Is the Time-Mean Northern Hemisphere Flow Baroclinically Unstable?

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 effectively neutralized. The wavy basic state is also rendered neutral when Rs reaches this value. It is argued that this magnitude of damping is within the range of observable parameters in the atmosphere. However, the precise position of the neutral point is sensitive to the relative magnitudes of temperature and vorticity damping. The latter is more efficient in stabilizing the system. For the wavy basic state, a second mode replaces the undamped mode as the fastest growing just before the neutral point is reached. This mode also resembles a midlatitude baroclinic wave train, but has a longer zonal wavelength. Zonal-mean transient fluxes of eddy temperature and momentum, and eddy kinetic energy calculated for this mode, show an improvement over the undamped and zonal-mean modes when compared with observations. It is argued that this improvement may be meaningful, particularly in an atmosphere that is close to neutral.