Even as the extent and thickness of sea ice in the Arctic plunges to ever-lower levels, the seasonal sea ice that encircles the frozen continent of Antarctica has stubbornly refused to follow suit. In fact, if anything, the trend is slightly in the other direction. The years 2012 to 2014 saw record highs in Antarctic wintertime sea ice extent, before a return to average levels in 2015. This is despite the fact that water temperatures in the Southern Ocean have been increasing.
Scientists have puzzled over the reason for this; among the theories that have been advanced are that the ozone hole above Antarctica may be resulting in strengthened winds conducive to sea ice formation, or that melting glaciers and ice shelves on the continent are causing a freshening of the sea surface, and thus a higher freezing point (as saltwater freezes at a lower temperature than freshwater).
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A new study may have solved the mystery of Antarctica's resilient sea ice. Its authors argue 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.
Unlike in the Arctic, Antarctic sea ice cover almost completely disappears during summer - its extent in February was, on average, approximately 17 percent of its September peak - and rebuilds anew each fall and winter. By analyzing radar data from NASA's QuikScat satellite to trace the paths of Antarctic sea ice movements and map its different types, Son Nghiem of NASA's Jet Propulsion Laboratory and colleagues found that, early in the growth season, sea ice is pushed offshore by winds, forming a protective shield of older, thicker ice that circulates around the continent.
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These persistent winds, which flow off the continent and are shaped by Antarctica's topography, pile ice up against the ice shield, enhancing its thickness, and create areas of open water nearer to the continent where new ice can form - "ice factories" - in the authors' parlance.
They found that the northern boundary of this "great shield" of ice remains behind a 30-degree F temperature line surrounding Antarctica, which corresponds with the southern front of a boundary between cold and warm waters called the Antarctic Circumpolar Current.
They further found that the path of this current follows the underwater bathymetry: Areas where the seafloor boasts prominent features such as ridges closely align with the path of the current and the shield of ice, but off the coast of West Antarctica, the current spreads out over the deep, smooth seafloor and the extent of sea ice varies considerably from year to year.
The behavior of Antarctic sea ice, the authors concluded, is therefore "not a paradox, as some have suggested" at all. "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.