A Test for Annular Modes

The use of empirical orthogonal functions (EOFs) has grown popular as a tool to determine underlying variability from the rapidly increasing volume of climate data. It has been noted that the dominant or first EOF of geopotential heights, in each hemisphere at levels from the surface through the troposphere and into the midstratosphere, appears to be zonally symmetric. It has also been suggested that all of the first EOFs, throughout the atmospheric column are barotropically coupled and annular. Moreover, such modes of variability in both hemispheres are thought to be analogous to each other. To define annularity more objectively and to facilitate comparisons both temporally and spatially, a framework has been formulated within which modes of variability may be tested for their degree of zonal symmetry or annularity. Motivated by previous choices, pressure–height fields in each hemisphere are tested for annularity, one near the surface and the other in the midstratosphere. Periods chosen coincide with times when the troposphere and stratosphere are actively coupled. According to the test for annularity on the first mode of variability, these fields can be ranked in order of degree of annularity: the first EOF of Northern Hemisphere (NH) December–January–February (DJF) 50-hPa geopotential height is annular; the first EOF of Southern Hemisphere November 50-hPa geopotential height is weakly annular; the first EOF of Southern Hemisphere November 850-hPa geopotential height is weakly nonannular; and the first EOF of NH DJF sea level pressure is nonannular.