= Meridional-wave instability of a two-layer Sverdrup flow
{title2=$0<l^2<2k_I^2$}
For purely meridional <wavevectors>, $k=0$, $l\ne0$, the growth rate of a uniform meridional upper-layer flow $V$ over a resting lower layer is
$$
\gamma=\frac{|lV|}{2}\sqrt{\frac{2F-l^2}{l^2+2F}}.
$$
Growth requires $V\ne0$ and $0<l^2<2F$. The perturbation has no northward velocity, so <planetary vorticity> advection does not explicitly stabilize it. At fixed normalized <wind stress curl>, $V=W/\beta$ means stronger $|\beta|$ nevertheless reduces the shear and growth rate.
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