Variability in a Nonlinear Model of the Atmosphere with Zonally Symmetric Forcing

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Abstract
The variability in a two-level nonlinear atmospheric model is examined. The model domain is spherical. The sole forcing is a zonally symmetric parameterization of the December-February insulation. An extended 1500 day run is carefully analyzed. Despite the absence of any asymmetric forcing the model produces a large amount of low-frequency variance and very red temporal spectra in subtropical and polar latitudes. The spatial signature of these low-frequency disturbances is that of quasi-stationary Rossby wave trains. It is suggested that the ubiquity of quasi-stationary Rossby wave trains in this model with no stationary asymmetric forcing is a consequence of energy cascade from the scale of baroclinic instability. The inertial cascade of energy toward larger spatial scales that is characteristic of geotrophic turbulence is terminated at a wavenumber where wave dispersion becomes as important as advection. This is precisely the scale at which Rossby waves are stationary. Hence, the cascade of energy from the scale of baroclinic instability to larger scales deposits energy preferentially into quasi-stationary Rossby waves. The tropical variance in this model is dominated by an abundance of mixed Rossby-gravity waves that are driven from the extratropics. Most extratropical waves are seen to be dissipated at their low-latitude critical line if they propagate into the tropics.

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