See how this 9,000-mile freight train of warm water may fuel a super El Niño

 

A freight train of warm water is crossing the Pacific Ocean.

This wave of warmth is 7.5 degrees Celsius above average (13.5 degrees Fahrenheit) and is increasing the chances for a super El Niño later this year.

(Ben Noll/the Washington Post; Copernicus Marine Service)

From WP by Ben Noll

 

The key to just how intense an El Niño may become this year lies hundreds of feet down in the Pacific Ocean. 

The key to the intensity of a coming El Niño lies hundreds of feet down in the Pacific Ocean.
That’s where a freight train of record-warm water is chugging along.
This train, called a Kelvin wave, is carrying ocean waters that have reached 7.5 degrees Celsius (13.5 degrees Fahrenheit) above average in parts of the deep ocean — a huge amount of warming for the ocean, which warms and cools much slower than land.

This undersea wave of warmth could contribute to one of the strongest El Niño events on record later this year, with cascading effects expected on global climate patterns into 2027, including increasing risks for drought, flooding rain and record heat and humidity.

And because of a recent sequence of La Niña events as well as climate change, there’s more warm water available to this forming El Niño than to ones in the past.
El Niño is a warming of surface ocean waters in the east-central tropical Pacific that develops every few years, whereas cooler water in that area signals a La Niña.
This year, sea temperatures there could surge 3 degrees Celsius (5.4 degrees Fahrenheit) above average and break records.

What this undersea wave of warm water looks like

A freight train of warm water that's reached 7.5 degrees Celsius above average is crossing the undersea Pacific Ocean, with chances rising for a super El Niño.
(Video: Ben Noll; Copernicus Marine Service)
Kelvin waves shift some of the world’s warmest ocean water from the West Pacific to the western shores of South America, a distance of around 9,000 miles.
Experts are comparing this wave of immense warmth to the ones that contributed to some of the strongest historical El Niño events.

“The current Kelvin wave is impressive and, by some measures we look at, it is rivaling the one we saw in 1997,” said Michelle L’Heureux, a physical scientist for NOAA’s Climate Prediction Center.
The super El Niño from 1997 into 1998 — which caused estimated global losses of up to $96 billion at the time — is one of six such super El Niño events that have occurred based on records that extend back to 1850.
That also includes the strongest El Niño on record from 1877 to 1878 that wiped out millions of people.
But the oceans are now much warmer than they were in the past, providing an extra boost to the El Niño forming now.

What powers this train of warmth

Kelvin waves — named after Lord Kelvin, the scientist who discovered them in 1879 — are powered by winds that blow some of the planet’s warmest ocean waters from west to east across the Pacific.

And for one to develop as big as this year’s, the ingredients can take years to come together.

Large volumes of warm water

The first key element is having large volumes of water available, which come from a remote place called the West Pacific Warm Pool.
The West Pacific Warm Pool, east of Indonesia, is a global heat engine, fueling towering thunderstorms that affect global weather patterns.

Sultry waters that typically reside in the West Pacific Warm Pool, where ocean temperatures exceed 82 degrees Fahrenheit year-round, shift to the east during El Niño events. 

(Ben Noll/the Washington Post; NOAA)

Water temperatures in this zone are typically among the highest in the world.
That’s because toasty tropical winds blowing from the east — called trade winds — cause lots of warm water to pile up.
This effect means the ocean surface is usually about 1 to 3 feet higher near Indonesia than it is off the coast of Ecuador.

Record-breaking amounts of warm water built across the upper 1,000 feet of the West Pacific in 2025, caused by the planet’s long-term warming trend as well as five La Niña events in six years.
In addition to this, L’Heureux pointed to a 5,000-mile marine heat wavein the North Pacific that’s contributing to a potential super El Niño this year.

Where the upper ocean was record warm or very unusually warm in 2025

Showing a low-resolution version of the map.
Make sure your browser supports WebGL to see the full version.

 

Considering average temperatures in the upper 1000 feet of ocean from 1958 to 2025
Source: ORAS5

Winds of change

Every couple of years, these easterly trade winds weaken.
In extreme cases, they reverse direction and start blowing from the west.

That’s known as a westerly wind burst.
These important, heat-shifting winds blow from west to east at about 15 mph for a few weeks.

Westerly wind bursts provide the energy needed to form Kelvin waves.
These winds create stress on the ocean surface, forcing warm water downward and eastward as the wave propagates across the Pacific Ocean.
This reduces the separation between the warm surface water and cooler deep waters.
This subsurface temperature boundary, called the thermocline, migrates closer to the ocean surface as heat is transported from from the West Pacific Warm Pool toward South America – which eventually causes weather patterns to change.

A strong westerly wind burst last December was among the first clues that an El Niño could form in 2026.
Then, a record-breaking wind burstwas triggered by triplet cyclones in the Pacific during April.
That’s the one that caused this undersea freight train of warmth to develop, greatly increasing the odds for a super El Niño this year.

Kelvin wave in transit

 
Kelvin waves are not like the waves that curl and crash at the beach.
Instead, they slowly slosh beneath the surface of the Pacific Ocean, taking two to three months to cross the basin.
That means the big one now chugging across the Pacific will soon reach the western shores of South America.
In Peru, local scientists are monitoring the arrival of this undersea warmth.

“We haven’t seen anything like this since 1997,” wrote Peru-based forecaster Abraham Levy.
When it arrives, it can cause a weakening of upwelling – a process by which winds churn cooler, deeper ocean water to the surface.
Because the cool water feed shuts off, surface ocean waters begin to warm and El Niño starts to form.
However, L’Heureux noted that despite this intense Kelvin wave, the eventual strength of El Niño remains uncertain.

Here comes El Niño

 

An El Niño-like pattern of warmer than average sea surface temperatures is emerging in the equatorial Pacific Ocean as of May 17.
There are four key El Niño monitoring regions, but the one that has the biggest influence on global climate patterns is called Niño 3.4.
(Ben Noll/the Washington Post; NASA)

As this Kelvin wave completes its journey and causes ocean temperatures to increase in the equatorial Pacific Ocean, the atmosphere is expected to respond.
Thunderstorms that once frequently rumbled in the West Pacific will shift east, influencing the location and strength of high- and low-pressure cells near the tropics and storm-carrying jet stream winds, with weather-related impacts spreading across the world.

Those thunderstorms also release heat that comes from the ocean, which is why concerns are mounting that global temperature records – as well as atmospheric moisture records – will be broken in 2027, possibly by a wide margin.
“The reason why that matters so much is because what happens in the tropical Pacific doesn’t stay in the tropical Pacific,” climate scientist Daniel Swain said in a recent video update about El Niño, referring to the wide-reaching effect these warming waters can have.

About this story
The subsurface ocean temperature anomaly graphics, which show conditions from the ocean surface to a depth of around 1000 feet near the equator, were created using around one terabyte of data downloaded from the Copernicus Marine Service.
Ocean temperature anomalies were calculated relative to the averages between 1993 and 2025.

 

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