Arctic sea ice starting melting quickly in late April
By Summit Voice
SUMMIT COUNTY — After staying near average levels during much of April, the Arctic sea ice extent started a rapid decline late in the month, marked by the meltdown of freshly formed thin ice that can’t persist from year to year.
The linear rate of decline for April ice extent over the satellite record is 2.6 percent per decade.
For the month, the ice extent averaged 5.69 million square miles. Because of the very slow rate of ice loss through the last half of March and the first three weeks of April, ice extent averaged for April ranked close to average out of 34 years of satellite data, according to the latest update from the National Snow and Ice Data Center.
That was the highest average ice extent for April since 2001, only 104,000 square miles below the 1979 to 2000 average extent. The ice cover remained unusually extensive in the Bering Sea, continuing a pattern that persisted for much of the winter. Ice extent was also slightly higher than average in Baffin Bay and part of the Sea of Okhotsk. As in recent winters, ice extent was well below normal in the Barents Sea, compensating for the extensive ice in the Bering Sea.
April air temperatures over most of the Arctic were higher than usual, particularly over the central Arctic Ocean. Over the Bering Sea and parts of the East Greenland and Norwegian seas, temperatures ranged from average to slightly below average.
From the NSIDC update:
“While ice conditions approached the 1979 to 2000 average levels for this time of year, the high ice extent will have little influence on how much ice melts this summer. Much of the ice cover is recently formed thin ice that will melt out quickly. Research has shown that sea ice extent in spring does not tell us much about ice extent the following summer. More important to the summer melt is the thickness of the ice cover, and summer weather.”
At the other end of the Earth, Antarctic sea ice has been mostly above average from December through March, with a slower-than-average decline in November and the highest extent in the Ross Sea and Weddell Sea.
The main reason for the high sea ice extent probably was caused at least in part by unusually strong winds that circled the continent of Antarctica during most the southern summer.
From the bulletin:
These circumpolar winds tend to push the ice out from the continent, increasing the extent of the ice, although not necessarily the volume. Air temperatures in December and January were close to average over most of the sea ice-covered water. Researchers approximate the circumpolar wind intensity by an index called the Southern Annular Mode.
A positive value for SAM indicates strong circumpolar winds around the continent; negative values indicate weaker winds. This index was at a record high for the two months of December 2011 and January 2012, at the same period of the higher-than-normal seasonal extents. For more information on Antarctic sea ice, see the NSIDC Icelights article: Sea ice down under: Antarctic sea ice and climate.