Back in the spring, a number of experts suggested we could see a strong El Niño in late 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 huge deal.

But in the months since, scientists have gotten a surprise: El Niño hasn't been unfolding as expected. In fact, there's a good chance it might not appear at all in 2014.

The odds of El Niño have fallen from 80% in the spring to 58% today

The National Weather Service's Climate Prediction Center has dropped the odds of El Niño appearing this winter down to 58 percent (a few months ago, those odds were at 80 percent). And if El Niño does form, forecasters now think it will be a "weak" one — not a strong event, as once thought.

It's a notable shift, and scientists are still trying to figure out what, exactly, happened. One possibility is that the Pacific Ocean has been extremely warm all over this year, making it tougher for El Niño to form. (Also, in fairness, NOAA never said a strong El Niño was inevitable, and some scientists had been emphasizing caution about predictions all along — because surprises were inevitable.)

The stakes are quite high: El Niño can be terrible news for some countries (it brings 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 understand how it 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:

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

As the trade winds blow west, warm water piles up near indonesia

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: During "neutral" conditions, sea levels are 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:

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

In El Niño, the usual pattern gets 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 ripple 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:

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.)

Ok, 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.

The unusually warm ocean may have made it harder for a temperature gradient to form

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 messy. Atmospheric conditions over the Pacific Ocean didn't shift 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.)

In August, Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, told me that the waters in the western Pacific hadn't cooled off as quickly as expected — so we didn't seen the sort of west-to-east temperature gradient that can sustain a strong El Niño.

One possible reason for that? The Pacific Ocean everywhere has been extremely warm all over this year. "When the ocean surface is warm all over, there's no strong temperature gradient for the atmospheric component to build from," writes Angela Fritz of Capital Weather Gang:

The ocean is unusually warm all over

(Ocean temperatures have been getting steadily warmer over time due to global warming, though there's still debate about how a hotter world might affect the strength/frequency of El Niño events.)

Could we still get an El Niño in 2014?

That's possible, though far from certain. The National Weather Service's Climate Prediction Center now predicts a 58 percent chance of El Niño by the end of the year — which is still above the long-term average of how often we get El Niño conditions:

That said, the models also suggest that it's unlikely to be a strong El Niño. "If El Niño does emerge, the forecaster consensus favors a weak event," the National Weather Service said.

It's a reminder that there's a lot about El Niño we still don't know

Regardless of the outcome, some experts have warned that the 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," William Kessler, an oceanographer at NOAA's Pacific Marine Environment Laboratory, told me back in August. "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," Kessler said. "And what this year might be showing is that, in some situations, maybe there isn't."

Likewise, over at NOAA's El Niño blog, Emily Becker points out that weak El Niño events — as this one is likely to be — seem to be getting harder and harder to forecast in recent years: "Many scientists think we have seen a shift in low-frequency variability since around the year 2000. These shifts may make ENSO prediction harder for today's state-of-the-art climate models to predict."

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):

Some countries get hurt by El Niño: A 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 can hurt Australia and India but benefit the United States

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:

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.

Note: This story has been updated with new information since it first appeared in August — when El Niño forecasts were starting to weaken considerably.