Internal vacillations in stratosphere-only models

An investigation is made of steady-state and vacillating regimes, in an idealized numerical model of the winter stratosphere, to improve understanding of possible mechanisms for variability in the real stratosphere. Variability obtained here is internal to the model in the sense that all external parameters are kept constant throughout the model integrations. The model used allows arbitrary height and latitude structure in waves and zonal-mean flow, so the investigation extends previous work on steady and vacillating regimes in channel models that include only a single mode to represent the latitudinal structure.

The variation of model behavior with two parameters, the "strength" of the radiative basic state and the amplitude of the lower-boundary wave forcing, is investigated. For each radiative, slate considered, two qualitatively different vacillating regimes and two different steady states are obtained for different values of wave forcing. The vacillating regimes are interpreted in terms of wave and mean flow diagnostics, and the controlling mechanisms are shown to depend upon the latitudinal degrees of freedom. The steady-state regimes are found to be both obtainable under identical external conditions, that is, the model exhibits multiple-flow equilibria. The property of multiple-flow equilibria is known to exist in the simpler height-only channel models, but appears not to have been found before in models with both height-and latitude structure.