Seals With Sensors Help Solve the Mystery of Antarctica’s Giant Sea Ice Hole

An elephant seal with a CTD sensor on its head. An individual seal is typically only tagged once in its lifetime, and sensors usually only stay on for a few months until the animal molts.
Photo: C. Guinet (CEBC/CNRS, France)

Scientists studying the vast, remote waters encircling Antarctica frequently enlist the help of elephant seals, sticking sensors on their heads that collect data on temperature and salinity. Now, these flippered field assistants, along with robotic floats that collect similar intel, have been conscripted to help solve an icy mystery.

For the past few years, researchers have been puzzling over a nearly 4,000 square-mile hole, or polynya, that emerged in Antarctica’s sea ice in the winter of 2017. A recent study using satellite data and climate models gave us some closure. But in new research published Monday in Nature, seals and robots are offering a more complete picture of what went down in both in 2017 as well as 2016, when a smaller polynya appeared in the same region of the Lazarev sea.

The earlier study showed that storms played an important role in the 2017 polynya, the likes of which had not been seen in this region since the 1970s. But as Ethan Campbell, a PhD student at the University of Washington and lead author of the new study explained to Earther, strong storms spin up in the Southern Ocean all the time, and it isn’t every day they trigger ice holes over twice the size of Rhode Island.

An elephant seal chilling out with a CTD sensor on its head.
Photo: M. Hindell (IMAS, Australia

“There are always storms in this area, it’s one of the most stormy areas in the world,” Campbell said. “If that was the only reason a polynya would occur, you’d see them every day.”

Some researchers suspected there was more to this story, and that answers could be found in the ocean. Fortunately, there was plenty of ocean data for Campbell and his colleagues to draw on. Since 2004 scientists have been affixing so-called CTD sensors onto the heads of elephant seals to collect oceanographic data. The sensors work for several months or up to a year, falling off when the seals haul ashore for their annual molt (if not sooner). Complementing the seals is data from thousands of robotic floats that have been bobbing about Earth’s oceans about 20 years under the banner of the Argo program.

And there was one other source of ocean intel at the researchers’ disposal. By luck, two newer, souped-up Argo floats that were recently deployed as part of the Southern Ocean Carbon and Climate Observations Modeling (SOCCOM) program happened to surface inside the polynya in 2017, allowing direct measurements of the ocean conditions at the time. Combining all of this, along with information from boat expeditions and satellites, the researchers were able to show that the waters where the polynya appeared were definitely behaving oddly in recent years.

As Campbell explained, when the Southern Ocean is covered in ice in the wintertime, the typical situation underneath it is one of cool, fresh water floating atop warmer, denser, saltier water. That cool surface water forms a sort of protective barrier that helps keeps the ice frozen. But preceding the the winters of 2016 and 2017, their data show a vigorous upwelling of warmer, deeper water toward the surface, weakening the barrier. As powerful storms blew over, the researchers think that even more of that warm, salty water was stirred to the surface, where it was able to melt the overlying ice and create large, self-sustaining polynyas.

Photo: Dan Costa (University of Santa Cruz, USA

“What we see in 2016 and 2017 is this barrier was even weaker [than usual],” Campbell said. “The ocean was literally poised to overturn, to mix vertically in this feedback of circulation. That’s the reason these large storms had a significant impact.”

As for the reason the ocean was more mixed up than usual, the new study points to a large scale climate pattern, known as the Southern Annual Mode (SAM). When the SAM is in a positive phase, the belt of winds that whip around Antarctica (known as the westerlies) contract closer to the continent, causing more ocean mixing and weakening its natural stratification. And it just to happens that the SAM veered sharply positive in 2015, right before the polynyas began to emerge.

Fabien Roquet, an oceanographer at the University of Gothenburg who maintains the database of oceanographic information collected by elephant seals and other marine mammals (called MEOP) that the study used for its baseline, called it a “very interesting paper.” He was especially impressed by the fact that the researchers managed to get data from several floats that emerged directly inside the polynya while it was open.

He was, of course, also happy that seals got to play a supporting role. In the 15-odd years elephant seals have been tagged for research, Roquet said, they’ve become a key source of data in the oceans south of 60 degrees latitude, and have been particularly helpful in collecting observations around the ice shelves that buttress Antarctica’s coast, which they can access in the wintertime when even robotic floats are unable to.

That data is constantly helping researchers build a better understanding of how the oceans work and how they’re changing, but as the new study shows it can also be useful in sleuthing out the occasional mystery.

“When you put a tag on a seal you don’t know where it’s gonna go and what kind of data you’re gonna get,” Roquet said. “I think in this area [the Southern Ocean] it’s becoming extremely common to have our data used to complete the picture.”

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