Sunday , April 11 2021

A new study reveals that atmospheric rivers are increasing snow mass in western Antarctica



PICTURE

PICTURE: Thwaites Glacier in 2019. Credits: view of the Kiya Riverman more

Credit: Kiya Riverman

A new study published today in the journal Letters on geophysical research used NASA’s laser ice-measuring satellite to identify atmospheric river storms as a key driver of increased snowfall in West Antarctica during the 2019 Australian winter.

These findings by scientists at the Scripps Institute of Oceanography at the University of California, San Diego, and colleagues will help improve the overall understanding of the processes driving change in Antarctica and lead to better predictions of sea level rise. The study was funded by NASA, with the additional support of the Rhodium Group’s Climate Impact Laboratory, a consortium of leading research institutions examining climate change risks.

Atmospheric rivers are phenomena that carry large amounts of water vapor in long, narrow “rivers” in the sky. They are known to be the main driver of rainfall along the west coast of the United States, making up 25-50 percent of annual rainfall in key parts of the West. Increasing research on atmospheric rivers reveals that they dominantly affect the western shores of most continents, due to the evaporation of oceans and storms that build high levels of humidity into the atmosphere.

NASA’s Ice, Cloud and Land Elevation Satellite (ICESat-2), launched into orbit in September 2018, provides a detailed view of ice and snow depths on a vast, frozen continent. The satellite works by sending 10,000 laser pulses per second to the Earth’s surface that measure the height of ice sheets, glaciers and even more by calculating the time it takes for a handful of these pulses to return to the satellite. Each light photon has a timestamp and these markers are combined with the GPS location to pinpoint its location and height on the ground. Every three months it measures a detailed series of trails above the Antarctic ice sheet.

“ICESat-2 is the first satellite that could measure snow over the Antarctic continent in such a precise way,” said Helen Amanda Fricker, a glaciologist at Scripps Oceanography and co-author of the study. “In winter, weather conditions prohibit a field team there from conducting field observations. ICESat-2 fills this gap in data on vast ice sheets and gives us a greater understanding of snow mass gains and losses on a seasonal scale.”

Looking at ICESat-2 data, scientists discovered an increase in altitude above the Antarctic ice sheet between April 2019 and June 2020 due to increased snowfall. Using a budget model of the atmosphere and snow, they found that 41 percent of altitude increased over western Antarctica during the winter of 2019 because occasional extreme rain events in short periods of time brought large amounts of snow. Of those events, 63 percent were identified as atmospheric rivers descending. These systems differed from other storms by significantly higher humidity levels measured in the lower parts of the atmosphere.

Atmospheric rivers that land in Antarctica originate from the subtropical, middle latitudes of the southern hemisphere. They travel long distances without continents to stop them, eventually landing in West Antarctica.

“We know that an increase in the frequency of atmospheric rivers is expected, so it’s important that scientists can measure how much they contribute to increasing snow mass or melting the surface,” said Susheel Adusumilli, lead author and PhD candidate at Scripps Oceanography. “Knowing how much snow is accumulating across the continent helps us better understand how mass is changing as a whole and informs our understanding of the potential for sea level rise from the Antarctic ice sheet.”

More than one hundred gigatons of ice are lost from the ocean from Antarctica every year, which contributes to the continuous rise in sea level. Most of this ice loss is caused by the increased flow of ice into the ocean due to the melting of the floating ice shelves that surround Antarctica. Understanding the balance of mass gains from snowfall in the interior of Antarctica and mass losses due to ocean warming is key to improving sea level rise projections.

Although this study followed ice mass in the short term, atmospheric rivers in Antarctica can also trigger large amounts of snowmelt. In fact, this study found that about 90 percent of summer atmospheric rivers and 10 percent of winter atmospheric rivers coincided with potential surface melting above the ice sheet of West Antarctica. Atmospheric melting driven by the river is due to the low clouds of these systems that can absorb and re-emit heat back to the surface. Scientists will need further studies to understand whether these events will create snow or melt, observing factors such as seasonality, humidity levels, cloud cover, or whether each is dependent on a storm.

“In the U.S., scientists are studying atmospheric rivers and investigating whether they could be useful for California’s water supply or dangerous, causing flooding,” said study co-author Meredith Fish, a postdoctoral fellow at Rutgers University and a former oceanography student at Scripps, where he studied at Center for Western Weather and Water Extremes. “What’s interesting about Antarctica is the question, will atmospheric rivers contribute to snowmelt or snow accumulation?”

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