Don’t expect a brawny El Niño to bust California’s historic drought

Scientists say a vast swath of warmer-than-normal water known as El Niño circulating in the eastern Pacific Ocean is poised to be one of the strongest of the past seven decades. It has the potential to play a key role in the formation of storms that could bring much-needed rain and snow to California and at least ease the effects of the state’s severe four-year drought.

But there’s that one word – “potential” – that commands attention for the state’s residents, farmers and water regulators, according to weather experts. They add that it will take more than one good winter for the region to escape the grasp of a sustained drought.

“A strong El Niño refers to how much warmer than average the equatorial Pacific Ocean is,” said Mike Halpert, deputy director of the National Oceanic and Atmospheric Administration’s Climate Prediction Center in Maryland. “The one we’re seeing now is currently strong. … It’s among the top two or three events we’ve seen going back to 1950.”

The warmest versions of El Niño to develop in the past seven decades occurred in 1997-98 and in 1982-83.

“In both of those events, California saw a lot of rain and snow,” Halpert said. “The hope is that this one delivers a similar type of pattern.”

No guarantees

Even with an El Niño that last month had sea surface temperatures warmer than normal and on the rise, there’s no such thing as “a sure thing” that California will get above-normal rain and snow this winter and spring.

“The conditions are what we’d call ‘favored,’ but you can’t guarantee anything about climate. There are an awful lot of things that go on in climate,” Halpert said. “A strong El Niño shifts the odds in our favor. But even if you go to Las Vegas and play craps with your own loaded dice, you’re not going to roll a seven every time.”

Forecasters anticipate that this El Niño will bring a wet year for California, “especially in Southern California, but that’s not necessarily where we want it to be,” Halpert said, noting that most of the state’s water storage falls as mountain snow in Northern California.

“You can get too much, too fast sometimes, and that creates flooding and mudslides. And you got a taste of that recently,” he added, referring to October mudslides that trapped trucks and cars on Interstate 5 over Tejon Pass south of Bakersfield and on Highway 58 over the Tehachapi Pass between Bakersfield and Mojave.

History shows that El Niño tends to be about an even-money proposition when it comes to dropping substantially more rain and snow on the Central Valley and the central Sierra Nevada. Both of the previous “very strong” El Niño events, in 1982-83 and 1997-98, packed a wet punch across the entire state. But other El Niño events have been duds.

“There have been prior strong El Niño events that did not bring the Valley above-average precipitation,” said Brian Ochs, a meteorologist at the National Weather Service office in Hanford. “Back in around 1965-66 (considered a strong El Niño), Fresno got below-average rainfall and there was snowpack in the Sierra that was average to below average.”

Since 1950, El Niño patterns have occurred 24 times. In 12 of those years, annual rainfall in Fresno has been below the 30-year norm of 11.5 inches. In the other 12, Fresno saw more rainfall than the long-term average. And in the Sierra, at Kaiser Pass in eastern Fresno County, the precious water content of the benchmark April 1 snowpack was actually below normal in 13 of the 24 El Niño years, and above normal 11 times.

That historical context is important for people to remember this year. But forecasters are getting more comfortable with the odds of more rain and snow this winter as the equatorial Pacific keeps warming up.

“A 50 percent chance of above-average rainfall around here is usually pretty confident,” Ochs said. “We’re pretty confident that we’ll see more precipitation than recent seasons, especially the past four years.”

What the future holds

Kevin Werner, a Seattle-based Western region climate services director for the National Centers for Environmental Information, said history can be instructive for people’s expectations for winter weather, with or without an El Niño.

“Historically, we characterize the climate in the Southwest and California as long periods of dry punctuated by much shorter periods of wet, and sometimes extreme wet,” Werner said. “Regardless of El Niño or climate change or anything else, that climate tends to be one that’s highly variable and tends to see a lot of dry days, dry months or even dry years. That’s the normal.”

“There are many other four- or five-year droughts in the paleo-climate data that go back to 1000 AD,” he added.

A scientific evaluation of tree rings, produced last year for the state Department of Water Resources, reinforces that historic climatic perspective. The width of trees’ annual growth rings tends to correspond to wet and dry years as indicated by river flows.

In the San Joaquin River watershed, an analysis of tree rings indicates that over the past 1,100 years, the region has endured 35 sustained droughts of at least four years in length before the current drought began. Five of those periods have occurred since 1900. The average length of those extended droughts was more than six years, and the two longest drought periods reflected in tree rings were 12 years, from 1450 through 1461, and 13 years, from 1471 through 1483.

Weather experts say that even a gully washer of a winter cannot fully heal California’s drought wounds. That’s because breaking a drought of this magnitude requires more than just rain falling by the bucket-load and snow burying the mountains.

“It’s possible in one year do a pretty good job of filling your reservoirs,” said Halpert. “But when you look at drought, you’re looking at time scales of short-term drought and long-term drought. … A long-term drought goes more into the groundwater. Over the last few years, your area has survived by draining the groundwater – and that’s not going to recharge in one year.”

The underground water table takes years to recharge, “and it doesn’t take nearly as long to drain it,” Halpert added. “That’s a challenge that folks in California are going to be dealing with for a long time.”

This story was originally published December 4, 2015 at 5:05 AM with the headline "Don’t expect a brawny El Niño to bust California’s historic drought."