A disturbing warming event is taking place in the oceans

by The Insights

Illustration: MBARI

The dark red areas on the map above show that the Pacific waters off South America are currently very warm. This is an unusual “coastal El Niño” that is unrelated to the larger El Niño with global climate implications, says biological oceanographer Francisco Chavez of the Monterey Bay Aquarium Research Institute. A classic El Niño is a band of warm water that develops across the Pacific. This contrasts with La Niña that we have experienced in recent years, which is a band of cold water in the Pacific.

Models suggest there is a 62% chance of a classic El Niño developing by June or July, with a four in 10 chance of a strong El Niño. But that’s not a sure thing because El Niño is the consequence of complex atmospheric dynamics – essentially, the wind blowing hot water from Asia. “There’s still a lot of uncertainty,” Chavez said. “Predicting the real El Niño is difficult because the atmosphere is chaotic.”

Each time El Niño arrives, it will have consequences. On the upside, there is generally less hurricane activity in the Atlantic when El Niño is active in the Pacific. But the results for precipitation are mixed: for Peru, El Niño tends to create more precipitation, but to the east in the Amazon rainforest, it can bring devastating drought. And all that extra heat in the Pacific could dramatically increase global temperatures. “There is a chance that 2023 will be the hottest year on record,” says Rohde. “If an El Niño develops, as we now think, 2024 will likely be warmer than 2023.”

In the ocean itself, warmer waters—due to El Niño or simply long-term global warming—can become less biologically productive. Some organisms that reach their thermal limit may migrate to colder waters, transforming both the ecosystems they leave and the new ones in which they take refuge. But others, like corals, are blocked. These animals are particularly sensitive to heat, and blanch in response, releasing their symbiotic algae which provide them with energy.

The ocean food web also depends on the natural circulation of water, which is partly influenced by temperature. When cold water from the depths rises to the surface, it brings up nutrients that fertilize the phytoplankton. These microscopic plants grow in sunlight and become an essential food source for tiny animals called zooplankton. But as the water warms at the surface, it stratifies, forming a sort of cap that rests on the cooler waters below. “The bigger the cap, the harder it is to break it. By heating the ocean, you’re essentially going to decrease the amount of nutrients that are there,” Chavez says. “A longer-term concern is this: to what extent will this global warming modify the natural processes of fertilization, such as the upwelling of water? Will the ocean become more desert over time?

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