The Dynamic State of the Ocean and Atmosphere during Megadroughts in the American West

Thursday, 17 December 2015: 17:45
2003 (Moscone West)
Sloan Coats, Columbia University of New York, Palisades, NY, United States, Jason E Smerdon, LDEO of Columbia University, Palisades, NY, United States, Benjamin Cook, NASA Goddard Institute for Space Studies, New York, NY, United States, Richard Seager, Lamont Doherty Earth Obs, Palisades, NY, United States, Edward R Cook, Lamont -Doherty Earth Observatory, Palisades, NY, United States and Kevin J Anchukaitis, Woods Hole Oceanographic Institution, Woods Hole, MA, United States
Multidecadal droughts are a prominent feature of the Common Era paleoclimate record in the American West, particularly during the Medieval Climate Anomaly. These megadroughts represent an important out-of-sample target for validating the ability of Coupled General Circulation Models to adequately characterize drought risk over the near-term future. Such model validations, however, require a substantially improved understanding of the atmosphere-ocean state during megadroughts. Here we use spatiotemporal tree-ring reconstructions of Northern Hemisphere hydroclimate to infer the atmosphere-ocean dynamics that underlie megadroughts, and find that these features are consistently associated with ten-to-thirty year periods of frequent cold conditions in the tropical Pacific Ocean. Nevertheless, a prominent hypothesis that there were multiple centuries of persistently La Niña-like conditions during the Medieval Climate Anomaly is inconsistent with our analysis of the tree-ring reconstructions. Instead, warm conditions in the Atlantic likely played a necessary role in amplifying drying during this period. These results provide a framework for evaluating the performance of General Circulation Models, which simulate megadroughts under a range of different atmosphere-ocean states, including stochastic atmospheric variability, and clarify the dynamical mechanisms that will determine the risk of megadroughts in the future.