Stability of atmospheric circulation patterns and its relation to extreme events

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We propose to study the sensitivity of the large-scale atmospheric circulation on internal atmosphere dynamical forcings and forcings arising from changes in surface boundary conditions. Methods will include rigorous data analysis and a hierarchy of modeling approaches: From conceptual, to intermediate complexity, to atmospheric general circulation, and finally fully-coupled earth system models. The focus of the proposed research will be towards the Hadley circulation and anomalous mid-latitudinal circulation patterns observed in the Northern Hemisphere during several recent boreal summers. The latter has been linked to the occurrence of atmospheric blocking and associated hemispheric-scale extreme weather events. Shifts in large-scale circulation patterns have the ability to cause abrupt climate change over large, continentalscale regions. Possible changes in the location of the major dry regions and in the frequency and/or intensity of extreme weather would have severe humanitarian and economic impacts. Large-scale atmospheric circulation plays a key role in these phenomena but uncertainties in its exact role are large. The proposed research will attempt to reduce these uncertainties using multiple modeling approaches and data analysis techniques.