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. As indicated ny the title of this session, there has been growing acceptance of the idea that the dominant or first EOF of geopotential heights, in each hemisphere at levels from the surface through the troposphere and into the mid-stratosphere, appears to be zonally symmetric or 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, we formulate a framework within which modes of variability may be tested for their degree of zonal symmetry or annularity. We test two pressure/height fields in each hemisphere for annularity, one near the surface and the other in the mid-stratosphere. 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 and 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 non-annular and the first EOF of NH DJF sea level pressure is non-annular.