A team of scientists from the National Office of Oceanic and Atmospheric Administration (NOAA) has been studying marine heat waves at the sea surface for more than a decade, and they have concluded that they have effects devastating in the ecosystem, just like those suffered in the atmosphere.
Between 2013 and 2016, a marine heat wave known as 'The Blob' took place and altered the marine ecosystems of the west Pacific coast. With this example, the NOAA team has presented an investigation whose results have been published in the journal Nature Communications. The study used a combination of observations and computer modeling to generate the first comprehensive assessment of marine heat waves at the bottom of producing waters across the North American continental shelves.
Lead author of the research, Dillon Amaya, said, "This is the first time we've been able to really dive deeper and assess how these extreme events play out along the shallow seafloor."
Marine heat waves dramatically impact the health of ocean ecosystems around the world. As a consequence, they interrupt the productivity and distribution of the organisms that inhabit the ocean; whether they are small like plankton or as big as whales. It is for this reason that researchers have been working to be able to not only identify and track these heat waves, but also the intensity and duration, as well as being able to predict the moment in which they occur.
The ocean has absorbed 90% of the excess heat from global warming, and this has led to a temperature increase of about 1.5ºC in the last century. In addition, in the last ten years heat waves have been 50% more frequent. For this decade-long study, scientists had to use the limited data available to have a baseline to start from.
And they are limited, since the bottom water temperature was unknown. So the NOAA team used a data product called "reanalysis," which started with available observations and "filled in the blanks" using computer models that simulated ocean currents and the influence of the atmosphere.
The scientists found that the intensity and duration of bottom sea waves (BMHW) experience a strong temperature variation, ranging from -0.5ºC to 3ºC. In addition, the NOAA team highlights that "BMHWs can be more intense and persist longer than surface heat waves (SMHW)."
While both phenomena often coexist, "BMHWs can also exist without a SMHW." "That means it may be happening without fisheries managers realizing it until the impacts start to show," Amaya said.
"Intense bottom temperature changes can have unique and dramatic impacts on the productivity and organization of demersal species found along the continental shelf," the study authors note.
Seafloor water temperature anomalies are related to declining Pacific cod abundance in the Gulf of Alaska, expansion of invasive lionfish across the southeastern United States, coral bleaching, or changes in the patterns of disease occurrence in lobster, among others.
"We know that early detection of marine heat waves is needed for proactive management of the coastal ocean," says one of the research authors, Michael Jacox. "It is now clear that we need to pay more attention to the ocean floor, where some of the most valuable species live and can experience very different heat waves than on the surface," he adds.
