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Eddy-forced coherent structures as a prototype of atmospheric blocking

SUMMARY The relevance of nonlinear free solutions of the equivalent barotropic vorticity equation known as ‘modons’ or ‘vortex pairs’ is reconsidered as a prototype of atmospheric blocking. It is argued that modons are highly appropriate reference solutions from which to consider the dynamics of blocks accounting for their local nature, their stability and longevity. Nonlinear resonance studies with an equivalent barotropic numerical model on a P-plane channel demonstrate the ease and vigour with which vortex pairs can be excited by a vorticity-forcing function of the appropriate form. Localized nonlinear structures which can be identified with modons exist on blocking timescales even in parameter ranges which allow stationary Rossby waves. The circumstances in which synoptic systems can provide an appropriate forcing function are investigated by studying the interaction between a modon and a train of travelling vortices. It is shown that the anomalous transfer of potential vorticity induced by the straining of synoptic systems propagating in a diffluent jet is in a sense to maintain the modon against dissipation. Time sequences of the potential vorticity field from the equivalent barotropic model bear a remarkable resemblance to isentropic potential vorticity maps observed during a blocking episode and suggest that much of the detail can be understood in terms of the passive deformation of baroclinically inactive weather systems.