How rivers in the sky melted Antarctic ice
A few years ago, a giant hole opened up in the Antarctic sea ice, capturing attention around the world. Not since the 1970s had such a chasm appeared in the mid-ocean ice of the Weddell Sea.
Scientists showed in previous research that ocean processes and cyclones contributed to the hole, called a polynya. But a recent study has revealed a new piece of the puzzle: atmospheric rivers.
Most polynyas in the Southern Ocean occur along Antarctica’s coast. These temporary ice-free zones are oases for penguins, seals, and other Antarctic wildlife. The Weddell polynya, however, formed much farther from shore.
Though they are just massive holes in the ice, polynyas can affect regional and global climates. Understanding the factors that contribute to their creation—especially of an anomalous open-ocean polynya like the large Weddell polynya—can then lead to more accurate predictions of their behavior in a warming climate, the study says.
In her previous work, lead author Diana Francis, an atmospheric scientist at Khalifa University in the United Arab Emirates, found that cyclones played a role in creating the polynya. However, since these storms are relatively common and don’t always result in such major openings in the ice, she continued to search for another contributor; that’s when she landed on atmospheric rivers.
Atmospheric rivers are long streams in the atmosphere that carry moisture from the tropics toward the north and south poles. They can be hundreds of kilometers wide, thousands of kilometers long, and carry more water vapor than the world’s largest rivers. Francis and her colleagues found that a series of them crossed the Weddell Sea in the days before and after the massive polynya opened in 2017. They carried an exceptional amount of water vapor—itself a potent greenhouse gas—that warmed and weakened the sea ice and helped intensify the cyclones that followed. The atmospheric rivers also brought large amounts of warm snow that likely enhanced the melt, Francis says.
Looking back at historical events, Francis and her team found that atmospheric rivers were also associated with the last big polynya in the Weddell Sea, in 1973–1974, and with another smaller hole in 2016.
Sarah Gille, an atmospheric scientist and physical oceanographer at Scripps Institution of Oceanography at the University of California San Diego who was not involved in the work, calls Francis’s study “transformative.”
“We tend to think the oceans are the real driver of [polynya formation]. The paper suggests a much more complex set of processes may precondition the ocean and allow a polynya to exist,” she says.
Atmospheric conditions may even enhance the oceanic processes involved in polynya formation. The blanket of snow the atmospheric rivers delivered, for instance, may have acted as an insulator, trapping heat from the ocean and magnifying the ice melt from below, explains Ethan Campbell, a graduate student at the University of Washington, who has studied the Weddell polynya.
The rarity of open-ocean polynyas means there isn’t much data to help scientists understand whether they are as important for marine animals as polynyas closer to shore, says Mia Wege, a marine predator ecologist with South Africa’s University of Pretoria.
Marine animals, which only have a certain amount of time to feed and build up their body mass for the breeding season, tend to return to the same foraging areas over and over again, Wege says.
She wouldn’t expect a new polynya to suddenly draw a lot of predators to the area. But if it starts to open up more consistently, marine animals may eventually learn that there’s a new spot to find food—particularly in the more productive spring season, Wege says.
Marilyn Raphael, a geographer at the University of California, Los Angeles, says she’s interested in what further research might reveal about the role atmospheric rivers play in Antarctic sea ice variability more broadly.
Changes in the sea ice can have implications for the global climate, and previous research has shown that climate change is expected to make atmospheric rivers stronger and more common.
“The Antarctic sea ice system is so complex, and there are so many things that influence its growth, its advance, its retreat,” Raphael says. “Any bit of information that will help explain what we’re seeing would be welcomed.”