Internal Wave-Driven Turbulent Mixing: an Argo-Based finescale strain approach versus the global internal wave model "IDEMIX"

Nonlinear wave-wave interactions transfer energy in the internal wave field from the large generation to the small dissipation scales and thereby link internal wave energetics to turbulent mixing. This mixing is considered an essential contributor to driving the large-scale overturning circulation, but cannot be resolved in ocean general circulation models. In order to represent it consistently, recently developed parameterizations involve internal wave dynamics, taking into account that breaking internal waves are thought to be a major source of small-scale turbulence.

The model IDEMIX (``Internal Wave Dissipation, Energy and Mixing'') predicts the propagation and dissipation of oceanic internal gravity waves as well as the corresponding diapycnal diffusivities based on a simplification of the spectral radiation balance of the wave field and can be used as a mixing module for global numerical simulations. The model is validated against finestructure estimates of turbulent kinetic energy dissipation rates obtained from Argo-float CTD-profiles following the approach by Kunze et al. (2006).

These estimates are sensitive to the shear-to-strain ratio $R_{\omega}$, which has to be set to a constant value when using CTD-data only, or to the version of the Garrett-Munk model, that features as a reference in the parameterization, but the related uncertainties lie within the general uncertainty range of the method (factor 2-4). The spatial variation is influenced both by bottom topography and surface winds and features a noticeable seasonal cycle. The IDEMIX-model in general reproduces both the magnitude and the spatial variations of the observed dissipation rates. Sensitivity experiments show that the dissipation rate's strength and pattern (especially in the Gulf Stream) cannot be explained without taking meso-scale eddy energy into account. The observed seasonal cycle, too, can in the model only be explained by the seasonal variations in eddy kinetic energy dissipation. A detailed fine-tuning of the IDEMIX-module will be attempted based on parameters like the symmetrization time scale of the internal wave field, using a global ocean general circulation model and the newer version of IDEMIX, that not only describes the internal wave continuum but treats near-inertial waves and internal tides separately.