Brief Communications

Nature 432, 290-291 (18 November 2004) | doi:10.1038/432290b; Published online 17 November 2004

Atmospheric science: Early peak in Antarctic oscillation index

Julie M. Jones1 & Martin Widmann1

The principal extratropical atmospheric circulation mode in the Southern Hemisphere, the Antarctic oscillation (or Southern Hemisphere annular mode), represents fluctuations in the strength of the circumpolar vortex and has shown a trend towards a positive index in austral summer in recent decades, which has been linked to stratospheric ozone depletion1, 2 and to increased atmospheric greenhouse-gas concentrations3, 4. Here we reconstruct the austral summer (December–January) Antarctic oscillation index from sea-level pressure measurements over the twentieth century5 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 Antarctic oscillation index 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.

  1. Institute for Coastal Research, GKSS Research Centre, 21502 Geesthacht, Germany

Correspondence to: Julie M. Jones1 Email: jones@gkss.de

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