Transitions between Baroclinic Flow Regimes

Abstract

we use truncated spectral Eady models with two Ekman layers of different strength to investigate the baroclinic flow transitions observed in annulus experiments. Our analysis is both analytical and numerical As the dissipation parameter is varied in a single y-mode model a sequence of transitions occur between the axisymmetric flow, steady waves, single-, double-, and multiple-period amplitude vacillations, and aperiodic flow regimes. A model with two y-modes describes transitions between axisymmetric flow, steady waves, amplitude vacillation and structural vacillation. From these models we find that the transition from axisymmetric flow to steady waves is due to the baroclinic instability of the basic parallel flow, while the transition from steady waves to amplitude vacillation is due to the instability of the steady, sinπy-wavy flow with respect to a sinπy-mode perturbation with the same zonal wavenumber. The transition from amplitude vacillation to structural vacillation is due to the instability of the vacillating, sinπy-wavy flow with respect to a sin2πymode perturbation with the same zonal wavenumber. Our work suggests that most vacillations in the atmosphere may be viewed as a mixed vacillation, which in our model occurs in the transition region between amplitude and structural vacillations.