The National Oceanic and Atmospheric Administration on Thursday said again that El Niño, a warming of temperatures in the Pacific, is 80 percent likely to strike this winter—though how intense it could get is still unclear.
That uncertainty leaves a critical question unanswered: Could El Niño bring to America the same heavy rainfall this year that it has in the past?
Nowhere is the need for an answer more acute than in California, where extreme drought covers 80 percent of the state and water supplies have dwindled to one-fifth of normal levels. This year is likely to be the driest in state history.
"El Niño would be the one way that we could...really breathe a sigh of relief and say, 'Okay good, this winter should alleviate this drought,'" said Mike Dettinger, a California-based research hydrologist at the U.S. Geological Survey and research associate at the Scripps Institution of Oceanography. "People are watching it with real interest."
Two exceptionally strong El Niños—one in 1982-83 and one in 1997-98—caused torrential rains in California.
But on average, El Niños have brought heavier-than-normal precipitation to California and the Southwest only a third of the time. And instead of delivering drought relief, weak El Niños can deliver drought.
"We've gotten our driest years and our wettest years from El Niño," Dettinger said.
In a new analysis, he found that only an El Niño that ranks among the strongest in the last 80 years or so would bring enough rain to return California's water levels to normal. Fifteen percent of historic El Niños would have been wet enough to do that.
"There was a lot of blather about how El Niño won't save us, but I've been working on El Niño for long enough to know that an El Niño could save us," Dettinger said. "In terms of the intensity—which is the critical thing for us—that's going to take some time."
El Niño is a warming of the Pacific Ocean that strikes every three to seven years. In a normal climate without El Niño, equatorial winds push warm sea water to the western Pacific Ocean. Under an El Nino scenario, those winds down die and the warm water lingers in the eastern Pacific heating up the air above the ocean. This triggers a feedback loop, in which the hotter air creates even warmer water, which produces even hotter air. The extra heat sets off extreme weather across the globe—from heat waves to storms and floods.
A recent study published in Nature Climate Change showed that global warming could double the number of the most extreme episodes by the end of the century.
Whether the El Nino occurs this year—and if it does, how strong it will be—depends on the equatorial winds. Will they subside long enough to spawn El Niño and give it strength?
Dettinger said it would take another month or two for scientists to know for certain, though forecasters from the National Weather Service are predicting a weak-to-moderate El Niño for this winter. NOAA's Climate Prediction Center won't declare an official El Niño until the increase in sea surface temperature, measured by buoys in the eastern Pacific, averages more than 0.5 degrees Celsius for five consecutive three-month periods.
Doug Parker, director of the California Institute for Water Resources, said the state's residents should continue to prepare for the worst, and not count on an El Niño to save them. The institute is housed at the University of California and is part of a federal network of water research stations.
"I think we really need to be prepared for more drought," Parker said. "If you take El Niño out of the equation, there's a pattern of dry years happening so there's a higher probability that next year will be a dry one." He said that almost all of California's water supply usually comes from four to five storms per year. The state has experienced one major storm in 2014.
Climate change is expected to worsen California's woes, according to federal scientists, causing precipitation to decrease in the spring and droughts to become more frequent and severe, even while increasing flooding in low-lying areas like Sacramento from rising seas.
Parker said he is primarily concerned with replacing the water that Californians are using. Though Gov. Jerry Brown asked residents in January to cut their water use by 20 percent, the state's most recent status report shows residents have only achieved a five percent reduction.
"The key is that a lot of our drought management comes from the groundwater and that's a great resource during the drought, but you have to put that water back in the ground," said Parker. "It's how we're going to get through the next drought."
Amy Nordrum is an intern at InsideClimate News.