El Niño
El Nino happens when weakening trade winds (which sometimes even reverse direction) allow the warmer water from the western Pacific to flow toward the east. This flattens out the sea level, builds up warm surface water off the coast of South America, and increases the temperature of the water in the eastern Pacific.An El Nino condition results from weakened trade winds in the western Pacific Ocean near Indonesia, allowing piled-up warm water to flow toward South America.Since fish can no longer access this rich food source, many of them die off. That is why, these conditions are called "El Nino", or "the Christ Child", which is what Peruvian fisherman call the particularly bad fishing period around December. More importantly, the different water temperatures tend to change the weather of the region.
La Nina
If, on the other hand, the surface trade winds strengthen and with it the east-west slopes and along with it the east-west oceanic temperature gradients the resulting weather pattern leads to an anti-El Niño, that is often referred to as La Niña. Such events are characterized by a high positive Southern Oscillation Index (i.e. an increased westward pressure gradient over the equatorial Pacific), stronger surface trade winds over the central Pacific, and cooler SSTs in the eastern equatorial Pacific. Such a weather pattern, on the other hand, is associated with increased cyclone activity in the western Pacific, off shore of eastern Australia, the Phillipines, and the western Atlantic region.
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