Emergence of modons from collapsing vortex structures on the beta-plane

The evolution of unstable barotropic vortices is studied numerically. Exact solutions to the equation of potential vorticity conservation under the "r igid lid" condition, as well as nonsteady-state configurations, are set as initial states in the evolutionary experiments. The examined "shielded modo n" structures usually collapse within one to several synoptic periods and r adiate vortex pairs propagating westward and eastward. The latter are shown to be modons of Larichev and Reznik. The westward dipoles are identified a s "nonlocal modons," that is, vortical cores of stationary nonlinear Rossby waves. In the case of standing Stern modons, some small initial perturbati ons induce slow westward drift and subsequent collapse of the vortex struct ure due to the Rossby wave radiation, others lead to their transformation i nto Larichev and Reznik's modons. This conclusion is supported by the resul ts of a numerical integration of the linear stability problem.