Rossby Wave Critical Layers on a Jet

Abstract

It is argued using simple examples that while free, barotropic Rossby wave critical layers on unbounded stable monotone profiles are difficult to excite with smooth initial data, they are easily excited on stable jets when the background absolute-vorticity gradient is sufficiently weak. After sufficient time the critical layer, which is located at the jet maximum, becomes locally nonlinear in the absence of dissipation, regardless of the initial amplitude of the disturbance. Even though the nonlinear terms are smallest at the critical layer, they dominate due to the smallness of the linear terms there. We also speculate on how other nonmonotone profiles might be treated.