We study the generation, propagation, and dissipation of wind-generated near-inertial waves (NIWs) in a global 1/25° Hybrid Coordinate Ocean Model (HYCOM) simulation with realistic atmospheric forcing and background circulation during 30 days in May-June 2019. The time-mean near-inertial wind power input and depth-integrated energy balance terms are computed for the total fields and the fields decomposed into vertical modes to differentiate between the radiative and (locally) dissipative components of NIW energy. Only 30.3% of the near-inertial wind input projects onto the first 5 modes, whereas the sum of the NIW energy in the first 5 modes adds up to 58% of the total NIW energy. Almost all of the depth-integrated NIW horizontal energy flux projects on the first 5 modes. The global distribution of dissipation and decay distances of NIW modes confirm that lower latitudes are a sink for NIW energy generated at higher latitudes. The locally dissipated fraction of NIW energy, qlocal, is found to be uniform throughout the global ocean with a global mean value of 0.79. The horizontal NIW fluxes diverge from areas with cyclonic vorticity and converge in areas with anticyclonic vorticity, i.e., anticyclonic eddies are a sink for NIW energy fluxes, in particular for higher modes. Most of the residual energy that does not project onto modes propagates downward in anticyclonic eddies. The global near-inertial wind power input is 0.21 TW for the 30 days, of which only 19% is transmitted below 500 m depth.

We study the generation of resonantly growing mean flow by weakly non-linear internal wave beams. With a perturbational expansion, we construct analytic solutions for 3D internal wave beams, exact up to first order accuracy in the viscosity parameter. We specifically focus on the subtleties of wave beam generation by oscillating boundaries, such as wave makers in laboratory set-ups. The exact solutions to the linearized equations allow us to derive an analytic expression for the mean vertical vorticity production term, which induces a horizontal mean flow. Whereas mean flow generation associated with viscous beam attenuation -known as streaming -has been described before, we are the first to also include a peculiar inviscid mean flow generation in the vicinity of the oscillating wall, resulting from line vortices at the lateral edges of the oscillating boundary. Our theoretical expression for the mean vertical vorticity production is in good agreement with earlier laboratory experiments, for which the previously unrecognized inviscid mean flow generation mechanism turns out to be significant. † Email address for correspondence: f.beckebanze@uu.nl arXiv:1805.12356v2 [physics.flu-dyn]

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