TRR 181 Seminar "Argo versus IDEMIX: evaluating the internal wave model IDEMIX using finestructure methods"

The internal wave model IDEMIX describes the generation, propagation and dissipation of internal gravity wave energy. By relating the vertical diffusivity and the turbulent kinetic energy (TKE) dissipation rate to the energy transferred out of the internal wave spectrum, the model provides an energetically consistent description of ocean mixing induced by breaking internal gravity waves. Coupled to a global ocean model, it is evaluated against Argo-based finestructure estimates of TKE dissipation rates using the Gregg-Henyey-Polzin parameterization following Whalen et al, 2012/2015. A novel method to calculate the internal wave energy levels from finescale strain or potential density information alone is presented and additionally used for the evaluation of IDEMIX. 

These fields' magnitudes and geographic variations are well reproduced by IDEMIX, but only if a fraction of the dissipated mesoscale eddy energy is added to the internal wave field at the bottom---an idealized representation of lee wave generation via eddy-topography interaction. The detailed spatial structure is less well reproduced in IDEMIX, suggesting a need to improve the forcing functions. 

The oceanic internal wave field is mainly forced by the sloshing of the barotropic tide over rough bottom topography. The associated energy transfer can be calculated based on linear theory following Bell (1975a,b) and, for a vertical normal mode treatment of the internal tides, Llewellyn Smith and Young (2002). 

These concepts describe the energy conversion in terms of the tidal velocity amplitude and the topographic slope and form the backbone of global products with realistic topography and stratification, which are i.a. used as external forcing in internal wave models such as IDEMIX. Inherent in these approaches is the assumption that the energy conversion is the same in all horizontal directions, although in reality, the geometry of topographic obstacles as well as the tidal flow strength vary. We here present a new method which allows us to resolve the horizontal direction of the internal tide generation. This is achieved by computing the energy conversion in terms of the energy flux rather than, as done in previous studies, the integrated energy sources. 

Sensitivity studies for idealized and realistic topography corroborate the quality of the new method. In idealized simulations, internal wave parameters in IDEMIX are notably modified when resolving the direction of the tidal forcing compared to the current standard of using the same average energy input in all horizontal directions.