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El Niño and La Niña

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Describe the environmental changes and effects that result from El Niño and La Niña events. Created by Khan Academy.

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    At , why does the northern US and Canada become drier than usual in an El Nino year? Wouldn't the increased temperature lead to more rain in the region?
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  • blobby green style avatar for user Peter Patterson
    At , why does South Asia and the Pacific island experience warmer conditions? Isn't the warm water staying around the east coast of the Pacific if the wind is weaker?
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Video transcript

- [Instructor] Every few years you might hear about El Nino in the news. And this also might come with powerful images of flooding and rainfall, but it is not just a storm. It's actually a climate pattern that takes place in the Pacific Ocean. And we'll get a little bit more into what that actually means. Now, fun fact about how El Nino is named is that a long time ago, South American fishermen noticed one December that the Pacific Ocean was actually warmer than it normally is. And this brought about an abundance of fish. So because they were grateful, they named this event El Nino to correlate with the commemoration of Christ in this part of the world during Christmas time. So that's a little trivia you can keep in your back pocket, but back to what we hear about El Nino in the news, you might also notice that it isn't talked about every single year and that's because El Nino comes around every two to seven years on average. And scientists are still not sure exactly what triggers El Nino, but they know what signs to look for once it is approaching or once we're in that event. So even though it's true that El Nino can bring about heavy rainfall and flooding, it can also cause severe drought. So it's really important to note that different regions around the world experience different effects of El Nino. And we'll see a few examples of those. So sometimes an El Nino year is actually, a little bit helpful and might bring about some much needed rain, but other times bigger and more severe El Nino events can bring devastating weather events all across the globe. So we can look at the different effects by looking at the biggest El Nino on record, during the 1997 to 1998 season. And we can see how different regions were affected. So for example, in California, we saw very destructive mudslides. In Ecuador, there was heavy rainfall. And in Indonesia there was actually extreme drought and fire. And overall, this El Nino event was really large and very destructive, there are estimates of about $36 billion in damage to infrastructure. So you might be asking yourself, how can the same climate pattern caused such drastically different effects around the globe? So to understand this better, we have to look at what a quote unquote normal year looks like. So this is a map of the Pacific Ocean. And normally there are winds that are pushing warm water towards the West. So towards Asia and the Pacific islands and warm water accumulates on this side, on the other side on the East side, near Central and South America, we have an accumulation of cool water. And this is also due to a process called upwelling, where cold water from the bottom of the ocean is pushed up. And during these conditions in general, you would see less rain on this side. Or you'll see more rain on this side. Okay, so now let's look at what changes during an El Nino year. So during an El Nino year, you still have trade winds pushing water West, but they're much weaker and that's the key here, that your trade winds are weakened, which means less water is pushed towards Asia and the Pacific islands. So compared to a normal year, you actually have cooler water that starts to accumulate on this side of the Pacific Ocean. Meanwhile, on the other side, since less water is being pushed away from Central and South America, you get more warm water accumulating. So now you can think back to those South American fishermen and why they were experiencing warm water along their coasts during an El Nino year. So this map does a really great job of showing the temperature gradient, and how this warm water is staying along the coast of Central and South America during an El Nino year. Meanwhile, you have cold water accumulating towards Asia. And this causes a lot of changes that affect weather patterns. So in the US the Pacific jet stream moves South of its neutral position. And because of that, we start to see that the Northern US and Canada are actually warmer and drier than they usually are. The US Gulf Coast and Southwest regions of the US are wetter, and are there more increased risk for flooding. And South Asia and the Pacific islands experience warmer and drier conditions that lead to drought on the other side of the Pacific Ocean. So now you start to see how one event El Nino, this one climate pattern can cause both flooding on one side of the Pacific Ocean, while also causing drought on the other side. And finally, we'll end with La Nina, because you might hear this come up too when talking about El Nino. So La Nina is essentially the opposite. So if you've got El Nino down, you can start to understand what happens during La Nina event. So in this situation, our trade winds are getting stronger than our normal years. So they're pushing water West, but they're doing so even more than we normally would expect. So this warm water that accumulates towards Asia actually accumulates more West than it would during our normal years. And on the other side, we have even more upwelling causing this cold water to accumulate. So the next time you hear El Nino or LA Nina in the news, you'll know that they're not just talking about a really big storm, but they're actually talking about climate patterns that can affect weather all across the globe.