El Niño, nature's most powerful influence on weather around the globe, has been in a lull for two years. But indications suggest that could change as early as fall.
Since spring 2012, the eastern tropical Pacific Ocean has not warmed enough to create an El Niño. Nor has it cooled to form a La Niña. Instead, it has lingered in an in-between state some experts call "La Nada."
Though it is too early to predict with much certainty, scientists say their observations and computer models show increasing signs of El Niño's return, which might portend more rain for California.
"We're getting multiple indications, and we're kind of due for one," said Gerald Meehl, senior scientist at the National Center for Atmospheric Research in Boulder, Colo.
An El Niño cycle begins every two to seven years, when weak trade winds in the Pacific allow warmer water to pile up along the west coast of South America. That generates storms in the tropical Pacific farther east than usual, changing winds and altering precipitation patterns across the globe.
During El Niño conditions, the jet stream usually dips south over North America, steering storms to the California coast and across the southern U.S., while La Niña is associated with drier conditions, especially in the interior Southwest.
If El Niño reemerges, experts say, it could tame the Atlantic hurricane season, bump up global temperatures in 2015 and potentially bring more rain to parched California by next winter. But such conditions are not necessarily the answer to drought, experts warned, because they don't always result in wetter weather.
"Only the really big ones do," said Michael Anderson, California's state climatologist.
Of the six winters with strong El Niño conditions since 1950, four produced above-average rainfall in California, according to records from the National Climatic Data Center of the National Oceanic and Atmospheric Administration. However, in the winters during two strong El Niños in 1965-66 and 1991-92, rainfall was below normal.
For now, the U.S. government's El Niño alert system remains inactive, but the long-term forecast has inched upward in recent months in favor of El Niño. Last month, the National Weather Service's Climate Prediction Center gave a 49% probability of El Niño returning by fall, compared with a 45% likelihood of conditions remaining neutral and a 6% chance of La Niña. An update is expected this week.
One group of scientists has sounded an unusually early alarm. A study by an international group of researchers published last month gave El Niño a 75% chance of returning in late 2014, but made no predictions about its strength.
The pronouncement was criticized by some climate scientists, who cautioned that El Niño forecasts before spring are not reliable. Ocean and atmospheric conditions are so unstable in the spring that they act as a barrier to accurate predictions, said Anthony Barnston, chief forecaster at the International Research Institute for Climate and Society at Columbia University.
"Even if things look like they're headed to El Niño before that barrier period, things can change easily," Barnston said. By June or July, "it will be more obvious whether it's accelerating quickly or kind of feeble and waffling around the borderline."
The last El Niño, in 2009-10, was moderate and followed the next season by La Niña.
Maury Roos, chief hydrologist for the California Department of Water Resources, rattled off a series of winters when El Niño had failed to provide relief from dry conditions. California's exceptional drought of 1976-77 occurred during weak El Niño conditions, he said. And some La Niña episodes came with heavy rains.
The severe El Niño of winter 1997-98, however, stayed true to forecasts and more than doubled rainfall in Southern California. Powerful storms hammered the coast, triggering landslides and floods that caused more than $550 million in damage. And 1998 went down as one of the warmest years on record.
Since then, the eastern tropical Pacific has remained in the cool phase of another climate cycle known as the Pacific Decadal Oscillation, which typically lasts for decades, said Richard Seager, a climate scientist at the Lamont-Doherty Earth Observatory at Columbia University.
"At some point there will be a decent El Niño again," Seager said. "But there is no reason to think that this largely cooler period will end any time soon."