Ocean heat waves often lurk out of sight

Heat waves don’t just strike on land — they can also occur in the ocean. And roughly a third of marine heat waves aren’t detectable at the ocean’s surface, a new study reports. The findings, published in the December Nature Geoscience, suggest that far more of these potentially harmful events might be occurring than previously believed.

Ocean heat waves can have a slew of adverse effects on marine ecosystems because many forms of life cannot rapidly adapt to changes in temperature. For instance, cod populations were devastated from 2013 to 2015 when a marine heat wave lingered off the West Coast of North America. Given the interconnectedness of marine food webs, seabirds suffered, too (SN: 1/15/20).

Marine heat waves are often identified with satellite observations that measure the temperature of the ocean surface. But these data leave the ocean depths unmonitored.   

To literally take a deeper look, statistician Furong Li and colleagues turned to computer simulations of the ocean’s temperature, salinity and currents, among other parameters, created from both satellite and subsurface data stretching back to the early 1990s. Such simulations are a powerful way of studying the ocean on a global scale, says Li, of the Ocean University of China in Qingdao.

The researchers pinpointed ocean heat waves in the simulations by looking for layers of water that remained unusually warm — up to a few degrees Celsius above surrounding levels — for at least five days. Such events can be caused by changes in atmospheric circulation, for instance, or shifts in ocean currents.

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).