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Identifying two steps in the internal wave energy cascade


In addition to their own convective breaking, the energetic baroclinic tides at a generation site stimulate the weaker non-linear interactions that might comprise the more-typical open ocean energy cascade. Observations of two classes of nonlinear interactions are presented. The first transfers energy from the semidiurnal tide to near- diurnal subharmonics. The second transfers energy from low -frequency, high-shear waves to high frequencies. Evidence is shown for wave-wave interactions between the low-mode, semidiurnal tide and high-mode, near-inertial motions. Profiles of velocity and density were collected aboard the Research Platform FLIP, at two sites during the Hawaii Ocean Mixing Experiment (HOME). In HOME Nearfield, significant bicoherences are observed between the dominant low-mode semidiurnal tide and opposing pairs of near- diurnal waves of vertical scale O(100 m). Growth rates of diurnal waves during each fortnightly cycle agree with theoretical predictions for the Parametric Subharmonic Instability (PSI) mechanism. At the Farfield location, 430 km from the generation site, near-diurnal waves are also observed but are not significantly bicoherent with the semidiurnal tide. Triple correlations between low frequency vertical shears and high frequency Reynolds stresses, -, are used at both HOME sites to estimate energy transfers from low frequencies, including both wind-generated motions and PSI subharmonics, to high frequencies. Energy bispectra show significant energy transfers to pairs of waves with nearly identical frequency. However, wavenumber resonances do not fit either the Induced Diffusion (ID) or eikonal models of interaction. Peak transfer rates in the Nearfield are of order 1 x 10⁻⁷ W kg⁻¹, while transfer rates in the Farfield are 3-4x smaller. Nearfield energy transfers are larger than local turbulence dissipation, but fall within an order of magnitude. Farfield energy transfers and turbulence dissipation agree within a factor of 3 throughout the measurement profile. The suggestion is that the HOME observations of energy transfer from the semidiurnal tide to near-diurnal waves via PSI, and subsequently from low to high frequency waves, represent a skeleton of the open-ocean energy cascade

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