Quantifying the roles for structural uncertainty and intrinsic climate variability in determining the atmosphere-ocean conditions that drive a persistent and severe drought in western and central North America

Previous research using coupled global climate models (CGCMs) and observations has shown that large-scale modes of atmosphere-ocean variability are important drivers of persistent and severe droughts over western and central North America. In a recent study, a framework for characterizing the spatiotemporal propagation of droughts demonstrated that in CGCMs a wide range of atmosphere-ocean conditions can drive droughts with spatiotemporal characteristics similar to an observed event between 2011–2014. We build off this framework, utilizing initial condition large ensembles (ICLEs) to determine whether the previous results are due to structural uncertainty or intrinsic climate variability (or some combination). Utilizing 570 simulations across 19 CGCMs, we identified 222,716 droughts using a Markov Random Field and machine learning-based drought identification algorithm. Focusing on the 671 droughts most similar to the observed event, we find little consistency in the atmosphere-ocean conditions through their life cycles, with the exception of persistent atmospheric ridging in the north-central Pacific Ocean. To better understand this inconsistency, the atmosphere-ocean conditions during droughts were compared within each individual CGCM, to quantify intrinsic climate variability, and across different CGCMs, to quantify structural uncertainty. Structural uncertainty is found to dominate, as it is larger than intrinsic climate variability for 15 of the 19 CGCMs. Nevertheless, within CGCMs there is still substantial intrinsic climate variability, and thus intrinsic climate variability is large but structural uncertainty is even larger. The substantial uncertainties that we identify undermine confidence in the ability of CGCMs to simulate persistent and severe drought events and motivates future efforts to understand the implications for hydroclimate projections over the coming decades.