Observations of semidiurnal M2 internal tidal parametric subharmonic instability in the northeastern South China Sea

Near-diurnal vertically-standing waves with high vertical wavenumbers k(z) were observed in the velocity and shear fields from a set of 75-d long ADCP moored in the northeastern South China Sea (SCS) away from the "critical" latitude of 28.8 degrees. These enhanced near-diurnal internal waves followed a fortnightly spring-neap cycle. However, they always happened during semidiurnal spring tides rather than diurnal springs although strong diurnal internal tides with the fortnightly spring-neap cycle were prevailing, suggesting that they were generated via subharmonic instability (PSI) of dominant semidiurnal M-2 internal tides. When two semidiurnal internal tidal waves with opposite vertical propagation direction intersected, both semidiurnal subharmonic and super harmonic waves were largely intensified. The observed maximum diurnal velocity amplitudes were up to 0.25 m/s. The kinetic energy and shear spectra further suggested that frequencies of daughter waves were not always perfectly equal to M-2/2. The superposition of two daughter waves with nearly equal frequencies and nearly opposite k(z) in a PSI-triad leaded to the vertically-standing waves.