Understanding the Response of the Austral Jet Stream to Changes in Greenhouse Gases and Stratospheric Ozone

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The first goal of this research is to assess and attribute the response of the Southern Hemisphere jet stream to anthropogenic forcing in comprehensive climate models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). The Principal Investigator (PI) suggests a simple method to partition the impact of ozone and greenhouse gas induced changes on the jet stream will be sugg. It will allow to better quantify uncertainty in climate projections, separating differences in the thermal response to greenhouse gases and ozone from differences in the sensitivity of the jet stream to changes in atmospheric temperature. Analysis of CMIP5 models to previous generation climate models from the Coupled Model Intercomparison Project Phase 3 (CMIP3) and Chemistry Climate Model Validation Activity 2 (CCMVal2) will be undertaken to assess the impact of model improvements on climate projection.The second goal of this research is to understand the mechanism(s) causing changes in the jet stream, using a series of controlled experiments with an idealized general circulation model. Preliminary analysis of CMIP3 and CCMVal2 models suggests that the jet is sensitive to the temperature gradient in the upper troposphere and lower stratosphere, so that the response is similar when the tropics are warmed or the high latitudes are cooled. This hints at a common mechanism behind greenhouse gas and ozone induced changes. In addition, stratospheric ozone induced changes have a strong seasonal footprint. The PI will explore interactions between the background seasonal cycle in the stratosphere and troposphere with a seasonally localized ozone-like forcing. Observed trends in the midlatitude circulation of the atmosphere are stronger in the Southern Hemisphere summer than in other seasons (or in the Northern Hemisphere), due to the combined effect of greenhouse gases and ozone. This has had significant impacts on precipitation throughout the Southern Hemisphere, even the tropics. Thus this project will increase our understanding of how changes in model configuration, e.g. representation of the stratosphere, impact a model's response. This research will involve a graduate student and postdoctoral scientist, providing opportunity for their development as a research scientists.