The Most Unstable Long Wavelength Baroclinic Instability Modes

There has been an apparent inconsistency between the comparatively large growth rates given for long-wavelength baroclinic instability modes for jet flows on the sphere and the small values given by the Charney and Green models. It is shown that this discrepancy is due to the consideration of a fixed meridional structure in the quasi-geostrophic theories. When this restriction is removed there is good agreement. For reasonable parameters, the stabilizing effect of the β-parameter is no more important for the most unstable mode at wavenumber 1 than it is at wavenumber 5. The most unstable wavenumber 1 still has essentially the structure of an Eady mode.