El Nino Impacts: Weaker In The Past, Stronger In The Future?

Bette Otto-Bliesner used NCAR's recently developed climate system model(CSM) to study the behavior of ENSO, which affects much of the globe,and the North Atlantic Oscillation (NAO), a more regional cycle thatalters temperatures and precipitation across the northwest Atlantic andEurope. She found that the impacts of ENSO on Northern Hemispherewinters (for instance, a general warming across Canada and the northernU.S. during El Nino) were up to 50% weaker 6,000 years ago. Her researchwas funded by the National Science Foundation (NSF), NCAR's primarysponsor.

To reproduce the climate of 4000 B.C., Otto-Bliesner altered theincoming solar radiation to account for the planet's greater tilt and ashift in perihelion (the earth's closest annual approach to the sun) atthat time. The result is accentuated seasonality, with up to 6% moresolar input during the Northern Hemisphere summer and a correspondingreduction in the winter. As a control run, Otto-Bliesner used the CSM tosimulate present-day conditions. In both the 4,000 B.C. and present-dayruns, the model captured the ebb and flow of El Nino and La Nina, butthe impacts of ENSO were considerably weaker in the 4,000 B.C. run.

What about the future of El Nino? According to NCAR senior scientistKevin Trenberth, ENSO's impacts may be enhanced by human-producedclimate change. El Ninos have been unusually frequent since the mid-1970s. The same period has seen a dramatic rise in global temperature.The year 1998 set a global record and was one of the United States' twowarmest years since records began in the late 1800s.

Trenberth has found that the global mean temperature peaks three to fourmonths after the peak in El Nino. "It is no coincidence that theexceptional warmth in the first seven months of 1998 occurred as thePacific Ocean lost heat following the peak of the 1997-98 El Nino inDecember 1997," notes Trenberth. During El Ninos, warm waters spreadacross the tropical Pacific, evaporating large amounts of water vaporthat release heat when the vapor condenses into clouds and rain. Thus,El Nino events tend to transfer heat from ocean to atmosphere, warmingthe globe about 0.1 degree Celsius for each standard deviation ofdeparture from average temperatures in the Southern Oscillation index,according to Trenberth.

Trenberth theorizes that much of the additional heat trapped byincreasing amounts of greenhouse gases may be going into the oceans. Itis later released through El Ninos that are larger, more frequent, orless efficient in releasing the ocean-stored heat. The atmosphericwarming induced by El Nino also helps to further dry out regions alreadyprone to drought during El Nino, such as Indonesia, Australia, and partsof Africa and Brazil. "Naturally occurring droughts, such as from ENSO,are likely to set in quicker, plants will wilt sooner, and the droughtsmay become more extensive and last longer with global warming."Trenberth's work was sponsored by the National Oceanic and AtmosphericAdministration's Office of Global Programs, the National Aeronautics andSpace Administration, and NSF.

NCAR is sponsored by NSF and managed by the University Corporation forAtmospheric Research, a consortium of more than 60 universities offeringPh.D.s in atmospheric and related sciences.

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Writer: Bob Henson

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