Vortex stretching in layered quasi-geostrophic flow
= Vortex stretching in layered quasi-geostrophic flow
The stretching contribution to upper-layer <potential vorticity> is $-F(\psi_1-\psi_2)$, with the opposite sign below. An interface displacement thickens one layer and thins the other. In a parcel's <potential-vorticity equation>, its material change balances changes of relative and <planetary vorticity> and any forcing. The two layer derivatives cannot in general be replaced by one common advection operator.