Understanding El Niño

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The term El Niño--Spanish for "The Christ Child"--refers to an interval of especially warm ocean temperature that intermittently appears around Christmas in the equatorial Pacific. The phenomenon is associated with weather changes around the globe, including in the Pacific Northwest, where it causes our winters to be especially mild. The effect occurs with a frequency that varies from two years up to a decade.

UW scientists Edward S. Sarachik, David S. Battisti, and collaborators are credited with being the first to explain why El Niño-like events occur in a numerical model developed by Cane and Zebiak at the Massachusetts Institute of Technology. The Cane-Zebiak model successfully simulates the evolution of El Niño and is being used to predict aspects of the event in the tropical Pacific a year in advance.

UW researchers have played an important role in developing a theoretical understanding of the mechanisms involved. El Niño events occur when the easterly (east to west) winds that normally blow across the equatorial Pacific begin to weaken. Ordinarily, the winds blow from the Galapagos Islands off the coast of Ecuador toward Indonesia, dragging surface water along with them. When the surface water moves away, colder, nutrient-rich water moves up from below to replace it, a phenomenon known as upwelling. The upwelling typically occurs all along the west coast of South America. But when the winds weaken, this upwelling decreases. The event triggers a change in the upper layer of the ocean. The sea level flattens out, as water is no longer pushed westward by the easterly wind. Sea level drops in the west and rises in the east; surface water surges eastward along the equator. An impulse of warm water reaches the eastern end of the Pacific basin, and turns northward and southward along the coast, raising the sea level, causing fish to move elsewhere, and disrupting the food chain.

UW research to elucidate this mechanism paved the way for understanding why El Niño is predictable; and it also defined what kind of ocean observing system would be required in order to make successful forecasts of the event--measurements such as wind speeds, ocean currents, sea level, and ocean temperature as a function of depth.

The work of UW scientists John D. Horel and John M. Wallace was instrumental in establishing the link between El Niño and mild winters in the Northwest. As illustrated in the figure, winds at the jetstream level, five to fifteen miles above sea level (shown with small arrows), change their course between normal and El Niño winters. A ridge of high pressure over North America's west coast during El Niño keeps temperatures above normal in the shaded region and fends off storms that otherwise would pass through the Northwest. El Niño also brings heavier than normal rainfall to much of the southern U.S

Wallace was principal author of a Report to the Nation about El Niño; the report is available at no cost to schools throughout the country.footnote 1

  1. "El Niño and Climate Prediction," Reports to the Nation on Our Changing Planet, Spring 1994, No. 3, University Corporation for Atmospheric Research, Office for Interdisciplinary Earth Studies, P.O. Box 3000, Boulder, CO 80307-3000.

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