Superensemble Climate Simulations for attribution of 2013-15 Western US drought

Abstract:

As of summer 2015, California is in its fourth year of drought, Oregon in its second year, and Washington in its first, with serious economic and ecological consequences. Scientists are using various approaches to evaluate the rarity, severity, and causes of this drought. This talk describes one approach to evaluating the influences of various causal factors. Using a framework for generating a superensemble of regional climate simulations called climateprediction.net, created by scientists at University of Oxford, we have generated tens of thousands of simulations of the period November 2013 to May 2015, to focus on the most acute period of drought in these three states. One set used observed sea surface temperatures (SSTs), one used average SSTs from 2000-2010, and one “natural” set used the observed SSTs but with the anthropogenic warming component subtracted and greenhouse gas concentrations set to pre-industrial values. Within the three sets, the simulations differ only in the atmospheric initial conditions used. Distinctly different likelihoods of drought emerge in the Pacific states in the three sets of simulations. This talk will describe differences in temperature, precipitation, snow water equivalent, atmospheric flow, and energy and water balance terms in the various sets of simulations, and how they differ from California to Washington.