Remote Forcing of Internal Waves in Regional Ocean Models

Regional oceanic general circulation models with nested grids are an essential approach to allow the use of higher grid resolutions. High resolution is required for the study of smaller scale processes, such as submesoscale currents and the internal wave field, in particular the baroclinic tide. Limits of available computing power determine the size of the computational grid, setting the maximum size of the modeled domain. A serious problem arises because internal waves can propagate over long distances, providing a remotely forced component for the wave field in any model that doesn't cover a complete oceanic basin. Current approaches in regional oceanic modeling are not adequate. The often used approach of radiative-restoring boundary conditions that work well for flows at sub-inertial time-scales are inadequate to force sufficiently energetic high-frequency internal waves. Prescribed (or ``clamped'') boundary conditions are susceptible to spurious reflection of energy at the boundaries, leading to an overestimate of internal wave energy inside the regional model domain. We propose a combination of a dynamic tuning between propagating and externally prescribed boundary conditions based on matching the vertically-integrated, high-frequency pressure flux across an open boundary. We show that this approach provides accurate internal wave energy fluxes at the boundaries of the regional domain. This innovation is both necessary and sufficient to investigate the dynamics of the internal wave field continuum and its interactions with submesoscale flows at the high spatial resolutions that regional oceanic models allow.