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. An accelerating Brewer–Dobson circulation (BDC) is a robust signalof climate change in model predictions but has been questioned bytrace gas observations. We analyse the stratospheric mean age of air andthe full age spectrum as measures for the BDC and its trend. Ageof air is calculated using the Chemical Lagrangian Model of theStratosphere (CLaMS) driven by ERA-Interim, JRA-55 and MERRA-2reanalysis data to assess the robustness of the representation ofthe BDC in current generation meteorological reanalyses. We findthat the climatological mean age significantly depends on thereanalysis, with JRA-55 showing the youngest and MERRA-2 theoldest mean age. Consideration of the age spectrum indicates thatthe older air for MERRA-2 is related to a stronger spectrum tail,which is likely associated with weaker tropical upwelling and strongerrecirculation. Seasonality of stratospheric transport is robustlyrepresented in reanalyses, with similar mean age variationsand age spectrum peaks. Long-term changes from 1989 to 2015 turnout to be similar for the reanalyses with mainly decreasing mean ageaccompanied by a shift of the age spectrum peak towards shortertransit times, resembling the forced response in climate modelsimulations to increasing greenhouse gas concentrations. For theshorter periods, 1989–2001 and 2002–2015, the age of air changes areless robust. Only ERA-Interim shows the hemispheric dipole patternin age changes from 2002 to 2015 as viewed by recent satelliteobservations. Consequently, the representation of decadalvariability of the BDC in current generation reanalyses appears lessrobust and is a major uncertainty of modelling the BDC.

How robust are stratospheric age of air trends from different reanalyses?