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Parameterization of submesoscale symmetric instability in dense flows along topography
  • Elizabeth Yankovsky,
  • Sonya Legg,
  • Robert W. Hallberg
Elizabeth Yankovsky
Princeton University, NOAA-GFDL

Corresponding Author:eyankovsky@gmail.com

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Sonya Legg
Princeton University
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Robert W. Hallberg
NOAA/Geophysical Fluid Dynamics Laboratory
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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.
Jun 2021Published in Journal of Advances in Modeling Earth Systems volume 13 issue 6. 10.1029/2020MS002264