Shows changes in sea surface temperatures (SST) from the calculated normal average in the equatorial pacific.
In a normal year, the trade winds blow westward and push warm surface water near Australia and New Guinea. When warm water builds up in the western Pacific Ocean, nutrient-rich cold water comes up off the west coast of South America and fosters the growth of the fish population.
During an El Niño event, the trade winds weaken and warm, nutrient-poor water occupies the entire tropical Pacific Ocean. Heavy rains that are tied to the warm water move into the central Pacific Ocean and cause drought in Indonesia and Australia. This also alters the path of the atmospheric jet stream over North and South America.
Caption: Arrows in the first image show how the normal Walker cell circulates air over a steep temperature gradient in the Pacific Ocean between warm western and cold eastern waters in January 1997. By March 1998, the lack of an east-west sea surface temperature difference has collapsed the Walker circulation cell. As shown in the second image, the collapse produced lower clouds (fewer thunderstorms) in the west and higher clouds (more thunderstorms) in the east during the 1998 El Niño as compared to normal years. (Images courtesy of NASA SVS).
Warm El Niños and cold La Niñas follow each other against the backdrop of the ocean seasons. During a La Niña, the trade winds are stronger and cold, nutrient-rich water occupies much of the tropical Pacific Ocean. Most of the precipication occurs in the western tropical Pacific Ocean, so rain is abundant over Indonesia.
Image above: The image shows what happens when a very strong El Nino strikes surface waters in the Central equatorial Pacific Ocean. The sequence shows warm water anomalies (red) develop in the Central Pacific Ocean. Winds that normally blow in a westerly direction weaken allowing the easterly winds to push the warm water up against the South American Coast. Click on image to enlarge. Credit: NASA
Image above: This image shows colder than normal water (blue) anomalies in the central equatorial Pacific associated with La Nina. Stronger than normal trade winds bring cold water up to the surface of the ocean. Click on image to enlarge. Credit: NASA
El Nino can have impacts on weather at various locations around the globe. Off the east coast of southern Africa, drought conditions often occur. In countries such as Zimbabwe, the effects of drought can be devastating. Southeast United States (including Florida) has increased rainfall.
This is a coupled system between the hydrosphere and the atmosphere and hence it is difficult to determine which is a cause. There may be other less understood factors affecting these systems. More study is needed.
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