Modons with Uniform Absolute Vorticity

Abstract

In recent observational studies of atmospheric blocking it was found that the blocking region is characterized by relatively low and uniform values of the isentropic potential vorticity. It will be shown that the barotropic vorticity equation on a sphere has modon solutions with similar properties. In these solutions the absolute vorticity is relatively low and uniform in a circular subregion of the sphere. Inside this region, to be associated with the blocking region, the streamfunction has a north?south oriented dipole structure. Outside it the stream-function gives rise to a wavy westerly background flow. The spatial flow pattern and linear stability properties of a representative example are discussed and compared with examples of previously obtained modons. The merits and drawbacks of the new solutions as possible descriptions of atmospheric blocking are considered in the light of the studies mentioned above.