Ocean eddies pour extreme heat and cold into the deep ocean, teeming with life.

The mesopelagic twilight zone is rich in life. Clockwise from the top: mesopelagic jellyfish, pit vipers, lanternfish, larvae, copepods, and squid. Credit: Wikimedia/Drazen et al. CC BY-NC-ND
On land, heat waves and cold waves are well known. However, the deep ocean also experiences long periods of high and low temperatures.
Marine heat waves and cold waves can seriously damage marine ecosystems and habitats such as coral reefs. These extreme conditions can also cause sudden losses to fisheries, forcing species to migrate or die.
Research published today in Nature shows that nearly half of all heat and cold waves that reach the ocean’s twilight zone (depths of 200 to 1,000 meters) are caused by large eddy currents, or swirling currents, carrying warm or cold water. It was shown that
As ocean temperatures rise, heat waves associated with eddies are becoming more intense, and so are cold waves. These pose a potential threat to the vast amount of life in the Twilight Zone, home to the world’s most abundant vertebrates and the largest migration on Earth.
Monitoring the deep sea is difficult.
About 90% of the heat trapped by greenhouse gases escapes into the ocean. As a result, marine heatwaves are arriving more frequently, particularly off the east coast of Australia, Tasmania, the northeast Pacific coast of the United States, and the North Atlantic Ocean.
Researchers have long relied on satellite measurements of sea surface temperatures to detect these extreme ocean temperature events. Surface temperature is directly affected by the atmosphere. But the depth is different.
Satellites cannot measure the temperature below the water surface, making monitoring the deep ocean very difficult.
Instead, we have several long-term moorings in oceans around the world: measurement buoys suspended in the deep ocean. These are extremely valuable because they continuously record temperature and can detect extreme temperature changes.
Recent decades have seen welcome progress in the form of the Argo float, a robotic diver that dives to depths of 2,000 meters and surfaces, sampling temperature and salinity as it travels.
Combining data from these two sources with conventional measurements from ships made our study possible.
heat wave in vortex
This data yielded 2 million high-quality temperature measurements, or “profiles,” of the world’s oceans over 30 years. We used this rich data to uncover the role of eddy currents.
Ocean whirlpools are giant swirling loops, sometimes hundreds of kilometers in diameter and more than 1,000 meters in diameter. It is so large that it can be seen even in satellite photos.
These powerful currents can push warm surface water deeper and lift cooler water at depth, causing rapid temperature changes. Vortices can travel long distances before disappearing, carrying bodies of cooler or warmer water with them.
We discovered their role in driving deep heat waves and cold waves by examining their respective temperature profiles and cross-matching them with eddies present at the same time and location.
This showed that eddies play a major role in causing marine heat waves and cold waves in oceans deeper than 100 meters deep, especially in mid-latitude oceans in the north and south of the tropics.
The East Australian Current moves warm water south to the east coast, creating many eddies. More than 70% of deep-sea heatwaves in this region actually occur within ocean eddies.
When these ocean current eddies rotate counterclockwise, they tend to bring warm water deeper into the ocean, creating ocean heat waves. But when they rotate clockwise, cold deep water is carried higher up, bringing with it colder waves.
We found that deep temperature extremes associated with eddies are more frequent in major oceanic boundary currents, such as the East Australian Stream and the Kuroshio Current in the Pacific Ocean, and the Gulf Stream in the Atlantic Ocean. Deep ocean heatwaves also occur in the Leeuwin Current off the coast of Western Australia. The stronger the eddy currents, the more likely they are to cause extreme temperatures at depth.
Eddy currents are the main cause of almost half of deep ocean heat waves and cold waves. Other factors include ocean temperature fronts caused by strong ocean currents and large ocean waves.
What does this mean for marine life?
Heat trapped by greenhouse gases is transferred to the ocean every day.
You would expect marine heatwaves to increase, and that’s exactly what happened. However, the cold wave has not gone away. In fact, as the climate changes, extremes of both heat and cold are increasing in the deep ocean.
Our study suggests that eddies act to magnify the warming rate of ocean heat waves and the cooling rate of cold waves. Rising temperatures across the ocean can make eddies stronger, causing larger temperature changes over longer vertical distances.
Because satellites can detect ocean eddies, this research can be used to predict when deep ocean heat waves or cold waves will occur. This helps identify which ecosystems may be affected by extreme heat or cold and assess what damage they cause.
The ocean layer affected by these extreme events is called the twilight zone and is between 200 meters and 1,000 meters deep. These deep layers are home to many important fish species and plankton. In fact, there is more fish biomass in this zone than in the rest of the ocean combined. Bristlemouths, a type of small fish, are probably the most abundant vertebrates on Earth, and their number can reach quadrillion, or trillions.
At night, vast numbers of fish, crustaceans, and other creatures migrate toward the surface, creating the largest animal migration on Earth. During the day, many pelagic fish head at dusk to avoid sharks, whales, and other surface predators.
The heat and cold brought on by vortices aren’t the only threats to the Twilight Zone. Marine heatwaves can cause decreased oxygen levels and nutrients in the water. We will need to know what threat the combination of these changes poses to life in the twilight.
Further information: Qingyou He et al., Common occurrence of subsurface heat waves and cold waves in oceanic eddies, Nature (2024). DOI: 10.1038/s41586-024-08051-2, doi.org/10.1038/s41586-024-08051-2
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Source: Ocean eddy currents leak extreme heat and cold into life-filled depths (October 19, 2024) from https://phys.org/news/2024-10-ocean-eddy-currents-funnel-extreme Retrieved October 20, 2024.html
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