Atmospheric Chemistry and Physics Age of stratospheric air in the ERA-Interim

Abstract. The Brewer-Dobson mean circulation and its vari-ability are investigated in the ERA-Interim over the period 1989-2010 by using an off-line Lagrangian transport model driven by analysed winds and heating rates. At low and mid-latitudes, the mean age of air in the lower stratosphere is in good agreement with ages derived from air-craft, high altitude balloon and satellite observations of long-lived tracers. At high latitude and in the upper stratosphere, we find, however that the ERA-Interim ages exhibit an old bias, typically of one to two years. The age spectrum exhibits a long tail except in the low tropical stratosphere which is modulated by the annual cy-cle of the tropical upwelling. The distribution of ages and its variability is consistent with the existence of two separate branches, shallow and deep, of the Brewer-Dobson circula