SENSITIVITY OF A MULTILAYER QUASI-GEOSTROPHIC BETA-CHANNEL TO THE VERTICAL STRUCTURE OF THE EQUILIBRIUM MERIDIONAL TEMPERATURE-GRADIENT

A multi-layer, quasi-geostrophic, Boussinesq model on a beta-channel i s used to study the sensitivity of the mid-latitude storm-track to cha nges in the vertical structure of the zonal mean equilibrium meridiona l temperature gradient. Motivation for this study is taken from the ob servation that the doubled CO2 climatology of general-circulation mode ls presents a weaker gradient in the lower troposphere and a stronger gradient in the upper troposphere than in a 'control' climatology. It is observed that the dynamics of the quasi-geostrophic model are gener ally more sensitive to the lower than to the upper tropospheric temper ature gradient as a consequence of the importance of shallow eddies. I n order to give a good representation of the eddy dynamics it is neces sary to use a sufficiently high resolution, which, in the case chosen, proved to be between 10 and 20 tropospheric layers. If only three lay ers are used the general features of the response are still reproduced , but the model does not represent the sensitivities of the eddy merid ional momentum-flux convergence adequately. It is argued that this is connected with the particular choice taken when parametrizing the surf ace Ekman friction. When a higher-resolution model is used, the eddy m omentum-flux convergence proves to be more sensitive to changes in the upper than in the lower meridional equilibrium temperature gradient b ut is generally much less responsive to these changes than other quant ities like the surface eddy potential-vorticity flux or eddy kinetic e nergy. For each variable the two sensitivities become close to each ot her as the baroclinicity of the equilibrium now is increased because o f the prevalence of deep eddies in the dynamics, sensitive only to the total radiative equilibrium wind vertical gradient. For very high val ues of the baroclinicity of the equilibrium flow the sensitivities app roach a constant value in accordance with a simple scaling argument.