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El Niño, explained: Why this year’s could be one of the strongest on record

A very basic definition of El Niño

• El Niño is a weather phenomenon that occurs irregularly in the eastern tropical Pacific every two to seven years. When the trade winds that usually blow from east to west weaken, sea surface temperatures start rising, setting off a chain of weather impacts.
• El Niños can be strong or weak. Strong events can temporarily disrupt weather patterns around the world, typically making certain regions wetter (Peru or California, say) and others drier (Southeast Asia). Some countries suffer major damage as a result.
• El Niños also transfer heat stored in the deeper layers of the ocean to the surface. When combined with global warming, that can lead to record hot years, as in 1998.
• “El Niño” got its name in the 1800s from Peruvian fisherman, who first noticed a mysterious warm current that would appear around Christmas. They called it the “little boy” or “Christ child.”

Why this year’s El Niño could be a huge deal

The last truly massive El Niño appeared in 1997-’98 and ended up causing an estimated $35 billion in destruction and 23,000 deaths around the world. (It also inspired that famous Chris Farley sketch.) Now we may be on the verge of a similar-size event:

Early-August status of the 1997 and 2015 El NIño events. Satellite imagery shows the departure from average sea-surface height for a given time of year, which is correlated with warmth in the upper ocean. (NASA/JPL via Weather Underground)

That, in itself, is a surprise. Back in March, NOAA’s Climate Prediction Center announced that a weak El Niño had formed in the Pacific, but many experts initially thought it might just fizzle out in the summer. Instead, El Niño kept getting stronger, with ocean temperatures in the eastern tropical Pacific continuing to soar. Some forecasters now think this could turn into one of the strongest El Niño events in memory when it peaks later this fall or winter.

“We’re predicting this El Niño could be among the strongest El Niños in the historical record, dating back to 1950,” said Mike Halpert, deputy director of the Climate Prediction Center, in a recent press call. We’ll see if this latest forecast holds up.

If it does, countries across the globe will have to brace themselves. In the past, major El Niño events have brought unusually hot, dry weather to Australia that can cramp wheat yields and amp up wildfires. It can bring hotter, drier weather to India that hurts agriculture. It can bring heavy rain and destructive flooding to Peru, washing away houses and spreading cholera. In 1997, El Niño dried out Indonesia so badly that it led to huge forest fires whose smoke disrupted daily life in Singapore.

Yet El Niño isn’t all bad. In the United States, it could potentially bring needed rain this winter to ease California’s drought (though also mudslides and flooding). Historically, El Niño has also served up milder US winters and helped tamp down hurricanes in the Atlantic Ocean.

One important caveat here is that every El Niño is a bit different — and some have unexpected impacts. As NOAA’s Emily Becker points out, strong El Niño events usually bring rain to California (as in 1982-’83), but occasionally they don’t (as in 1965-’66):

(NOAA Climate Prediction Center)

Another story to watch is whether a strong El Niño could help make 2015 or 2016 the hottest year on record. This one seems increasingly plausible.

Global temperatures are already going up over time, thanks to all the carbon dioxide we’re adding to the atmosphere. According to NASA, 2014 was already the hottest year on record. But there was no El Niño that year — and El Niño years tend to be a bit hotter than average, as heat gets transferred from the ocean to the surface. So the combination of El Niño and rising CO2 could help 2015 and even 2016 break records:

(NOAA)

Bottom line: There’s still a lot of uncertainty here, but El Niño could very well be the biggest weather story of late 2015, with potentially far-reaching impacts.

How El Niño actually works, step by step

To see how El Niño works, it helps to understand what the equatorial Pacific looks like under normal, or “neutral,” conditions:

1) Neutral conditions in the equatorial Pacific Ocean

Normally, the tropical Pacific features strong trade winds that blow warm ocean water from east to west, where it piles up near Indonesia. Meanwhile, back east along South America, frigid water deep down in the ocean gets pulled up closer to the surface, cooling the area around Peru. Here’s a diagram:

(William Kessler/NOAA/PMEL)

As a result, during “neutral” conditions, sea levels are about half a meter higher near Indonesia than they are in Peru. And the surface water near Indonesia is about 8°C warmer (14.4°F) than it is near Peru. That temperature difference creates a convective loop in the atmosphere that, in turn, reinforces the trade winds.

This ends up affecting a lot more than just this stretch of ocean. Because the Pacific is so vast, this system is a major driving force in the global climate. The large, warm pool of water near Indonesia causes the air above it to rise, creating rainfall in the region. And this system shapes the jet streams that guide weather and storms around the world.

That’s how it works under normal conditions, anyway. But things look a little different when El Niño comes along.

2) Now along comes El Niño

Every few years, those prevailing Pacific trade winds that blow east to west can weaken. (Scientists are still debating the nuances of exactly why this happens.)

When the trade winds weaken, all that warm water that was piled up near Indonesia starts sloshing back eastward, pulled back down by gravity. What’s more, the underwater layer known as the thermocline starts sinking. As a result, there’s less cold water rising up from the deep ocean near South America — so the waters near Peru start warming up. Here’s another diagram:

(William Kessler/NOAA/PMEL)

This causes sea surface temperatures in the east and central Pacific to start rising and the trade winds to weaken even further. What’s more, rainfall starts following that warm pool of water as it travels eastward. That’s why El Niño is usually associated with drier weather in places like Indonesia and Australia, as well as heavier rains in places like Peru (or California). The rain is essentially moving east.

Scientists officially declare an El Niño when sea surface temperatures in the equatorial Pacific Ocean (known as the Niño 3.4 region) rise 0.5°C above their historical baseline for three months in a row — and once atmospheric conditions and rainfall patterns shift accordingly.

Again, because the Pacific is so vast, an El Niño can have large ripple effects on weather around the world, especially during the winter months. Here’s a look at the changes that have historically accompanied El Niño events:

Typical effects of an El Niño during the winter:

(NOAA)

A strong El Niño can weaken monsoons in the Indian Ocean, for example. It can also cause the jet stream to start stretching from the Eastern Pacific across the southern United States, bringing rainfall and storms with it. Still, a lot depends on how strong the El Niño actually is — and occasionally there are aberrations and exceptions to the rule. More on that below.

El Niño’s return in 2015 — and why scientists are talking about a “Godzilla” event

Ever since early 2014, scientists have been expecting this latest El Niño to form. But, in a sign of how slippery the system can be, El Niño kept defying predictions and not showing up.

Finally, in March 2015, after a number of false starts, scientists at NOAA’s climate prediction center were ready to declare that a weak El Niño was underway. Specifically, sea surface temperatures in that Niño 3.4 region (roughly in the center of the chart below) had been at least 0.5°C above their baseline since September. And, importantly, atmospheric conditions were responding in turn, with more rain over the central Pacific and less rain over Indonesia:

Sea surface temperature departures from average (based on 1981–2010) at the end of February 2015. NOAA map by Emily Becker, Climate Prediction Center.

At the time, however, NOAA’s forecasters said that this El Niño looked “weak,” with possibly minimal effects on global weather patterns, and only had a 50 to 60 percent chance of lasting through the summer.

Then, somewhat unexpectedly, El Niño got stronger and stronger. By August 2015, sea surface temperatures had soared to more than 1.2°C above baseline in the Niño 3.4 region, and scientists were seeing the resulting telltale atmospheric changes. Here’s a chart from July and August — notice how the anomalous warm area has moved eastward since March:

Sea surface temperatures in the tropical Pacific: departure from the 1981-2010 average. (NOAA)

NOAA’s Climate Prediction Center is now estimating that there’s a 90 percent chance El Niño will persist through the fall/winter. And when it peaks, signs suggest that this could be an extremely strong event, rivaling the strongest El Niños since detailed records began in 1950. Some forecasters have even dubbed this one a potential “Godzilla.”

Over at NOAA’s ENSO blog, Emily Becker offers a more detailed breakdown of why forecasters are betting on a powerful, possibly record-setting, El Niño. Keep in mind that forecasts often go awry, that surprises occur regularly, and we can’t be perfectly certain of how things will turn out. Still, she writes, “We have a relatively confident forecast for a strong event.”

El Niño could bring rain to California — but may not end the drought

As noted above, El Niño tends to be associated with changes in weather patterns around the world, especially during the Northern Hemisphere winter. The most tantalizing possibility is that a strong El Niño could bring rain to California, potentially alleviating the state’s drought.

But even here, nothing is yet assured. El Niño only affects US weather indirectly, by altering atmospheric circulation and shifting the North Pacific jet stream. (See here for a lucid explanation by Columbia University’s Anthony Barnston.) This is an intricate chain of events, and small kinks at certain points can affect the ultimate outcome.

As such, Becker cautions people to think not in terms of certainties but in terms of probabilities. Here’s an example of how El Niño might shift the odds of a wet winter for California (she notes that this isn’t a prediction, just an illustration):

An example of how a strong El Niño could shift the odds for the amount of seasonal precipitation. Official outlooks from the Climate Prediction Center are available here.

In other words, thanks to El Niño, California has a greater chance of more precipitation this winter, but not a 100 percent chance.

What’s more, even if rain does come, that may not be enough to completely erase the massive water deficit that California has built up over the past five years. The state likely needs record precipitation to end the drought, and it also needs the right mix of rain (to recharge the reservoirs) and snow in the Sierra Nevada mountains (to melt during the spring and summer).

Also, be warned: Heavy rain after a drought can bring floods and mudslides. So California needs to be ready for some negative impacts, as well.

El Niño tends to hurt some countries, and benefit others

It’s not quite right to say that El Niño events are “bad” or “good.” They tend to have different impacts on different regions.

One recent study from the University of Cambridge found that on average, El Niño events hurt economic activity in Australia, Chile, Indonesia, India, Japan, New Zealand, and South Africa. The reasons varied: drought and reduced crop yields in Australia and India, forest fires in Indonesia, less-productive fisheries in Peru.

But that study also found that on average, El Niño tends to boost the economies in Argentina, Canada, Mexico, and even the United States, at least in the very short term. Again, many factors were at play: In addition to bringing needed rain to California and Texas, El Niño was associated with less tornado activity in the Midwestern United States and fewer hurricanes in the Atlantic Ocean.

Here’s a table of the estimated economic impacts on a broad selection of countries:

(Cashin et al, 2014)

Again, every big El Niño is different and has its own idiosyncrasies. So think of this table as more a rough guide than gospel.

El Niño could help make 2015 or 2016 the hottest years on record

Thanks to global warming, the Earth’s average surface temperature has been going up over time. But there’s a lot of variation from year to year. El Niño years tend to be a bit hotter than average. La Niña years (when those trade winds strengthen rather than weaken) tend to be a bit cooler than average. Like so:

(NASA)

So what’s going on here? As humans load more greenhouse gases in the atmosphere, we’re trapping more and more heat on the Earth’s surface. But more than 90 percent of that extra heat is absorbed by the oceans. So subtle interactions between the ocean and the atmosphere can make a big difference for surface temperatures.

When conditions in the Pacific are neutral, more of that heat is trapped beneath the ocean surface. When a strong El Niño forms, more of that heat is transferred to the surface. That’s why the Earth’s average surface temperatures reached new highs in 1998: you had the combination of global warming and an extremely strong El Niño.

What was remarkable about 2014 is that it was likely the hottest year on record even without an El Niño event — a sign that Earth keeps getting warmer overall. Meanwhile, 2015 has so far been on track to be even hotter than 2014.

Now throw a potentially record-setting El Niño into the mix, and we’re looking at a potential shattering of records. Back in January, NASA’s Gavin Schmidt explained at a press conference that temperatures typically peak about three months after an El Niño event. Given that forecasters expect this current El Niño to last until next spring, it’s entirely possible we could see 2015 or 2016 break the temperature record. We’ll have to wait and see.

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