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[AOS11-05] A new parameterization of turbulent mixing enhanced over rough seafloor topography
Keywords:Parameterization of turbulent mixing, Rough seafloor topography, Tidal flow, Geostrophic flow, Internal lee waves, Southern Ocean
In the present study, I propose a new parameterization of tidal mixing enhanced over a rough seafloor with the revised formulation of its vertical decay scale. Of special importance is that internal waves emanating from the seafloor transform from internal tides to internal lee waves as tide-topography interaction strengthens. Taking into account this fact, I formulate the vertical decay scale of the energy dissipation rate off the seafloor assuming that it can be substituted by a “vertical mean free path” of the internal lee waves emanating from the seafloor and then resonantly interacting with the background Garrett-Munk internal wave field. The resulting formula predicts that the vertical extent of the mixing hotspot off the seafloor increases in proportion to U0 squared, but independently of kH, where U0 is the amplitude of the barotropic tidal flow and kH is the dominant horizontal wavenumber of the seafloor. Encouraged by the fact that this theoretical prediction is consistent with the results of the eikonal calculations by Hibiya et al. [2017], as well as the results of microstructure measurements in the Southern Ocean by Sheen et al. [2013], I propose a new parameterization of tidal mixing enhanced over the rough seafloor by incorporating the above formulated vertical decay scale of energy dissipation rates.
As is well known, tidal mixing in the global ocean is classified into the “far-field” mixing and the “near-field” mixing; it has long been believed that the far-field mixing is associated with nonlinear interactions between propagating low-mode internal tidal waves and the background internal wave field in the ocean interior, while the near-field mixing is associated with nonlinear interactions between high-mode internal tidal waves and the background internal wave field near the seafloor. However, even if tidal flow over the rough seafloor in the deep ocean is barely sufficient to generate internal tidal waves, it can be locally amplified, for example, over the rough seafloor at the top of the ocean ridge so that internal lee waves might occur. In such a case, we can expect that resulting tidal mixing region extends further upward from the seafloor, even possibly reaching the thermocline; thus compensating the shortfall of turbulent mixing required to sustain the global overturning circulation. The intensity and the vertical extent of the near-field tidal mixing employed in the current global ocean circulation models should therefore be reconsidered on the basis of the present study.
As is well known, tidal mixing in the global ocean is classified into the “far-field” mixing and the “near-field” mixing; it has long been believed that the far-field mixing is associated with nonlinear interactions between propagating low-mode internal tidal waves and the background internal wave field in the ocean interior, while the near-field mixing is associated with nonlinear interactions between high-mode internal tidal waves and the background internal wave field near the seafloor. However, even if tidal flow over the rough seafloor in the deep ocean is barely sufficient to generate internal tidal waves, it can be locally amplified, for example, over the rough seafloor at the top of the ocean ridge so that internal lee waves might occur. In such a case, we can expect that resulting tidal mixing region extends further upward from the seafloor, even possibly reaching the thermocline; thus compensating the shortfall of turbulent mixing required to sustain the global overturning circulation. The intensity and the vertical extent of the near-field tidal mixing employed in the current global ocean circulation models should therefore be reconsidered on the basis of the present study.