= Quasi-geostrophic wave pseudomomentum
{title2=$\mathcal A=\overline{q'^2}/(2Q_y)$}
For small <quasi-geostrophic approximation> waves on a locally uniform basic zonal flow with nonzero meridional <potential-vorticity gradient> $Q_y$, this signed quadratic <wave activity> obeys $\mathcal A_t+\nabla\cdot\mathbf F=\overline{q'\mathcal D}/Q_y$, where $\mathbf F$ is the <Eliassen–Palm flux> and $\mathcal D$ is a PV source. Multiply the linear PV equation by $q'/Q_y$ and use the <Taylor identity for quasi-geostrophic flux> to obtain it. In a locally uniform conservative wave packet, $\mathbf F=\mathcal A\mathbf c_g$, so the flux follows the <group velocity>, with its sign determined by $Q_y$.
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