Anisotropic internal tide forcing in the consistent internal wave mixing scheme IDEMIX

Breaking internal gravity waves cause small-scale turbulent mixing, which changes water mass properties, affects biogeochemical cycles, and contributes to driving the large-scale overturning circulation. Ocean general circulation models do not resolve this process and thus rely on a parameterization. The state-of-the-art IDEMIX (Internal wave Dissipation, Energy and MIXing) model predicts the propagation and dissipation of internal wave energy based on external forcing functions that represent the main generation mechanisms, notably the internal tide generation at the sea floor and the near-inertial wave generation at the sea surface. By linking small-scale mixing to internal wave energetics, IDEMIX allows the consistent parameterization of wave-induced mixing in ocean models. Its basic incarnation treats all internal waves as part of a horizontally homogeneous continuum and was shown to successfully reproduce observed turbulent kinetic energy dissipation rates and internal wave energy levels. In a newer configuration (IDEMIX2), the internal wave field is compartmentalized, distinguishing between a high-mode continuum on the one hand and low-mode near-inertial wave and internal tide compartments, whose horizontal propagation is explicitly resolved in wavenumber angle space, on the other hand. We present the evaluation of the IDEMIX2 model with a particular focus on the impact of applying an anisotropic internal tide forcing. So far, parameterizations of internal tide-driven mixing have not taken the strong anisotropy of the internal tide generation process into account. We demonstrate the need for doing so, showing a notable impact on the modeled internal wave energetics and predicted mixing when changing from the previous isotropic to the new anisotropic tidal forcing in IDEMIX2.