Explainer: El Niño and La Niña

Normally, the temperature of the Pacific Ocean’s surface waters is about 7.8° Celsius (14° Fahrenheit) higher in the Western Pacific than the waters off South America. This is a result of the trade winds, which blow from east to west along the equator. Those winds allow an upwelling of deep, cold water off the northwest coast of South America to move west, piling up on the other side of the ocean. But during an El Niño event, every few years, the trade winds lose strength and can reverse direction. Now cold ocean water from off of the coast of South America piles up near the nearby coastline. Before long, people around the world may notice big and prolonged changes in local weather patterns. 


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.”

The warming in the central and eastern Pacific Ocean triggers changes in air pressure across the ocean. Air pressure is the force of the weight of the atmosphere pushing down on a place. Scientists call these pressure changes the Southern Oscillation (oscillation means fluctuation). They are triggered by the temperature changes brought by El Niño. So the phenomenon’s full name is El Niño-Southern Oscillation, or ENSO. Climate scientists usually detect an El Niño toward the end of a year. Its major effects, however, typically are not felt until the following year.

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, see substantial climate impacts from ENSO events.

Power Words

atmospheric pressure The pressure exerted by the weight of the atmosphere.


climate The weather conditions prevailing in an area in general or over a long period.


drought  An extended period of abnormally low rainfall; a shortage of water resulting from this.


ENSO Short for El Niño-Southern Oscillation: It is a natural cycle of changing temperatures in the ocean and atmosphere, near the equator in the Pacific Ocean. During ENSO events, atmospheric pressure also changes in affected areas



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