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Observations of parametric subharmonic instability‐induced near‐inertial waves equatorward of the critical diurnal latitude
Author(s) -
Xie XiaoHui,
Shang XiaoDong,
van Haren Hans,
Chen GuiYing,
Zhang YuanZhi
Publication year - 2011
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 - 10.1029/2010gl046521
Subject(s) - instability , inertial wave , subharmonic , latitude , geophysics , parametric statistics , geology , atmospheric sciences , physics , geodesy , mechanics , wave propagation , nonlinear system , longitudinal wave , optics , mechanical wave , mathematics , statistics , quantum mechanics
Moored current observations of 75 days duration in the northeastern South China Sea (∼20°N) suggest that parametric subharmonic instability (PSI) of semidiurnal ( D 2 ) internal tides can not only generate waves of frequencies close to D 2 /2, but also excite near‐inertial waves whose frequencies are different from D 2 /2. Time series of shear amplitudes clearly show a 14‐day cycle. Although near‐inertial and near‐diurnal motions dominate the shear, this cycle is in phase with the fortnightly spring‐neap cycle of D 2 ‐waves. After separation of near‐inertial and near‐diurnal waves using band‐pass filters, shear magnitudes for both motions still follow this 14‐day cycle, rather than that of diurnal internal tides or variations of the local wind field. This strongly suggests that PSI equatorward of the critical latitude for D 2 /2 waves (∼29°) not only transfers D 2 ‐energy to D 2 /2 waves, but also to high‐mode near‐inertial waves. Near‐inertial waves ( f ) and another subharmonic ( D 2 ‐ f ), together with D 2 waves, compose a PSI‐triad following strong interaction.