The 'Ridiculously Resilient Ridge' Is to Blame for California's Drought

If you flipped around Mark Twain's old adage about San Francisco's weather, you'd end up with, "The warmest summer I ever spent was a winter in San Francisco." It's a fair assessment: As rest of the US does its best Siberia impression, it's been an alarmingly warm winter season in California.

In the midst of polar vortexes and state-sized blizzards, we in San Francisco have been hula-hooping barefoot and sunbathing in the park. But the sun-drenched glamor comes with a large side of discomfort: California is wading deep into its third year of a nasty drought.

The first rain of the year came Feb. 2, bringing 0.85 inches to the San Francisco International Airport and a collective sigh of relief for Californians, who have been urged by Gov. Jerry Brown to cut back water usage by a fifth.

It's easy to chock up the unseasonable warmth as a vague side effect of climate change. What's tougher is to identify a culprit, which is what makes the observations of Stanford PhD student Daniel Swain so intriguing. In December, Swain took to his blog to point out a 2,000-mile long partition of high pressure off the west coast of Canada that blocks cold weather from the Arctic from moving south.

He calls it the Ridiculously Resilient Ridge, and he says it's mostly to blame for the US's wild weather of late. It's a typical feature this time of year. What is concerning is that it hasn't budged in 14 months.

"We can say that this is very rare to unprecedented in our historical record," said Swain, who studies at Stanford's Department of Environmental Earth System Science. "It's like a boulder in a stream. It just sits in place and doesn't move and doesn't allow any storms to slip underneath."

The ridge isn't simply blocking Arctic jet streams from entering California air space. It's refracting them east into a trough of frigid temperatures gripping the Midwest and east coast.

"The warmth in California and the cold weather in the rest of the country—those events are not unrelated," he said. "The trough on the other side of this ridge is partly what allowed the polar vortex to meander from its typical home in the Arctic into Michigan."

You can track Swain's revelation of the RRR, as he calls it, by scanning his entries on California's drought on his blog, WeatherWest. First, in November, he casually observed that sunny CA is on pace to experience the driest year on record and makes note of the massive blob of "unusually high geopotential heights [that] has been extraordinarily persistent over the past 12 months."

On Dec. 1, he observed some light precipitation rolling down the West Coast that might bring some much needed rainfall and snow to California's Central Valley.

Then, on Dec. 13, he reported—almost marveled—about how a cold front that has Canada in a "hard freeze" and has its sights set on the East Coast will actually leave California "in the midst of a slow but substantial warming trend." What followed was a solid month of weather warm enough to bring San Franciscans outside at 10 AM for Sunday yoga in the park.

"It's hard to complain about the fact that it's 75 degrees in January and you can put on shorts and go hike on the Pacific Crest," Swain said. "But knowing the context of it is kind of scary."

Swain worries that the nice weather distracts from the scary environmental conditions in which the Golden State finds itself.

"It's harder to convey the urgency of a drought compared to a flood or hurricane where you have these vivid images and they require action on very short timescales," he said. "Part of the value of giving a name to this malevolent feature in the atmosphere is that it becomes part of the broader conversation about drought."

It's tough to say how long the ridge will stick around. There isn't much humans can do to disrupt its reign, but a strong jet stream from the North Pole might help dissolve it.

Even tying it directly to global warming is tricky, Swain said. At best, he said, it "provides us with a new data point when we study the relationship between climate change and extreme weather."

"We're looking at that right now," he said. "Stay tuned. Hopefully there will be a report on this in the near future."