Observations and Models of Low-Mode Internal Waves in the Ocean
The generation of internal gravity waves in the ocean is largely driven by tides, winds, and interaction of currents with the seafloor. Models and observations indicate a global energy supply for the internal wave field of about 1 TW by the conversion of barotropic tides at mid-ocean ridges and abrupt topographic features. Winds acting on the oceanic mixed layer contribute 0.3–1.5 TW, and mesoscale flow over rough topography adds about 0.2 TW. Globally, 1–2 TW are needed to maintain the observed stratification of the deep ocean by diapycnal mixing that results from the breaking of internal waves. Ocean circulation models show significant impact of the spatial distribution of internal wave dissipation and mixing on the ocean state, e.g., thermal structure, stratification, and meridional overturning circulation. Observations indicate that the local ratio of generation and dissipation of internal waves is often below unity, and thus, the energy available for mixing must be redistributed by internal tides and near-inertial waves at low vertical wavenumber that can propagate thousands of kilometers from their source regions. Eddy-permitting global ocean circulation models are able to quantify the different sources of energy input and can also simulate the propagation of the lowest internal wave modes. However, the variation of the internal wave energy flux along its paths by wave–wave interaction, topographic scattering, and refraction by mesoscale features as well as its ultimate fate by dissipation remains to be parameterized.
We thank Brian Dushaw (Univ. Washington) for providing estimates of mode 1 internal tides from altimetry and Carsten Eden (Univ. Hamburg) for the energy fluxes from IDEMIX. Janna Köhler received funding from the Deutsche Forschungsgemeinschaft (DFG, grant Ko4814/1-1).
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