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Observations of enhanced nonlinear instability in the surface reflection of internal tides
Author(s) -
Xie Xiaohui,
Shang Xiaodong,
Haren Hans,
Chen Guiying
Publication year - 2013
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1002/grl.50322
Subject(s) - internal wave , instability , harmonics , internal tide , superposition principle , geology , reflection (computer programming) , nonlinear system , physics , geophysics , mechanics , quantum mechanics , voltage , computer science , programming language
Enhanced vertically standing waves formed by the superposition of two upward and downward going near‐diurnal ( D 1 ) waves are observed during one semidiurnal ( D 2 ) spring tide in an approximately 75 day long velocity record from the northeastern South China Sea. Bicoherence estimates suggest that the enhanced D 1 waves are likely due to nonlinear parametric subharmonic instability of D 2 internal tides. The timescale for energy growth by an order of magnitude is about 2.5 days for these waves. In addition to subharmonics, higher harmonics D 4 (= D 2  +  D 2 ) and a mean flow are generated by a different nonlinear interaction during the same D 2 spring tide. The separation of coherent from incoherent internal tidal signals and a rotary spectral decomposition in the vertical direction reveal that D 2 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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