The Extraordinary California Drought of 2013-2014: Character, Context, and the Role of Climate Change

Nearly the entire state of California experienced exceptionally dry conditions during 2013 and early 2014. Statewide 12-month accumulated precipitation was less than 34% of average over this period, leading to a wide range of adverse impacts upon both human and natural systems. The California drought occurred in tandem with a persistent region of positive geopotential height (GPH) anomalies associated with high-amplitude ridging over the northeastern Pacific Ocean (i.e. the so-called "Ridiculously Resilient Ridge"), which displaced the jet stream well to the north of its climatological mean position and prevented winter storms from reaching California during the canonical wet season. Remarkably, the extreme dipolar flow pattern induced by this recurrent anticyclone over the northeastern Pacific occurred during two consecutive winter seasons, which helped to accelerate the onset of severe drought conditions in California. We find that the extreme GPH values during 2013 were observationally unprecedented over a vast region covering most of the North Pacific Ocean and adjacent land areas, and that California experienced both its driest calendar year and driest consecutive 12-month period in at least 119 years. We also assess the role of climate change in affecting the likelihood of occurrence of the large-scale conditions linked to the California drought, and find that global warming has very likely increased the likelihood of extreme GPH events in this region. Together, the complexity and severity of the observed drought impacts, coupled with our finding that global warming has increased the probability of large-scale conditions similar to those associated with the 2013/14 drought, suggest that understanding the link between climate change and persistent North Pacific ridging events will be crucial in characterizing the future risk of severe drought in California.