Variability of the Antarctic Oscillation during the Twentieth Century

The principal extratropical atmospheric circulation mode in the Southern Hemisphere, the Antarctic Oscillation (or Southern Hemisphere Annular Mode), has shown a trend towards a positive index in austral summer in recent decades, which has been linked to stratospheric ozone depletion and to increased atmospheric greenhouse gas concentrations. We reconstruct the austral summer (December-January) Antarctic Oscillation Index (AAOI) from sea-level pressure measurements over the twentieth century and find that large positive values, and positive trends of a similar magnitude to those of past decades, also occurred around 1960, and that strong negative trends occurred afterwards. This positive AAOI and large positive trend during a period before ozone-depleting chemicals were released into the atmosphere, and before marked anthropogenic warming, together with the later negative trend, indicate that natural forcing factors or internal mechanisms in the climate system must also strongly influence the state of the Antarctic Oscillation (Jones and Widmann 2004).

Jones, J. M. and M. Widmann, Early peak in Antarctic oscillation Index. Nature, 432, 290-291.