The strength of global thermohaline circulation is intimately bound up with the distribution and intensity of diapycnal mixing in the deep ocean.Ocean general circulation models demonstrate that the spatially averaged diapycnal diffusivity of 10-4 m2s-1 is required in the deep ocean to support the global thermohaline circulation.The power required to reproduce diapycnal diffusivity of the order of 10-4 m2s-1 is believed to be provided at large scales by tide-topography interactions and wind stress fluctuations.While the forcing at large scales must be balanced by dissipation at small scales, it remains unknown how the energy thus supplied atlow-vertical-wavenumber and low-frequency cascades through the localinternal wave spectrum down to small dissipation scales.
We examine the physical mechanism by which the energy supplied atlarge scales is transferred down to dissipation scales inthe three dimensional internal wave field.For this purpose, we employ ″Eikonal Approach″ in which thepropagation of wave packets within the prescribedthree-dimensional internal wave field is traced (Henyey et al., 1986).The calculation shows that the energy dissipation rate within theundisturbed Garrett-Munk (GM) internal wave spectrumamounts to 4.34 × 10-9 W kg-1 yieldingdiapycnal diffusivity 0.3 × 10-4m2s-1, much less than the value required tomaintain the global thermohaline circulation. This value remains almost unchanged even if the energy level atlow-vertical-wavenumber and low-frequency within the GM internal wave spectrum is increased.Significantly enhanced energy dissipation occurs only when the energy level at high-vertical-wavenumber and near-inertial frequency within the GM spectrum is increased.
This indicates that dissipation processes in the deep ocean aredominated by the resonant interaction, parametric subharmonicinstability which transfers energy from low-vertical-wavenumberwaves with frequencies over 2f (f is the local inertial frequency) to high-vertical-wavenumber near-inertial waves. |
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