LOW-FREQUENCY VARIABILITY AND WAVE-NUMBER SELECTION IN MODELS WITH ZONALLY SYMMETRICAL FORCING

The authors consider a two-layer quasigeostrophic model with linear su rface drag and forcing that relaxes to a zonal baroclinically unstable equilibrium state consisting of a meridionally confined temperature g radient. It is observed that the most energetic wave in the time-mean climate has near zero frequency and is not driven by upscale nonlinear energy transfers. This wave has a zonal scale near the long-wave cuto ff of the equilibrium state, and its energy balance is mainly between baroclinic generation and dissipation. This maintenance mechanism is d ifferent from that suggested by beta-plane, two-dimensional, and quasi geostrophic turbulence arguments and may be relevant to the dynamics o f zonally asymmetric low-frequency variability in the atmosphere, part icularly in the Southern Hemisphere.