 Stationary zonal-wavenumber-two Rossby-wave condition

In the uniform-current beta-plane model, $U=\beta /(k_2^2+l^2)$ with $k_2=2/(a\cos \varphi )$. This is a model-dependent balance between eastward advection and westward intrinsic propagation, not a unique speed set by zonal wavenumber alone.