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Abstract
We present a hierarchy of idealized models of the North Atlantic Oscillation (NAO) and Annular Modes, the dominant patterns of intraseasonal variability in the extratropical atmosphere. The models isolate the dynamics governing the spatial and temporal structure of the patterns, and provide a framework for interpreting measures of the variability produced by Empirical Orthogonal Function and teleconnection analysis.
A barotropic model on the sphere indicates that the patterns are a consequence of the chaotic "stirring" of the atmosphere by baroclinic eddies. Such stirring, represented by a simple random forcing, leads to variability of the zonal flow via variability in the eddy momentum flux convergence. Zonal structure of the forcing dictates zonal structure of the variability, suggesting that the NAO follows from the localized synoptic variability of the North Atlantic storm track. Intraseasonal variability is more persistent than the eddies that force it, as integration of eddy fluxes by the equations of motion strengthens power at lower frequencies.
The patterns' spatial structure is investigated with analytic, purely stochastic models of the zonally averaged zonal wind and surface pressure. The meridional structure stems from geometric constraints and the conservation of mass and zonal momentum. A two-dimensional extension of the model reveals that symmetry of the statistics, not necessarily the motions, is sufficient for the existence of an annular pattern, explaining the presence of annular modes in systems lacking hemispherically coherent motions.
Conclusions from the simpler models are verified with a dry, primitive equation general circulation model. Annular mode and NAO-like patterns are found by varying the zonal structure of synoptic variability with idealized topography and heating anomalies approximating land-sea contrast. The NAO arises from the confluence of topographic and thermal forcing, and is best understood in terms of the eddy life cycle. A parameter sensitive coupling between eddies and the large-scale flow extends the persistence and zonal coherence of the intraseasonal variability. The mainly barotropic circulation anomalies influence the baroclinicity by shifting the critical latitudes, thus shaping the eddy momentum fluxes. Sensitivity of the coupling to asymmetric forcing suggest that localized NAO-like variability is more dominant when eddy-mean flow interactions are weakened.
