Steps toward a quantitative explanation of differences in Chemistry Climate Model predictions of future ozone evolution

The composition of the stratosphere is changing as the chlorofluorocarbons decrease due to the Montreal protocol and it amendments. The climate of the stratosphere is also changing due to the increase in greenhouse gases including CO2 that cool the stratosphere. Stratospheric ozone loss depends on the amount of chlorine and other reactive species as well as the temperature, and chemistry climate models (CCMs) are the current tool used to simulate the 21st century evolution of stratospheric ozone. Eighteen CCMs participated in the 2010 CCMVal exercise, a SPARC initiative designed to evaluate CCMs through comparisons with observationally derived diagnostics that were chose to test the degree to which CCMs correctly represent atmospheric processes. The simulated ozone response and the performance on these diagnostic evaluations are not uniform across the CCMs. An essential remaining challenge is to develop a strategy to use the information obtained from the diagnostic evaluation of the CCMs to reduce uncertainty in prediction. As a first step, we analyze the ozone evolution in the upper stratosphere, where photochemical terms dominate the ozone tendency. In these simulations, upper stratospheric chlorine levels are largely controlled by the assumed boundary conditions, but other factors including the temperature and its response to greenhouse gas increase and the level of reactive nitrogen species contribute to the spread in model predictions. The goal of the present work is to use observations and diagnostic information to develop a quantitative explanation for the differences among predictions by the collection of CCMs.