Forced Coherent Structures and Local Multiple Equilibria in a Barotropic Atmosphere

Abstract

We consider the response of the barotropic vorticity equation on a zonally infinite f-plane or beta-plane to a weak localized vorticity source accompanied by weak Ekman damping. By performing an expansion about the unforced, undamped problem, we derive a solvability condition determining when a slight modification of a solution to the inviscid problem can occur as a solution to the weakly forced and damped problem. This condition states simply that forcing must balance dissipation in the average along each closed streamline. Much of the degeneracy of the inviscid problem is removed by the solvability condition. The above considerations are used to show that under a fairly general set of circumstances the weakly forced system possesses a high amplitude equilibrium state (identified with blocking) and a low amplitude equilibrium state. The high amplitude response is maintained by a local nonlinear resonance phenomenon, and requires the existence of a suitable solution to the inviscid problem, such as the “modon” solution. In contrast to cases previously discussed, the multiple equilibrium mechanism we treat is not dependent on global resonance. Explicit examples of local multiple equilibria are constructed through numerical integrations on the f-plant and on a zonally infinite beta-channel. Through introduction of a reasonable amount of meridional confinement, a high amplitude solution can be obtained on the beta-plane without the use of a small radius of deformation. It is suggested that transient eddy fluxes may be able to play the role of the forcing required in our model. A tentative comparison with a blocking event is presented, indicating a number of problematic aspects of the theory.