A new way of studying Antarctica’s weather, not from Earth’s surface but from space, is revealing a phenomenon that could help determine how quickly the enormous ice sheet melts in a warming world.
The study, published on Tuesday in Geophysical Research Letters, focuses on “atmospheric rivers,” huge belts of water vapor that form over tropical and subtropical oceans, then ride the winds that encircle the planet, delivering sometimes copious amounts of rain and snow. One famous atmospheric river, called the Pineapple Express, is responsible for much of the water supply for the West Coast of the United States.
Using data from NASA’s Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) mission, launched into orbit in late 2018, a team of researchers found that atmospheric rivers were a major driver of precipitation, mostly snowfall, across West Antarctica in 2019, helping to replenish mass that the ice sheet is quickly losing. With warming seas expected to send bigger, longer-lasting atmospheric rivers to Antarctica’s shores in the future, the research points to an understudied and little-understood process that could help to slow the ice sheet’s meltdown—or accelerate it, depending on the timing of the storms.
“Just from the first few months of [ICESat-2] data, we found these huge increases in snowfall” that coincided with the presence of atmospheric rivers over the area, says lead study author Susheel Adusumilli, a PhD candidate at the University of California San Diego’s Scripps Institution of Oceanography. “Which was a total surprise.”
Spotting snowstorms from space
Antarctica is losing more than 100 billion tons of ice a year as glaciers flow into the sea and large icebergs calve off their fronts. Just last week an iceberg twice as big as Chicago broke off an Antarctic ice shelf.
Ice losses are speeding up due to the upwelling of warm, deep ocean water melting the frozen continent’s floating ice shelves from below. That allows the glaciers they hold back to flow into the sea more quickly, an effect that likely will be exacerbated by climate change. However, Antarctica also receives billions of tons of snowfall every year. This fresh snow eventually gets buried and compacted into new ice, helping offset ocean-driven losses.
The tug-of-war between ice melt and ice replenishment will determine how fast the Antarctic ice sheet—Earth’s largest—shrinks in a warming world, and how much sea level rise it contributes. But measuring snowfall across Antarctica is notoriously difficult; there aren’t enough weather stations or observers.
Now, researchers are starting to fill in the meteorological gaps using ICESat-2. The polar-orbiting satellite is measuring the height of Earth's ice sheets at unprecedented resolution—down to the width of a pencil—by shooting pulses of laser light at the surface and timing how long it takes individual photons to return to the satellite.
Because the satellite flies the same tracks over Earth’s ice sheets every few months, if the ice height changes in a particular area due to a major snowstorm or a melt event, ICESat-2 will spot it.
“With ICESat-2, which is highly precise, we thought it’d be great to be able to measure these large changes in snowfall that are occurring,” Adusumilli says.
For their new study, Adusumilli and his colleagues looked at some of the earliest data ICESat-2 collected, between April 2019 and June 2020. Within this window, the researchers noticed large increases in the height of the West Antarctic ice sheet between May and October 2019 (during the winter). Using a modeling tool called re-analysis that produces “hindcasts” of past weather, they found that 41 percent of those height increases—which totaled over eight feet in some coastal areas—were due to brief but intense precipitation events.
Of those extreme events, 63 percent could be tied to atmospheric rivers striking the continent, which the researchers distinguished from other storms in their models based on their high moisture content. Unlike the atmospheric rivers that impact the West Coast, which form in the tropics near Hawaii, the sky rivers dumping snow on Antarctica are forming just north of the Southern Ocean, which encircles the continent, according to Meredith Fish, postdoctoral researcher at Rutgers University and co-author of the study.
Only a handful of studies have investigated atmospheric rivers in Antarctica. One 2014 analysis of data from a weather station showed that atmospheric rivers dumped significant quantities of snowfall over East Antarctica in 2009 and 2011, while another study inferred the impact of atmospheric rivers on snow melt in West Antarctica using models. (Atmospheric rivers can melt snow and ice when their precipitation falls as rain, but also because the low clouds associated with them absorb and re-emit heat from Earth’s surface.)
The sheer amount of atmospheric river activity detected in the new study bolsters the case that the weather phenomenon is an important process for Antarctic researchers to study.
“Antarctica is a desert and, like all deserts around the world, is sensitive to extreme precipitation events,” says Jonathan Wille, a postdoctoral researcher at the Grenoble Alpes University in France who led the earlier study on atmospheric river-fueled melting in West Antarctica. “Just like how atmospheric rivers can cause flooding in non-polar deserts, this study demonstrates how these rivers can cause massive quick increases in snowfall accumulation outside normal accumulation patterns.”
Forthcoming research by Wille and his colleagues finds that since 1980, atmospheric rivers have been responsible for a majority of extreme precipitation events over East Antarctica, “driving annual snowfall trends.” On the whole, Wille says the evidence to date suggests atmospheric rivers are “a net positive” for Antarctica, helping the ice sheet gain mass and offset ocean-driven ice losses.
Climate change wildcard
However, that could change. Climate models suggest atmospheric rivers could be bigger and last longer over Antarctica as Earth warms, and the timing of those future storms could dictate their effect on the ice sheet.
While most of the atmospheric rivers spotted in the new study occurred in winter, driving snow accumulation, the authors also detected atmospheric rivers in the summer. Ninety percent of those summer storms coincided with potential surface melt events on the ice sheet, which the authors suspect were fueled by local, cloud-induced heating, not by rain. “Their impacts whether it’s summer or winter are very different,” Fish says.
“What we don’t know is which effect will be more important as atmospheric rivers bring both extra heat and moisture to Antarctica. Will they cause more surface melt and possibly add to the ice shelf hydrofracturing? Or will they bring more extreme snowfall events” that add mass to the ice sheet? To learn more “we need more measurements at high precision,” says Irina Gorodetskaya. She is a scientist at Portugal’s University of Aveiro’s Center for Environmental and Marine Studies, who first made a connection between extreme snowfall and atmospheric rivers in East Antarctica.
That’s why Adusumilli and his colleagues are continuing to analyze ICESat-2 data as it becomes available. In results that are not yet published, they’ve seen “a big atmospheric river influence” on 2020 snowfall similar to 2019, he says. Eventually, the researchers hope to stitch together a high-resolution picture of snowstorms and atmospheric rivers across all of Antarctica, which modelers can use to improve their predictions.
“This new dataset is giving us an amazing way to monitor these atmospheric river events and a clear way to measure snowfall, which is one of the most difficult observables on the ice sheet,” Adusumilli says.