Non-linear effects in internal-tide beams, and mixing

A non-linear non-hydrostatic model (MIT-gcm) is used to study the generation and propagation of internal tides. The model domain covers a continental slope and neighbouring parts of the deep ocean and shelf. Uniformity in the along-slope direction is assumed. We focus on the non-linear evolution of the internal tide once generated. In particular, we show that in the main region of generation, over the upper part of the slope, small-scale features occur, indicative of breaking and mixing. Far from the generation region, non-linear processes are important in the reflection of the beam at the bottom, where higher harmonics are generated. This implies an energy transfer toward higher frequencies and the resulting shape of the energy spectra is consistent with observations. Turbulent and mixing processes are analysed by employing an adiabatic sorting method; thus, we calculate the development in time of the available potential energy, the variation in the background potential energy due to irreversible processes, and the distribution of the Cox number (the local turbulent diffusivity normalized by the background diffusivity) over the slope. With rotation, the transfer of energy to higher harmonics is reduced.