Using Trace Gas Observations to Determine Trends in the Stratospheric Circulation and in Age of Air

Climate models predict the strengthening of the stratospheric circulation (or the Brewer-Dobson Circulation, BDC) and verifying this prediction with climate models has proved challenging. Truly comparing like-to-like between models and observations is made more difficult because the BDC is not directly observable, and so any observational metric must be a proxy for the vertical velocity that shows such robust trends in models. Any disagreement between observations and models could also be because the internal variability of the circulation of the stratosphere is large enough that a forced trend would not be detectable with the current length of record. Age of air describes how long an air parcel has been in the stratosphere since entering at the tropical tropopause, and it is often used as a proxy for the strength of the circulation even though age at a given location can be influenced by more than just the BDC. A better proxy is the difference between tropical and extratropical age of air (or age difference) on at a given level in the stratosphere. The long record of in situ observations is limited to the extratropics, however, and so age difference is only available during the satellite record. We have examined how to use trace gas observations (especially N₂O, CO₂, and SF₆) to calculate trends in age of air for both in situ observations and satellite observations and for age difference in the satellite observations. We have also examined when a forced trend might be distinguishable from the natural variability, accounting for the observational limitations. To calculate age of air from N₂O, the compact relationship between N₂O and age of air is needed, and how well this relationship is characterized affects the uncertainty of the trend detection. Observing the BDC trend predicted by the models will require additional observations, and we discuss what future observational requirements might look like in order to meet this goal.