Polar vortex reversal experiment in a rotating shallow water

A rotating shallow water experiment with hemispheric beta effect was pursued. In the experiment, the steady planetary waves with zonal wave number one or two in initial westerly polar vortex are excited at lower latitudes. As a result of polar-ward propagation of the planetary waves, the initial polar vortex was reversed into easterly polar vortex at higher latitudes. One of experimental parameters is the Rossby number which was defined as the ratio of the strength of initial polar vortex to the system rotation. The larger value of the Rossby number had, the weaker reversal was happened, in both cases that the forcing wave number is one or two. In other word, the weak polar vortex is easily reversed, and the reversal of strong vortex is hard to happen. This property of the polar vortex reversal suggests a positive feedback mechanism which may explain the robustness of the Arctic Oscillation pattern with intra-seasonal, inter-annual and inter-decadal time scales. That is, in the negative (weak westerly) phase of the Arctic Oscillation, once the reversal would happen, the strength of the revived polar vortex would be small. Accordingly, the secondary reversal would happen much more easily. As a result, the negative phase of the Arctic Oscillation as a temporal mean state may have a tendency to be maintained. Vice versa, the positive (strong westerly) phase also may be maintained.