A Comparison of the Weakly Nonlinear Instability of Westerly and Easterly Jets in a Two-Layer Beta-Plane Model

Abstract

Abstract The nonlinear evolution of disturbances that emanate from unstable westerly and easterly jet profiles is examined with a two-layer quasi-geostrophic β-plane channel model. Significant differences arise between the solutions for westerly and easterly jets. For unstable narrow (width less than the deformation radius) westerly jets, the disturbance undergoes a sequence of life cycles characterized by barotropic growth and barotropic decay. Unstable easterly jets also give rise to a series of life cycles in which the disturbance grows and decays in a combined baroclinic/barotropic manner. In each of these cases, the disturbance structure remains close to its unstable normal mode form throughout the life cycle. This contrasts with earlier results of Feldstein and Held who find that disturbances emanating from unstable wide (width greater than the deformation radius) westerly jets undergo a change in meridional structure, enhanced lateral radiation, and a single life cycle consisting of baroclinic grow...