Observations of enhanced nonlinear instability in the surface reflection of internal tides
Xie, X.; Shang, X.; van Haren, H.; Chen, G. (2013). Observations of enhanced nonlinear instability in the surface reflection of internal tides. Geophys. Res. Lett. 40(8): 1580-1586. dx.doi.org/10.1002/grl.50322
In: Geophysical Research Letters. American Geophysical Union: Washington. ISSN 0094-8276; e-ISSN 1944-8007, more
Enhanced vertically standing waves formed by the superposition of two upward and downward going near-diurnal (D1) waves are observed during one semidiurnal (D2) spring tide in an approximately 75day long velocity record from the northeastern South China Sea. Bicoherence estimates suggest that the enhanced D1 waves are likely due to nonlinear parametric subharmonic instability of D2 internal tides. The timescale for energy growth by an order of magnitude is about 2.5days for these waves. In addition to subharmonics, higher harmonics D4 (=D2+D2) and a mean flow are generated by a different nonlinear interaction during the same D2 spring tide. The separation of coherent from incoherent internal tidal signals and a rotary spectral decomposition in the vertical direction reveal that D2 waves with opposite vertical propagation directions in the region of internal tide reflection from the surface may be responsible for the pronounced nonlinear instability.
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