Among the first observational papers to document the existence of the Northern Hemisphere annular mode was a study by Lorenz, published in 1951, based on correlations between zonally averaged sea-level pressure and zonal wind at various latitudes. During the past decade the northern and southern hemisphere annular modes have emerged as important patterns of variability of the atmospheric general circulation on time scales ranging from months to thousands (and perhaps even millions) of years. They are hemispheric in scale, and during certain seasons they are linked to the stratospheric circulation. They owe their existence to the feedbacks between the basic state zonal wind field and synoptic scale baroclinic waves. Variations in the annular modes can be forced by a variety of processes, including stratospheric warmings, episodic increases in stratospheric aerosols following volcanic eruptions, stratospheric ozone depletion, sea surface temperature anomalies over the tropical Indian Ocean, and perhaps even by variations in solar activity.