Will climate change temper El Niño's tantrums?

December 09, 2002

SAN FRANCISCO--The broad-scale warming expected from increased greenhouse gases may actually sap the strength of a typical El Niño, according to researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. In contrast, the average El Niño during the last ice age may have packed more punch than today's. The scientists have examined the past and future behavior of El Niño using a sophisticated computer model of global climate. They present their results this week at the annual meeting of the American Geophysical Union in San Francisco, December 6-10.

More tepid El Niños to come?

NCAR scientist Esther Brady is lead author of a study that uses the NCAR Climate System Model to track how global air and ocean circulation could evolve at increasing levels of carbon dioxide, the most prevalent of the industrial greenhouse gases. The scientists simulated Earth's climate with atmospheric carbon dioxide at one, two, and six times its preindustrial level of about 280 parts per million.

As greenhouse gases increase and global air temperatures rise, Brady's results show a significant weakening of the average El Niño event. El Niño typically shifts warm water from the western Pacific toward the central and eastern tropics, as east-to-west trade winds weaken. Her simulations show an increase in cold upwelling off the coasts of Ecuador and Peru. This helps keep the eastern tropical Pacific from warming up as much as the west, sharpening the oceanic contrast that feeds the trade winds and helps keep El Niño at bay. Brady also found that greenhouse warming in the model led to a decoupling of the link between Pacific trade winds and the underlying sea-surface temperatures. This ocean-atmosphere link is believed to help drive the cycle of El Niño and its cool-water counterpart, La Niña.

Although this cycle might weaken on average in a greenhouse-warmed world, any given El Niño could still be intense, Brady notes. Even in the most extreme simulation, with six times the present-day level of carbon dioxide, large El Niños occur--but fewer overall.

Simulating El Niño's past

It turns out there's a history of diminished El Niño events in a warming world, according to another Climate System Model study. Led by NCAR's Bette Otto-Bliesner, this project examined the period around 11,000 years ago, when global temperatures were rebounding from the last ice age. The average El Niño during this period in the computer simulation was about 20% weaker than today. The main factor responsible for the decrease is a slow shift in Earth's asymmetric orbit around the Sun. Nowadays, Earth's orbit comes closest to the Sun in early January, but 11,000 years ago, the closest approach came in the Northern Hemisphere summer, the season when most El Niños are just beginning to intensify. Along with other factors, the near-Sun approach may have provided enough extra heating to warm the western Pacific, while the eastern Pacific--where upwelling of cold water dominates--remained chilly. Driven by this intensified contrast, the east-to-west trade winds would strengthen, hindering developing El Niños.

Looking even further back in time, Otto-Bliesner and colleagues found that a more vigorous El Niño may have held sway when the last ice age was at its peak. Simulations for 21,000 years ago show the typical El Niño about 20% stronger than today. In the model, cold water sinks as it drifts from ice-covered southern oceans into the tropical Pacific. The thermocline--an oceanic boundary that separates surface warmth and subsurface chill--is thus strengthened, and the effect, says Otto-Bliesner, is to ramp up the average intensity of both El Niños and La Niñas.

Previous studies have differed on how intense El Niño events might have been in the past. She adds that both weak and strong El Niños show up in each era studied thus far, and more work is needed to arrive at a solid history. "The observational record is pretty short. El Niño may be changing already, but I don't think we really know that yet."

Background: How El Niño works

A tight coupling between ocean and atmosphere produces the weather and climate impacts of El Niño and its counterpart, La Niña. During El Niño, the trade winds that usually blow from east to west across the tropical Pacific weaken, and the strong upwelling that normally keeps waters cool off Peru and Ecuador diminishes. This allows warmer water to extend across the tropical Pacific, rather than being confined to the west near Indonesia. Tropical showers and thunderstorms follow the warm waters eastward, toward South America. The air rising within these displaced storms helps steer upper-level winds and shape climate across much of the globe. In contrast, during La Niña, the trade winds strengthen, upwelling increases, and the eastern tropical Pacific is cooler than normal. This helps trigger a different set of climate impacts, some of them the opposite of those expected during El Niño. The entire system of ocean-atmosphere linkages is known as the El Niño-Southern Oscillation (ENSO).
-end-
Writer: Bob Henson

On the Web:
El Niño and La Niña Fact Sheet: http://www.ucar.edu/communications/factsheets/elnino

National Center for Atmospheric Research/University Corporation for Atmospheric Research

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