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Global Climate Change Digest

A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999

FROM VOLUME 12, NUMBER 3, MARCH 1999

NEWS...
El Niños Past and Future


Item #d99mar40

In its Press Release 1999-1, the University Center for Atmospheric Research cited the work of Bette Otto-Bliesner and Kevin Trenberth presented at the American Meteorological Society annual meeting in Dallas, Tex., in January. Otto-Bliesner used NCAR's climate system model to study the behavior of El Niño/Southern Oscillation (ENSO) and the North Atlantic Oscillation. Her simulations indicated that the impacts of ENSO on Northern Hemisphere winters (e.g., a general warming across Canada and the northern United States) 6000 years ago were only 50% their magnitude today. To reproduce the climate of 4000 B.C., Otto-Bliesner altered the incoming solar radiation to account for the planet's greater tilt and a shift in perihelion (the earth's closest annual approach to the sun) at that time. The result is accentuated seasonality, with up to 6% more solar input during the Northern Hemisphere summer and a corresponding reduction in the winter. The modeling technique was verified against current-day observations. In both the 4,000 B.C. and present-day runs, the model captured the ebb and flow of El Niño and La Niña, but the impacts of ENSO were considerably weaker in the 4,000 B.C. run.

Trenberth found that the global mean temperature peaks three to four months after the peak in El Niño. "It is no coincidence that the exceptional warmth in the first seven months of 1998 occurred as the Pacific Ocean lost heat following the peak of the 1997-98 El Niño in December 1997," noted Trenberth. During El Niños, warm waters spread across the tropical Pacific, evaporating large amounts of water vapor that release heat when the vapor condenses into clouds and rain. Thus, El Niño events tend to transfer heat from ocean to atmosphere. Trenberth theorizes that much of the additional heat trapped by increasing amounts of greenhouse gases may be going into the oceans. It is later released through El Niños that are larger, more frequent, or less efficient in releasing the ocean-stored heat. The heat transferred to the atmosphere by condensation and precipitation helps to further dry out regions already prone to drought.

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