Macroturbulent Equilibration in a Thermally Forced Primitive Equation System

Abstract

A major question for climate studies is to quantify the role of turbulent eddy fluxes in maintaining the observed ocean–atmosphere state. It has been argued that eddy fluxes keep the midlatitude atmosphere in a state that is marginally critical to baroclinic instability, which provides a powerful constraint on the response of the atmosphere to changes in external forcing. No comparable criterion appears to exist for the ocean. This is particularly surprising for the Southern Ocean, a region whose dynamics are very similar to the midlatitude atmosphere, but observations and numerical models suggest that the currents are supercritical.

This paper aims to resolve this apparent contradiction using a combination of theoretical considerations and eddy-resolving numerical simulations. It is shown that both marginally critical and supercritical mean states can be obtained in an idealized diabatically forced (and thus atmosphere-like) Boussinesq system, if the thermal expansion coefficient is varied from large atmosphere-like values to small oceanlike values. The argument is made that the difference in the thermal expansion coefficient dominantly controls the difference in the deformation scale between the two fluids and ultimately renders eddies ineffective in maintaining a marginally critical state in the limit of small thermal expansion coefficients.