The broad-scale warming expected from increased greenhouse gases may actually sap the strength of a typical El Nino, according to researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. In contrast, the average El Nino during the last ice age may have packed more punch than today's. The scientists have examined the past and future behavior of El Nino 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 Ninos 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 Nino event. El Nino 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 Nino 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 Nino and its cool-water counterpart, La Nina.
Although this cycle might weaken on average in a greenhouse-warmed world, any given El Nino could still be intense, Brady notes. Even in the most extreme simulation, with six times the present-day level of carbon dioxide, large El Ninos occur--but fewer overall.
Simulating El Nino's past
It turns out there's a history of diminished El Nino 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 Nino 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 Ninos 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 Ninos.
Looking even further back in time, Otto-Bliesner and colleagues found that a more vigorous El Nino may have held sway when the last ice age was at its peak. Simulations for 21,000 years ago show the typical El Nino 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 Ninos and La Ninas.
Previous studies have differed on how intense El Nino events might have been in the past. She adds that both weak and strong El Ninos 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 Nino may be changing already, but I don't think we really know that yet."
Background: How El Nino works
A tight coupling between ocean and atmosphere produces the weather and climate impacts of El Nino and its counterpart, La Nina. During El Nino, 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 Nina, 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 Nino. The entire system of ocean-atmosphere linkages is known as the El Nino--Southern
On the Web:El Nino and La Nina Fact Sheet: http://www.ucar.edu/communications/factsheets/elnino
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