"The Dynamics of the North Atlantic Oscillation and Annular Modes"

Abstract

The North Atlantic Oscillation (NAO) and Annular Modes are the dominant patterns of intraseasonal variability in the extratropical atmosphere.  The NAO in particular characterizes a significant fraction of the wintertime variability in Eastern North America and Europe, and has been recognized in some form since the eighteenth century.  I'll begin with a simple model based on random walks to explain why these patterns of variability always dominate statistical analyses of the extratropical circulation.  I'll then move to a more realistic model, a dry general circulation model of the atmosphere, to probe the dynamics of the variability.  Annular Mode and NAO-like patterns are created with the addition of idealized topography and heating anomalies approximating land-sea contrast.  We find that the NAO arises from the confluence of topographic and thermal forcing, and is best understood in terms of the eddy life cycle.  We also find a parameter sensitive coupling between eddies (day-to-day weather) and the large-scale flow that extends the persistence of the variability on timescales of 10-100 days.  The feedback loop, however, is very sensitive to zonal asymmetries, consistent with the limited persistence of the observed NAO and annular modes.

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