 Shorter Rossby-wave propagation inside a stationary long wave

For a stationary $n=2$, $l=0$ wave in a uniform zonal current, a perturbation $n>2$ has phase speed $U(1-4/n^2)$ and zonal group speed $U(1+4/n^2)$. Phase speed relative to the air is westward; packet propagation relative to the ground is faster downstream.