El Niño Southern Oscillation, or ENSO, refers to the effects of a band of
sea surface temperatures which are anomalously warm or cold for long periods of time that develops off the western coast of South America and causes climatic changes across the tropics and subtropics. http://www.elnino.noaa.gov/In normal, non-El Niño conditions (top panel of schematic diagram), the trade winds blow towards the west across the tropical Pac:ific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador.
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| (a) | (b) |
| (a) Schematic diagram of normal El Niño conditions in the Pacific Ocean, and (b) temperature on the Equator at 110W |
The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry. The observations at 110 W (left diagram of 110 W conditions) show that the cool water (below about 17 degrees C, the black band in these plots) is within 50m of the surface.
During El Niño (bottom panel of the schematic diagram), the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. The observations at 110W show, for example, that during 1982-1983, the 17-degree isotherm dropped to about 150m depth. This reduced the efficiency of upwelling to cool the suface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region. The weakening of easterly tradewinds during El Niño is evident in this figure as well. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.
The extremes of this climate pattern's oscillations cause extreme weather (such as floods and droughts) in many regions of the world. Developing countries dependent upon agriculture and fishing, particularly those bordering the Pacific Ocean, are the most affected.
During El Nino conditions the waters of the Pacific Ocean off the west coast of South America become abnormally warm, and moist rising water vapor forms thunderstorm clouds, which in turn cause the upper atmosphere to become warmer than usual.
As that warm air branches out, it moves upward in altitude toward the poles and then sinks back into the tropics in loops that are called Hadley cells. All this change disrupts weather conditions across the planet, and that in turn tends to cause a cascade of weird and disturbing events, ranging from heavy rains and flooding to food shortages. One 2011 study even found that El Niño was associated with an increased risk of civil war in tropical countries.