How can we best “observe” changes in the Brewer-Dobson circulation?

Climate simulations are now performed with chemistry-climate models that encompass the troposphere and stratosphere since it has been recognized that the stratosphere plays a role in surface climate variability. The Brewer-Dobson circulation is a fundamental aspect of the stratosphere and yet it cannot be measured directly. How, then, can we best evaluate how well this new generation of climate models perform in simulating variability of the Brewer-Dobson circulation? Here we describe a suite of trace gases that can accurately and uniquely indicate variability in both components of the Brewer-Dobson circulation, the mean meridional circulation and mixing, on seasonal to multi-decadal time scales. We demonstrate the potential of this suite of trace gases using some of their sporadic measurements over the past nearly four decades and an idealized model of the stratosphere. We suggest that regular, long-term balloon-based measurements of these trace gases in certain strategic locations would be a realistic means of “observing” detailed changes in the Brewer-Dobson circulation to be used to compare to whole atmosphere chemistry-climate model simulations.