Internal tide generation from linear theory: Supercritical slopes, directionality, and ocean mixing implications

The main forcing of the ocean’s internal gravity wave field is the interaction of the barotropic tide with the rough seafloor. This process is inherently anisotropic: the orientation of the topographic obstacles and the direction of the tidal currents determine the amount and direction of the generated internal wave of tidal frequency, the internal tide. Available global estimates of the internal tide generation, however, do not take this directionality into account. We present estimates of the global M2-tide generation into the first 10 vertical normal modes using a new method based on linear theory that resolves both magnitude and direction. Linear theory breaks down once the slope of the topographic obstacle exceeds that of the generated tidal beam. We discuss the role of such supercritical slopes at continental shelves and in the open ocean. Finally, we will use the anisotropic M2-tide generation as forcing of the internal wave model IDEMIX, the backbone of an energetically consistent parameterization of wave-induced turbulent mixing for ocean general circulation models. Both wave energy levels and turbulent kinetic energy dissipation differ substantially compared to the reference scenario with the previously used isotropic tidal forcing. This underlines the importance of resolving the directionality of the internal tide generation in parameterizations of wave-induced turbulent mixing.