Response of Baroclinic Life Cycles to Barotropic Shear

Cyclonic barotropic shear of incrementally increasing magnitude is imposed on an idealized midlatitude jet, and the life cycles of baroclinically unstable wavenumber 6 perturbations growing on these jets are studied. When the barotropic shear parameter passes a critical value, the life cycle makes an abrupt transition from anticyclonic to cyclonic behavior. The abrupt transition in behavior is most evident in the barotropic decay of eddy kinetic energy, in the structure of the eddies as seen in potential vorticity maps, and in the nature of the zonal flow accelerations produced during the life cycle. It is suggested that the abrupt transition can be interpreted as arising from a positive feedback between eddy propagation and eddy-induced zonal flow accelerations. Dependences on the zonal scale of the eddy are also investigated. Wavenumber 8 exhibits cyclonic behavior, while wavenumber 4 exhibits anticyclonic behavior for all values of cyclonic shear considered. This dependence on zonal scale is consistent with predictions of linear WKB wave propagation theory. The sharpness of the transition in life cycle behavior has implications for medium-range predictability and for the generation of lowfrequency variability in the atmosphere.