Collaborative Research: P2C2--Assimilation of Cool and Warm Season Moisture Reconstructions and Atmospheric Conditions Over North America for the Past Millennium

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Droughts and floods have had a major impact on the economy and ecosystems in the United States (US) during the instrumental period. All of these hydroclimate extremes exhibited noticeable seasonal variability, suggesting that the seasonality is an important component of variability and forcing behind drought and wetness regimes. There is robust evidence from paleoclimate records that the U.S. was struck by more intense, long lasting, and widespread drought and wet episodes during the last two thousand years. However, many of the available proxy records cannot resolve the seasonality of droughts and pluvials. Therefore, the discrete seasonality of hydroclimate variations needs to be specified in proxy reconstructions to the extent possible to provide the most realistic estimation of past climate. The research outlined in this project is a plan to estimate discrete cool and warm season surface moisture and associated atmospheric states on an annual basis over North America during the last millennium using an offline data assimilation approach. These new estimations will then be used to understand the atmospheric circulation and large-scale forcing of cool and warm season droughts and pluvials on interannual, decadal, and centennial time scales.The research will reconstruct gridded cool and warm season moisture and three dimensional atmospheric states across North America during the last millennium using paleoclimate data assimilation (PDA) techniques. The explicit seasonal reconstructions will be based on the North American Seasonal Drought Atlas (NASDA) and a subset of existing annual tree ring chronologies with restricted seasonal climate responses. The paleoclimate data assimilation uses dynamical models to infer spatial relationships within and between climate fields. Therefore, the estimated seasonal moisture and atmospheric states are physically and dynamically consistent. The cool and warm season moisture and atmospheric states will all be exactly dated and annually resolved. The new reconstructions will provide an unprecedented opportunity to quantify the cross-season coherency and casual mechanisms of major drought and pluvial events during the last millennium. This project will examine whether the physical processes that initiated, sustained and/or diminished past droughts or pluvials in North America could vary at different time scales. Specifically, the new reconstructions will be used to test hypotheses concerning: 1) the ocean-atmospheric forcing of cool and warm season hydroclimate over North America, 2) the climate dynamics responsible for persistent droughts and pluvials on seasonal basis, 3) the linkages between cool and warm season hydroclimate variability, and 4) the causal mechanisms for individual hydroclimate extremes.The potential Broader Impacts (B.I.) include the development of an online and interactive atlas of cool and warm season moisture and atmospheric states for each year over the past millennium. This atlas will include maps of cool and warm season moisture, sea level pressure, 500hPa geopotential height and other key variables. Users from academic communities and the general public will be able to access the detailed seasonal atmospheric information for any particular year or group of years. The seasonal reconstructions will also provide an objective framework to evaluate the impacts of hydroclimate extremes on social and ecological changes during the prehistoric, colonial, and modern eras. The researchers will collaborate with the Arkansas State Climate Office to promote the awareness of potential water issues associated with droughts and pluvials. This project will also support one graduate studentThis 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.