Study Helps Explain Sea Ice Differences at Earth’s Poles

antarcticWhy 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? Since the late 1970s, its extent has been relatively stable, increasing just slightly; however, regional differences are observed. A NASA/NOAA/university team found that two persistent geological factors — the topography of Antarctica and the depth of the ocean surrounding it — are influencing winds and ocean currents, respectively, to drive the formation and evolution of Antarctica’s sea ice cover and help sustain it.

“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.

To tackle this cryospheric conundrum, Nghiem and his team adopted a novel approach. They analyzed radar data from NASA’s QuikScat satellite from 1999 to 2009 to trace the paths of Antarctic sea ice movements and map its different types. They focused on the 2008 growth season, a year of exceptional seasonal variability in Antarctic sea ice coverage.

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.

To tackle this cryospheric conundrum, Nghiem and his team adopted a novel approach. They analyzed radar data from NASA’s QuikScat satellite from 1999 to 2009 to trace the paths of Antarctic sea ice movements and map its different types. They focused on the 2008 growth season, a year of exceptional seasonal variability in Antarctic sea ice coverage.

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.

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