December 6, 2013
The Health Of Antarctic Glaciers Relies On Salt Intake: NASA
April Flowers for redOrbit.com - Your Universe Online
The discovery, published in Nature Communications, involves cold, extra salty water—brine—that forms within openings in sea ice. The findings will add to our understanding of how ice sheets interact with the ocean, and may improve our ability to forecast and prepare for future sea level rise.
“I was curious why Totten was changing so fast when the glacier just next to it wasn’t changing much,” said Ala Khazender of NASA's Jet Propulsion Laboratory (JPL).
Khazender and his colleagues developed a hypothesis that reductions in the volume of brine would increase Totten's thinning and melting by combining satellite observations with ocean numerical modeling. The hypothesis was supported by further research.
Although scientists are still puzzling out the mechanisms behind the regional variations being observed, most attribute ice loss in Antarctica to the well-documented rise in temperature of the surrounding ocean. The study findings highlight the key role of processes occurring on small geological scales in determining how global climate change can affect the stability of ice sheets.
The team used observations from NASA's ICESat-1, which measures how much ice surfaces are rising or falling over time. This data revealed that Totten Glacier, which currently discharges enough ice into the surrounding ocean to fill Lake Erie in just over a week, was thinning rapidly. Meanwhile, the nearby Moscow University Glacier and its floating ice shelf were showing very little change. The researchers wanted to understand the difference.
“We were convinced that the answer must be in the ocean,” Khazendar said.
The ocean water around Antarctica is warmer than both the continent's icy surface and the polar air. Contact with the warmer ocean water below them causes ice shelves, the floating front edges of glaciers that extend tens to hundreds of miles offshore, to melt more than sunlight does. Although the melting bottoms of ice shelves are a part of Antarctica's natural water cycle, when glaciers start to melt unusually quickly, it is a sign that something is off balance.
Khazander collaborated with scientists from JPL; UCLA; the University of California, Irvine; and Utrecht University in the Netherlands to combine ICESat remote sensing observations from 2003 to 2008 using ocean numerical computer models to gain insights into the interaction between the ice shelves and their ocean basin.
Polynyas (poe-LEEN-yahs), are large, annually recurring openings in the winter sea ice cover that are found in the ocean basin around Antarctica. The sizes and numbers of polynyas vary markedly from winter to winter, though there is no overall trend in this region. According to computer simulations, these year-to-year variations in the polynyas greatly affected the glacier’s melting rate.
Large quantities of sea ice form in polynyas, only to be swept away by the winds that formed the openings in the first place. Freezing seawater expels its salts, which produces a layer of very dense, briny water at the freezing temperature. Polynyas form from this cold, dense brine then sink to the seafloor, where they can flow into the cavities under the ice shelves, just as warmer ocean water could.
The scientific team theorized that when the cold brine pooled beneath Totten Ice Shelf, it mixed with the water there and lowered its temperature. This slowed the glacier's melt rate. If their theory is correct, a reduction in cold brine would mean the glacier's winter melt rate would increase.
Passive microwave measurements from the Defense Meteorological Satellite Program were examined by the team, revealing that in the latter part of the study period, the extent of polynyas (and therefore the production of cold brine) decreased significantly. A comparison of ICESat observations showed that at the same time, the thinning of Totten Glacier increased. This supported the hypothesis of the research team.
Khazender points out that if there are more winters with reduced polynya extents, the cavity under Totten can fill with warmer ocean water rather than cold brine. “If that happens, the glacier’s flow could be significantly destabilized, causing it to discharge even more ice into the ocean,” he said.
”With the widespread changes seen in Antarctic sea ice conditions over the last few years, this process could be affecting other glaciers around Antarctica and the volume of ice they discharge into the ocean,” he added.