A century ago, tree-ring records revealed prolonged "megadroughts" that occurred repeatedly in western North America during the medieval period and that dwarfed any droughts observed during the 19th and 20th centuries. In the 1990s and early 2000s it was discovered that at least one medieval megadrought in California coincided with prolonged drought conditions in Patagonia and subsequent work supported the El Nino-Southern Oscillation (ENSO) as a driver of low-frequency hydroclimatic variability in both hemispheres. Nevertheless, paleoclimate records from North and South America have not been used to formally test the degree to which megadroughts in these regions have co-occurred. Using hydroclimate and dynamical information from the Paleo Hydrodynamics Data Assimilation product, we show that megadroughts have indeed appeared in southern South America over the last millennium. We find that these megadroughts are driven primarily by ENSO and not other modes of climate variability or radiative forcing. We also show that megadroughts in North and South America have co-occurred more often than would be expected by chance and that these coincident megadroughts were driven by an increased frequency of strong La Nina states. Our results illustrate the significant risk of coupled megadroughts in these two important agricultural regions. These results also provide paleoclimatic context for the hemispherically synchronous, ENSO-driven drought impacts and crop failures observed during the instrumental era.

About the Speaker:

Nathan Steiger is an Associate Research Scientist at the Lamont-Doherty Earth Observatory of Columbia University. He works to understand the historical variability of the climate system and its relevance to human societies. In particular, he conducts research on the physical mechanisms of severe droughts, pluvials, and other climate extremes.
                   
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