Oceanic drivers of widespread summer droughts in the United States over the Common Era

We examine oceanic drivers of widespread droughts over the contiguous United States (herein pan-CONUS droughts) during the Common Era in what is one of the first analyses of the new Paleo Hydrodynamics Data Assimilation (PHYDA) product. The canonical understanding of oceanic influences on North American hydroclimate suggests that pan-CONUS droughts are forced by a contemporaneous cold tropical Pacific Ocean and a warm tropical Atlantic Ocean. We test this hypothesis using the paleoclimate record. Composite analyses find a robust association between pan-CONUS drought events and cold tropical Pacific conditions, but not with warm Atlantic conditions. Similarly, a self-organizing map analysis shows that pan-CONUS drought years are most commonly associated with a global sea surface temperature pattern displaying strong La Nina and cold Atlantic Multidecadal Oscillation (AMO) conditions. Our results confirm previous model-based findings for the instrumental period and show that cold tropical Pacific Ocean conditions are the principal driver of pan-CONUS droughts on annual timescales. Plain Language Summary Widespread summer droughts across the contiguous United States (pan-CONUS droughts) pose unique challenges because of their potential to strain multiple water resources simultaneously, and the financial damages from these droughts are significant. For example, pan-CONUS droughts in 1988 and 2012 cost an estimated $40 and $30 billion, respectively. We provide a millennium-length perspective on the causes of these droughts using a new paleo reconstruction product that merges climate model information with multiple climate proxies including tree rings, ice cores, and corals. We characterize pan-CONUS droughts from 850 to 1850 CE and demonstrate how ocean variability has helped cause these events. We find that La Nina events in the tropical Pacific are the principal oceanic influence on these droughts, while variability in the Atlantic has not played a significant role. These findings are at odds with previous work that has estimated a role for the Atlantic in causing pan-CONUS droughts but support conclusions from a more recent model-based study of the causes of pan-CONUS droughts over the instrumental record. Our findings are important for predictions of pan-CONUS droughts and for determining how the occurrence of these droughts may change in the future due to increases in greenhouse gas emissions.