Tidal conversion into vertical normal modes by continental margins

The largest source of internal waves in the ocean is tidal currents flowing across rough bottom topography, resulting in internal tides. When these waves break, they release some of their energy to the background ocean in the form of diapycnal mixing. The distribution of this internal tide-driven mixing has a significant impact on the ocean state. The parameterization of internal tide-driven mixing in climate models is based on estimates of the generation of internal tides, or tidal conversion. Semi-analytical methods based on linear wave theory have been used to compute the tidal conversion from the observed bottom topography, ocean stratification, and tidal currents. However, such linear calculations fail completely at steep continental slopes. Here, we construct a computationally inexpensive method to estimate the tidal conversion into vertical normal modes by continental slopes and shelves, and apply it at the global scale. It uses the usual observational data as inputs but relies on a reduced-physics numerical model rather than on linear theory to compute the tidal conversion. The method also resolves the onshore and offshore energy fluxes. The output is validated with the conversion diagnosed from a realistic simulation. The results are useful for parameterizing the subsequent propagation and breaking of the internal tides and their resulting diapycnal mixing.