ENSO-driven coupled megadroughts in North and South America over the last millennium

Geological evidence from the last millennium indicates that multidecadal megadroughts may have occurred simultaneously in California and Patagonia at least once. However, it is unclear whether or not megadroughts were common in South America, whether or not simultaneous megadroughts in North and South America occurred repeatedly, and what would cause their simultaneous occurrence. Here we use a data-assimilation-based global hydroclimate reconstruction, which integrates palaeoclimate records with constraints from a climate model, to show that there were about a dozen megadroughts in the South American Southwest over the last millennium. Using dynamical variables from the hydroclimate reconstruction, we show that these megadroughts were driven by the El Nino/Southern Oscillation (ENSO). We also find that North American Southwest and South American Southwest megadroughts have occurred simultaneously more often than expected by chance. These coincident megadroughts were driven by an increased frequency of cold ENSO states relative to the last millennium-average frequency. Our results establish the substantial risk that exists for ENSO-driven, coupled megadroughts in two critical agricultural regions. Cold ENSO states can lead to the simultaneous occurrence of megadroughts in southwestern North and South America, according to a hydroclimate reconstruction of the last thousand years assimilating palaeoclimate records with climate model constraints.