What causes it? Basically, a change in wind patterns. According to NASA, the Pacific receives more sunlight than any region on Earth, most of which is stored in the water in the form of heat. Typically, the Pacific trade winds blow from east to west, pulling the warm surface water westward, where it accumulates in a large, deep pool around Indonesia and Australia. As that happens, the deeper, colder waters in the eastern Pacific rise to the surface. That's the basic circulation pattern.
But every so often, the winds pushing that natural conveyor belt weaken. There are different reasons why this may happen, but subtle shifts in the Earth's orbit around the sun is one major factor, according to a 2014 study by University of Wisconsin researchers, who used computer models to look at El Niño over more than a 20,000-year period.
In any case, the result is that not as much water gets moved around. That causes the waters of the equatorial central and eastern Pacific to get warmer. As the waters warm, they have a feedback effect, weakening the winds even more. That causes the ocean to get even warmer, and makes an El Niño grow more powerful.
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So why is that a big deal? Changing the temperature in one part of the ocean can have all sorts of strange effects. In the east, the water expands, raising the sea level by as much as a foot, according to NASA. And slowing the conveyor belt that brings cold water to the surface also decreases the supply of nutrients that feed phytoplankton, the tiny organisms near the surface that fish and marine mammals depend upon for food. That can be a big problem for coastal areas where many people depend on fishing for their livelihood.
As the warm water moves eastward, more heat and moisture rise into the atmosphere there, which disrupts the usual global weather patterns and shifts the jet stream over the western Pacific eastward. In the eastern Pacific, that means more storms, which translate into more rainfall in places like California. (That's one reason that residents of the drought-ravaged state may see El Niño as a savior.)
At the same time, the atmospheric shifts caused by El Niño can also lower the probability of hurricanes in the Atlantic and cause severe drought in Australia, Indonesia and parts of southern Asia. A really powerful El Niño, such as the one in 1982-83, can cause billions in damage from floods, storms, crop-ruining droughts and forest fires across the planet.
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El Niño can drive up global atmospheric temperature, too, but predicting exactly how much is tricky, because that number is affected by numerous other factors as well, according to NOAA's Climate.gov. But scientists say that its presence raises the possibility that 2015 will set a new temperature record.
Finally, we should mention that El Niño actually is part of a bigger phenomenon called the El Niño-Southern Oscillation cycle, or ENSO. There's also another phase, called La Niña, in which the Equatorial central and eastern Pacific is colder than usual. That results in cooler temperatures in the Pacific Northwest and warmer winters in the Southeast.