What will El Nino bring this winter? - Weather

El Nino has returned, but it is too early to tell how it will impact this winter's weather, according to David Arnold, professor of meteorology, Ball State University, Muncie, Ind. The phenomenon is caused by a shift in wind direction and water temperature in the equatorial Pacific Ocean. El Nino can bring about a change in typical weather conditions, sometimes with disastrous results.

Arnold stresses that "It is important to realize that interpreting the impact of any El Nino event is not always a straightforward matter since each one is somewhat unique. Considerable variability exists between El Nino years. Subtle differences between some El Nino winters may produce unexpected results A seemingly minor change in the character of the mid-level water circulation pattern can bring about much-different conditions at the surface than ware anticipated. For example, the El Nino event of 1976-1977 marked the first in a series of three of the coldest consecutive winter seasons in the Midwest," he indicates.

An El Nino condition exists when an unusual warm pool of sea-surface temperatures exists over the Pacific in the vicinity of the International Dateline. This ocean-surface temperature anomaly has been shown to affect the large scale atmospheric circulation pattern that dictates "spells" of weather over a good portion of the Northern Hemisphere during the cold season.

Arnold notes that how the warm and dry winter forecast by the National Oceanic and Atmospheric Administration (NOAA) works out is dependent on the performance of long-range forecasting methods. "The accuracy of these methods is not as good as wa are accustomed to with our daily weather forecasts. Atmospheric scientists would not be as confident in this coming winter's prediction as they would be for the weather this coming weekend."

Arnold figures that even if NOAA's winter forecast pans out, periods of cold and snowy weather might still occur during the coming winter. "We could easily experience a few vary cold periods, mixed in with slightly longer periods of warmer and drier than average weather, and still meet the forecast of a warm and dry winter," he notes.

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