Folded resonance and seasonal vacillation in a thermally-forced baroclinic wave model

Abstract

A three component model, somewhat more general than a recent model of Lorenz, is studied in order to illustrate the possibilities for quasi-resonant interaction of long baroclinic waves and thermal-monsoonal forcing. The model is shown to be capable of exhibiting a "folded resonance" equilibrium portrait suggestive of that which seems to be required to account for the observed bimodality of the atmospheric long-wave amplitude. One of the modes is associated with a stable, equilibrated, thermally forced wave, while the other is associated with a stable limit cycle about an unstable equilibrium. This limit cycle is activated in winter, leading to vacillatory fluctuations characterized by a long residence in a state of high amplitude and fixed phase that may correspond to one of the two modes observed.