California drought patterns becoming more common

http://www.eurekalert.org/pub_releases/2016-04/ssoe-rot032416.php

Public Release: 1-Apr-2016
Rise of the ridiculously resilient ridge: California drought patterns becoming more common
Stanford's School of Earth, Energy & Environmental Sciences

Atmospheric patterns associated with droughts in California have occurred more frequently in recent decades, Stanford scientists say.

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"The current record-breaking drought in California has arisen from both extremely low precipitation and extremely warm temperature," said Diffenbaugh, an associate professor of Earth System Science at Stanford's School of Earth, Energy & Environmental Sciences and a senior fellow at Stanford's Woods Institute for the Environment. "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."

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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. In 2014, the Stanford 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 global warming.

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 to this question.

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

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 said. "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 said. 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 said. " "This means that California is indeed experiencing more warm and dry periods, punctuated by wet conditions."


tags: severe weather, extreme weather