Abstract
We examine some of the effects of “partial equilibration” and an Ekman lower boundary condition on baroclinic instability by considering the linear quasi-geostrophic stability of two classes of zonal flows. These flows, which are simple models of the partially equilibrated basic states that may result when a growing baroclinic wave modifies the constant shear zonal wind profile of the Charney problem, have reduced vertical shear and negative or zero potential vorticity gradients at low levels. The unstable eigenmodes of increasingly equilibrated zonal wind profiles have, in general, increased amplitudes in the mid- and upper troposphere, and the growth rates of these instabilities have decreased sensitivity to the presence of an Ekman lower boundary condition. To the extent that the nonlinear equilibration of a baroclinic wave can be modeled by wave-mean flow interactions, our results are consistent with the upper-level maxima in eddy amplitudes and fluxes found in both observations and nonlinear numerical ‘life-cycle’ calculations. The wave overreflection properties of the basic states provide insight into many of the characteristics of the instabilities.
