Concerning the effect of surface drag on the circulation of a baroclinic planetary atmosphere

Extended runs of a multi‐level spectral numerical model of atmospheric flow on a hemisphere have been used to investigate the effect of surface drag variations on the nature of the time‐averaged flow. It is shown that the most drastic effect of reducing the surface drag is to permit strong horizontal, barotropic shears in the zonal mean flow, which inhibit baroclinic instability. This superficially paradoxical result is implicit in many earlier calculations, most clearly in the lifecycle description of nonlinear baroclinic instability, but appears not to be widely appreciated. The kinetic energy spectra are interpreted in terms of the wavenumber of the most baroclinically unstable normal modes K R , the Rhines blocking wavenumber K β , and a limiting wavenumber based on surface drag K D . The changes in K R and the smaller changes in K β as drag is reduced account satisfactorily for the changes found in the kinetic energy spectra.