An empirical investigation of nonlinear energy transfer from the M 2 internal tide to diurnal wave motions in the Kauai Channel, Hawaii

• Published in: Geophysical research letters 2014-01, Vol.41 (2), p.505-512
• Main Authors: Chou, Sherry H., Luther, Douglas S., Guiles, Martin D., Carter, Glenn S., Decloedt, Thomas
• Format: Article
• Language: English
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2013GL058320

Current profiles are examined for evidence of nonlinear energy transfers from the M 2 internal tide to diurnal waves. The 6 month records, unlike shorter records, produce well‐resolved velocity and shear spectra that consistently exhibit maxima at the diurnal tides O 1 and K 1 , with a minimum at the intermediate M 2 subharmonic, M 2 /2. The ratio of velocity spectral energy at M 2 /2 and M 2 is quantified, providing a needed modeling benchmark. Bispectra and bicoherences imply a negligible [−M 2 /2, −M 2 /2, −M 2 ] triad interaction, but possibly a significant interaction for the [−O 1 , −K 1 , −M 2 ] triad. Numerical simulations, however, indicate that O 1 and K 1 signals are from internal tides. Tests with synthetic data, linear tides plus random noise, reveal that bispectrum and bicoherence estimators can yield significant values, thus misleading results. Therefore, resolving the diurnal tides from M 2 /2 is essential to meaningfully assess nonlinear transfer of energy from M 2 to diurnal waves. Strong semidiurnal and diurnal internal tide beams are observed in Kauai Channel Energy at M2 subharmonic (M2/2) is weak at all depths, including within M2 beam Bispectra can falsely indicate nonlinear interactions if diurnal tides are present