On the asymmetry of baroclinic instability between easterly and westerly shear

Abstract

A complementary analysis of Charney-model baroclinic instability for a zonal current with easterly shear is presented. In contrast to an infinite number of unstable modes (the Charney, Green, and Burger modes) in westerly shearing currents, in easterly shearing flow, only a single unstable mode exists, corresponding to the Charney mode. For the same magnititude of vertical shear, the maximum growth rate in easterly shear is substantially smaller while the preferred wave length is considerably longer than the corresponding counterparts in westerly shear. The steering level of the most unstable mode is higher than one density scale height for easterly shear, while lower than one half density scale height for westerly shear. The asymmetry of baroclinic instability arises from the asymmetric variation of the basic state potential vorticity gradient with respect to the sign of the vertical shear due to the combined effects of the meridional variation of the Coriolis parameter and the vertical variation of density stratification.