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El Niño’s future


Last summer, a network of buoys straddling the equator in the eastern Pacific registered abnormally high sea-surface temperatures — signaling that El Niño, a driver of global climate, had resurfaced after a four-year hiatus. The warm pool of water triggered changes in jet streams that brought unseasonable warmth to the northern United States west of the Rockies this winter, and a deluge of moisture to drought-ridden states along the Gulf Coast.

Scientists from the National Oceanic and Atmospheric Administration adjust sensors on a buoy in the equatorial Pacific. A network of buoys spanning from the coast of Ecuador westward to Indonesia record ocean temperatures and winds — data critical for predicting and monitoring El Niño and La Niña events. Photo courtesy of NOAA/Department of Commerce.

While forecasters can now predict El Niño events up to a year before they reach their peak, the impacts of long-term climate change on El Niño remain difficult to pin down. Some climate models suggest that global warming could lock the climate into a permanent state of El Niño or into its counterpart La Niña, which typically brings storms to the northern United States in the winter. Gone would be the cyclic alternation between El Niño and La Niña, and the associated cycle in atmospheric pressure called the Southern Oscillation — known collectively as ENSO — that define modern climate.

A paper in the Feb. 7 Science looks back to the Eocene, a time 55 to 35 million years ago when the global climate was 10 degrees Celsius warmer than it is now, and found evidence for ENSO cycles similar to those today. Past robustness suggests that ENSO will not break down under future warming, the authors say.

“The study fundamentally challenges some of the papers that have suggested that the future of El Niño is dire in a greenhouse-warmed climate,” says geologist Donald Rodbell of Union College in Schenectady, N.Y.

The researchers adapted the Community Climate System Model of the National Center for Atmospheric Research to match Eocene conditions. The publicly available model simulates ocean, land and atmosphere interactions. Going back to the Eocene meant tailoring the model by shifting plate locations, changing ocean basin morphologies and doubling the atmospheric concentrations of the greenhouse gas carbon dioxide.

“The Eocene provides a particularly exacting test of the ENSO shutdown idea,” says co-author Rodrigo Caballero, an atmospheric scientist at the University of Chicago. “It features the warmest temperatures of the past 60 million years. … If ENSO didn’t shut down then, it becomes very difficult to argue that it has shut down at any later time.”

Theories predicting a permanent El Niño suggest that global warming will disrupt a delicate feedback between the ocean and the atmosphere, says co-author Matthew Huber, a climatologist at Purdue University. Today, an El Niño event ends when trade winds blowing from east to west across the Pacific pick up speed. The winds push the warm pool of water off the coast of Ecuador to the west. Cold water from below upwells, replacing the departed warm water. Temperature differences between the eastern and western Pacific, in turn, strengthen the trade winds. That positive feedback draws up more cold water in the east that eventually dissipates the anomalous surface water warmth that defines El Niño. A greenhouse atmosphere could raise the temperature of the upwelling water, which would undermine the east-west temperature gradient that drives the feedback that pulls the climate out of El Niño.

In the Eocene model, atmospheric warming does raise the temperature of the oceans. However, the majority of the added warmth gets locked into highly saline water that sinks to the bottom of the ocean. The top kilometer of ocean water remains relatively cool. Because trade winds pull water up from several hundred meters below the surface — not from the very deep ocean water — the model sustains the cold-water upwelling necessary to break El Niño events. In the simulations, El Niño develops and disappears once every three to five years, as observed today.

Sedimentary records from two lakes appear to support the simulations: one from Wyoming, reported by Maurizio Ripepe from the University of Camerino, Italy; the other from Germany, reported by J Mingran, from GeoforschungsZentrum in Potsdam, Germany. Annual bands of sediment laid down during the Eocene suggest that floods and heat waves pulsed through those regions at roughly the frequencies predicted by the Eocene model.

However, both the model and the field data must be taken with more than a grain of salt, says Princeton geophysicist George Philander. “The Earth climate has strong asymmetry relative to the equator, and most models fail to reproduce it. … They can’t really get El Niño [today] correct.” Until the simulations model the variability of today’s climate more accurately, back calculations of variability in the distant past remain questionable, Philander says. And the lake sediments were so far away from the equatorial Pacific during the Eocene that other climate variables likely masked any true El Niño signals. “The last place you would go to look for El Niño signals today would be Germany,” says Philander.

Yet very few annual records from the Eocene exist, Caballero says. “One of the aims of our paper was to generate sufficient interest to stimulate the data people to actually go out and search for such records.” An Eocene record obtained directly from the tropics — from sediments in South America, for example — would provide an ideal test for the model, he adds.

Greg Peterson


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