Researchers: California drought patterns becoming more common

Researchers at Stanford University have found that atmospheric patterns associated with droughts in California, on the U.S. West Coast, have occurred more frequently in recent decades.

Noah Diffenbaugh, an associate professor of Earth system science at Stanford School of Earth, Energy & Environmental Sciences and a senior fellow at Stanford Woods Institute for the Environment, believes that "the current record-breaking drought in California has arisen from both extremely low precipitation and extremely warm temperature."

"We find clear evidence that atmospheric patterns that look like what we've seen during this extreme drought have in fact become more common in recent decades," said Diffenbaugh, who led a team of researchers to analyze the occurrence of large-scale atmospheric circulation patterns that have occurred during California's historical precipitation and temperature extremes.

By focusing on the northeastern Pacific Ocean and far western North America, encompassing the winter "storm track" region from which the vast majority of California precipitation originates, the team investigated whether atmospheric pressure patterns similar to those that occurred during California's historically driest, wettest, warmest and coolest years have occurred with different frequency in recent decades compared with earlier in the Golden State's history.

The researchers used historical climate data from U.S. government archives to investigate changes during California's October to May "rainy season," identified the specific North Pacific atmospheric patterns associated with the most extreme temperature and precipitation seasons between 1949 and 2015, and revealed a significant increase in the occurrence of atmospheric patterns associated with certain precipitation and temperature extremes over the 67-year period.

In a new study published online in Science Advances, they reported, in particular, robust increases in the occurrence of atmospheric patterns resembling what has occurred during the latter half of California's ongoing multi-year drought.

"California's driest and warmest years are almost always associated with some sort of persistent high pressure region, which can deflect the Pacific storm track away from California," said Daniel Swain, the study's first author and a graduate student in Diffenbaugh's lab.

Blocking ridges are regions of high atmospheric pressure that disrupt typical wind patterns in the atmosphere. Scientists concluded that one such persistent ridge pattern - which Swain named the Ridiculously Resilient Ridge - was diverting winter storms northward and preventing them from reaching California during the state's drought.

"We found that this specific extreme ridge pattern associated with the ongoing California drought has increased in recent decades," Swain said.

In 2014, the Stanford researchers published findings that showed that the increasing occurrence of extremely high atmospheric pressure over the same part of the Northeastern Pacific is "very likely" linked to global warming. Endit