El Niño is coming. Will it help make 2015 the hottest year on record?

In the spring of 2014, a number of experts suggested that a huge El Niño would arrive later that year, perhaps similar to the monster event in 1997–‘98 that wreaked havoc on weather patterns across the globe, bringing big droughts and floods.

Things didn’t quite pan out that way. El Niño is only just now surfacing in the Pacific Ocean in early 2015, scientists from NOAA’s Climate Prediction Center announced on March 5 — much later than expected. That means sea-surface temperatures in the eastern tropical Pacific are elevated, and local rainfall patterns are shifting in response.

And, despite the apocalyptic predictions from a year ago, this looks like a very weak El Niño so far, with only a 50 to 60 percent chance of lasting through summer, NOAA said. If that prediction pans out, it’s unlikely to alter global weather patterns very drastically. That’s good news for some people (fewer floods in Peru, for instance), but bad news for others.

In particular, NOAA forecasters note, this weak El Niño is probably emerging too late in the season to bring heavy rain to drought-stricken California, as many people in the state once hoped. At most, El Niño might bring a bit more spring rain to the Gulf Coast:

The one big question, meanwhile, is whether global warming and this weak El Niño could still combine to make 2015 the hottest year on record. That’s possible, though not yet certain.

Global temperatures are already going up over time, thanks to all the carbon dioxide we’ve added to the atmosphere. According to NASA, 2014 was likely 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 more heat gets transferred from the ocean to the surface. So 2015 could potentially see even higher highs:

This will be a story to watch throughout the year. Below is a more detailed rundown of how, exactly, El Niño works, why it took so long to show up, and how it could alter weather around the world.

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 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 ocean water from east to west. All that warm water piles up in the west, near Indonesia. 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.

The 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 they are near Peru. That temperature difference creates a convective loop in the atmosphere that, in turn, reinforces the trade winds.

Because the Pacific Ocean 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.

But every so often, along comes El Niño to disrupt the usual pattern:

2) El Niño conditions:

For reasons that are still being debated, those prevailing Pacific trade winds can get disrupted every few years.

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 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.

Scientists typically declare an official El Niño when 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 and rainfall patterns shift accordingly.

A strong El Niño often has large ripple effects all around the world, especially in the winter — 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:

Typical 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.)

That’s the big question. Back in the spring of 2014, it really did look like a strong El Niño would emerge later in the year.

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 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.) So they weren’t ready to declare an official El Niño.

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 in 2014. “When the ocean surface is warm all over, there’s no strong temperature gradient for the atmospheric component to build from,” wrote Angela Fritz of Capital Weather Gang.

On March 5, 2015, scientists at NOAA’s climate prediction center were finally ready to declare that a weak El Niño was underway.

Specifically, they noted that sea surface temperatures in the Niño 3.4 region have been at least 0.5°C above their historical average since September:

And, more crucially, the atmosphere was finally responding in turn — with more rain over the central Pacific, and less rain over Indonesia. That had been the missing element thus far. (See this post by Emily Becker for a more detailed explanation.)

NOAA’s forecasters note, however, that this is a very “weak” El Niño that is forming far later in the year than it normally does. And it only has a 50 to 60 percent chance of persisting through the summer.

For now, NOAA’s scientists think the impacts will be pretty minimal. “Due to the weak strength of the El Niño, widespread or significant global weather pattern impacts are not anticipated,” the agency noted in a press release.

“However, certain impacts often associated with El Niño may appear this spring in parts of the Northern Hemisphere, such as wetter-than-normal conditions along the U.S. Gulf Coast.” Becker offered this map showing historical March-May precipitation conditions in the United States:

In particular, Becker notes that the news isn’t very good for California: “Only about 3 of the past 10 El Niño years exhibited above-average rainfall in California during March-April-May,” she notes. “Especially since the rainy season in the West is winding down by March, it is unlikely that these current El Niño conditions will lead to substantial, drought-breaking rains.

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 tend to be a bit cooler than average. Like so:

Why is that? As humans put 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.

During La Niña events, more of that heat is trapped beneath the ocean surface. When a strong El Niño hits, more of that heat is essentially transferred to the surface. That’s why the Earth’s average surface temperatures reached new highs in 1998 — due to 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 the Earth keeps getting warmer overall. That suggests that an El Niño in 2015 could push temperatures up to new highs.

In January, NASA’s Gavin Schmidt explained at a press conference that temperatures typically peak about three months after an El Niño event. And, he added, it’s possible that a weak El Niño could help 2015 set a record. But we’ll have to wait and see.

Further reading: NOAA’s ENSO blog is a terrific source of information for updates on El Niño.