We present a simple model for the basic spatial and temporal structure of the large-scale modes of the North Atlantic Oscillation and annular modes. We show the spatial structure of these phenomena results as a consequence of the stirring effects of baroclinic eddies and demonstrate explicitly how such stirring, as represented by a simple random forcing in a barotropic model, leads to a variability in the zonal flow via a variability in the eddy momentum flux convergence. Control over the spatial distribution of the stochastic forcing enables the model to capture both NAO and annular mode-like variability, indicating that both are manifestations of the same phenomenon.

The simplest model, with no feedback between the jets and the stirring, does not capture all of the dynamics. In particular, the dynamics do not generate variability on very long timescales (e.g. 100-1000 days) on their own. The model rather appears to rely on low frequencies in the stochastic forcing, leading to a white spectrum at these timescales. While it is unclear whether the power in the observed NAO/annular mode patterns does continue to redden for increasingly long timescales or whether it flattens out and whitens, incorporating feedback lessens the dependence of the model on the nature of the stochastic forcing and allows for the generation of enhanced variability at low frequencies.