Atmospheric patterns resembling those that appeared during the latter half of California’s ongoing multiyear drought are becoming much more common, a new study finds.
“The current record-breaking drought in California has arisen from both extremely low precipitation and extremely warm temperature,” says Noah Diffenbaugh, associate professor of earth system science at Stanford University. “In this new study, 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.”
Diffenbaugh and colleagues 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 California’s history.
The scientists focused on the northeastern Pacific Ocean and far western North America, encompassing the winter “storm track” region where the vast majority of California precipitation originates.
hey used historical climate data from US government archives to investigate changes during California’s October to May “rainy season.” They identified the specific North Pacific atmospheric patterns associated with the most extreme temperature and precipitation seasons between 1949 and 2015. Their analysis shows a significant increase in the occurrence of atmospheric patterns associated with certain precipitation and temperature extremes over the 67-year period.
In particular, they found robust increases in the occurrence of atmospheric patterns resembling what has occurred during the latter half of California’s ongoing multiyear 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,” says Daniel Swain, first author of the study published in the journal Science Advances and a graduate student in Diffenbaugh’s lab.
“Since California depends on a relatively small number of heavy precipitation events to make up the bulk of its annual total, missing out on even one or two of these can have significant implications for water availability.”
FEWER ‘AVERAGE’ YEARS
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 (the Triple R)—was diverting winter storms northward and preventing them from reaching California during the state’s drought. In 2014, researchers published findings that showed that the increasing occurrence of extremely high atmospheric pressure over this same part of the Northeastern Pacific is “very likely” linked to climate change.
The group next wanted to investigate whether the particular spatial pattern associated with the Triple-R has become more common—a question not asked in the original 2014 study. The new study provides a more direct answer.
“We found that this specific extreme ridge pattern associated with the ongoing California drought has increased in recent decades,” Swain says.
Despite the fact that the number of very dry atmospheric patterns in California has increased in recent decades, the number of very wet atmospheric patterns hasn’t declined.
“We’re seeing an increase in certain atmospheric patterns that have historically resulted in extremely dry conditions, and yet that’s apparently not occurring at the expense of patterns that have historically been associated with extremely wet patterns,” Swain says. “We’re not necessarily shifting toward perpetually lower precipitation conditions in California—even though the risk of drought is increasing.”
That might sound contradictory, but it’s not, the scientists say. Imagine looking at a 10-year period and finding that two of the years are wet, two are dry, and the rest experienced precipitation close to the long-term average. Now imagine another decade with three very dry years, three very wet years, and only four years with near-average precipitation.
“What seems to be happening is that we’re having fewer ‘average’ years, and instead we’re seeing more extremes on both sides,” Swain says.” “This means that California is indeed experiencing more warm and dry periods, punctuated by wet conditions.”