Megadrought Risk and ENSO Over the Last Millennium

Abstract:

Proxy evidence indicates that over the last millennium, 'megadroughts' of greater length and severity than observed in the instrumental record have occurred with some frequency. However, the mechanisms for these events are complex and not completely understood: stochastic atmospheric variability, teleconnections from the tropical oceans, and land surface feedbacks all contribute to megadrought initiation and persistence. The NCAR Community Earth System Model (CESM) Last Millennium Ensemble presents an excellent opportunity for examining the relationship between oceanic variability and drought in a probabilistic framework, as the LME contains 30 separate realizations of the 850-2005 period with varying combinations of external forcing factors. The LME is used to investigate the link between El Nino/Southern Oscillation (ENSO) variability and hydroclimate on both interannual and multidecadal timescales, in several drought-prone regions throughout the world; ENSO is quite strong in the CESM, and its influences on interannual hydroclimate variability are correspondingly large. The ENSO-megadrought link is less straightforward on multidecadal timescales, and the influences of both ENSO and low-frequency oceanic modes on the occurrence of megadrought conditions is examined. Changes to these relationships under 20th century greenhouse warming are also investigated, as well as comparisons with 21st century simulations using the CESM Large Ensemble. Implications for future megadrought risk are discussed.