The Role of the Mean-State Atmospheric Circulation in Future Projections of Regional Hydroclimate and Cloud Feedbacks

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Efforts to mitigate climate risk rely on model simulations to determine how climate will change on a regional basis, for instance whether it will rain more or less in a given region as the world warms. But in many regions climate models do not produce consistent answers, for instance in the US Southwest some models project more summer rainfall while others project less. The lack of consensus could mean that either outcome is in fact equally likely, and the range of projections across the ensemble of available climate models is an accurate representation of the range of likely outcomes. Alternatively, perhaps some portion of the range should be discounted because the projections in that range are influenced by specific model biases which have a clear dynamical connection to the projected changes. For instance a recent study by the Principal Investigator (PI) connects projections of future precipitation over the Southwest to the moisture flow around a mid-tropospheric ridge over the central US in present-day simulations from the same models. The models which produce the strongest precipitation increases show excessive ridging which is not consistent with present-day observations, thus casting doubt on their projections of rainier summers in the Southwest.Research under this award continues the PI's effort to identify present-day atmospheric circulation features which affect future projections of regional precipitation change, as well as projections of regional cloud cover change which can amplify or reduce the warming effect of greenhouse gas emissions. The work uses statistical analysis of model simulations to identify relationships between future projections and present-day circulation features, followed by specialized simulations using the Community Atmosphere Model (CAM), the atmospheric component model of CESM, the Community Earth System Model. The simulations use "nudging" to produce present-day simulations which mimic several climate models, paired with simulations in which a uniform warming is applied to the ocean surface so that the effects of the differing present-day circulations can be assessed.The project is of societal as well as scientific interest given the need for better information to guide efforts to assess and respond to climate risk. The project provides support and training to a postdoctoral associate and a graduate student, thereby promoting the future workforce in this research area. In addition, the project hosts a summer undergraduate student through the Virginia-North Carolina Alliance for Minority Participation's summer research program.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.