Cite this paper:
Qian LIU, Xiaodong SHANG, Xiaohui XIE. Observations of semidiurnal M2 internal tidal parametric subharmonic instability in the northeastern South China Sea[J]. Journal of Oceanology and Limnology, 2021, 39(1): 56-63

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

Qian LIU1, Xiaodong SHANG2,3, Xiaohui XIE4
1 School of Naval Architecture and Ocean Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China;
2 State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;
3 Southern Marine Science and Engineering Guangdong Laboratory, Guangzhou 511458, China;
4 State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310000, China
Near-diurnal vertically-standing waves with high vertical wavenumbers kz 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°. 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 M2 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 M2/2. The superposition of two daughter waves with nearly equal frequencies and nearly opposite kz in a PSI-triad leaded to the vertically-standing waves.
Key words:    internal tides|South China Sea (SCS)|resonant interactions|rotary spectra|parametric subharmonic instability   
Received: 2019-05-20   Revised: 2019-09-19
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