Collaborative Research: Multiscale modeling of internal tides at topographic generation sites: turbulence and wave energetics:

Turbulent processes associated with internal waves occurring at or near the generation sites of internal tides are key ingredients in maintaining and regulating the global ocean circulation which is a crucial component of the climate system affecting simultaneously the uptake of carbon dioxide into the ocean and the meridional transport of heat. Accurate computation of the magnitude and spatial distribution of turbulent dissipation is necessary for the development of physically based parameterizations of conversion and dissipation rates in the near-field. Qualitative changes in turbulence occur when geometry, barotropic forcing and environmental parameters change. The spatial and temporal scales of the physical processes that drive the turbulent energy dissipation during the generation of internal waves span several orders of magnitude. To address these knowledge gaps, a multi-scale approach is necessary to span the disparity between scales: from the scale of the outgoing low-mode internal tide (vertical scale is of order one kilometer, horizontal scale is of order tens of kilometers, time is of order hours) through the nonlinear formation of higher wave number modes to, finally, the turbulence events (spatial scale of meters and time scale of minutes). The integration of models across disparate scales is not only relevant to ocean sciences but also of great interest in many areas of science and engineering, e.g., the representation of turbulent boundary layer processes in medium- and long-term weather forecasting. The broader community interested in developing or applying parameterizations will have access to the simulation data and the numerical model code. Two graduate students will be trained and gain valuable experience in applying cutting edge numerical tools to a complex ocean problem.

A numerical investigation of the generation process of internal waves by barotropic tidal flow over an isolated topographic feature scales with the relevant non-dimensional parameters will be conducted. The driving hypothesis is that only the inclusion of turbulence in a realistic way can provide a correct description of the dissipation rates during generation and near-field propagation of internal waves at these sites. The scale-separation will be handled through a novel hierarchical approach that combines Large Eddy Simulation (LES) at small scales with the Stratified Ocean Model with Adaptive Refinement (SOMAR) for the large scales. The LES model, equipped with a sophisticated subgrid-scale model, is capable of providing a faithful description of turbulence, without the need of tunable parameters. The non-hydrostatic SOMAR is specifically optimized to deal with the anisotropy of the internal wave problem. The goal is to implement a two-way nested SOMAR-LES model so that the LES is driven with realistic forcing, and SOMAR receives realistic turbulent feedbacks. We will do so for a model triangular ridge at oceanic scales over a wide range of key non-dimensional parameters: overall Excursion number, obstacle criticality, inner Excursion number and length of critical slope. The simulations will be analyzed to ascertain (i) the dependence of internal wave energetics on the non-dimensional parameters, and (ii) a better understanding of stabilities, turbulence and associated dissipation rates in the near-field.