Atmospheric rivers drenching California
30 Pineapple Express storms have soaked state since Oct. 1
Why is it raining so much this year in California?
Two words: Pineapple Express.
In a typical year, California has between 10 to 15 “atmospheric river” storms — the fire hoses that rampage in from Hawaii and account for up to 50 percent of the state’s annual rainfall — and nearly all of its floods. But since the rainy season began on Oct. 1, there already have been 30. And there’s two months of winter to go.
“We are well beyond double what is normal,” said Marty Ralph, director of the Center for Western Weather and Wa- ter Extremes at UC San Diego.
Ralph, one of the nation’s experts on atmospheric rivers — which are often called Pineapple Express storms when they come from the tropics — calculated the big boost in atmospheric river storms last week for the state Department
of Water Resources. He found that from Oct. 1 to Feb. 16, there was an average of one atmospheric river storm every 4.6 days that made landfall in California. Nine of them were categorized strong or extreme.
There have been other years with a similarly large number of atmospheric rivers, including 2006 and 2010, but scientists don’t know for sure why they’re seeing so many this year.
“The storm track has set up so that we have gotten these bursts of very active weather for a week or so followed by a dry period, and then the storm track has reverted back,” Ralph said. “Why that happens, it’s a mystery.”
One thing different this winter than in the two previous two: The massive wall of high-pressure air that was parked off the West Coast, diverting storms away from California and into Canada, has largely broken up and gone. Without that “Ridiculously Resilient Ridge,” there’s nothing to stop Pineapple Express storms — which can be 250 miles wide, 1,000 miles long and carrying 20 times as much water as the Mississippi River at its terminus with the Gulf of Mexico — from drenching once-parched California.
Why did the ridge go away? Nobody really knows that either.
“What we’ve seen describes California’s climate. It’s a climate precipitated by extremes,” said Daniel Swain, an atmospheric researcher at UCLA. “The precipitation is driven by the presence or absence of atmospheric rivers, and right now we have an overabundance of them. A few too many.”
With every soaking storm, California’s historic five-year drought is fading further into the background. On Thursday, federal scientists reported that 83 percent of California is no longer in drought conditions.
The latest round of storms wrecked Oroville Dam’s spillway, flooded downtown San Jose and closed Interstate 80 in the Sierra Nevada under record-breaking blizzards. As a result, just nine of California’s 58 counties, all of them in Southern California, have any significant drought conditions, the U.S. Drought Monitor reported.
Swain noted that unusual extremes in Pacific Ocean water temperatures may have played a role in the behavior of the ridge. Or its persistence may have been random bad luck, from California’s standpoint.
“We are trying to figure out what it is in the Earth’s system that can be a precursor to this sort of thing,” he said. “Is it the state of the Pacific Ocean? This year there wasn’t an El Niño or much of La Niña either. Clearly it can’t just be that.”
While both Swain and Ralph say that the science is clear that the climate is changing and that the Earth is steadily warming, they emphasize that there isn’t enough evidence to tie this year’s boost in atmospheric rivers to climate change.
But climate change is increasing the likelihood that future droughts will be hotter and more severe, and future wet winters will have more flooding, many scientists say. That’s because the warmer the ocean water, the more evaporation takes place, providing more moisture for atmospheric river storms.
“Climate change may have shifted the odds already, but I am dubious about just jumping ahead to assume so,” said Michael Dettinger, a research hydrologist for the U.S. Geological Survey and a researcher at Scripps Institution of Oceanography in La Jolla.
“Basically climate change is not required to explain this winter,” Dettinger said. “California is historically capable of this sort of extreme. However, we should be paying attention to how our systems work this year, because climate change will make this sort of year a lot less anomalous than in the decades past.”
If that happens, the state and federal government will need to figure out ways to better maintain their levees and reservoirs to withstand summer and winter on steroids.
“When the government talks about the risk of a particular financial policy or foreign policy, you consider a range of outcomes,” Swain said. “Is the risk of drought and floods in California increasing? The answer is yes.”
So far, the parade of atmospheric rivers has created the wettest winter ever measured in the Northern Sierra, with precipitation 229 percent of the historic average. On Thursday, eight key weather stations from Lake Tahoe to Mount Shasta measured an average of 76.2 inches of precipitation since Oct. 1. A normal year is 50 inches, and this year’s total is even running above the monster winters of 1997-98 and 1982-83.
Nearly every major city in California, meanwhile, already has exceeded its annual average rainfall for the year, including Los Angeles, San Francisco, San Jose, Oakland and Sacramento.
The state’s largest reservoirs are collectively at 122 percent of average, and dam operators at Shasta, Oroville, Folsom and other enormous lakes are releasing water at a furious pace to create space to capture more stormwater and melting snow in the weeks and months ahead to try to reduce flood risk.
Gov. Jerry Brown’s administration has said it will wait until April to decide whether to rescind or amend the emergency drought declaration he issued in January 2014.
“Is the drought over?” Ralph asked. “By most measures I would say yes. The dilemma is that if you look at groundwater, the drought has left a reduction in some places that will be tough to refill. But if you define drought by precipitation, yeah it’s over.”