Dr. Andreas Engel, Institute for Atmosphere and Environment, J.W. Goethe University FrankfurtDate:
Aniara30 years of stratospheric mean age observations: is the Breweer Dobson circulation changing?
The increase of greenhouse gas abundances in the atmosphere is associated with an increased radiative forcing, leading to a warming of the troposphere and a cooling of the stratosphere. A secondary effect of increasing levels of greenhouse gases is a possible change in the stratospheric circulation with substantial feed-backs on chlorofluorocarbon lifetimes, ozone and on the climate system. Model calculations have shown that the mean age of air in the stratosphere is a good indicator of the strength of the residual circulation and that mean age is expected to decrease. Here we present a data set of 30 years of balloon borne observations of stratospheric trace gases. This data set is used to derive mean age of air from stratospheric SF6 and CO2 measurements. We show that mean age has shown a slight increase, in contrast to model predictions. Taking into account an assessment of the uncertainties, we conclude that the derived positive trend is significant on the 1 sigma level (68% confidence interval), but not on the 90% confidence interval. This data set puts an important constraint on the range of model predictions which would be consistent with observations. Created 2008-12-18 10:21:50 by Uwe RaffalskiLast changed 2009-01-15 08:53:52 by Uwe Raffalski