Quantifying the Austral Jet Stream Response to Stratospheric Ozone and Greenhouse Gases

We explore the impact of anthropogenic forcing on the austral jet stream in comprehensive climate models, focusing on the summer, the period of the year when stratospheric ozone loss and recovery have been shown to impact the troposphere. We find that the multimodel mean response of the jet stream to greenhouse gases and ozone is fairly comparable across three recent multimodel datasets: the Chemistry Climate Model Validation Activity 2 (CCMVal2) and Coupled Model Intercomparison Projects, Phases 3 and 5 (CMIP3 and CMIP5). A simple model of the response based on temperature trends in the polar stratosphere (which are primarily driven by stratospheric ozone changes) and tropical upper troposphere (which are driven by increased greenhouse gas forcing) allows us to attribute the jet response. The simple framework suggests that trends in the jet are driven by changes in the temperature gradient in the upper troposphere and lower stratosphere, so that the jet responds similarly when the tropics warm or the polar stratosphere cools. Consistent with previous studies, the analysis suggests that ozone has dominated changes in temperature to date, and that ozone and greenhouse gases will largely offset each other in the future. There is considerable spread, however, among the various climate models. The simple model allows us to separate uncertainty that stems from differences in the temperature response (related to differences in climate sensitivity and ozone forcing), and differences in the circulation response to a given temperature change. This latter effect, which we term a ``circulation sensitivity'' to differentiate it from climate sensitivity, is related to differences in stratosphere-troposphere coupling and large scale dynamics.