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El Niño Could Usher in a Decade of Stronger Events

By Luc Cohen, Reuters

In Buffalo, early December meant breaking a 116-year-old record for a lack of snow. In Duluth, Minn., a newspaper reported that the temperature was 40 degrees above zero, not below. And in Miami, beachgoers stayed indoors during what had become the third-wettest December in local history, just eight days into the month.

What's going on with the weather?

It's the phenomenon called El Niño, which is happening now as ocean water temperatures rise above normal across the central and eastern Pacific, near the equator. Its effects will leave the U.S. Northeast warmer than usual, the Midwest drier, and the West and the South wetter. And scientists have a message for everyone bracing for one of the strongest El Niño events on record: get used to it.

Snow melts into the South Yuba River near Big Bend, Calif., Dec. 4, 2015.
Credit: Reuters/Max Whittaker

While El Niño oscillates on a more or less yearly cycle, another dynamic in Pacific Ocean water temperatures, known as the Pacific Decadal Oscillation (PDO), has the potential to accelerate global warming and increase the severity of El Niño episodes, scientists said. The last time the PDO was, as it may be now, in a prolonged positive, or "warm" phase, it corresponded with two of the strongest El Niños on record.

"When you really have a monster El Niño, it could be enough to flip the PDO into a new phase for a decade or so," said William Patzert, a climatologist at NASA's Jet Propulsion Laboratory in California. "Keep your eyeballs peeled because maybe we're in for a decadal shift."

Previous warm phases have also coincided with increased precipitation on the U.S. West Coast, signaling potential relief for California from a severe drought.

Before January 2014, the world experienced a 15-year period of mostly negative values for the Pacific oscillation, according to data maintained by Nathan Mantua, an atmospheric scientist at the National Oceanic and Atmospheric Administration's (NOAA) Joint Institute for the Study of the Atmosphere and Oceans.

That period saw only weak or moderate El Niño events. During the 21 years before that, the Pacific oscillation values trended mostly positive, a period that coincided with the 1982-83 and 1997-98 El Niño events, two of the strongest on record.

Now, scientists are beginning to wonder if the 15-year period of relative El Niño calm is coming to a close, marking the start of a warmer, stormier era akin to the 1980s and 90s.

The PDO index has been positive for 22 months through October, the longest such streak since a 26-month positive period between 2002 and 2004. Scientists are not sure if the current streak marks a longer-term turnaround or just a temporary blip like the 2002-2004 streak.

"It's more likely that we'll have a change in phase and we'll remain in positive territory," said Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colorado, noting that while a decadal shift was far from a guarantee, the odds in favor are approximately 2-to-1.  

A Warmer Base State

In many ways, the weather of the 15 years before 2014 has resembled that of the mid-1940s to mid-1970s, the last prolonged period of a negative Pacific oscillation cycle, with drought in the American West and very few El Niños, Patzert said.

The recent period saw several moderate La Nina events, a counterpart to El Niño defined as cooler than normal sea surface temperatures in the eastern and central tropical Pacific that dumps rain on Australia and Indonesia but leaves the Southwest United States dry, including episodes in 1998-99, 1999-2000, 2007-08 and 2010-11.

El Niño's heat in the Pacific Ocean.
Credit: NOAA

The warmer sea surface temperatures in the northern Pacific during the positive PDO phase tend to amplify El Niño's effects, Trenbirth said.

Several scientists said the current El Niño could contribute to more positive PDO conditions at the moment and in the future. 

"The key ingredient is the strong El Niño," said NASA's Veronica Nieves, noting that strong episodes have historically triggered decadal shifts. She has submitted a paper to an academic journal arguing that the Pacific may be in store for another 20 years or more of warmer sea surface temperatures.

To be sure, the two-year period of positive Pacific oscillation values that happened from 2002 to 2004, which saw weak and moderate El Niños, is still fresh in scientists' minds, preventing them from being certain that the world is truly on the cusp of a decadal shift.

But so far in these past two years, the values have been more sharply positive than the 2002-04 streak. This has implications beyond El Niño: the recent decade has been referred to as a 'hiatus' in global warming, with the negative PDO value seen as limiting global temperature gains.

"If [PDO] transitions back into positive, we'd see a resumption in these more rapid rates of global warming," said Gerald Meehl, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo. "Having that shift in the background base state means that the peaks of the El Niño are going to be higher."

Reporting by Luc Cohen, editing by John Pickering

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