value of f/omega(M2) at depths from 30 to 150 m.