The network of nearly 2,000 North American tree-ring chronologies is an outstanding high-resolution paleoclimate data set and has been used to reconstruct the long-term soil moisture balance across the continent. However, these existing reconstructions represent an average of the differing seasonal climate signals encoded in the annual ring width chronologies and do not provide discrete reconstructions of winter and summer conditions. The precise seasonality of ancient droughts and wet periods needs to be specified to provide the most realistic reconstructions of past climate and the most useful constraints for climate model simulations. Many North American tree-ring chronologies encode distinct seasonal climate signals that can be used for the separate reconstruction of cool and warm season moisture amounts over large sectors of the continent. The research will use hundreds of new and existing tree-ring chronologies to reconstruct both cool and warm season moisture levels across much of tropical, subtropical, and temperate North America. The seasonal reconstructions will be developed on a 0.5° grid for every year during two fixed time periods: AD 1500-present and AD 1000-present. They will be based on existing chronologies, re-measurements of existing tree-ring collections to derive seasonally explicit earlywood and latewood width chronologies, and selective field collections of new chronologies.
Separate cool and warm season moisture reconstructions will not be possible everywhere, but are likely to be most continuous over tropical and subtropical North America where climate forcing from Pacific sea surface temperatures is particularly strong and changes sign with latitude and season. The new reconstructions will be used with climate model simulations to test hypotheses concerning the ocean-atmospheric forcing of cool and warm season climate over North America, the climate dynamics responsible for decadal droughts and wet periods in each season, and the influence of anthropogenic trace gas and land surface changes on seasonal droughts and wet periods. The reconstructions and climate simulations will also be used to identify ancient analogues for the extreme decadal droughts and wet periods witnessed during the modern era.
This collaborative research project will involve the University of Arkansas, Lamont-Doherty Earth Observatory of Columbia University, the University of Memphis, and NASA's Goddard Institute for Space Studies. The research will provide the long, spatially detailed reconstructions of cool and warm season moisture anomalies that currently do not exist but are needed to provide more realistic seasonal constraints on model simulations of North American climate variability. The new reconstructions will have major interdisciplinary applications by providing an objective exactly dated framework for evaluating the impacts of seasonal climate extremes on social and ecological change during the prehistoric, colonial, and modern eras. The reconstructions will be served on a web-based interface with interactive analytical capabilities to leverage the widest possible use of the seasonal reconstructions [the North American Seasonal Drought Atlas (NASDA)]. Students will be provided with authentic field and laboratory experience in scientific research and publication. This project will support an early career scientist, will initiate a new collaboration with NASA scientists, and will strengthen a long-term collaboration with the Mexican Forest Research Service (INIFAP, Torreon, Mexico).
