It’s official: El Niño is back. By late fall 2026, forecast models give a 2-in-3 chance of a strong-to-very strong El Niño affecting the weather, climate and ocean temperatures across the planet.
El Niño is the climate system’s biggest player and one side of the El Niño-Southern Oscillation, or ENSO. It’s the heads to La Niña’s tails.
During El Niño, a swath of ocean stretching 6,000 miles (about 10,000 kilometers) westward off the coast of Ecuador warms for months on end, typically by 2 to 4 degrees Fahrenheit (about 1 to 2 degrees Celsius). A few degrees may not seem like much, but in that part of the world, it’s more than enough to completely reorganize wind, rainfall and temperature patterns all over the planet.
Alexis Rosenfeld/Getty Images
I’m a climate scientist who studies the oceans. With an El Niño expected to strengthen through the summer and fall, water temperatures will heat up even more. It’s time to start preparing.
How does El Niño affect the planet?
No two El Niño events are exactly alike, though we’ve seen enough of them that forecasters have a pretty good idea of what’s likely to happen.
People tend to focus on El Niño’s impact on land, justifiably. The warm water affects air currents that leave areas wetter or drier than usual. It can ramp up storms in some areas, like the southern U.S., while tending to tamp down Atlantic hurricane activity.
El Niño can also wreak havoc on the many marine ecosystems that support the world’s fishing industries, including coral reefs and seagrass meadows.
Specifically, El Niño tends to trigger intense and widespread periods of extreme ocean warming known as marine heat waves.
Global ocean temperatures are already near record highs, so El Niño-induced marine heat waves could push many sensitive fisheries to a breaking point.
What is a marine heat wave?
A marine heat wave is just that: a “wave” of extreme heat in the ocean, not dissimilar to an atmospheric heat wave on land.
At their smallest, marine heat waves can inundate local bays and coves with hotter-than-normal water for a few days or weeks. At their largest, marine heat waves like the Northeast Pacific Warm Blob of 2013-2014 can grow to gargantuan proportions, with regions three times the size of Texas experiencing ocean temperatures 4 to 6 F (about 2 to 3 C) above average for months or even years.
Dillon Amaya
Warm water might not seem like a big deal, especially to surfers hoping to leave their wetsuits at home. But for many marine organisms that are highly adapted to specific water temperatures, marine heat waves can make living in the ocean feel like running a marathon.
For example, some fish increase their metabolism in warm waters by so much that they burn energy faster than they can eat, and they can die. Pacific cod declined by 70% in the Gulf of Alaska in response to a marine heat wave. Other impacts include bleached corals, widespread harmful algal blooms, decimated seaweeds and increased marine mammal strandings. All told, billions of U.S. dollars are lost to marine heat waves each year.
Marine heat waves flare up for a variety of reasons. Sometimes, ocean currents shift warm water around. Sometimes, surface winds are weaker than normal, leading to less evaporation over the ocean and warmer waters. Sometimes, cloudy places just aren’t as cloudy for a few months, which lets more sunlight in and heats up the ocean. Sometimes, both weaker winds and fewer clouds happen at the same time, producing record-breaking marine heat waves.
How does El Niño fit in?
In the climate system, El Niño is king. When it dons its fiery crown, the entire planet takes notice, and the oceans are no exception. But the likelihood of increased marine heat wave activity during El Niño depends on where you are.
Along the U.S. West Coast during El Niño, surface winds that normally blow from the north tend to subside. This weakens evaporation and slows upwelling of colder, deeper water. That increases the chances of coastal marine heat waves. California waters are already extremely warm. El Niño could make things even hotter for longer.
Peruvian fishers have for centuries weathered periods of extreme ocean warming that drive fish away. It wasn’t until the 1920s that scientists realized that these South American marine heat waves were related to the Pacific-wide ENSO.
In the Bay of Bengal east of India, interactions between El Niño and a tropical air flow pattern known as the Walker Circulation elevate the risk for marine heat waves.
Seafloor heat waves are another risk
Even if marine heat waves aren’t obvious at the ocean surface, that doesn’t mean all is well down below.
In a 2023 study, my colleagues and I showed that marine heat waves also unfold along the seafloor of coastal regions. In fact, these “bottom marine heat waves” are sometimes more intense than their surface counterparts. They can also persist much longer. For example, a 1997-1998 bottom marine heat wave off the U.S. West Coast lasted an extra four to five months after surface ocean temperatures had already cooled.
Events like this can be related to El Niño and put a lot of stress on bottom-dwelling species. Bering Sea snow crab landings were down 84% in 2018 after a marine heat wave reached the seafloor.
We’re in (for) hot water
With El Niño on the horizon, what can we expect for this year?
The good news is seasonal forecast models can skillfully predict marine heat waves three to six months in advance, depending on the region. And forecasts tend to be most accurate during El Niño years.
NOAA
The latest forecast predicts several marine heat waves developing as El Niño ramps up, with damaging heat reaching close to half the global ocean by the end of 2026. The California and Mexican coasts in particular have a very high likelihood of strong marine heat waves, and the Indian Ocean and parts of the Southern Ocean are also likely to see damaging heat.
These predictions are far enough out that conditions could change. Time will tell whether they hold (hot) water, but we would do well to prepare.
This article incorporates details from an article originally published April 18, 2023.
