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La Nina stokes Southwestern forest fires.

La Nina Stokes Southwestern Forest Fires

As wildfires rage through the western United States each year, officials long for a way to determine months in advance whether the next fire season will bring severe blazes. In years to come, forest managers in the Southwest may look for such a warning in the middle of the Pacific Ocean.

Two researchers have found a strong correlation between Pacific weather conditions and the spread of fires months later in Arizona and New Mexico. The most extensive forest fires in this region have struck predominantly during a weather phenomenon known as a La Nina, which occurs when surface water in the central Pacific turns unusually cold for a year or more. In contrst, relatively minor southwestern fires have developed during El Nino periods, when the mid-Pacific turns warmer than normal, report Thomas W. Swetnam from the University of Arizona and Julio L. Betancourt from the U.S. Geological Survey.

The Tucson-based scientists sifted through fire statistics going back to 1905 for national forests in the Southwest and compared them with wintertime fluctuations in the Southern Oscillation -- a record of air pressure over the central Pacific that can serve as an indicator for El Nino or La Nina conditions.

To gauge the extent of pre-1905 fires, the researchers also studied fire scars on 315 trees throughout the Southwest, obtaining a bioloical record stretching back to 1700. Since the Southern Oscillation record goes back no farther than the 1860s, they used historical information from Peru (typically pounded by heavy rains during an El Nino) to identify El Nino periods before that.

The Southern Oscillation strongly influences weather in the tropics, and its effects ripple through many other parts of the world. During El Nino years, the Southwest usually experiences wet falls and wet springs, with reduced burning during late spring, when fires are most common there. Conversely, the region often suffers severe winter-spring droughts followed by extensive fires during strong La Nina years, researchers report in the Aug. 31 SCIENCE.

The La Nina-fire connection does not hold up perfectly for each year, they note. For example, the Southwest suffered abnormally large fires this year even though the Pacific air pressure pattern remained near normal last winter. But Swetnam says the relationship can still prove useful for officials planning "prescribed burns" or organizing fire-fighting efforts. "If we have a La Nina situation in winter months, I would say caution is advisable," he asserts.

Daniel Winner, assistant director of fire management for the U.S. Forest Service in Albuquerque, N.M., agrees that the Southern Oscillation may help officials prepare. "It looks like this will give us more lead time to predict severe conditions," he says. With the advance warning, managers might have ample time to request increased funding and resources, he notes.

Swetnam says the new finding also have important implications for theories about the development of forests and other ecosystems. Traditionally, ecologists have maintained that a forest's character depends largely on internal factors, such as competition among the tree species. But this study and others suggest that external factors, such as Climate, also play a significant role in shaping an ecological community.
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Title Annotation:Pacific Ocean weather phenomenon
Author:Monastersky, Richard
Publication:Science News
Date:Sep 1, 1990
Words:526
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