Surface Quasigeostrophic Turbulence in Variable Stratification
Abstract
Numerical and observational evidence indicates that, in regions where mixed layer instability is active, the surface geostrophic velocity is largely induced by surface buoyancy anomalies. Yet, in these regions, the observed surface kinetic energy spectrum is steeper than predicted by uniformly stratified surface quasigeostrophic theory. By generalizing surface quasigeostrophic theory to account for variable stratification, we show that surface buoyancy anomalies can generate a variety of dynamical regimes depending on the stratification's vertical structure. Buoyancy anomalies generate longerrange velocity fields over decreasing stratification and shorterrange velocity fields over increasing stratification. As a result, the surface kinetic energy spectrum is steeper over decreasing stratification than over increasing stratification. An exception occurs if the nearsurface stratification is much larger than the deepocean stratification. In this case, we find an extremely local turbulent regime with surface buoyancy homogenization and a steep surface kinetic energy spectrum, similar to equivalent barotropic turbulence. By applying the variable stratification theory to the wintertime North Atlantic, and assuming that mixed layer instability acts as a narrowband smallscale surface buoyancy forcing, we obtain a predicted surface kinetic energy spectrum between k−4/3 and k−7/3, which is consistent with the observed wintertime k−2 spectrum. We conclude by suggesting a method of measuring the buoyancy frequency's vertical structure using satellite observations.
 Publication:

Journal of Physical Oceanography
 Pub Date:
 December 2022
 DOI:
 10.1175/JPOD220040.1
 arXiv:
 arXiv:2110.04242
 Bibcode:
 2022JPO....52.2995Y
 Keywords:

 Physics  Atmospheric and Oceanic Physics;
 Physics  Fluid Dynamics
 EPrint:
 19 pages, 9 figures