February 5, 2003
The widespread warming expected from increased greenhouse gases may dilute the strength of a typical El Niño occurrence, according to researchers at the National Center for Atmospheric Research. Using a sophisticated computer model of global climate, NCAR scientists examined the past and future behavior of El Niño. It is believed that the average El Niño event during the last ice age may have packed more punch than today’s typical El Niño.
More mild El Niños to come?
NCAR scientists used 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. NCAR scientist Esther Brady and others simulated Earth’s climate with atmospheric carbon dioxide at one, two and six times the preindustrial level of about 280 parts per million.
As greenhouse gases increase and global air temperatures rise, Brady’s results demonstrated 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 noted. 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
Another NCAR climate system model study indicated a history of diminished El Niño events in a warming world. 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 percent weaker than today. The main factor responsible for the decrease is a slow shift in Earth’s asymmetric orbit around the sun.
Today, 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 nearest-sun approach of the Earth 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, thereby 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 percent 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, said 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. Otto-Bliesner pointed out 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,” she said.