Meteorologist Tom K. Priddy observes the effects of El Niño from the University of Kentucky College of Agriculture. He is director of the college's Agricultural Weather Center, which provides information to the National Weather Service and issues a weekly crop and weather report. He offers the following answer.

El Niño is the term used for the period when sea surface temperatures are above normal off the South American coast along the equatorial Pacific. Spanish for "the boy child" or "Christ child," due to its typical occurrence near December, El Niño has been observed since antiquity by Peruvian fishermen along the equatorial Pacific and along the western coast of South America.


Image: NOAA

DIAGRAMS show normal and El Niño conditions in the Pacific Ocean.

This warming of the ocean is also called a "teleconnection" because it affects weather in other parts of the world. It is now known that El Niño events can cause far-reaching global disruption in the general circulation of the Pacific Ocean and atmosphere. Climate records suggest that El Niños have occurred back to the turn of the century--and probably would show it earlier before that if consistent climate data existed.

Fisherman off the coast of Peru have long known to expect possible changes in fishing patterns every few years, usually with the approach of December. Normally, strong trade winds drive ocean surface currents toward the west along the equatorial Pacific. Cold water, upwelling from deep ocean currents, provides nutrient-rich food for anchovy, the fishermen's preferred catch. El Niño occurs when the trade winds along the equatorial Pacific become reduced or calm for many weeks. Then the upwelling of cold, deep ocean waters slows or stops, allowing sea surface temperatures to increase much above normal. The warm water drives the fish to deeper waters or farther away from usual fishing locations. This happens about every 3 to 7 years.

How this apparently local event can affect the short-term climate and weather in other parts of the world has only recently become better understood. When El Niño occurs, much more than fishing patterns are changed off the coast of South America. Very warm waters in the equatorial Pacific pump more moisture into the air, causing an increase in showers, thunderstorms and tropical storms over a much larger area. The area affected can be so large and deep in the atmosphere that major wind currents in the upper air are affected. Since major wind currents steer the weather systems in the middle latitudes across North America, typical storm paths in the U.S. are shifted.

It is this shifting of major upper air wind currents by El Niño that causes weather and short-term climate changes in other parts of the globe. Places such as Australia, Indonesia, Brazil, India and Africa could experience drought conditions because moisture-bearing storms are shifted away from these areas. Likewise, Argentina, South China, Brazil and Japan can receive an increase in moisture-bearing storms that cause long periods of heavy rains and flooding.

Additionally, there is a decrease of tropical storms (hurricanes) in the Gulf of Mexico and Western Atlantic and an increase of tropical storms in the Pacific. This year, for an example, only one named storm occurred during August and September in the Gulf of Mexico and Western Atlantic. At the same time, the Eastern Pacific has suffered 16 named hurricanes, for a total of 22 tropical (numbered and named) storms.

For North America during the winter months, the storm track is shifted so that an increase in precipitation occurs along the southern tier of U.S. states and Mexico. Warmer-than-normal temperatures invade northern latitudes of Canada, Alaska and the northern tier of U.S. states. The effect of El Niño on the central U.S. varies. El Niño events have provided both extremes of weather and some have had no effect at all. Typically, though, El Niño reduces the amount of snowfall in central sections of the states. These changes can have far-reaching economic impacts.

Is there more to this story? You bet. Teleconnections are very complex. For example, research has also shown that cold periods of cold sea-surface temperatures in between El Niño events can influence have an effect on global weather and short-term climate.