A NUMERICAL-METHOD FOR FINDING FORM-PRESERVING FREE SOLUTIONS OF THE BAROTROPIC VORTICITY EQUATION ON A SPHERE

A numerical method is presented that allows one to obtain steady or zo nally moving (i.e., form-preserving) solutions of the barotropic vorti city equation on a sphere. The solutions are characterized by nonlinea r functiOnal relationships between the streakfunction (the streamfunct ion in a steady or zonally moving frame of reference) and the absolute vorticity. Three new examples of steady solutions are obtained that r esemble atmospheric blocking. The examples have approximately the same global structure but differ in the degree of uniformity of the stream function and the absolute vorticity in the blocking region. From a num erical linear stability analysis it is concluded that the example in w hich the absolute vorticity in this region is the most uniform is to b e preferred as a description of atmospheric blocking. This conclusion is consistent with recent observational evidence.