Pacific Ocean ENSO cycle a key player in global climate
By Bob Berwyn
The shift from a powerful El Niño to the cooler La Niña phase of Pacific Ocean temperatures will temporarily end the planet’s recent record streak of record-warm years, according to climate scientists who see the cyclical ocean changes as a key factor in the long-term global climate change equation.
Nearly all record-warm global years since 1950 (when accurate records start) have come during during El Niños, when the Pacific Ocean releases heat to the atmosphere and intensifies global warming caused by heat-trapping greenhouse gas pollution. The 2015-16 El Niño was one of the strongest on record, but it has now ended, according to the National Oceanic and Atmospheric Administration, which says sea surface temperatures in the central and equatorial Pacific have cooled to average in the past few weeks.
It’s unlikely there could be a record-warm year during La Niña, said Andrew Hoell, a research meteorologist with NOAA.
“It would be a low probability outcome judging from the temperature anomaly record. 2015 was so warm that it will take a few years to get back to that level,” Hoell said. I’ll use the 2009-2010 El Nino as an analogue. The 2010 global average temperature anomaly was 0.72 due to El Nino. The next two years were La Nina, and were much cooler at 0.60 and 0.63 anomalies.
“2013 was ENSO neutral, and the global average temperature anomaly was 0.65. 2014 was ENSO neutral early in the year, trending toward weak El Nino late in the year, and the global average temperature anomaly was 0.74. El Nino was strong in 2015 and the global average temperature anomaly was 0.87,” he said.
Hoell said researchers are watching how ENSO cycles affect climate patterns regionally.
“A big question for us is how the synchronous warming of SST and La Nina events force droughts globally. For example, the La Nina events of 1999-2013 were associated with strong droughts across much of the Northern Hemisphere … Western Asia, the Western United States and East Africa. We aim to understand the components of these drought that are natural (i.e. La Nina) and the components of these droughts that are anthropogenically forced (i.e. warming oceans). We also aim to understand how the synchronous warming of SST and La Nina events project into the future, and what their global precipitation impacts will be,” he said.
The El Niño Southern Oscillation (ENSO) cycle is a complex interplay of winds and ocean currents that sloshes huge masses of water back and forth in the Pacific. When the easterly trade winds die down, warm water pooled near Indonesia (the world’s warmest ocean region, on average) spreads eastward toward the coast of South America. La Niña emerges when the water in that region cools to below average.
Researchers study the interaction of climate change and ENSO because the cool phase can offer temporary relief from year-to-year temperature increases. The respite can moderate some global warming impacts, giving heat-struck corals a chance to recover and bringing relief from drought in some regions. By contrast, the warm El Niño phase exacerbates global warming impacts. The cycle, about four years long, is the main reason global temperatures don’t increase at a constant rate, despite the steady buildup of greenhouse gases.
Strong El Niños can speed sea ice melt in some areas, and drive deadly heatwaves and crop-killing droughts. A recent study in the journal Nature Climate Change showed how the recent El Niño helped drive atmospheric CO2 levels to a new record in 2015 by drying out tropical forests, preventing them from absorbing greenhouse gases. That set the stage for big wildfires that directly emitted more CO2 to the atmosphere.
All this is bad news for the climate if, as some studies suggest, the Pacific Ocean is moving toward a more permanent El Niño-like state because of human-caused global warming. And one scientist who studies climate on the geological time scale said that, at the very least, there’s evidence that the El Niño-La Niña cycle (ENSO) is strengthening. Combined with human-caused global warming, that leads to “tipping points” in various earth systems.
“More and more ecosystems will be crossing such tipping points in the future,” said Kim Cobb, an atmospheric scientist and paleoclimatologist at the Georgia Institute of Technology. Decades-long warming in the world’s oceans, caused by the greenhouse gas buildup, in combination with the 2015-16 El Niño wiped out a coral reef Cobb has been studying for 18 years.
“We’ll be studying this El Niño’s long-term effects, if and how the reef recovers, for the next decade or so. But events like this illustrate the destruction of natural climate extremes superimposed on long-term climate change,” she said.
Short global cool-down?
Sea surface temperature increases associated with El Niños generally spike for a year and can linger for two years, often followed by a shift to cooler-than-average temperatures–the La Niña phase of the ENSO cycle that’s beginning now.
And there’s never been a record-warm year during La Niña, said Dr. Phil Jones, director of the Climatic Research Unit at the University of East Anglia (UK). That means Earth will get a short reprieve from an unprecedented series of record-warm months and years that started in 2014 and will continue through the end of 2016.
“The key thing to realize is … the way that global temperature responds to El Niño is not at the same time,” Jones said. “The impact of La Niña will not be felt much in 2016, but there will be some effect. Global temperatures … are going to drop off the rest of the year, but 2016 will still be warmer than 2015, which was the warmest year on record,” he said.
“The interesting feature for me is, how much cooler with 2017 be?” he said, adding that the emerging La Niña is likely to prolong the California drought and bring more moisture to Australia and Indonesia, where the 2015 El Niño caused drought that led to huge wildfires.
“The La Niña effect is going to come, it’s going to be in 2017, and it will take the values back down about 0.15 degrees Celsius,” Jones said, describing that as the average amount by which La Niña cools global temperatures. El Niño, by contrast, warms global temperatures by about 0.20 degrees Celsius.
Even though global surface temperatures may cool slightly during 2017, greenhouse gases continue to trap an ever-increasing amount of the sun’s incoming heat. And during La Niña, winds and ocean current combine to push that heat deep into the ocean, according to Lisa Goddard, director of the International Research Institute for Climate and Society at Columbia University. Eventually, the warmer water in the deep ocean will re-emerge and intensify future El Niños.
Sea ice impacts
Climate researchers will also be watching the upcoming shift to La Niña because of short-term climate implications, including sea ice at both poles. Scientists aren’t completely sure how the cycle affects sea ice formation and melting, but because sea ice is an important indicator of long-term climate change, researchers want to know how ENSO–a tropical phenomenon–influences climate in high latitudes.
“The jury is still somewhat out for the Arctic,” said Dirk Notz, a sea ice researcher with the Max-Planck-Institute for Meteorology in Hamburg, Germany. But some studies have been able to link El Niño with regional changes in sea ice formation, he said.
“A 2004 study found that El Niño causes less ice in the Beaufort Sea but more ice in the
Northwestern Passage … The decrease in the Beaufort sea ice is linked to the warming of the Pacific sector of the Arctic, which then melts ice there,” Notz said.
“For the Antarctic, changes also differ quite a bit regionally,” he said. NOAA’s National Snow and Ice Data Center linked the recent El Niño with a slowdown in Antarctic sea ice growth last October. Warm Pacific Ocean water spreading southward weakens winds that normally blow sea ice outward from Antarctica, Notz said.
Effects on sea ice, corals, drought and heatwaves are all good reasons to study the El Niño-La Niña cycle, including this one, said Cobb.
“Of course, it is of fundamental importance to understand if ENSO is changing in response to climate change. My own lab’s research hints that it is intensifying in recent decades,” she said, referring to a study that examined ancient coral skeletons to reconstruct climate patterns, including ENSO cycles.
“And now, with the huge 2015/2106 event added to our long-term record, our results are even stronger. If this is true, it obviously implies that the global effects of this last year’s El Nino event may become a more frequent occurrence – part of the growing tab of climate change that becomes more clear and pressing every year. Our work would imply that both El Nino and La Nina events are increasing in magnitude,” she said.
Other researchers say there’s not enough of a long term record yet to be able to say how the ENSO cycle will respond to global warming, but that those changes “have the potential to be one of the largest manifestations of anthropogenic climate change,” according to an international research consortium established to study El Niño and La Niña. The CLIVAR project, launched in 1995, aims to examine the links between ENSO and climate change.
This winter’s impending La Niña may provide some answers.
“This may be the first major, classical La Nina event since 1998-2000, which was fairly destructive,” said Cobb. “I’ll be interested in how the predictions … square with what materializes this winter, and of course, what the global temperatures will be should a major (La Niña) event materialize.
“Closer to my heart, I’ll be back down at Christmas Island this fall and winter to document the early recovery on the reef, and hope that a major La Nina event will give the reef a much-needed boost in the form of 1990’s-like temperatures to aid its recovery,” she said.