Is the Time-Mean Northern Hemisphere Flow Baroclinically Unstable?

Abstract

Abstract The dynamical stability of the Northern Hemisphere wintertime mean atmosphere is investigated in a linearized primitive equation model. In the absence of any damping on the perturbation, exponentially growing modes are found for the zonal-mean and zonally varying basic states. Their growth rates are 0.41 and 0.38 days−1, respectively. Both have the form of midlatitude baroclinic wave trains. Three distinct idealized profiles of linear damping are then imposed on the perturbation vorticity and temperature. The damping is strongest below 800 mb and weak or nonexistent in the rest of the troposphere. It is specified to be proportional at all levels to a single parameter, Rs, the strength of damping at the surface. For the zonal-mean basic state, as Rs is increased linearly, the growing modes decrease their growth rates almost linearly, and change their structure only slowly. For an average damping timescale in the boundary layer of about one day (Rs = 2 days−1), the growing baroclinic modes are effe...