Hidden Ocean Heat Waves: Unseen but Frequent Phenomenon

It's not just on land that heat waves can occur, they can strike in the ocean too. According to a recent study, approximately one-third of marine heat waves are not detectable at the ocean's surface. These findings were published in the December issue of Nature Geoscience, and suggests that there might be more of such harmful events happening than previously known.

Heat waves in the ocean can be severely harmful to marine ecosystems, as many forms of life are not equipped to quickly adapt to rapid temperature changes. A prominent example occured from 2013 - 2015, when cod populations off the North American west coast dwindled due to a lingering marine heat wave. This event not only affected the fish, but also had repercussions on seabird populations due to the interconnectivity of marine food webs.

Marine heat waves are generally identified via satellite observations that measure the temperature at the ocean's surface. However, this method leaves ocean depths unmonitored.

To gain a more in-depth understanding, statistician Furong Li and her team made use of computer simulations of numerous ocean parameters, such as temperature, salinity, currents etc. These simulations, created from both satellite and subsurface data collected since the early 1990s, served as a powerful means of studying the ocean on a global scale, Li stated.

The team was able to pinpoint ocean heat waves in the simulations by identifying water layers that remained unusually warm - a few degrees Celsius above surrounding levels - for a minimum of five days. Such occurrences can be induced by changes in atmospheric circulation or shifts in ocean currents.

Through their analysis, Li and her team identified several hundred marine heat waves per year. However, they found that about one-third of these events remained hidden and were never visible in the top 10 meters of water. "We discovered a great number of marine heat waves hidden below the sea surface," Li elaborated.

According to Mike Jacox, an oceanographer at the National Oceanic and Atmospheric Administration not involved in the research, researchers might be missing a lot of these hidden events. "Just looking at the surface temperature might not be enough," he stated.

Therefore, marine life might be dealing with considerably more ocean heat waves than what was previously thought. This could in turn cause additional stress on marine ecosystems. In 2020, Jacox and his team found that marine animals seeking to escape from these marine heat waves would have to swim hundreds of kilometers on average. In cases where creatures can't move, marine heat waves can prove to be fatal. For instance, coral bleaching is often linked to warming in the Great Barrier Reef.