Remote Forcing of Internal Waves in Regional Ocean Models

Jeroen Molemaker, Pierre Damien, Devin Dollery

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.