Nathan J. Steiger

TL;DR: In this article, the authors investigated the effect of large tropical volcanic eruptions over the last millennium using two state-of-the-art data assimilation products, the Paleo Hydrodynamics Data Assimilation product (PHYDA) and the Last Millennium Reanalysis (LMR), and simulations from the National Center for Atmospheric Research Community Earth System Model-Last Millennium Ensemble (NCAR CESM-LME).

TL;DR: It is confirmed that La Niña conditions significantly influence southwest U.S. drought over the past millennium but only account for, by one metric, ~13% of interannual drought variability in that region, which suggests a substantial role for internal atmospheric variability.

TL;DR: The authors examined oceanic drivers of widespread droughts over the contiguous United States during the Common Era in what is one of the first analyses of the new Paleo Hydrodynamics Data Assimilation (PHYDA) product.

Abstract: 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.

TL;DR: In this article, the authors study the last millennium of glacier-length variability across the globe using a simple dynamic glacier model, which they force with temperature and precipitation time series from a 13-member ensemble of simulations from a global climate model.