Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

Monthly Update sign up
Mailing List signup

WEATHER: What forces affect our weather?
The Water Cycle

NOVA Online: Tracking El Niño
Find out how El Niño works—and what the arrival of La Niña means to the world's weather.

El Niño Theme Page
Information about El Niño and La Niña from the National Oceanic and Atmospheric Administration.

Reports to the Nation on Our Changing Planet: El Niño and Climate Prediction
An overview of El Niño's effects on global climate patterns. Includes a discussion of how El Niño was identified and where it got its name.

El Niño: Warming up the Waters

In 1997 and 1998, the press was filled with reports of a weather phenomenon called El Niño. El Niño was blamed for severe flooding, drought, crop shortages, coral bleaching, and even the spread of the deadly hantavirus in some areas. What is El Niño, and how could it have such a widespread impact?

El Niño's name, which means "Christ child" or "boy child" in Spanish, comes from the time of year it is likely to occur: around Christmas. It's not a hurricane or tropical storm, but a series of warm water currents that can have surprisingly serious effects. El Niño is an ocean-driven weather pattern, and it recurs every few years. It was first noticed by fishermen off the coasts of Peru and Ecuador, who observed that the warmer currents of an El Niño year tended to deplete fish populations. Warmer water temperatures reduce the amount of algae, which thrives in cooler temperatures. With less algae to feed on, fish populations decline.

Making the connection: Water currents and weather events

In 1957, Jacob Bjerknes, a scientist studying El Niño, discovered the connection between changing water currents and weather events such as increased rainfall. Every few years, western-flowing trade winds weaken, allowing warm waters to flow farther east. As these warm currents reach South America, moisture builds up in the atmosphere. Thunderstorms increase, causing heavy flooding and mudslides in some normally dry regions such as the desert areas of Peru. Until Bjerknes's discoveries, scientists thought that Peru was the only area affected by El Niño. Further research uncovered the fact that El Niño can cause weather changes in many parts of the globe.

La Niña

Scientists have also discovered that El Niño has a sibling: La Niña. La Niña means "little girl" in Spanish. Like El Niño, it is a recurring weather pattern, but it has the opposite effect. La Niña brings cooler water currents, usually increasing fish populations but also bringing colder winters to warm areas.

La Niña and El Niño tend to alternate every few years, and in fact they are part of one phenomenon. In the 1920s, a scientist named Sir Gilbert Walker observed that barometric pressure readings in the Pacific showed a seesaw effect. When pressure increased in the eastern Pacific, pressure dropped in the western Pacific. As pressure changed, surface currents changed, causing either cooler or warmer waters to circulate. Scientists now refer to this seesaw effect as the El Niño Southern Oscillation, or ENSO.

[Back to "The Water Cycle"] [Next Topic: "Powerful Storms"]

 "Weather" is inspired by programs from Planet Earth.


© Annenberg Foundation 2017. All rights reserved. Legal Policy