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Abstract:
We review the possible role of spontaneous emission and subsequent capture of internal gravity waves (IGWs) for dissipation in oceanic flows under conditions characteristic for the ocean circulation. Dissipation is necessary for the transfer of energy from the essentially balanced large-scale ocean circulation and mesoscale eddy fields down to smaller scales where instabilities and subsequent small-scale turbulence complete the route to dissipation. Spontaneous wave emission by flows is a viable route to dissipation. For quasi-balanced flows, characterized by a small Rossby number, the amplitudes of emitted waves are expected to be small. However, once being emitted into a three-dimensional eddying flow field, waves can undergo refraction and may be ``captured.'' During wave capture, the wavenumber grows exponentially, ultimately leading to breakup and dissipation. For flows with not too small Rossby number, e.g., for flows in the vicinity of strong fronts, dissipation occurs in a more complex manner. It can occur via spontaneous wave emission and subsequent wave capture, with the amplitudes of waves emitted in frontal systems being expected to be larger than amplitudes of waves emitted by quasi-balanced flows. It can also occur through turbulence and filamentation emerging from frontogenesis. So far, quantitative importance of this energy pathway---crucial for determining correct eddy viscosities in general circulation models---is not known. Toward an answer to this question, we discuss IGWs diagnostics, review spontaneous emission of both quasi-balanced and less-balanced frontal flows, and discuss recent numerical results based on a high-resolution ocean general circulation model.
