Equilibration of an atmosphere by geostrophic turbulence

Abstract

A major question for climate studies is to quantify the role of turbulent eddy fluxes in maintaining the observed atmospheric mean state. It has been argued that eddy fluxes keep the mid-latitude atmosphere in a state that is marginally critical to the deepest mode of baroclinic instability, which provides a powerful constraint on the response of the atmosphere to changes in external forcing. A similar criterion does, however not hold in the Southern Ocean, a region whose dynamics are otherwise very similar to the mid-latitude atmosphere. This thesis resolves this apparent contradiction, using a combination of theoretical considerations and eddy-resolving numerical simulations. It is shown that the adjustment of the extra-tropical troposphere to states of marginal criticality does not follow from a fundamental constraint, but is rather the result of the particular parameters characterizing Earth's troposphere. Both marginally critical and strongly supercritical zonal mean flows can be obtained in planetary atmospheres if external parameters are varied. We argue that changes in the equilibrated mean state over a wide range of simulations can better be understood in terms of a balance between the diabatic forcing and the eddy driven overturning circulation. Using a diffusive closure for the eddy flux of potential vorticity, we can relate the eddy-driven overturning transport to properties of the mean flow, and derive scaling relations for both the baroclinicity and vertical stratification of the equilibrated state.