California crippling drought linked to climate change: Scientists

“We have seen this amazingly persistent region of high pressure over the northeastern Pacific for many months now, which has substantially altered atmospheric flow and kept California largely dry.”

Blocking ridges occur periodically at temperate latitudes, but the Triple R was exceptional for both its size and longevity. While it dissipated briefly during the summer months of 2013, it returned even stronger by fall 2013 and persisted through much of the winter, which is normally California’s wet season.

“At its peak in January 2014, the Triple R extended from the subtropical Pacific between California and Hawaii to the coast of the Arctic Ocean north of Alaska,” said Swain, who coined the term “ridiculously resilient ridge” last fall to highlight the unusually persistent nature of the offshore blocking ridge.

Like a large boulder that has tumbled into a narrow stream, the Triple R diverted the flow of high-speed air currents known as the jet stream far to the north, causing Pacific storms to bypass not only California but also Oregon and Washington. As a result, rain and snow that would normally fall on the West Coast was instead re-routed to Alaska and as far north as the Arctic Circle.

An important question for scientists and decision-makers has been whether human-caused climate change has influenced the conditions responsible for California’s drought. Given the important role of the Triple R, Diffenbaugh’s team set out to measure the probability of such extreme ridging events.

The release notes that the team first assessed the rarity of the Triple R in the context of the twentieth-century historical record. They found that the combined persistence and intensity of the Triple R in 2013 was unrivaled by any event since 1948, which is when comprehensive information about the circulation of the atmosphere is first available.

More directly to address the question of whether climate change played a role in the probability of the 2013 event, the team collaborated with Bala Rajaratnam, an assistant professor of statistics and of environmental Earth system science and an affiliated faculty member of the Stanford Woods Institute for the Environment. Rajaratnam and his graduate students Michael Tsiang and Matz Haugen applied advanced statistical techniques to a large suite of climate model simulations.

Using the Triple R as a benchmark, the group compared geopotential heights — an atmospheric property related to pressure — between two sets of climate model experiments. One set mirrored the present climate, in which the atmosphere is growing increasingly warm due to human emissions of carbon dioxide and other greenhouse gases. In the other set of experiments, greenhouse gases were kept at a level similar to those that existed just prior to the Industrial Revolution.

The interdisciplinary research team found that the extreme geopotential heights associated with the Triple R in 2013 were at least three times as likely to occur in the present climate as in the preindustrial climate. They also found that such extreme values are consistently tied to unusually low precipitation in California and the formation of atmospheric ridges over the northeastern Pacific.

“We’ve demonstrated with high statistical confidence that the large-scale atmospheric conditions, similar to those associated with the Triple R, are far more likely to occur now than in the climate before we emitted large amounts of greenhouse gases,” Rajaratnam said.

“In using these advanced statistical techniques to combine climate observations with model simulations, we’ve been able to better understand the ongoing drought in California,” Diffenbaugh added. “This isn’t a projection of 100 years in the future. This is an event that is more extreme than any in the observed record, and our research suggests that global warming is playing a role right now.”

— Read more in Daniel L. Swain et al., “The Extraordinary California Drought of 2013/2014: Character, Context, and the Role of Climate Change,” Explaining Extreme Events of 2013 from a Climate Perspective, Special Supplement to the Bulletin of the American Meteorological Society 95, no. 9 (September 2014): S3-S7