PNNL

Abstract

Central Great Plains precipitation deficits during May-August 2012 were the most severe since at least 1895, eclipsing the Dust Bowl summers of 1934 and 1936. Drought developed suddenly in May, following near-normal precipitation during winter and early spring. Its proximate causes were reduction in atmospheric moisture transport into the Great Plains from the Gulf of Mexico. Processes that would provide air mass lift and condensation were mostly absent, including a lack of frontal cyclones in late spring followed by suppressed deep convection in mid-late summer owing to large-scale subsidence and atmospheric stabilization. In this sense, the drought resulted mostly from natural variations in weather. Seasonal forecasts did not predict the summer 2012 central Great Plains drought development, which therefore arrived without early warning. Climate simulations and empirical analysis suggest that neither ocean surface temperatures nor changes in greenhouse gases induced a substantial reduction in summertime rainfall over the central Great Plains during 2012. Diagnosis of historical data, climate simulation data, and seasonal forecasts paint a picture of an extreme drought that may not have had extreme forcing as its cause and that had limited long lead predictability. Yet, diagnosis of the retrospective climate simulations also reveals a regime shift toward warmer and drier summertime Great Plains conditions during the recent decade, most probably due to natural decadal variability. As a consequence, the probability for a severe summer Great Plains drought may have increased 5-fold in the last decade compared to the 1980s and 1990s, and the models show that the so-called tail-risk for severe drought was heightened in summer 2012.