Suppression of Baroclinic Instability in Horizontally Sheared Flows

Baroclinic instability in the presence of a meridionally sheared barotropic component to the basic flow is studied using a two-level model. The inclusion of a linear shear results in merldionally confined normal modes with growth rates much reduced compared to the unsheared case. Similar results hold for more complicated situations, such as a baroclinic jet with barotropic shear. The results are applied to life cycle calculations in which it is shown that the nonlinear decay phase of a baroclinic disturbance sets up barotropic shears whichtend to suppress further baroclinic developments. An example of the Southern Hemisphere winter mean flow is found to be strongly stabilized by virtue of a strong sheared barotropic part of the zonal flow.