Collaborative Research: An integrated model-proxy approach to understanding Western US hydroclimate change since the last glacial period

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Understanding what processes drive changes in rainfall along the U.S. West Coast is essential for predicting future water availability in this densely populated and agriculturally productive part of the country. However, given the short duration of direct meteorological observations (~100 years), it is beneficial to reconstruct past changes in rainfall over longer time periods (i.e., thousands of years) in order to uncover the potential sensitivity of water resources in the Western U.S to future climate change. This research project combines measurements of ancient groundwater and state-of-the-art climate model experiments to both quantify and understand the dynamical drivers of hydrological change along the American West Coast during the last ice age (i.e., the Last Glacial Maximum or LGM, ~25,000 years ago). This project also includes a comprehensive public outreach component, involving the creation of a museum exhibit for display at the National Center for Atmospheric Research (NCAR). In collaboration with the University of Colorado Boulder, the researchers will lead an interdisciplinary course for students to design and prototype a museum exhibit that brings to life the radically different atmospheric circulation and rainfall patterns present during the LGM.The first research component of this project involves a field campaign to collect groundwater samples from a network of wells within the Columbia Plateau Aquifer system in northeastern Oregon and southeastern Washington, which will provide a much-needed quantitative constraint on LGM hydroclimate in the data-poor Pacific Northwest. A new analytical technique for high-precision measurements of dissolved noble gas concentrations and Krypton and Xenon isotope ratios within the groundwater will be employed to reconstruct past temperature and regional water table depth. The second research component of the project involves climate model experiments using NCAR’s Community Earth System Model version 1.2 (CESM1) to build upon existing fully equilibrated climate simulations to study the influence of different LGM boundary conditions (ice sheet albedo/topography, greenhouse gases, orbital forcing, etc) on North Pacific atmospheric circulation and the intensity, variability, and landfalling orientation of moisture-rich storms known as Atmospheric Rivers (ARs). The project also includes a series of time-slice experiments, simulating both deglacial and glacial conditions, to examine the transient response of ARs and western North American hydroclimate to shrinking and growing continental ice sheets. Finally, detailed comparisons will be made between these model simulations and the proxy reconstructions of Pacific Northwest water table depths.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.