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

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 degrees. 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 800K 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.