Parameterization of submesoscale symmetric instability in dense flows
along topography
Abstract
We develop a parameterization for representing the effects of
submesoscale symmetric instability (SI) in the ocean interior. SI is an
important contributor to water mass modification and mesoscale energy
dissipation throughout the World Ocean. Dense gravity currents forced by
surface buoyancy loss over shallow shelves are a particularly compelling
test case, as they are characterized by density fronts and shears
susceptible to a wide range of submesoscale instabilities. We present
idealized experiments of Arctic shelf overflows employing the GFDL-MOM6
in z* and isopycnal coordinates. At the highest resolutions, the dense
flow undergoes geostrophic adjustment and forms bottom- and
surface-intensified jets. The density front along the topography
combined with geostrophic shear initiates SI, leading to the onset of
secondary shear instability, dissipation of geostrophic energy, and
turbulent mixing. We explore the impact of vertical coordinate,
resolution, and parameterization of shear-driven mixing on the
representation of water mass transformation. We find that in isopycnal
and low-resolution z* simulations, limited vertical resolution leads to
inadequate representation of diapycnal mixing. This motivates our
development of a parameterization for SI-driven turbulence. The
parameterization is based on identifying unstable regions through a
balanced Richardson number criterion and slumping isopycnals towards a
balanced state. The potential energy extracted from the large-scale flow
is assumed to correspond to the kinetic energy of SI which is dissipated
through shear mixing. Parameterizing submesoscale instabilities by
combining isopycnal slumping with diapycnal mixing becomes crucial as
ocean models move towards resolving mesoscale eddies and fronts but not
the submesoscale phenomena they host.