Describe the El Niño Southern Oscillation. What global environmental effects characterize an El Niño event? What global environmental effects characterize a La Niña event?

Question

What will be an ideal response?

Answer 1

The atmospheric and oceanic conditions during an El Niño-Southern Oscillation (ENSO) warm phase occur when the high pressure along the coast of South America weakens, reducing the difference between the high- and low-pressure regions of the Walker Circulation Cell. This, in turn, causes the southeast trade winds to diminish. In very strong El Niño events, the trade winds actually blow in the reverse direction. Without the trade winds, the Pacific Warm Pool that has built up on the western side of the Pacific begins to flow back across the ocean toward South America, creating a band of warm water that stretches across the equatorial Pacific Ocean. The warm water usually begins to move in September of an El Niño year and reaches South America by December or January. During strong to very strong El Niños, the water temperature off Peru can be up to 10°C (18°F) higher than normal. In addition, the average sea level can increase as much as 20 centimeters (8 inches), simply due to thermal expansion of the warm water along the coast. As the warm water increases, sea surface temperatures across the equatorial Pacific, temperature-sensitive corals are decimated in Tahiti, the Gala?pagos, and other tropical Pacific islands. In addition, many other organisms are affected by the warm water. Once the warm water reaches South America, it moves north and south along the west coast of the Americas, increasing average sea level and the number of tropical hurricanes formed in the eastern Pacific. La Nin?a events are associated with sea surface temperatures and weather phenomena opposite those of El Nin?o. Indian Ocean monsoons, for instance, are typically drier than usual in El Nin?o years but wetter than usual in La Nin?a years.