In response to global warming, the Brewer–Dobson circulation inthe stratosphere is expected to accelerate and the mean transport time of airalong this circulation to decrease. This would imply a negative stratosphericage of air trend, i.e. an air parcel would need less time to travel from thetropopause to any point in the stratosphere. Age of air as inferred fromtracer observations, however, shows zero to positive trends in the northernmid-latitude stratosphere and zonally asymmetric patterns. Using satelliteobservations and model calculations we show that the observed latitudinal andvertical patterns of the decadal changes of age of air in the lower to middlestratosphere 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 thenorthern tropics. This net change is consistent with long-term trends frommodel predictions.
展开▼
