How to Ocean heat waves often lurk out of sight.

Not only can heat waves occur on land, but they can also happen in the ocean. Furthermore, a recent study finds that about one-third of marine heat waves are undetectable at the ocean's surface. The results, which were released in December in Nature Geoscience, imply that a lot more of these potentially dangerous occurrences may be happening than previously thought.

Because many types of life cannot quickly adjust to changes in temperature, ocean heat waves can have a wide range of negative consequences on marine ecosystems. For example, between 2013 and 2015, a maritime heat wave that persisted off the North American West Coast severely damaged cod stocks. Because marine food webs are interrelated, seabirds also suffered.

Satellite measurements of the ocean's surface temperature are frequently used to detect marine heat waves. However, these statistics do not track the ocean's depths. The statistician Furong Li and colleagues used computer simulations of the ocean's temperature, salinity, and currents, among other characteristics, to literally take a closer look. These simulations were made using satellite and subsurface data dating back to the early 1990s. According to Li of the Ocean University of China in Qingdao, these kinds of models are an effective means of researching the ocean globally. By searching for water layers that stayed abnormally warm for at least five days—up to several degrees Celsius above surrounding levels—the researchers were able to identify ocean heat waves in the simulations. Such occurrences may be brought about,

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Li and her collaborators spotted several hundred marine heat waves per year. But the real surprise was finding that about 1 in 3 of those events consistently lurked out of view and were never visible in the uppermost 10 meters of the water. “We discovered a great number of marine heat waves hidden below the sea surface,” Li says.

Researchers might accordingly be missing a lot of these events, says Mike Jacox, an oceanographer at the National Oceanic and Atmospheric Administration in Monterey, Calif., not involved in the research. “Just looking at the surface temperature might not be enough.”

Marine life might therefore be contending with significantly more ocean heat waves than previously thought. And that means additional stress on marine ecosystems. In 2020, Jacox and his colleagues showed that animals seeking to escape marine heat waves would have to swim hundreds of kilometers on average (SN: 8/10/20). And when creatures are unable to move, marine heat waves often prove deadly. Warming in the Great Barrier Reef, for example, has been linked to coral bleaching (SN: 4/7/20).

Every year, Li and her associates see several hundred maritime heat waves. The major surprise, though, came when it was discovered that roughly one in three of those occurrences constantly lurked out of sight and was never visible in the top ten meters of the water. According to Li, "we found a significant number of marine heat waves hidden below the sea surface." Thus, Mike Jacox, an oceanographer at the National Oceanic and Atmospheric Administration in Monterey, California, who is not involved in the research, believes that many of these events may be missed by researchers. "It might not be sufficient to simply look at the surface temperature." Therefore, it's possible that marine life is facing a lot more ocean heat waves than previously believed. And that puts marine ecosystems under even more stress. 2020 saw Jacox