The Destabilization of Rossby Normal Modes by Meridional Baroclinic Shear*
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The Rossby normal modes of a two-layer fluid in a meridional channel of width L-* are altered by the presence of a meridional flow with a small vertical shear. The stability of the modes in the presence of the weak shear is considered. It is found that the joint presence of the Rossby modes and the vertical shear leads to baroclinic instability even for arbitrarily small values of the shear. The results are used to explain previous numerical calculations of the persistent instability of meridional flows when the ratio betaL(D)(2)/V > 1, where V is the magnitude of the shear, beta is the planetary vorticity gradient, and L-D is the deformation radius. If the flow were zonal it would be stable for such weak shears. The growth rates are weak when betaL(D)(2)/V >> 1 and each unstable mode exists in a narrow range of meridional wavenumber. The asymptotic results qualitatively agree with the earlier numerical results at moderate values of the same parameter.