EL NIÑOTEMPLATE John Doe and Mary Taylor  1.   DEFINITION Variations in climate and weather can significantly affect people's lives. Humans are linked to their ecosystem through climatic events such as extremely cold winters, persistent droughts, or emergencies such as flooding, heat waves, or forest fires.  2.   HISTORY There is a historical record of an El Niño event going back to 1567. Peruvian fishermen along the westernmost shores of South America were the first to give a name to the climatic anomaly. In a normal year, the waters were cold and flowed from south to north; however, every few years, the waters reversed their flow and warmed up. Environmental News Network Fig. 1  Historical record of El Niño years from the 1500's to 1995.    3.   CAUSES The main cause of El Niño involves the full complexity of oceanic-atmospheric interactions on a global scale. While the cause is not fully known, scientists have a good understanding of how it evolves once it has begun. ICESS-ESRG, University of California, Santa Barbara Fig. 2  During normal years, trade winds blow from east to west, warming the ocean surface along the western Pacific. The north-south component of the large-scale tropical airflow is the Hadley Cell circulation. The Hadley Cell circulation is comprised of two cells which transport heat through the atmosphere from tropical to subtropical latitudes. ICESS-ESRG, University of California, Santa Barbara Fig. 3  During El Niño years, trade winds reverse and blow from west to east, warming the normally cold surface waters of the eastern Pacific.    4.   PATTERNS On the average, an El Niño event occurs every 2 to 7 years, but only irregularly, and not as predictably as the astronomically controlled tides. The return period of El Niño is varied; its intensity and duration are also varied.   World Meteorological Organization Fig. 4  1982-99 time sequence of sea-surface-temperature anomalies in the central equatorial Pacific Ocean.   El Niño often begins early in the year and peaks between the November and following January, but two consecutive events do not behave in the same way. The intervening weak and moderate events do not bring disastrous consequences.    5.   IMPACTS The ocean and atmosphere disturbances created by El Niño can have devastating impacts on the environment. Changes in the "normal" weather patterns can bring floods to the driest lands on earth, creating lakes where desert plains normally existed.  6.   SUMMARY El Niño is a global climatic anomaly caused by a weakening of the trade winds along the tropical Pacific, which leads to warmer-than-normal sea-surface waters along the eastern tropical Pacific. The name of El Niño originated with Peruvian fishermen, who noticed that reduced fisheries were associated with the warmer-than-normal waters. The phenomenon tended to peak around Christmas; thus, they named it "El Niño," for "The Child Jesus." http://elnino.sdsu.edu 030221