Abstract
Stability properties of topographically forced baroclinic Rossby waves and zonal flows are investigated by the use of a two-layer, quasi-geostrophicβ-channel model. Two kinds of instabilities are found when the vertical shear of the zonal flow exceeds the minimum critical shear for the conventional baroclinic instability of the zonal flow: One is the topographic instability which is identical with that examined by Charney and DeVore (1979) and Mukougawa and Hirota (1986a) in the barotropic model. This instability appears in the near-resonant flow. The other is the baroclinic instability composed of synoptic disturbances with a horizontal modulation by effects of the forced wave. This is found to correspond to the storm-track type instability of free baroclinic Rossby waves investigated by Frederiksen (1978, 1982).
Examination of various effects of these unstable modes on the basic flow reveals that the roleof the synoptic disturbances on the transition of the weather regime is not so important as suggested by Reinhold and Pierrehumbert (1982). Alternatively, other unstable modes, such as those due to the topographic instability, are expected to cause the regime transition because their effects on the basic flow are completely different from those of the baroclinic instability.
