Saturation of the internal tides and induced mixing in the abyssal ocean

The presence of celestial bodies induces a barotropic current in the ocean, known as the barotropic tide. This barotropic tide interacts with the seafloor in the presence of stratification, resulting in the radiation of internal waves. This process is known as tidal conversion and the resulting internal waves are known as the internal tides. These internal tides propagate from the seafloor, can become unstable and break. As they break, they contribute to vertical mixing; this could explain measurements of high diapycnal diffusivities over the rough bathymetry of the Mid-Atlantic Ridge where diapycnal diffusivity can reach several orders of magnitude higher than the background value 0.1 cm2/s. This tidal mixing is currently believed to be important for the dynamics of the large-scale ocean circulation as a whole, especially in the deep, abyssal ocean below 1500m depth or so. Tidal mixing at depth may also play an important role in homogenizing water masses of different origin, such as North Atlantic Deep Water and Antarctic Bottom Water.

As part of an ongoing effort to develop a parametrization of wave-induced abyssal mixing, we derive a heuristic model for nonlinear wave breaking and energy dissipation associated with internal tides. We then investigate the saturation and dissipation of internal tides for idealized and observed topography samples. One of the main results is that the wave-induced mixing could be more intense and more confined to the bottom than previously assumed in numerical models. Furthermore, in our model wave breaking and mixing clearly depend on the small scales of the topography below 10 kilometers or so, which is below the current resolution of global bathymetry. This motivates the use of a statistical approach to represent the unresolved topography when addressing the role of internal tides in mixing the deep ocean.