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Scientists were expecting a big El Niño this year. So where did it go?

Sea surface temperatures as of August 12, 2014. El Niño is characterized by unusually warm temperatures in the equatorial Pacific.
Sea surface temperatures as of August 12, 2014. El Niño is characterized by unusually warm temperatures in the equatorial Pacific.
NOAA

Earlier this spring, a number of experts suggested that we could see a strong El Niño in the fall or winter of 2014, perhaps similar to the monster event in 1997-98 that upended weather patterns across the globe. That might mean record temperatures, floods in some areas, droughts in others — a really big deal.

But in the months since, scientists have gotten a surprise: El Niño isn't unfolding quite as expected, and many of the bolder predictions from earlier this year have become more cautious.

The National Weather Service's Climate Prediction Center recently dropped the odds of an El Niño this fall or winter from 80 percent down to 65 percent. And Australia's Bureau of Meteorology says there's just a 50 percent chanceWhat's more, both agencies are now only forecasting a "weak" or "moderate" El Niño — and likely not a strong one.

(In fairness, it's worth noting that neither of those government agencies had ever said a strong El Niño was certain, and that quite a few scientists were emphasizing caution about predictions back in the spring.)*

It's a notable shift, and scientists are still puzzling over what exactly happened. And the stakes are fairly high: El Niño can be terrible news for some countries (they bring heavy flooding to Peru, for instance) but excellent news for others (an El Niño might bring much-needed rain to drought-ravaged California). So it's worth getting a better sense of what's going on here.

What is El Niño again? And why should I care?

El Niño is a phenomenon that occurs irregularly in the tropical Pacific Ocean every two to seven years and affects weather all around the world.

To get a clearer understanding of how it all works, we first need to see what the equatorial Pacific Ocean looks like under normal, or "neutral," conditions (which are more or less what we've been getting lately):

1) Neutral conditions in the equatorial Pacific Ocean:

(William Kessler/NOAA/PMEL)

Normally, the tropical Pacific Ocean features strong trade winds that blow from east to west.

As those trade winds travel west, they bring ocean water with them, which gets heated up by the sun. All that warm water then piles up in the western Pacific. Meanwhile, back east along South America, frigid water deep down in the ocean gets pulled up closer to the surface — cooling the waters around Peru.

End result: Sea levels are normally about half a meter higher in Indonesia than they are in South America. Sea surface temperatures near Indonesia are also about 8°C (or 14.4°F) warmer than near Peru. That gradient difference creates a convective loop in the atmosphere that, in turn, reinforces the trade winds.

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. And this system shapes the jet streams that guide weather and storms around the world.

So that's what "neutral" conditions look like. But every so often, along comes El Niño to disrupt all that:

2) El Niño conditions:

(William Kessler/NOAA/PMEL)

Every few years, for reasons that are still being debated, those Pacific trade winds can get disrupted.

When that happens, all that warm water that was piled up near Indonesia starts sloshing back toward the east, pulled back down by gravity. What's more, the underwater layer known as the thermocline starts sinking. That means there's less cold water rising up from the deep ocean near South America — so the waters near Peru start warming up.

This all causes sea surface temperatures in the east and central Pacific to start rising and the trade winds to weaken 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.

Once sea surface temperatures in the equatorial Pacific Ocean (namely, the Niño 3.4 region) rise 0.5°C above their historical average for three months in a row, and once atmospheric conditions shift accordingly, scientists typically declare an El Niño.

An El Niño usually has large knock-on effects all around the world, though it depends on how strong the El Niño actually is. For example, monsoons in the Indian Ocean can weaken. And the jet stream starts stretching from the Eastern Pacific across the southern United States, bringing rainfall and storms along:

Effects of El Niño in the winter:

El_nino_effects_december-february

NOAA

In addition, during an El Niño, the warmer tropical waters transfer heat into the atmosphere, which can raise global average surface temperatures. The very strong 1997-98 El Niño, combined with global warming, helped push global temperatures in 1998 to new highs. (The next record came in 2005, after a weaker El Niño.)

Okay, so why are scientists now lowering the odds of El Niño?

Earlier this year, it really did look like a strong El Niño would emerge.

A series of winds that begin in the Indian Ocean — known as the Madden-Julian Oscillation — began blowing eastward, counteracting and weakening those trade winds in the Pacific. That allowed some of that warm water piled up near Indonesia to start sloshing back toward the east.

As a result, by June, sea temperatures in the central equatorial Pacific (the Niño 3.4 region) had risen 0.5°C above their historical average. It looked like an El Niño was on the way.

But then… things got complicated. Atmospheric conditions over the Pacific Ocean weren't shifting as expected. Specifically, scientists weren't seeing the change in atmospheric pressure over both the eastern and western Pacific that you'd expect during an El Niño. (See this blog post from NOAA for a fuller explanation.)

So what happened? Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, points out that the waters in the western Pacific haven't cooled off as quickly as expected — so we haven't seen the sort of west-to-east temperature gradient that can sustain a strong El Niño. (It's not entirely clear why the water hasn't cooled, though the Pacific Ocean overall is warmer than usual.)

By August, NOAA noted that the atmosphere still hadn't responded to the change in ocean conditions — and the Niño 3.4 region of the Pacific has begun cooling again. As a result, somewhat fewer models are now predicting an El Niño this year.

Could we still get an El Niño in 2014?

Yes, that's quite possible.

As NOAA's Emily Becker points out, the National Weather Service's Climate Prediction Center still predicts a 65 percent chance of El Niño by the end of the year — which is twice above the long-term average of how often we get El Niño conditions. That means that, yes, they're still expecting El Niño to emerge — even if it's not a strong one.

Most model runs, in fact, predicted the current dip in temperatures in the Niño 3.4 region this summer — and they still expect an El Niño to form in the fall:

That said, the models also suggest that it's unlikely to be a strong El Niño. "A strong El Niño is not favored in any of the ensemble averages," the National Weather Service said in August, "and slightly more models call for a weak event rather than a moderate event."

Regardless of the precise outcome, some experts note that the recent shift in forecasts should be a reminder that there's still a lot about El Niño events we still don't know — particularly how, exactly, these events are triggered.

"Our whole system is set up based on the idea that there's some predictability here," says William Kessler, an oceanographer at NOAA's Pacific Marine Environment Laboratory. "Because there's a lot of value in that."

Farmers might, for instance, decide what to plant based on El Niño forecasts — or governments may decide to take water restrictions. "That's all predicated on the idea that there's some useful predictability here. And what this year might be showing is that, in some situations, maybe there isn't."

For example, Kessler notes, those westerly winds from the Indian Ocean abruptly stopped in April and May. If that turns out to be a big reason why a seemingly strong El Niño is now fizzling, that's disquieting, because those winds aren't very predictable.

So who benefits from El Niño (or no El Niño)?

It's not quite right to say that El Niño events are "bad" or "good." They tend to have different impacts in different countries (though it all depends on the strength of the El Niño event):

Ninooct_mar

Miami University of Ohio

Some countries get hurt by El Niño:recent economic study (pdf) from the University of Cambridge found that El Niño tends to hurt economic activity in Australia, Chile, Indonesia, India, Japan, New Zealand, and South Africa.

El Niño has its biggest impact in the Asia and Pacific region. It can cause hot, dry summers in Australia that feature fires and reduced wheat yields. What's more, an El Niño is typically associated with drier, hotter conditions in India, which can hurt the agricultural sector.

A strong El Niño can bring disaster in some areas. Back in 1997, El Niño so dried out Indonesia that it led to huge forest fires whose smoke severely disrupted Singapore. El Niño can also bring heavy rains and destructive flooding to Peru, washing away houses and spreading cholera.

Other countries can benefit from El Niño: But on the flip side, the paper found that El Niño can actually boost the economies in Argentina, Canada, Mexico, and the United States, at least in the short term.

Perhaps most notable, El Niño typically brings wetter winter weather to California and Texas, which are both currently plagued by drought. It is also associated with less tornado activity in the Midwest United States and fewer hurricanes hitting the eastern United States.

Here's a table of all the different impacts that the economists calculated:

Screen_shot_2014-08-13_at_2.01.23_pm

Cashin et al 2014

Now, climate scientists tend to stress that every El Niño is different, so don't take this as a hard and fast prediction about the impacts. Still, it suggests that El Niño tends to have winners and losers — which means that the same goes for a lack of El Niño.

Further reading: NOAA's ENSO blog is a terrific source of information for updates on El Niño. See, for instance, Michelle L'Heureux's post today on why forecasters aren't expecting El Niño to fizzle out entirely like it did in 2012.

* Update: I added in a clarification that the official forecasting agencies were being pretty cautious back in the spring.

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