North American Megadroughts in PMIP3 Last-Millennium Model Simulations

Some of the starkest features of proxy-estimated hydroclimate variability in the North American Southwest (NASW; 125°W-105°W, 25°N-42.5°N) are the severe and multidecadal drought periods that have existed in the region. These so called megadroughts are a prominent and well-established feature of the NASW's hydroclimate history. Given the prominence of these features in our recent past, it is imperative to consider whether Atmosphere-Ocean General Circulation Models (AOGCMs) are capable of simulating these events and if such features of past hydroclimate change are forced or the product of internal variability. Toward such ends, low-frequency hydroclimatic variability is investigated in six last-millennium (LM) simulations derived from AOGCMs as organized through the Paleoclimate Model Intercomparison Project Phase 3 (PMIP3) archive. These simulations are analyzed and compared to the North American Drought Atlas (NADA), a gridded tree-ring derived estimate of hydroclimate variability in North America that spans the last one to two millennia. Megadroughts that are similar in duration and magnitude to NADA-estimated droughts are found to be a robust feature of the LM AOGCM simulations. These droughts, nevertheless, are not the result of changes in exogenous forcing conditions. Moreover, characteristically similar droughts occur in control simulations from the same AOGCMs. Previous work with a single AOGCM showed megadroughts in the NASW to be the result of stochastic-atmospheric variability, but the larger ensemble of LM simulations analyzed in the current study indicates that simulated multidecadal droughts occur for a variety of dynamic reasons. In particular, models that simulate realistic teleconnections between the tropical Pacific Ocean and North American hydroclimate, as measured against observational data, are observed to simulate persistent droughts as a consequence of changes in tropical Pacific sea surface temperature (SST) gradients. Despite this connection, however, changes in SST gradients are shown to occur independent of radiative forcing conditions.