Study helps explain sea ice differences at Earth's poles
Why has the sea ice cover surrounding Antarctica been increasing slightly, in sharp contrast to the drastic loss of sea ice occurring in the Arctic Ocean? A new NASA-led study finds the geology of Antarctica and the Southern Ocean are responsible.
"Our study provides strong evidence that the behavior of Antarctic sea ice is entirely consistent with the geophysical characteristics found in the southern polar region, which differ sharply from those present in the Arctic," said Nghiem.
Antarctic sea ice cover is dominated by first-year (seasonal) sea ice. Each year, the sea ice reaches its maximum extent around the frozen continent in September and retreats to about 17 percent of that extent in February. Since the late 1970s, its extent has been relatively stable, increasing just slightly; however, regional differences are observed.
Over the years, scientists have floated various hypotheses to explain the behavior of Antarctic sea ice, particularly in light of observed global temperature increases. Are changes in the ozone hole involved? Could fresh meltwater from Antarctic ice shelves be making the ocean surface less salty and more conducive to ice formation, since salt inhibits freezing? Are increases in the strength of Antarctic winds causing the ice to thicken? Something is protecting Antarctic sea ice, but a definitive answer has remained elusive.
Their analyses revealed that as sea ice forms and builds up early in the sea ice growth season, it gets pushed offshore and northward by winds, forming a protective shield of older, thicker ice that circulates around the continent. The persistent winds, which flow downslope off the continent and are shaped by Antarctica's topography, pile ice up against the massive ice shield, enhancing its thickness. This band of ice, which varies in width from roughly 62 to 620 miles (100 to 1,000 kilometers), encapsulates and protects younger, thinner ice in the ice pack behind it from being reduced by winds and waves.
The team also used QuikScat radar data to classify the different types of Antarctic sea ice. Older, thicker sea ice returns a stronger radar signal than younger, thinner ice does. They found the sea ice within the protective shield was older and rougher (due to longer exposure to wind and waves), and thicker (due to more ice growth and snow accumulation). As the sea ice cover expands and ice drifts away from the continent, areas of open water form behind it on the sea surface, creating "ice factories" conducive to rapid sea ice growth.
To address the question of how the Southern Ocean maintains this great sea ice shield, the team combined sea surface temperature data from multiple satellites with a recently available bathymetric chart of the depth of the world's oceans. Sea surface temperature data reveal that at the peak of ice growth season, the boundary of the ice shield remains behind a 30-degree Fahrenheit (-1 degree Celsius) temperature line surrounding Antarctica. This temperature line corresponds with the southern Antarctic Circumpolar Current front, a boundary that separates the circulation of cold and warm waters around Antarctica. The team theorized that the location of this front follows the underwater bathymetry.
Study results are published in the journal Remote Sensing of Environment. Other participating institutions include the Joint Institute for Regional Earth System Science and Engineering at UCLA; the Applied Physics Laboratory at the University of Washington in Seattle; and the U.S. National/Naval Ice Center, NOAA Satellite Operations Facility in Suitland, Maryland. Additional funding was provided by the National Science Foundation.
NASA uses the vantage point of space to increase our understanding of our home planet, improve lives and safeguard our future. NASA develops new ways to observe and study Earth's interconnected natural systems with long-term data records. The agency freely shares this unique knowledge and works with institutions around the world to gain new insights into how our planet is changing.
Source: NASA [May 21, 2016]