Feedbacks between Eddy Heat Fluxes and Radiative Heating in an Energy-Balance Model

Abstract

The response of midlatitude temperature structure to changes in radiative forcing is examined in an analytical energy-balance model that includes parameterized eddy heat fluxes and linear radiative heating. The characteristics of heat-transporting baroclinic waves are determined within the model, while simultaneously allowing the waves to adjust the mean-state static stability and meridional temperature gradient. By changing the radiative equilibrium temperature, the forcing is varied through a very wide range in order to investigate qualitative, limiting behavior of the baroclinic eddy feedbacks. The flux parameterization is based on Charney's continuously stratified, ?-plane model of baroclinic instability and incorporates a parameter which is analogous to the supercriticality (i.e., degree of instability) of the Phillips' two-level model. A baroclinic adjustment hypothesis as proposed by other investigators suggests that interaction between baroclinic fluxes and radiative heating keeps the extratropical atmosphere near neutral baroclinic stability, i.e., zero supercriticality. Although the model considered here allows for this feedback, this behavior does not occur. The model meridional temperature gradient is primarily dependent on its radiative equilibrium value and is insensitive to changes in the static stability of radiative equilibrium. The negative feedback between meridional temperature gradient and eddy heat flux is enhanced as the meridional forcing increases and the eddy flux becomes more efficient. Static stability is very sensitive to changes in the meridional forcing reflecting the strong dependence of vertical heat flux on meridional temperature gradient. A comparison of observed seasonal variation of the baroclinic stability parameter with model results suggests that a stabilizing process like moist convection is important in determining the midlatitude static and baroclinic stability during Northern Hemisphere summer.