Polar accumulation of cyclonic vorticity

The drift of coherent vortices on a background gradient of potential vorticity has been previously studied in the case of uniform gradient. Here an extension is made to the case where the background gradient varies with a radial coordinate in an approximation to the variation of planetary potential vorticity on a rotating sphere. It is found that accumulation of cyclonic vorticity at the pole occurs provided the initial vortex anomaly exceeds the polar value of potential vorticity by approximately 12%. Although polar accumulation becomes slower as the deformation radius decreases, it persists for values as low as about 0.025 of the planetary radius. Polar accumulation of cyclonic vorticity is also found to persist in fully turbulent flows emerging from a large number of coherent vortex anomalies. In this case, a mixed zone in potential vorticity develops in a polar surf zone surrounding the polar cyclone, with a sharp jump at the surf zone edge defining a distinct subpolar jet whose structure depends on the deformation radius. The results are discussed in the context of the coherent polar cyclones and subpolar jets observed on the giant planets.