Decadal Drought Variability Over North America: Mechanisms and Predictability

The physical mechanisms and potential predictability of North American drought on decadal timescales are reviewed in a simple and straightforward manner amenable to a wide audience. During decadal droughts, the tropical oceans, most notably cold states of the Pacific but also warm states of the Atlantic, provide forcing that continually nudges the atmosphere toward circulation anomalies that favor high pressure over southern North America and dry conditions. However, even in these regions, and even more so in the northwest and northeast, the oceans exert less than dominant control and actual drought onset, evolution and termination can deviate due, presumably, to potent internal atmosphere variability. The ocean influence, however, justifies efforts to determine if the driving sea surface temperature anomalies in the tropical Pacific and Atlantic are predictable beyond the seasonal to interannual timescale. Evidence to date, based on initialized predictions with coupled models, is tantalizingly suggestive that useful predictability on these timescales may exist within the atmosphere-ocean system although relevance to North American decadal drought has not yet been demonstrated. These recent advances in drought science and prediction warrant continued research aimed at developing useful long term predictions of drought that can guide adaptation and minimize the associated widespread social and economic disruptions.