Collaborative Research: Stratospheric Age in a Changing Climate: Connecting Theory, Models, and Observations

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Changes in the atmospheric circulation in response to anthropogenic forcing are expected to affect stratospheric water vapor and ozone recovery, with direct effects on surface temperatures, precipitation, and winds. Policy makers and the public rely on model projections of these changes. This research will develop strategies to validate model simulations with analytic theory, to identify and correct model biases, and to establish rigorous test to assess their fitness for climate projection. This project will also support the education and training of two graduate students. providing them the opportunity to develop a physical understanding of the atmospheric circulation and practical experience with high performance scientific computing. This project will connect theory, modeling, and observations of stratospheric transport. The Brewer-Dobson Circulation describes the transport of mass and trace gases through the stratosphere. In concert with chemical processes, it sets the distribution of water vapor and ozone through the middle atmosphere. Models nearly unanimously predict the Brewer-Dobson Circulation to increase in response to anthropogenic forcing, but the circulation cannot be observed directly, due to the slow time scale of overturning (on the order of years). In situ and satellite-based estimates of the "age of air," a measure of the transport time from the surface to stratosphere which can be estimated from observations of trace gases, however, do not robustly detect a change in transport. Some even suggest a weakening of the circulation, although large uncertainties imply that model trends are not inconsistent. The first goal of this project is to establish a connection between the stratospheric circulation and age distributions with three-dimensional atmospheric models. In particular, the latitudinal structure of age, which can be calculated directly from measurements of trace gases such as CO2 or SF6, will be related to the mean circulation of mass, which is diagnosed in climate model integrations. The second goal is to use these insights to evaluate stratospheric transport in numerical models, connecting standard tests of dynamical cores to the climatological properties of the circulation. Theoretical constraints on age transport will be used to detect biases in climate models arising from deficiencies in their representation of the circulation and/or tracer transport. The third and final goal is to evaluate the new tracer-transport diagnostics in the context of climate change.