Fig. 2. Schematic beta gyre asymmetry in translating coordinates. At the vortex periphery there is a three way balance among the vortex motion, radial advection of axisymmetric PV by the asymmetric flow and tangential advection of asymmetric PV by the mean vortex. Nearer the center, the asymmetric PV is zero so that the balance is between the radial advection of axisymmetric PV and the vortex motion. In repositioned coordinates the term due to vortex motion is part of the interpolation to a new vortex position rather than part of the time marching so that the latter balance is impossible. Consequently, nonzero asymmetric PV accumulates in a region where it is subject to advective rotation and axisymmetrization.