The Relationship Between El Nino & La Nina

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  • 0:05 El Nino and La Nina
  • 1:01 What Causes ENSO Cycles?
  • 3:01 Effects on Land
  • 4:36 Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this video lesson, you will learn about El Niño and La Niña. You will understand the differences between them, as well as some of their effects on both ocean waters and weather patterns on land.

What Are El Niño and La Niña?

The ocean's children - you've probably heard of El Niño and La Niña before. They mean 'little boy' and 'little girl' in Spanish, and collectively they make up the ENSO cycle, which stands for El Niño-Southern Oscillation cycle. This cycle describes abnormally warm and cool ocean temperatures in the eastern tropical Pacific. The cycle is natural, and El Niño is the warm phase of the cycle, while La Niña is the cold phase of the cycle.

We get El Niño and La Niña episodes every 3-5 years, and El Niño happens more often than La Niña. They may last as little as nine months (about as long as it takes an actual little boy or girl to develop before being born) or as long as a couple of years. They usually form in the summer, between June and August, get strongest around December and then wither away between May and July of the next year.

What Causes ENSO Cycles?

If you've ever been to the Pacific Ocean along the West Coast of the U.S., you know that this water is very cold! This is because the winds along the equator in the Pacific blow from east to west, pushing the warm surface waters along with it. This leaves behind cooler water in the eastern part of the Pacific Ocean.

During an El Niño event, the winds blowing east to west are not as strong as normal, so a lot of that warmer water gets left in the eastern part of the ocean. What's really interesting is that, as the water gets warmer, it weakens the winds even more, which then leaves more warm water in the eastern part of the Pacific, which then weakens the winds even more and we get a self-perpetuating cycle.

So, what stops this cycle? Well, as El Niño grows large enough to reach the middle of the ocean, it creates Rossby waves. These are similar to tidal waves, where large amounts of water move in the same direction toward land. In a Rossby wave, the top part of this massive ocean wave moves really slowly in one direction and the bottom part of the wave moves in the other direction. How slowly does this wave move? About 100 times slower than a walking pace!

It takes months for Rossby waves to reach shore, and when they do, they get bounced back towards the ocean again. This slow-moving return wave from shore is called a Kelvin wave, and it brings enough cool water with it to cancel out the warming from the El Niño event. If enough cool water tags along with the Kelvin wave, we may get a La Niña event.

For this reason, a La Niña event often follows an El Niño, but this isn't always true. Though we tend to think of El Niño and La Niña as brother and sister, La Niña can occur independently. If those winds from before grow stronger instead of weaker, we get the opposite result. Colder waters are more prevalent in the eastern Pacific Ocean and we get La Niña instead of El Niño.

Effects on Land

Though they occur in the ocean, the effects of El Niño and La Niña are not confined to this space. And, just as this brother and sister duo have opposite effects on water temperature, they also have opposite effects on weather patterns on land. Because both events tend to build to their peaks during the winter months, this is usually when we see the greatest effects.

In the U.S., La Niña winters create drier conditions in the Southwest, wetter conditions in the Northwest and very cold conditions in the Northeast. El Niño produces the opposite effects: wetter conditions in the Southwest, drier conditions in the Northwest and stormy, snowy weather in the Northeast.

Other parts of the world, especially in the tropics of the Pacific, are also greatly affected by ENSO cycles. During La Niña, Australia and Indonesia are more likely to experience drought but will have heavier-than-normal rainfall during El Niño.

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