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

Abstract:

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.