Modeling the Tropical Atmosphere-Ocean System: Determining the Causes of Near Future Subtropical Drying

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Drying of the subtropics is a ubiquitous and robust result within model projections of 21st century climate. This drying impacts several areas with important agricultural economies and rising populations, most notably northern Mexico, the southwestern United States, southern Europe, North Africa and the Middle East. According to the models, this drying sets in immediately in the future. The projected amplitude is such that the U.S. Southwest will go over to a permanent Dust Bowl or 1950s drought level of aridity in the next few years to decades. While the future drought raises the specter of a return of conditions akin to Medieval mega droughts those have been linked to persistent La Nina-like states but future droughts appear to be a result of general warming.Research will be conducted to determine the causes of the drying of the Southwest and the subtropics using coupled General Circulation Models (GCMs) and simplified models. Both thermodynamic changes - whereby rising humidity leads to amplification of existing patterns of water vapor transport by the mean circulation, making dry areas drier and wet areas wetter - and changes in atmospheric circulation that shift patterns of precipitation and evaporation are involved. The investigators will a) determine within coupled GCMs the relative contributions of thermodynamical and dynamical mechanisms to subtropical drying and what changes in atmospheric circulation (MMC, transient eddies and stationary waves) are responsible for the dynamical contribution to drying indifferent areas,b) use diagnoses of coupled GCM and a hierarchy of simplified models to determine the causes of the changes in atmospheric circulation, examine the impacts of changes in tropical and extratropical SSTs and tropospheric temperatures, and study low level warming at higher latitudes and stratospheric cooling,c) determine the interactions between eddies, the mean circulation and water vapor transports that underlie shifts in precipitation and evaporation zones and subtropical drying, examining both winter and summer seasons, and d) contrast the dynamics of tropical forcing of naturally-occurring droughts with the dynamics of anthropogenic drought as a means to identify the causes of current and future drought.Intellectual merit of the research comes from understanding the fundamentals of how the tropical and global atmosphere circulation and hydrological cycle respond to external forcing which will lead to an improved understanding of the general circulation, in particular, the interactions between eddies, the mean flow and moist processes.Broader implications of the research concern the future of water resources in the arid lands of North America. The work will improve understanding of model projections of human-induced climate change and subtropical drying and, hence, aid assigning of confidence to the predictions. The PIs will communicate results to the climate research community and also to those making water-related decisions and work to educate a wider public on these matters through publications, open addresses, the media and the internet.