Edwin Gerber
Columbia University
Dept. of Applied Physics and Applied Mathematics
"The Dynamics of the NAO 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 have developed a hierarchy of idealized models to isolate the dynamics governing the spatial and temporal structure of these patterns. Today I will focus on a study with a dry, primitive equation general circulation model. Annular Mode and NAO-like patterns are created by varying the zonal structure of the synoptic variability with 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. With respect to the temporal structure, we find a parameter sensitive coupling between eddies and the large-scale flow that extends the persistence of the variability on timescales of 10-100 days. The intraseasonal circulation anomalies, which are themselves primarily barotropic, appear to influence the baroclinicity by shifting the critical latitudes, thus shaping the eddy momentum fluxes. The feedback loop, however, is very sensitive to zonal asymmetries in the synoptic variability.