Bridging ocean general circulation models and process ocean simulations

ICM-CRM Meeting 2023: New Bridges between Marine Sciences and Mathematics, 2-10 November 2023Ocean General Circulation Models (OGCMs) are attractive tools used to quantify the impacts of mesoscale (10-100 km) or submesoscale flows (1-10 km) over global or regional scales. Computational costs associated with resolving dissipative scales (cm) are minimized through subgridscale parameterizations. These parameterizations, however, typically fall short in quantifying turbulent mixing at depth because OGCMs are hydrostatic, and their vertical resolution coarsens with depth. These limitations prevent OGCMs from capturing the energy cascade from large to small scales. Non-hydrostatic process models, on the other hand, can resolve energy pathways to ocean turbulence but they are computationally prohibitive when used over domains large enough to capture large-scale background flows. We propose a novel one-way nesting approach that incorporates large-scale forcing from OGCMs into a non-hydrostatic process model at high vertical resolution. Realistic ocean simulations with OGCMs use finite differences formulations to solve for the equations of motion with irregular boundary conditions from bathymetry and coastlines. State-of-the-art OGCMs fully resolve mesoscale flows at a global scale, while smaller scales are often resolved with regional configurations of higher spatial resolution using dynamical downscaling. Conventional nesting techniques are one-way and use the same OGCM over smaller and smaller domains with the hydrostatic approximation, but this approximation breaks when turbulent-length scales start to be resolved. Spectral codes, on the other hand, are especially well suited to use in the non-hydrostatic regime as these have higher accuracy and computational efficiency than finite differences codes and they eliminate the need for subgrid-scale parameterizations. However, they are ill-suited to handle arbitrary boundary conditions, such as those provided by realistic ocean simulations from OGCMs in nested domains. Our solution to this problem is to remove discontinuous endpoint derivatives before applying spectral methods using analytically differentiable Bernouilli polynomials. In this way, we overcome OGCMs and non-hydrostatic process model limitations by performing high-resolution non-hydrostatic simulations in limited three-dimensional domains while retaining the impact of larger scales through boundary forcing with OGCM fields. Further information can be found at • J. S. Rogers, M. D. Rayson, S. S. Ko, K. B. Winters, O. B. Fringer. A framework for seamless one-way nesting of internal wave-resolving ocean model. Ocean Modelling, 143, Article number 101462 DOI (2019)Peer reviewe