IS THE TIME-MEAN NORTHERN-HEMISPHERE FLOW BAROCLINICALLY UNSTABLE

The dynamical stability of the Northern Hemisphere wintertime mean atm osphere is investigated in a linearized primitive equation model. In t he absence of any damping on the perturbation, exponentially growing m odes are found for the zonal-mean and zonally varying basic states. Th eir growth rates are 0.41 and 0.38 days(-1), respectively. Both have t he form of midlatitude baroclinic wave trains. Three distinct idealize d profiles of linear damping are then imposed on the perturbation vort icity and temperature. The damping is strongest below 800 mb and weak or nonexistent in the rest of the troposphere. It is specified to be p roportional at all levels to a single parameter, R-s, the strength of damping at the surface. For the zonal-mean basic state, as R-s is incr eased linearly, the growing modes decrease their growth rates almost l inearly, and change their structure only slowly. For an average dampin g timescale in the boundary layer of about one day (R-s = 2 days(-1)), the growing baroclinic modes are effectively neutralized. The wavy ba sic state is also rendered neutral when R-s reaches this value. It is argued that this magnitude of damping is within the range of observabl e parameters in the atmosphere. However, the precise position of the n eutral point is sensitive to the relative magnitudes of temperature an d vorticity damping. The latter is more efficient in stabilizing the s ystem. For the wavy basic state, a second mode replaces the undamped m ode as the fastest growing just before the neutral point is reached. T his mode also resembles a midlatitude baroclinic wave train, but has a longer zonal wavelength. Zonal-mean transient fluxes of eddy temperat ure and momentum, and eddy kinetic energy calculated for this mode, sh ow an improvement over the undamped and zonal-mean modes when compared with observations. It is argued that this improvement may be meaningf ul, particularly in an atmosphere that is close to neutral.