Articles | Volume 17, issue 18
Atmos. Chem. Phys., 17, 11177–11192, 2017
Atmos. Chem. Phys., 17, 11177–11192, 2017

Research article 21 Sep 2017

Research article | 21 Sep 2017

Shift of subtropical transport barriers explains observed hemispheric asymmetry of decadal trends of age of air

Gabriele P. Stiller1, Federico Fierli2, Felix Ploeger3, Chiara Cagnazzo2, Bernd Funke4, Florian J. Haenel1, Thomas Reddmann1, Martin Riese3, and Thomas von Clarmann1 Gabriele P. Stiller et al.
  • 1Karlsruhe Institute of Technology, IMK, P.O. Box 3640, 76021 Karlsruhe, Germany
  • 2National Research Council, Institute for Atmospheric Sciences and Climate, Via Fosso del Cavaliere 100, 00199 Rome, Italy
  • 3Institute for Energy and Climate Research – Stratosphere (IEK-7), Forschungszentrum Jülich, 52425 Jülich, Germany
  • 4Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain

Abstract. In response to global warming, the Brewer–Dobson circulation in the stratosphere is expected to accelerate and the mean transport time of air along this circulation to decrease. This would imply a negative stratospheric age of air trend, i.e. an air parcel would need less time to travel from the tropopause to any point in the stratosphere. Age of air as inferred from tracer observations, however, shows zero to positive trends in the northern mid-latitude stratosphere and zonally asymmetric patterns. Using satellite observations and model calculations we show that the observed latitudinal and vertical patterns of the decadal changes of age of air in the lower to middle stratosphere during the period 2002–2012 are predominantly caused by a southward shift of the circulation pattern by about 5°. After correction for this shift, the observations reveal a hemispherically almost symmetric decrease of age of air in the lower to middle stratosphere up to 800 K of up to −0.25 years over the 2002–2012 period with strongest decrease in the northern tropics. This net change is consistent with long-term trends from model predictions.

Short summary
The discrepancy between modelled and observed 25-year trends of the strength of the stratospheric Brewer–Dobson circulation (BDC) is still not resolved. With our paper we trace the observed hemispheric dipole structure of age of air trends back to natural variability in shorter-term (decadal) time frames. Beyond this we demonstrate that after correction for the decadal natural variability the remaining trend for the first decade of the 21st century is consistent with model simulations.
Final-revised paper