A non-linear response of the Antarctic Oscillation to stratospheric ozone depletion

The effect of stratospheric ozone loss on Southern Hemispheric atmospheric circulation is explored. The study is based on data firstly from a 6-member ensemble of atmosphere-only runs forced by observed oceanic, solar, and volcanic forcing, and secondly from a 4-member ensemble perturbed by stratospheric ozone depletion using trends estimated by SPARC. The study focuses on the Antarctic Oscillation, the leading mode of extratropical circulation in the Southern Hemisphere, which consists of a seesaw in atmospheric mass between highlatitudes poleward of 60° S, and the midlatitude band centred around 45° S. Studies using reanalysis data have shown that there has been a tendency towards the low phase of the Antarctic Oscillation, where there is low pressure over Antarctica and high surface pressure over the Southern Ocean.

In this study, the Antarctic Oscillation is represented by the first eigenvector from a Principal Component Analysis of December-February mean 500hPa geopotential heights. In the 'control' run, the Antarctic Oscillation explains 44% of the total variance, and is largely noise with only 11% of the variance explained by oceanic, volcanic and solar forcing. Whereas, in the run with stratospheric ozone depletion, there is a shift towards the low phase of the Antarctic Oscillation. The transition period is rather 'sudden', and occurs during 1986-1989, despite the steady rate of ozone loss per year during this period.

An investigation of the probability distribution function of the amplitudes of the Antarctic oscillation from the two ensembles, shows that the changes in stratospheric ozone increase the frequency of the low phase of the Antarctic Oscillation. This change in the frequency of this natural mode of atmospheric variability is typical of the non-linear view to climate change, as opposed to a linear perspective on climate change where signals are assumed to be distinct from natural variability. The seasonality of this response to stratospheric ozone depletion will also be discussed.