Modes of variability in a low-order two-level model

A low-order, two-level model in a beta-plane channel is integrated over long periods of time to study low-frequency mid-latitude variability. The model allows for nonlinear wave-wave and wave-mean flow interactions as well as zonally asymmetric forcing by topography. Time scales for the forcing and damping terms are varied, and, for closer analysis, values are chosen according to best estimates for the earth. A damping parameter, compensating for unresolved scales, is introduced at the shortest resolved length scale to prevent a spurious build-up of energy at this scale. Flow patterns are produced which are highly variable and seemingly chaotic, not suggestive of trajectories around multiple equilibria or distinct strong attractors in phase space. Analysis of the model integrations reveals the existence of low-frequency variability in the form of slowly-propagating Rossby waves, wave amplitude modulation and persistent anomalies in the height fields, which satisfy magnitude and duration criteria. Such anomalies have an equivalent barotropic structure and tend to occur in certain preferred regions and have preferred signs for larger threshold criteria. Similar anomalies were found in integrations without topography, but were evenly distributed geographically, fewer in number and not as long-lived.