On the Zonal Structure of the Annular Modes

The Northern Annular Mode (NAM) and Southern Annular Mode (SAM), as obtained from EOF analysis, have a dipolar meridional structure in the extratropics but relatively little zonal structure. Recent modeling and observational work suggest that the NAM and SAM patterns lack zonal structure because they represent the superposition, over a range of central longitudes, of individual events that are more zonally localized. To test this idea, we analyze annular-mode variability in the Northern and Southern Hemisphere extratropics in the NCEP/NCAR Reanalysis over the past 25 years. We apply EOF analysis to daily surface pressure in extratropical sectors of zonal width D and then examine the structure of the EOFs a function of D. (The original NAM and SAM are recovered for D = 360 degrees longitude). We find that the meridional structure of the modes is recovered for D = 135 longitude for the SAM and for D somewhat larger for the NAM. We identify the D-scale as the zonal scale of individual annular-mode events. The localized annular-mode structures capture much, though not all, of the vertical structure of the SAM and NAM. As a byproduct of this analysis, we also find a robust synoptic scale circulation structure for D= 5 to 10 degrees longitude. At lower levels, it is monopolar in streamflow, but at upper levels it exhibits a baroclinic wave packet structure. At the time of submission of this abstract, we are exploring the dynamics of these two patterns and their possible connection in terms of baroclinic wave packet dynamics, with reference to the work of Feldstein, S.-Y. Lee and their collaborators.