We examine the nonlinear evolution of barotropic β-plane jets on a periodic domain with a pseudospectral. A calculation of the linear growth rate yields an infected U-shaped curve on the β versus k0 plane which separates regions of stability and instability. This curve aids in clarifying the morphology of the nonlinear structures which evolve from monochromatic small-amplitude perturbations of wavenumber k0. At very small or zero β, we recover and further quantify previously obtained results, including formation of: dipolar vortex structures or bound pools of opposite-signed vortex regions at small k0; staggered streets isolated vortex pools at intermediate k0; and “cat-eyes” or staggered connected pools of vorticity at large but still unstable k0. As β is increased, the jet exhibits quite different evolutionary patterns. At low k0, where the laminar jet may be stable, we find a multistage instability. First, neutrally stable long-wavelength modes of small amplitude interact nonlinearly to produc... Abstract We examine the nonlinear evolution of barotropic β-plane jets on a periodic domain with a pseudospectral. A calculation of the linear growth rate yields an infected U-shaped curve on the β versus k0 plane which separates regions of stability and instability. This curve aids in clarifying the morphology of the nonlinear structures which evolve from monochromatic small-amplitude perturbations of wavenumber k0. At very small or zero β, we recover and further quantify previously obtained results, including formation of: dipolar vortex structures or bound pools of opposite-signed vortex regions at small k0; staggered streets isolated vortex pools at intermediate k0; and “cat-eyes” or staggered connected pools of vorticity at large but still unstable k0. As β is increased, the jet exhibits quite different evolutionary patterns. At low k0, where the laminar jet may be stable, we find a multistage instability. First, neutrally stable long-wavelength modes of small amplitude interact nonlinearly to produc...