Abstract

The Brewer-Dobson circulation carries trace gases, including CFCs and greenhouse gases, from the troposphere to stratosphere, and from equator to pole in the stratosphere. It is wave driven, and global models predict its speed to increase in the 21st century due to the action of increased greenhouse gases on temperature structure. Changes in its speed can be deduced from changes in the stratospheric NOy (produced by photo-oxidation of the N2O that originates in the troposphere). We analyse NO2 measurements from 1990 to 2007 made by a zenith-sky spectrometer in the Antarctic, and use a photochemical model to improve their accuracy and to deduce NOy from the observed NO2 . We focus on NO2 in summer because the ratio of NO2 to NOy is then least affected by stratospheric volcanic aerosol. The NO2 and NOy have large inter-annual variability, and a broad maximum in 2000-2001, with little overall trend. The patterns of variability signify similar changes in speed of Brewer-Dobson circulation but with opposite sign – a broad minimum near 2001. Multiple regressions show: – large correlations with solar and QBO indices – an error in trend of ±3.5 %/decade, not yet precise enough to constrain global models – an unexplained cycle of amplitude 15% and period >17 years.