Tuesday, October 2nd 2012, 7:25 AM EDT
Much has been written about the record low extent of Arctic sea ice this season. In July it stood at 7.94 million sq km – 2.12 million km below the 1979 – 2000 average, and in September it reached a record low of 3.14 million sq km.
When satellite monitoring of the Arctic ice began in 1979 it was clear that the data had caught a decline already in progress. There is some evidence that the decline may have been taking place since the 1950s, which is before the time that anthropogenic influences came into effect.
In the past few years it is possible that this roughly linear decline may have increased, or have reached a point whereby the behavior of the Arctic ice sheet has changed. 2007 was a record low year for ice extent and it may be that the ice melt-freeze cycle has become noisier. Figure 1.
It was with anticipation that many watched the dwindling sea ice from July onwards to see what minimum level would be reached in September. But then something unusual happened – am intense storm started of Alaska engulfing the Arctic region. The August 6th image, Figure 2, taken by the AQUA satellite shows the storm in all its glory. Such Summer storms are rare as polar lows are more usual in the Winter, there have been only eight similar events in the past 34 years. It had an unusually low central pressure of 965 hPa.
As a result of the decadal shrinkage this storm took place over ice that was in a weakened state. It would have had a significant impact on the ice pack. Firstly it would separate ice floes, widening the gaps between them and pushing them towards warmer water – the Laptev and Beaufort Seas, which were unusually warm at the time – that would accelerate their melting. The storm would also fragment the ice, especially in its thinner condition, that would speed-up the melt. It has even been suggested that the storm could turn some floes over exposing their darker, algae stained, bottom, again exacerbating their melting.
The cyclone would also have disrupted the ice through the wave action it induced. This is especially important given that over the past few decades the Arctic has lost much of its thick, long-term ice, which would have dampened the waves. Ice floes can be flooded with saltier water which melts the ice and can sometimes mislead satellite sensors into thinking they are seeing open water.
The storm will also increase mixing of the water column. The ice just below the ice pack is colder that that below it, as well as less salty. Some estimates are that about ten metres of the sea surface layer was churned by the vigorous wave action bringing warmer, saltier water up from the depths. The salty water will wash away the ice floes fresh melt layer preventing refreezing and accelerating its melting.
The effects of the storm, which lasted 5 days as opposed to a more usual 40 hours, was therefore to tear and move the ice as well as washing it with warmer water. All of these things would have affected the ice pack melting this season.
The effect can be seen on two satellite- based maps of the sea ice extent and thickness taken before and after the storm on August 3rd and 10th respectively. Figs 3 and 4. Darker colours represent thicker ice.
It might be that the storm’s effect can be seen by the change it caused to the rate of ice melt. Figure 5 shows the rate of melting increased during the storm which sends the melt trajectory on a lower path.
My conclusion is that 2012 was going to be among the lowest Arctic ice extent years but not exceptionally so being similar to the past few years. The storm disrupted the ice and caused extra melting pushing it down to a new record. It is rather like an Olympic sprinting record being set with the benefit of a strong following wind – it’s a fast time but it record status needs qualification. It is a pity that this important fact about this years enhanced Arctic melt was ignored when the alarmist headlines came to be written.