Mapping currents in the Southern Ocean is vital to monitoring climate change, but hard to conduct. So scientists turned to seals for help.
At the bottom of the planet is the Southern Ocean, its waters cold and roiling and sheathed with ice many months of the year.
The edge of the ice cover, which melts during summer and forms again in winter, is called the marginal ice zone, and it is incredibly difficult to study. Large icebreaking ships, which have traditionally been used for research in the region, cannot consistently observe small-scale ocean activity. And sea gliders — small, relatively cheap instruments that sink in the water and bob back up periodically — don’t work under the ice. “It’s a blind spot of knowledge in our climate system,” said Sebastiaan Swart, a marine biologist at the University of Gothenburg in Sweden.
What is known about the marginal ice zone is that it is an important storage system for carbon and heat emitted by humans. The global ocean as a whole stores more than 90 percent of Earth’s excess heat, and the Southern Ocean is the portal through which much of this heat is transferred from the atmosphere. This makes ignorance of the region particularly worrisome.
But Dr. Swart and Louise Biddle, a researcher also at Gothenburg, found a way around this methodological roadblock in a paper published in May. To do so, they turned to unique organic instruments that can gather consistent information from under the ice: southern elephant seals.
Seals in the Southern Ocean have been monitored for decades. Small sensors and trackers that are attached to their bodies and the tops of their heads, like tiny hats, transmit information from dives — depth, lateral distance, water temperature, salinity — that gets filed into open-access databases. A typical southern elephant seal is a masterful diver, and spends around 90 percent of its time underwater foraging for fish and squid, only surfacing for a couple minutes between expeditions to catch its breath before sinking back down to the inky depths.
Because of the frequency of these dives, seal data, like sea glider data, can reveal small eddies and flows in the water. These water fluxes result from many of the same forces, including winds and heat gradients, that create large currents like the Gulf Stream, but are far smaller and called submesoscale flows. Some are only the length of a football field and last no more than a day.
In short, the seals showed that water in the Southern Ocean moves a lot more under the ice, and particularly under thick ice, than many scientists had anticipated. Perhaps this has to do with the variable concentration of what Dr. Biddle called “pancake ice,” which creates heat variations in the mixed layer. Perhaps it has to do with certain wind and weather patterns. Either way, it is an important finding.
“If these submesoscales are to change in the future, they actually will really change how much heat and carbon is stored in the atmosphere or in the ocean,” Dr. Swart said. “And so they’re really, really important, cumulatively, to the habitable planet.