A new way to study the weather of Antarctica, not from Earth’s surface, but from space, reveals a phenomenon that could help determine how fast the enormous ice is melting in a warming world.
The study, published in Geophysical Research Letters on Tuesday, focuses on ‘atmospheric rivers’, large belts of water vapor that form over tropical and subtropical oceans, and then rides on the winds that surround the planet, which sometimes produces a lot of rain and snow. One famous atmospheric river, called the Pineapple Express, is responsible for most of the water supply for the West Coast of the United States.
Using data from the NASA mission Ice, Cloud and Land Elevation Satellite-2 (ICESat-2), which launched into orbit in late 2018, a team of researchers found that atmospheric rivers were a major driver for precipitation, mostly snowfall, over West Antarctica in 2019, which helps replenish the mass that the ice sheet is rapidly losing. As warming seas will send larger, long-lasting atmospheric rivers to the shores of Antarctica in the future, research points to an underestimated and incomprehensible process that could help slow down – or accelerate, the breakdown of ice, depending on the timing of the storms.
‘Only from the first few months of [ICESat-2] data, we found that this large increase in snowfall “coincided with atmospheric rivers in the area,” said lead author Susheel Adusumilli, a PhD candidate at the University of California, San Diego’s Scripps Institution of Oceanography. was a total surprise. ‘
Detect snowstorms from space
Antarctica loses more than 100 billion tons of ice a year as glaciers flow into the ocean and large icebergs calve from their front. Just last week, an iceberg twice the size of Chicago tore down an Antarctic ice shelf.
Ice losses accelerate due to the surge of warm, deep ocean water that melts the frozen ice shelves of the frozen continent from below. This allows the glaciers they hold back to flow faster into the ocean, which is likely to be exacerbated by climate change. However, Antarctica also receives billions of tons of snowfall annually. This fresh snow is eventually buried and compacted into new ice, which helps offset ocean-driven losses.
The tug of war between ice melting and ice replenishment will determine how fast the Antarctic ice sheet – the largest earth – shrinks in a warming world, and how much sea level rise it contributes. But measuring snowfall over Antarctica is quite difficult; there are not enough weather stations or observers.
Now researchers are beginning to fill in the meteorological gaps using ICESat-2. The polar satellite measures the height of the Earth’s ice sheets with an unprecedented resolution – up to the width of a pencil – by shooting pulses of laser light to the surface and determining how long it takes individual photons to return to the satellite. to stop.
Because the satellite flies the same tracks over the Earth’s ice sheets every few months, ICESat-2 will see it when the ice height changes in a particular area due to a major snowstorm or a melting event.
“With ICESat-2, which is very accurate, we thought it would be great to be able to measure these major changes in snowfall,” says Adusumilli.
For their new study, Adusumilli and his colleagues looked at the earliest data collected by ICESat-2, between April 2019 and June 2020. Within this window, the researchers noted that the height of the West Antarctic ice sheet rises sharply between May and October. was 2019 (during the winter). Using a modeling tool called re-analysis that has caused ‘obstacles’ to the weather in the past, they found that 41 percent of the elevation increases – which in some coastal areas were more than eight feet – were due to short but intense precipitation.
Of the extreme events, 63 percent can be linked to atmospheric rivers hitting the continent, which distinguishes the researchers in their models from other storms due to their high moisture content. Unlike the atmospheric rivers that affect the West Coast, which form in the tropics near Hawaii, the air rivers that snow on Antarctica form just north of the Southern Ocean, which surrounds the continent, according to Meredith Fish, postdoctoral researcher at Rutgers University and co. author of the study.
Only a handful of studies have examined atmospheric rivers in Antarctica. One 2014 analysis of weather station data showed that atmospheric rivers dumped significant snowfall over East Antarctica in 2009 and 2011, while another study concluded that the impact of atmospheric rivers on snow was used in West Antarctica. by using models. (Atmospheric rivers can melt snow and ice when they fall as rain, but also because the low clouds that accompany it absorb heat from the earth’s surface and radiate again.)
The large amount of atmospheric river activity detected in the new study reinforces the case that the weather phenomenon is an important process to study for Antarctic researchers.
“Antarctica is a desert and, like all deserts around the world, is sensitive to extreme precipitation events,” said Jonathan Wille, a postdoctoral researcher at Grenoble Alpes University in France, who led the earlier study on the melting of the river. in West Antarctica. . “Just as atmospheric rivers can cause flooding in non-polar deserts, this study demonstrates how these rivers can cause rapid increases in avalanches outside normal accumulation patterns.”
Future research by Wille and his colleagues finds that atmospheric rivers have been responsible for a majority of extreme precipitation areas in East Antarctica since 1980, causing the annual snowfall trends. Overall, Wille says the evidence so far suggests that atmospheric rivers are a “net positive” for Antarctica, helping to gain ice sheet mass and compensate for ocean-driven ice losses.
Climate change wildcard
However, this may change. Climate models suggest that atmospheric rivers may be larger and take longer over Antarctica as the earth warms, and the timing of future storms may determine its effect on the ice sheet.
While most of the atmospheric rivers observed in the new study occurred in winter, accumulating snow, the authors also detected atmospheric rivers in summer. Ninety percent of the summer storms coincided with possible surface melting events on the ice sheet, which according to the authors are fueled by local, cloud-induced heating, not by rain. “Its impact, whether it’s summer or winter, is very different,” Fish says.
‘What we do not know is what effect will be more important, as atmospheric rivers bring extra heat and moisture to Antarctica. Will it cause more surface melt and add the hydrofracture to the ice shelf? Or will they cause extreme snowfall events ”that add to the ice sheet mass? To learn more, “we need more high-precision measurements,” says Irina Gorodetskaya. She is a scientist at the Portuguese University of Aveiro’s Center for Environmental and Marine Studies, which first made a connection between extreme snowfall and atmospheric rivers in East Antarctica.
Therefore, Adusumilli and its colleagues continue to analyze ICESat-2 data as it becomes available. In results that have not yet been published, they have seen a major atmospheric river impact on snowfall 2020, similar to 2019, he says. Eventually, the researchers hope to compile a high-resolution picture of snowstorms and atmospheric rivers throughout Antarctica, which modelers can use to improve their predictions.
“This new data set gives us a great way to monitor these atmospheric river events and a clear way to measure snowfall, which is one of the most difficult to observe on the ice sheet,” says Adusumilli.