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Explainer: El Niño and La Niña

Hundreds of years ago, South American fishermen observed that every year around Christmas, coastal waters of the Pacific became warmer as a current flowed from north to south. This change often meant a smaller catch but more rainfall inland. And that translated to more abundant crops. They said the current came from El Niño (el NEEN yo) — Spanish for  “the boy.” But villagers were not referring to just any boy. Used at this time of year, their term referred to Jesus, “the Christ child.”

El Niños can bring heavy rainfall and flooding to the West Coast of South America. Meanwhile, Australia and Southeast Asia may face a drought and high risk of wildfires. In North America, scientists have also linked unusual weather events — including ice storms, droughts and mudslides — to the arrival of an El Niño.

Today, researchers use the term El Niño only for those periods when the surface water around the equator in the eastern and central Pacific warms for an extended period of time.  Scientists declare the development of an El Niño when they observe a temperature increase of at least 0.4 degree Celsius (0.72 degree Fahrenheit) for five months in a row in the eastern Pacific near the equator.

At other times, the surface water in the eastern Pacific instead may cool for long stretches of time. When the average temperature drops by at least 0.4° C (0.72° degree F), climate scientists will announce the arrival of a La Niña (lah NEEN yah). This is Spanish for “the girl.” In general, effects of a La Niña run opposite to those triggered by an El Niño: Now, Central and South America may face severe droughts while Australia floods.

Much of the rest of the world, including large portions of Africa and North America, may also see substantial climate impacts from ENSO events.

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