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Every three to five years, on average, the equatorial Pacific Ocean off the coast of South America warms up by a few degrees, leading to significant changes in rainfall around a large part of the globe. This phenomenon, known to the fishermen of Peru for centuries, brought with it torrential rains, flooding on land, and poor fishing at sea. Because the periodic warming generally occurred during Christmastime, the local fishermen named it “El Niño,” the Spanish name for Jesus Christ as a child. Other observers noted that droughts also seemed to come and go in cycles.

Today, we recognize the El Niño Southern Oscillation as one of the most important influences on atmospheric circulation and surface temperatures across the Pacific Ocean. During typical conditions in the tropical Pacific, the trade winds, which blow from the east, push warm surface water westward. As this water moves westward, cold water from the bottom of the ocean on the west coast of South America is drawn up to the surface, in a process called upwelling. However, as El Niño conditions develop, the trade winds weaken and the warm surface water flows in the opposite direction—eastward. When it reaches South America, this warm water moves north and south along the coast, pushing the normally cool surface water downward.

Such changes in atmospheric circulation and ocean surface temperatures influence the hydrologic cycle globally. The warmer than average sea surface temperatures during an El Niño are accompanied by low atmospheric pressure in the eastern Pacific Ocean. These physical conditions favor the production of storms in the eastern Pacific Ocean, as the water vapor in the warm rising air condenses to form storm clouds. When this happens, conditions are wet and cool across the southern portion of the United States and northern Mexico during December through February. Meanwhile, Southeast Asia and Australia are dry. So, while the southeastern United States faces torrential rains and flooding, Southeast Asia and Australia brace for drought and wildfires. The climatic conditions associated with El Niño generally persist for a year, sometimes two, before the climate swings back toward average conditions or beyond, to the opposite climatic extreme.

The opposite of the El Niño is now known as La Niña, which develops about as frequently as El Niño—approximately every 3 to 5 years. During a La Niña, the trade winds are stronger than usual and push warm water even farther west across the tropical Pacific. Meanwhile, thanks to increased upwelling, sea surface temperatures are lower than average in the eastern Pacific Ocean. Combined with cooler sea surface temperatures, La Niña brings higher atmospheric pressures. Under such conditions, cloud formation is inhibited over the eastern Pacific Ocean, whereas on the western Pacific Ocean more storms occur. December through February of a La Niña is characterized by dry and warm conditions across the southern United States and northern Mexico and wet conditions in Southeast Asia and northern Australia (Figure 6.5).

It took scientists most of the 20th century to piece together the global impact of El Niño and La Niña. Based on tree ring data, which correlates with precipitation amounts, and other evidence, scientists estimate that El Niño-like conditions have been occurring periodically for tens of thousands of years. Today, scientists can predict the onset of an El Niño year about nine months in advance.

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There is at present no clear consensus among climate scientists as to how climate warming (see Chapter 14, page 443) will influence the intensity and frequency of El Niño and La Niña. While some climate models predict higher frequencies and intensities of these events, other climate models predict the opposite. Regardless, the long geologic history of this system suggests that the El Niño will continue to affect Earth’s weather. From a water supply perspective, those regions under the influence of the El Niño experience much greater fluctuation in precipitation than do regions outside of its influence.