It was raining. That was what first startled Alyssa Atwood when she stepped off the plane on Christmas Island in August.
Torrential downpours had already turned the dirt roads into impassable waterways, leaving her research team questioning whether their trucks could even reach their lodge.
When she finally waded into the Pacific Ocean, it wasn’t quite as she remembered it.
“You almost don’t want to wear a wet suit, it’s so hot,” said Dr. Atwood, a paleoclimate scientist at Florida State University who recently returned from the island, a remote coral atoll that sits in the Central Pacific at the geographical bull’s-eye of El Niño.
The typical winter peak of El Niño is months away. Yet on Christmas Island, also known as Kiritimati, the ocean was already at least two degrees warmer than average, hovering between 30 and 30.5 degrees Celsius, and the reef was actively bleaching. It was an ominous sign of what forecast models suggest could become a record-breaking El Niño, and stark proof that the ocean had already entered uncharted territory amid what some have called a “super El Niño” or even a “Godzilla El Niño.”
It has already helped make this the hottest summer on record in the United States and boosted Pacific hurricane activity with rare, damaging storms near Hawaii. At the same time, it has kept hurricanes away from the Atlantic, making parts of the Caribbean drier than usual. And as winter approaches, experts are closely monitoring what El Niño’s most intense phase will mean for the day-to-day weather.
The dire warnings may leave everyday people around the world with a central question: How concerned should they be?
Who gets more rain? Who gets less?
First, a brief explanation: El Niño is a natural weather pattern that comes around every few years. It typically develops in the spring or summer, gets stronger through the fall, and peaks in the winter. It doesn’t, by itself, mean a wetter winter in California or a warmer Christmas in New York, but more often than not, when El Niño arrives, that’s what tends to happen. A stronger El Niño like this one can make it more likely.
Emily Becker, a climate scientist at the University of Miami, explained that seasonal forecasts, like the ones making headlines as the off-the-charts scale of this year’s pattern became clear, represent shifting probabilities rather than precise predictions. A winter forecast can offer a strong signal that December, January and February will likely be above or below average, she said, but not when a single storm will hit and how bad it will be.
Think of the El Niño pattern as a dance between the ocean and the atmosphere. As winds in the tropical Pacific weaken, a pool of warm water that otherwise would have stayed in the western Pacific near Indonesia instead sloshes east, dragging the ocean’s primary zone of thunderstorm activity with it. This shift acts like a boulder dropped into a stream, rippling upward through the atmosphere to alter the jet stream and disrupt weather patterns worldwide.
“I see El Niño as an artist that is providing some broad-brush preliminary sketches on what to expect for upcoming seasons,” said Michelle L’Heureux, an El Niño expert at the National Oceanic and Atmospheric Administration. “But the details of the final portrait will be determined by events we cannot predict months in advance.”
California, being on the Pacific, is often the first land mass affected by this new pattern. The state gets most of its annual precipitation in the winter, so an El Niño year leads to the kind of storms that live on in popular memory for decades. That’s what happened in 1997, when relentless storms left more than a dozen people dead and caused a half-billion dollars in damage.
But while a strong El Niño loads the dice, it does not offer a guarantee.
In 2015, another “Godzilla El Niño” year, forecasters expected winter storms to help break a multiyear megadrought. But the storms were forced around California by more regional weather patterns.
“Our return interval in California is two-thirds,” Dr. Becker said. In other words, California sees above-average rainfall in only two out of every three El Niño seasons. So there’s a one-in-three chance that the winter stays dry.
This time around, weather experts believe California is more likely to end up with more rain, said Jon Gottschalck, who works at NOAA’s Climate Prediction Center. But that forecast is based on the similar statistics used in 2015, and even Mr. Gottschalck cautions that California is often more difficult to predict with certainty than other parts of the country during El Niño.
Parts of the South, too, are likely to end up with more rain this winter, Mr. Gottschalck said. The jet stream routinely pins itself across the southern United States during El Niño, delivering a steady conveyor belt of Pacific moisture across Texas, Oklahoma and Kansas.
That could be a good thing, said Weston Anderson, a crop scientist.
“This El Niño may actually bring some needed relief to the area, with increased precipitation in the Southern U.S.,” Dr. Anderson said, noting that winter wheat crops in Kansas and Oklahoma stand to benefit significantly after years of La Niña-driven drought.
Unlike in California, the signal El Niño provides for above-average winter rainfall across the Gulf Coast, the Southeast and up the Eastern Seaboard of the United States is among the most reliable seasonal forecasters can read.
“The probability there’s pretty high,” Dr. Kirtman said.
What if you’re in the Northeast? It’s complicated.
While it’s extremely likely that there will be more storms, and that they will move up the East Coast as they do every winter, those systems don’t always translate into snowfall as they move into the Northeast, as they might in a year without El Niño.
Why? Because the northern half of the United States leans toward significantly warmer temperatures during El Niño. That means potential snowstorms often shift into rain or ice in the major Northeast cities, because it’s not always cold enough for snow to fall or accumulate.
“There’s a reasonably strong relationship between El Niño and a warmer winter throughout the northern tier,” Dr. Becker said.
But don’t put your winter coat back in the attic quite yet. A generally warm northern winter does not rule out sudden, severe blasts of Arctic cold.
Amy Butler, a research scientist at NOAA’s Chemical Sciences Laboratory, explained that El Niño frequently triggers what weather people call “sudden stratospheric warmings” high above the North Pole. These warmings can push the cold Arctic air farther south.
If you’re in New York, for example, it wouldn’t be impossible to be enjoying a warmer than usual Christmas Eve, only to need that heavy parka by the time the New Year rolls around.
This El Niño may enter uncharted territory.
As global temperatures continue to break records, scientists are grappling with how climate change, caused primarily by the burning of fossil fuels, is altering El Niño itself.
“El Niños have been acting like a stair step,” said Dr. Kim Cobb, a climate scientist at Brown . Global temperatures tend to spike during an El Niño, and when the cycle winds down, temperatures drop — only never back to the previous baseline. “That’s what frightens me about El Niño,” she said.
Dr. Cobb works with Dr. Atwood on Christmas Island, where they are watching corals bleaching before El Niño has even reached its winter stride. Super El Niños are recurring on tighter timelines, and fragile ecosystems no longer have sufficient time to recover between shocks.
That acceleration concerns her. “People should be planning for worst-case scenarios,” she said.
Dr. Michael McPhaden, the director of a NOAA buoy network that has monitored El Niño conditions since the 1990s, offers a different framing.
“Ignorance is the foundation of fear,” he said. “El Niño should not be feared. It should be understood for what it does, what it can do, and what you can do to prepare for it.”
That preparation is being tested like never before. Historical statistical models are fed data on past El Niño patterns to predict how the next one will behave. They have struggled to keep up with the extreme heat of the ocean this year.
There’s another type of projection, called a dynamic physical model, which calculates atmospheric physics based on current data rather than relying on past examples. Those models are forecasting a much stronger, more disruptive El Niño period.
“We are very much in uncharted territory,” Dr. Kirtman said. “What I feel completely confident in is that things are going to happen that are entirely unanticipated — things we didn’t even think of.”
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