The relationship of Age of Air and the Residual circulation

The strength of the Brewer-Dobson Circulation (BDC), as measured by the residual circulation, is projected by most state-of-the-art chemistry-climate models (CCMs) to increase. However, evidence of changes in the BDC from observations is weak. Among others, one problem is that the residual circulation strength cannot be measured directly, but only trace gas observations can be used to infer the transport of air through the stratosphere. The resulting measure of age of stratospheric air (AoA) is, however, not solely influenced by the residual circulation but also by two-way mixing. Therefore, the relation of changes in AoA and the residual circulation need to be better understood to reconcile the apparent discrepancies between model results and observations. To this end, global models that provide residual circulation data consistent with AoA are used to investigate both the climatological relation of the two quantities, and future changes. We use trajectories driven only by the residual circulation to derive a hypothetical 'age' that air would have if it was transported only by the residual circulation. This quantity is referred to as 'residual circulation transit time' (RCTT). The difference between AoA and RCTT is then interpreted as the additional aging of air caused by mixing processes. It is shown that this aging by mixing is positive throughout the lower stratosphere, only in the lowermost stratosphere at high latitudes, air is younger than expected from residual circulation transport only. The major contribution of the RCTT to AoA is found in low and high latitudes, where vertical transport by the residual circulation dominates, while the largest contribution of aging by mixing is found in subtropics and mid-latitudes, where horizontal mixing is strong. AoA is found to decrease throughout the lower and middle stratosphere during the period 2000-2050, along with an increase in residual velocities. However, the decrease in AoA is not caused entirely by a decrease in RCTT, but also changes in aging by mixing lead to younger air. The contribution of RCTT changes to AoA changes is distributed similar to the climatology, with strongest contributions in low and high latitudes.