CNBC Explains

What is El Niño, anyway?

Climate and weather forecasters are there is an 80 percent chance the climate pattern known as El Niño will continue through the rest of 2015.

But what is El Niño?

Rain pours from thunderclouds on the U.S. West Coast.
Frances Litman | Getty Images

El Niño is the name given to changes in the patterns of trade winds across the Pacific Ocean, which can cause unusual warming in ocean temperatures and all sorts of drastic weather changes in specific regions of the United States.

The name literally means "the boy" or "the little boy" in Spanish, and it is sometimes expressed as "Christ Child." It is said to have been first noticed by fishermen off the coast of South America, who gave it the religious name because the phenomenon tended to happen in December.

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The last strong El Niño took place in 1997-98, though lesser El Niños have occurred since then.

Normally, steady winds blow from east to west across the Pacific Ocean, pushing sun-warmed ocean water along with them toward Indonesia and Australia on the western edge of the ocean. These winds push so much water that sea levels are actually almost five feet higher near Indonesia than they are near Ecuador, according to the National Oceanic and Atmospheric Administration. (Tweet This)

How real is the risk from El Nino?
How real is the risk from El Nino?

Meanwhile, colder water rises to the ocean surface off the coast of South America, in place of the water being dragged by the winds toward Asia. This colder water brings cooler temperatures and carries nutrients that benefit marine life near the surface.

During El Niño, these winds slow down, sometimes halting or even reversing the flow of water across the Pacific. The colder water from the deep ocean stops rising to the surface, and temperatures across the Pacific rise.

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This can have all sorts of effects on weather, but it often brings drought along the western parts of the Pacific Ocean, especially in Indonesia and Australia. It also can lead to heavy rains and flooding in many regions of the Americas, including southern parts of the United States.

The NOAA Climate Blog notes that there have already been seven tropical cyclones in the western Pacific this season; the average number of such storms is two.

How US regions are affected

The United States tends to feel El Niño's effects from fall through early late winter or early spring, according to the NOAA Climate Blog's Mike Halpert. Usually it means more rain from October through March along the Gulf Coast region stretching from Texas through Florida.

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Stronger El Niño years can affect the Southwest and Western states as well, however. The severest El Niño known to history, 1982 to 1983, brought heavy rains and mudslides to California, according to the United States Geological Survey. Those stronger patterns are uncommon.

There is also a phenomenon called La Niña, which is when these ocean waters are unusually cool, rather than warm. Together, both climate patterns make up the El Niño/Southern Oscillation.