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Stimulated generation: extraction of energy from balanced flow by near-inertial waves

Published online by Cambridge University Press:  23 May 2018

Cesar B. Rocha Open the ORCID record for Cesar B. Rocha [Opens in a new window] ,
Gregory L. Wagner Open the ORCID record for Gregory L. Wagner [Opens in a new window]  and
William R. Young Open the ORCID record for William R. Young [Opens in a new window]
Cesar B. Rocha*
Affiliation:
Scripps Institution of Oceanography, University of California, San Diego, USA
Gregory L. Wagner
Affiliation:
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, USA
William R. Young
Affiliation:
Scripps Institution of Oceanography, University of California, San Diego, USA
*
†Email address for correspondence: crocha@ucsd.edu
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Abstract

We study stimulated generation – the transfer of energy from balanced flows to existing internal waves – using an asymptotic model that couples barotropic quasi-geostrophic flow and near-inertial waves with $\text{e}^{\text{i}mz}$ vertical structure, where $m$ is the vertical wavenumber and $z$ is the vertical coordinate. A detailed description of the conservation laws of this vertical-plane-wave model illuminates the mechanism of stimulated generation associated with vertical vorticity and lateral strain. There are two sources of wave potential energy, and corresponding sinks of balanced kinetic energy: the refractive convergence of wave action density into anti-cyclones (and divergence from cyclones); and the enhancement of wave-field gradients by geostrophic straining. We quantify these energy transfers and describe the phenomenology of stimulated generation using numerical solutions of an initially uniform inertial oscillation interacting with mature freely evolving two-dimensional turbulence. In all solutions, stimulated generation co-exists with a transfer of balanced kinetic energy to large scales via vortex merging. Also, geostrophic straining accounts for most of the generation of wave potential energy, representing a sink of 10 %–20 % of the initial balanced kinetic energy. However, refraction is fundamental because it creates the initial eddy-scale lateral gradients in the near-inertial field that are then enhanced by advection. In these quasi-inviscid solutions, wave dispersion is the only mechanism that upsets stimulated generation: with a barotropic balanced flow, lateral straining enhances the wave group velocity, so that waves accelerate and rapidly escape from straining regions. This wave escape prevents wave energy from cascading to dissipative scales.

JFM classification

Geophysical and Geological Flows: Internal waves Geophysical and Geological Flows: Ocean processes Geophysical and Geological Flows: Quasi-geostrophic flows

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Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 847 , 25 July 2018 , pp. 417 - 451
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© 2018 Cambridge University Press 

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