On the spatial and temporal structure of the NAO and annular modes

We investigate the spatial and temporal structure of the large-scale modes of intraseasonal variability in the atmosphere, in particular the North Atlantic Oscillation (NAO) and annular modes. Through a series of experiments with a dry primitive equation model, we probe the factors controlling the zonal structure of extratropical variability and identify the physical processes determining the timescales of the variability. We argue that both the NAO and annular modes are naturally found in the atmosphere, given the geometry of the midlatitude circulation and the conservation of mass and momentum by fluid motions. Both patterns arise as a nonlinear response to baroclinic stirring, and the differences in their zonal structure follow from the zonal asymmetries in baroclinic activity, that is, the presence of zonally localized storm tracks. This is demonstrated through a series of experiments where the initial zonal symmetry of the model is broken by the addition of topography and/or anomalies in the model's temperature forcing, which approximate the effects of land-sea contrast. While the spatial structure of the variability is quite robust, the temporal structure, particularly the persistence of the patterns, is more sensitive to model configuration, with the greatest sensitivity found in the stratosphere. Furthermore, variability is found to persist on timescales longer than would be suggested by a linear view of the system. Nonlinear interactions thus appear to provide the key role in setting the timescales of the NAO and annular modes.