Thursday, March 7th 2013, 7:37 AM EST
Last year’s record low of Arctic sea ice extent has been discussed widely. Some regard it as the latest example of decadally declining Arctic sea ice exceeding the record low of 2007 and still heading downward.
Arctic sea ice is usually at its greatest extent in March so it is only now that we can see the 2012 freeze-melt cycle completely. Looking at Fig 1 the profile of 2012’s cycle is unusual. The downward trend in August was suddenly doubled making it very different from the melting seen in previous years.
This significant change of slope is the result of what is now being called the “Great Arctic Cyclone of 2012.” A vast area of low pressure arose in Siberia on August 2nd. It crossed the Arctic Ocean and went onto Canada. By August 6th the storm’s pressure was 966 mb, the lowest ever recorded for an Arctic storm. (It is difficult to get data from Arctic storms. Surface pressure is measured only on a few Arctic islands, on a few land based stations around the Arctic Ocean and on buoys.)
The storm brought unusually warm waters to the East Siberian and Laptev seas and to the edge of the ice pack. It also pulled warmer air into the Arctic. It broke the ice pack into smaller pieces that are easier to melt. It also pushed the ice together making the pack smaller.
The conditions before the storm did not appear to be as good at promoting sea ice loss as they were in 2007. Weather before the storm was near average. In contrast, the summer of 2007 – the previous record sea ice low – had warm southerly winds along the shores of the East Siberian and Chukchi seas, favouring strong ice melting in these regions pushing ice away from the coast and leaving open water. The average pace of ice loss since late June had been just over 100,000 square kilometres (38,000 square miles) per day. However, this pace almost doubled for a few days in early August. On three consecutive days (August 7th , 8th , and 9th ), sea ice extent dropped by nearly 200,000 square kilometres (77,220 square miles).
As the storm began to wane by August 13th , sea ice extent was already among the four lowest summer minimum extents in the satellite record, with about five weeks remaining in the melt season. The Arctic sea ice extent was 5.09 million square kilometres (1.97 million square miles). This is 2.69 million square kilometres (1.04 million square miles) below the 1979 to 2000 average extent for that date, and is 483,000 square kilometres (186,000 square miles) below the 2007 figure.
Some believe that the August storm did not make much difference to the eventual sea ice melt. Scientists at the US National Snow and Ice Data Center say that the thinner Arctic ice cover in the summer of 2012 made the ice more vulnerable to the weather and that it would have melted anyway, and still produced a record low.
Using a computer model the researchers state that the storm would have had a big effect for about two weeks and a declining one thereafter.
But looking at Fig 1 suggests, to me at least, that the storm effect was somewhat longer. The shape of the 2012 melt is very different from previous years. In early August the declining curve is identical to several previous years indicating that it would probably be a year similar to 2011. The impact of the storm is obvious, the melt rate doubles. The 2012 curve does not return to those of previous years until mid-October.
2012 was an unusual year for Arctic sea ice melt with a two-month extra decrease induced by the storm. Therefore one should be careful about placing it into context as a record-breaking continuation of the general decline. The record low of 2012 was a result of a special circumstances.
This link from NASA may also explain more:
NASA take time out to explain the Arctic icepack break up in August