Annular modes in the extratropical circulation. Part I: Month-to-month variability

The leading modes of variability of the extratropical circulation in both h emispheres are characterized by deep, zonally symmetric or "annular" struct ures, with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 45 degrees lati tude. The structure and dynamics of the Southern Hemisphere (SH) annular mo de have been extensively documented, whereas the existence of a Northern He misphere (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 t he 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 b ased on data from the National Centers for Environmental Prediction-Nationa l Center for Atmospheric Research reanalysis and supplementary datasets. The structures of the NH and SH annular modes are shown to be remarkably si milar, not only in the zonally averaged geopotential height and zonal wind fields, but in the mean meridional circulations as well. Both exist year-ro und in the troposphere, but they amplify with height upward into the strato sphere during those seasons in which the strength of the zonal flow is cond ucive to strong planetary wave-mean how interaction: midwinter in the NH an d late spring in the SH. During these "active seasons," the annular modes m odulate the strength of the Lagrangian mean circulation in the lower strato sphere, total column ozone and tropopause height over mid- and high latitud es, and the strength of the trade winds of their respective hemispheres. Th e NH mode also contains an embedded planetary wave signature with expressio ns in surface air temperature, precipitation, total column ozone, and tropo pause height. It is argued that the horizontal temperature advection by the perturbed zonal-mean zonal wind field in the lower troposphere is instrume ntal in forcing this pattern. A companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades.