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. It tended to be at the same time as an ocean current flowed from north to south. This change often meant a smaller catch of fish. It also tended to signal when people could expect more rains inland. And those rains would tend to lead to bigger crop harvests.

The ancient fishermen said this 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.”

That climate 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 air pushing down on a place.) Scientists call these pressure changes the Southern Oscillation. (Oscillation means fluctuation.) Temperature changes triggered these pressure changes brought by an El Niño. So the phenomenon’s full name came to be El Niño-Southern Oscillation, or ENSO.

Climate scientists usually detect an El Niño towards the end of a year. Its major effects, however, may not be felt until well into the next year.

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

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 long time. How long? Typically for five months. And the warming needed must be at least 0.4 degree Celsius (0.72 degree Fahrenheit).

At other times, those surface waters in the eastern Pacific may cool for long spans of time. When the average temperature drops by at least 0.4° degree C, climate scientists may announce the arrival of a La Niña (Lah-NEEN-yah). This is Spanish for “the girl.” In general, the effects of a La Niña run opposite to those of an El Niño: Now, Central and South America may face severe droughts for a year or more while Australia might see floods.

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

Power Words

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

air pressure: The force exerted by the weight of air molecules.

average: (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

climate: The weather conditions that typically exist in one area, in general, or over a long period.

crop: (in agriculture) A type of plant grown intentionally grown and nurtured by farmers, such as corn, coffee or tomatoes. Or the term could apply to the part of the plant harvested and sold by farmers. 

current: A fluid — such as of water or air — that moves in a recognizable direction.

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

El Niño-Southern Oscillation: Also abbreviated ENSO. The term for a long-term swing in sea-surface temperatures in the tropical Pacific, lasting several years. Water temperatures in the eastern equatorial Pacific tend to vary from periods that are normal to ones that are either unusually cool (La Niña) or are warmer than normal (El Niño). These changes can affect climate across the planet, leading to times that are wetter or drier than normal. ENSO changes tend to track with the depth — or boundary — above which the ocean water varies greatly in temperature (a thermocline). When the thermocline is closer to the surface, it causes an upwelling of cold, nutrient rich water from low in the ocean. This tends to cool local temperatures at the ocean’s surface.

equator: An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.

fluctuation: (v. fluctuate) Some type of change in a pattern or signal that varies at irregular intervals and often by amounts that are hard to predict.

force: Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

La Niña: Extended periods when the surface water around the equator in the eastern Pacific cools for long stretches of time. Scientists will announce the arrival of a La Niña (lah NEEN yah) when the average temperature there drops by at least 0.4° C (0.72° degree F). Impacts on global weather during a La Niña tend to be the reverse of those triggered by an El Niño: Now, Central and South America may face severe droughts while Australia floods.

Pacific: The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.

pressure: Force applied uniformly over a surface, measured as force per unit of area.

risk: The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

weather: Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.

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