10:00 AM - 10:15 AM
[AOS15-05] Revisiting fine-scale parameterizations for enhanced tidal mixing over a rough ocean bottom
Keywords:Bottom-intensified turbulent mixing, Quasi-steady lee waves, Vertical group velocity, Nonlinear interaction time, Tidal flow amplitude, Ocean bottom roughness
Although an accurate representation of ocean mixing processes into global circulation models is essential for accurate climate predictions,, existing parameterizations of mixing over rough bathymetry have plenty of room for improvement. For example, they do not take into account the fact that, as tide-topography interactions strengthen (kHU0/Ω >1), the generated internal waves transform from linear internal tides to quasi-steady internal lee waves where U0 is the amplitude of the tidal flow dominating the background flow in the Garrett-Munk (GM) internal wave field, kH is the horizontal wavenumber of the bottom topography, and Ω is the semidiurnal tidal frequency.
In the present study, using a fixed value of the buoyancy frequency, we perform a series of eikonal calculations to examine the energy transfer from upward propagating quasi-steady internal lee waves to dissipation through nonlinear interactions with the background GM internal waves in a vertical two-dimensional plane. It is shown that the vertical structure of the mixing hotspot becomes dominated by U0 rather than kH; as U0 increases, the fraction of energy dissipated at the ocean bottom decreases and the energy dissipation region extends vertically upward off the ocean bottom. These calculated results can be interpreted in terms of the vertical group velocity, Cgz, and the life time, τ, of the upward propagating quasi-steady lee wave packet. For a fixed density stratification, as kH increases while keeping U0 constant, Cgz becomes larger but becomes smaller so that the vertical decay scale of the energy dissipation rate remains nearly constant, whereas Cgz becomes larger but remains unchanged as U0 increases while keeping kH constant so that the vertical decay scale of the energy dissipation rate rapidly increases. This means that the resulting mixing hotspot extends further upward as U0 increases, independent of kH. This is in contrast to the result of the previous study by Iwamae et al. [2009] and Iwamae and Hibiya [2012] who showed that the concentration of the mixing hotspot becomes more focused nearer the ocean bottom as kH increases, independent of U0, although a trade-off relationship is found between the fraction of energy dissipated at the ocean bottom and the vertical extent of the energy dissipation region off the ocean bottom. A possible explanation for this difference is that Cgz and τ are both inversely proportional to kH for linear internal tides.
The results of this study should be reflected in the parameterization of mixing over rough bathymetry to improve the accuracy of ocean general circulation models.
In the present study, using a fixed value of the buoyancy frequency, we perform a series of eikonal calculations to examine the energy transfer from upward propagating quasi-steady internal lee waves to dissipation through nonlinear interactions with the background GM internal waves in a vertical two-dimensional plane. It is shown that the vertical structure of the mixing hotspot becomes dominated by U0 rather than kH; as U0 increases, the fraction of energy dissipated at the ocean bottom decreases and the energy dissipation region extends vertically upward off the ocean bottom. These calculated results can be interpreted in terms of the vertical group velocity, Cgz, and the life time, τ, of the upward propagating quasi-steady lee wave packet. For a fixed density stratification, as kH increases while keeping U0 constant, Cgz becomes larger but becomes smaller so that the vertical decay scale of the energy dissipation rate remains nearly constant, whereas Cgz becomes larger but remains unchanged as U0 increases while keeping kH constant so that the vertical decay scale of the energy dissipation rate rapidly increases. This means that the resulting mixing hotspot extends further upward as U0 increases, independent of kH. This is in contrast to the result of the previous study by Iwamae et al. [2009] and Iwamae and Hibiya [2012] who showed that the concentration of the mixing hotspot becomes more focused nearer the ocean bottom as kH increases, independent of U0, although a trade-off relationship is found between the fraction of energy dissipated at the ocean bottom and the vertical extent of the energy dissipation region off the ocean bottom. A possible explanation for this difference is that Cgz and τ are both inversely proportional to kH for linear internal tides.
The results of this study should be reflected in the parameterization of mixing over rough bathymetry to improve the accuracy of ocean general circulation models.