= Closed-streamline threshold for a circular topographic anticyclone
{title2=$f\epsilon a/(2U)=1$}
For the <circular topographic anticyclone> in a positive uniform current $U$, the opposing disturbance speed is at most $Ca=f\epsilon a/2$. Below the displayed threshold $u$ remains positive everywhere, so no <streamline> can close. Above it an interior <stagnation point> at $(0,-U/C)$ is a center, while the exterior point $(0,-Ca^2/U)$ is a saddle. At equality the two meet at the rim. This is the onset criterion; once closed contours form, the <potential vorticity> of trapped fluid is not determined solely by its present upstream conditions.
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