Weather Wrecker: Teaching El Niño

Written by Andrew Clark, IGES


“A Still-Growing El Niño Set to Bear Down on U.S.”

The strongest El Niño event in decades is making headlines and your students have likely heard about this phenomenon, though they may not know what causes it or what impact it may have.

El Niño is one stage of a global cycle called the El Niño Southern Oscillation, or ENSO, in which the waters of equatorial Pacific Ocean go through warm and cool phases. When the waters of the eastern equatorial Pacific Ocean are warmer than average, ENSO is in the El Niño phase, and when the waters are cooler, ENSO is in the La Niña stage.

Grasping the complete picture of ENSO requires an understanding of the cycle and flow of energy and matter through the environment, in the forms of oceanic and atmospheric circulation.

The primary mover of the atmosphere is the input of energy from the sun, which is greater at the equator than towards the poles (Explore more in this lesson)The intensity of the equatorial sunlight causes air and moisture to rise into the atmosphere before cooling and sinking further away from the equator, in what is known as the Hadley Cell. The Coriolis Effect (Lesson) causes the trade winds in the Hadley cell to blow from east to west over the equatorial latitudes.

These trade winds blowing from east to west move the ocean from east to west, too! Surface water actually piles up in the western Pacific, while on the eastern side, cold subsurface water is drawn up to replace the water that has blown west in a process known as upwelling.(Lesson)

During the El Niño phase of ENSO, these trade winds weaken, which means that the warm surface waters are not blown west, and upwelling weakens. Warm water that was piled up in the western Pacific propagates eastward along the equator toward the central and eastern Pacific and weakens the trade winds further.

This uncommonly warm region of ocean and weakened trade winds sets up a cascade of effects throughout the ocean, atmosphere, and biosphere that can be felt worldwide.

Additional Resources

Latest El Niño/La Niña watch data from NASA
El Niño: Disrupting the Marine Food Web
ClimateBits: El Niño Info
ClimateBits: El Niño Video
El Niño updates and outlook from NOAA
El Niño on PBS Learning Media