El Niño and La Niña

El Niño and La Niña are terms that describe the biggest fluctuation in the Earth’s climate system, which can have consequences worldwide.

There are fewer droughts in some years than in others, and the regions impacted by droughts also change over time. This variation can be partly linked to El Niño and La Niña events, which result from changes in sea surface temperature in a band across the Pacific Ocean.

El Niño events occur every three to seven years, and La Niña events occur every three to five years. The time between each event is known as a normal year.

To understand El Niño and La Niña events, you must know what happens in the tropical Pacific during a normal year.

What happens during a normal year?

The tropical Pacific experiences trade winds, which blow along the surface from east to west. The winds push warm water near the surface in the same direction of travel, accumulating on the western side of the Pacific Ocean, around Australasia and Asia.  On the other side of the ocean, around South America, cold water rises from the ocean’s depths to replace the warm water being blown west. This process is called upwelling, creating a temperature difference across the tropical Pacific.

A normal year in the tropical Pacific

The warm ocean to the west heats the air above it, leading to low pressure and unsettled weather conditions. The warm, moist air cools and condenses, causing rainfall. The dry air then travels east in the upper atmosphere. This cool, dry air then descends in the eastern Pacific.