Barotropic Model Simulations of the North Pacific Retrograde Disturbances

Abstract

Branstator?Kushnir-type large-scale westward propagating waves are investigated using linear and nonlinear global barotropic models with an idealized zonally asymmetric basic state. Retrograde waves are found in the most unstable normal mode of the zonally asymmetric basic state with a jet in the Northern Hemisphere. West-ward propagating waves also exist in nonlinear equilibrium states under a wide range of supercriticality and in both periodic and chaotic regimes. The frequency of the most unstable mode remains as a peak in the frequency spectrum through the nonlinear equilibration process. That frequency matches the frequency of the westward propagating waves in the nonlinear equilibrium states. Local energetics analyses of the linear and nonlinear cases show that the barotropic energy conversion concentrated in the jet exit supplies the perturbation energy of the disturbances all over the globe. Under a traditional spherical-harmonic decomposition, the westward propagating waves consist of several spherical-harmonic components. In the weakly chaotic nonlinear equilibrium states, these components show higher regularity in time than the others and may possess higher predictability.