q 2000 American Meteorological Society Annular Modes in the Extratropical Circulation. Part I: Month-to-Month Variability*

• Main Authors: David W. J. Thompson, John, M. Wallace
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1998
• Subjects: Arctic; Midwinter
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.620.2667; http://www.atmos.colostate.edu/ao/ThompsonPapers/ThompsonWallaceJClim.pdf

The leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep, zonally symmetric or ‘‘annular’ ’ structures, with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude. The structure and dynamics of the Southern Hemisphere (SH) annular mode have been extensively documented, whereas the existence of a Northern Hemisphere (NH) mode, herein referred to as the Arctic Oscillation (AO), has only recently been recognized. Like the SH mode, the AO can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields. In this paper the structure and seasonality of the NH and SH modes are compared based on data from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis and supplementary datasets. The structures of the NH and SH annular modes are shown to be remarkably similar, not only in the zonally averaged geopotential height and zonal wind fields, but in the mean meridional circulations as well. Both exist year-round in the troposphere, but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave–mean flow interaction: midwinter in the NH and late spring in the SH. During these ‘‘active seasons,’ ’ the annular modes modulate the strength