El Niño is a disruption of the ocean-atmosphere system in the tropical Pacific with important consequences for weather around the globe. In "normal", non-El Niño conditions, the trade winds blow toward the west across the tropical Pacific. These winds pile up surface water in the western Pacific, so that the sea surface is about 0.5 meter higher at Indonesia than at Ecuador. The sea surface temperature is about 8 degrees C higher in the western Pacific, and cool temperatures prevale off South America (eastern Pacific), 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 in the western Pacific, and the eastern Pacific is relatively dry.

Among the consequences of El Niños are increased rainfall across the southern tier of the U.S. and Peru that has caused destructive flooding, and drought in the western Pacific that is sometimes associated with devastating brush fires in Australia. Observations of conditions in the tropical Pacific are considered essential for the prediction of short term (a few months to 1 year) climate variations. To provide necessary data, NOAA (National Oceanographic and Atmospheric Administration) operates a network of buoys which measure temperature, currents and winds in the equatorial band. These buoys transmit data which are available to researchers and forecasters around the world in real time (that is, as it happens).

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