Multiple Flow Equilibria in the Atmosphere and Blocking

Abstract

A barotropic channel model is used to study the planetary-scale motions of an atmosphere whose zonal flow is externally driven. Perturbations are induced by topography and by d barotropic analogue of thermal driving. The use of highly truncated spectral expansions shows that there may exist a multiplicity of equilibrium states for a given driving, of which two or more may be stable. In the case of topographical forcing, two stable equilibrium states of very different character may be produced by the same forcing: one is a “low-index” flow with a strong wave component and a relatively weaker zonal component which is locked close to linear resonance; the other is a “high-index” flow with a weak wave component and a relatively stronger zonal component which is much farther from linear resonance. It is suggested that the phenomenon of blocking is a metastable equilibrium state of the low-index near-resonant character. The existence of the two types of equilibria has been confirmed by numerical integr... Abstract A barotropic channel model is used to study the planetary-scale motions of an atmosphere whose zonal flow is externally driven. Perturbations are induced by topography and by d barotropic analogue of thermal driving. The use of highly truncated spectral expansions shows that there may exist a multiplicity of equilibrium states for a given driving, of which two or more may be stable. In the case of topographical forcing, two stable equilibrium states of very different character may be produced by the same forcing: one is a “low-index” flow with a strong wave component and a relatively weaker zonal component which is locked close to linear resonance; the other is a “high-index” flow with a weak wave component and a relatively stronger zonal component which is much farther from linear resonance. It is suggested that the phenomenon of blocking is a metastable equilibrium state of the low-index near-resonant character. The existence of the two types of equilibria has been confirmed by numerical integr...