Tidal Mixing over Rough Topography: Sensitivity to Topographic Length Scale and Steepness

Tidal flow over topography generates internal waves at the tidal frequency known as internal tides and the breaking of these waves leads to enhanced dissipation and diapycnal mixing over topography. This tidally driven mixing influences the large-scale circulation, affecting ocean heat and salt distribution, carbon uptake and sea level. Developing improved parameterizations of tidal mixing for general circulation models is therefore crucial. Climate model simulations are sensitive to the vertical profile of dissipation (Melet et al., 2013), an unknown in current parameterizations. At rough abyssal topography, the dominant process leading to internal wave breaking appears to be nonlinear wave-wave interactions which transfer energy to smaller vertical scales (Nikurashin and Legg, 2011). Here a parameter space study of tidal mixing over rough topography, focusing on the dependence of the vertical profile of dissipation to Coriolis frequency and the topographic wavelength, is conducted using two-dimensional MITgcm numerical simulations. Four different Coriolis frequencies are considered for six different topographic wavelengths, encompassing both subcritical and supercritical topographies. Existing parameterizations are shown to incorrectly reproduce the simulated vertical structure of the enhanced dissipation over topography. In addition, although wave-wave interactions leading to dissipation are known to peak at the critical latitude before decreasing once again towards the poles, a peak in dissipation at the critical latitude is observed for only some of the six topographic wavelengths. Further simulations attempt to distinguish the effects of topographic steepness on the wave breaking and dissipation from that of topographic wavelength. The effects of Coriolis frequency, topographic wavelength and topographic steepness need to be included in parameterizations of tidal mixing over rough topography to more accurately simulate climate states.