Regeneration record for winter Arctic sea ice
June 2013: the extent of Arctic sea ice reached its annual high on 15 March last and since then has begun a slow decline towards its end-of-summer low. Ever since it has been possible to assess them in a reliable and systematic manner from space, these two extremes of the annual cycle of the extent of the sea ice have shown a continued decline. However, the end-of-winter high shows a much slower declining trend than the end-of-summer low, as the recent change for the 2012-2013 period clearly shows, when placed in the historical context of the last decade
Maximum winter Arctic sea ice extent in March 2008 (left), and in march 2013 (right) - © NSICD
Following a very similar trend to that observed following the minimum extent of September 2007, which was then a record for the satellite observation period, the Arctic sea ice, having broken an absolute record for minimum extent on 16 September 2012, seems to have recovered to maintain an end-of-winter extent close to that of the previous years, in the order of 15 million km2. As in March 2008, this seasonal recovery of the ice cover shows the ice’s ability to absorb disturbances by adjusting its freezing rate: the thinner the ice cover, the faster its growth, and thus ice formation is more effective when the ocean is free of ice. The record seasonal increase in the extent of sea ice between the summer minimum and winter maximum was indeed reached in March 2013, with growth of 11.7 million km2, which represented a slight increase in the growth of 11 million km2 recorded between September 2007 and March 2008.
This form of seasonal recovery of the ice cover partly explains why the declining trend of the extent of end-of-winter Arctic sea ice does not exceed 2.5% (of the average extent over the 1979-2000 reference period), while the trend for the end-of-summer minimum is now over five times greater (13%). This reduced but nonetheless continued decline in the Arctic winter cover explains why March 2013 ranks in 5th position among the least icy winters in the satellite observation period. In addition to its low extent, the Arctic ice cover is showing increasingly clear signs of structural changes. For the first time since 2008, the North Pole was covered with first-year ice at the end of winter 2013. At the same time, the surface area of perennial ice (i.e. ice which has survived at least one melt season) continues to shrink and only accounted for 30% of the ice cover in March 2013 compared to 60% in the 1980s. Furthermore, initial estimates of the sea ice's thickness from the CryoSat-2 satellite’s altimetry observations confirm the gradual thinning of the ice, which had already been suggested by field measurements over the past decades. This thinning affects the thickest ice, which is located to the north of Greenland and considered to be the reservoir of the Arctic marine cryosphere.
Compared sea ice concentration in march 2008 and march 2013 - © NSICD
Fluctuations in the winter ice cover mainly reflect variations in extent of the sea ice margins in the coastal seas or in the subpolar seas of the Pacific and North Atlantic. These variations remain modest as they are heavily restricted by the geometry of the Arctic Basin, enclosed between Eurasian, Greenland and North American territories, or by the presence of oceanic fronts in subpolar regions. The decline of the winter ice cover in recent years has been associated with ice edge retreat in the Atlantic area east of Greenland (Greenland and Barents Seas) and advance in the Bering Sea area. The gradual retreat of winter ice in the Barents Sea area could thus be linked to more heat transported to the area by the Atlantic waters.
Although year to year fluctuations in the winter sea ice cover remain much (almost five times) smaller than those of the summer ice, the spatial distribution of the ice edge shows clear differences from one year to the next. In certain winters like 2010 and 2013, the sea ice edge altogether retreated much to the west of Greenland (Labrador Sea) and very little, or even advanced, to the east of Greenland while in other years, such as 2008 and 2012, the opposite occured. These variations are linked with variations in atmospheric circulation represented by the Arctic Oscillation Index. Thus winter 2013, like winter 2010, was characterized by a persistent negative Arctic Oscillation Index, while 2008 and 2012 resulted in a positive Index. When the index is negative, the wide gyre of the Beaufort Sea is strengthened and the old, thick ice from the Canadian sector of the Arctic Ocean tends to be caught in the gyre which results in maintaining the ice cover. The impact of the index’s polarity on the future ice cover is nonetheless complex. Based on the position of centres of atmospheric pressure, the Arctic Oscillation in its negative phase can lead either to the reduction of the export of ice towards the Atlantic sector through the Fram Strait to the benefit of the Chukchi and Bering Seas, or to its increase. Furthermore, in February 2013 strong easterly winds linked with the negative Arctic Oscillation Index hit the coasts of northern Canada and Alaska, heavily fracturing the ice in the sector and doubtless making it more vulnerable to transport by winds and currents.
Winter sea ice cover decreasing tendency since 1979 - © NSICD
Finally, it must be noted that it is difficult to assess the overall influence of the atmosphere on the ice as the ice, in turn, exerts an influence on the atmosphere. A negative Arctic Oscillation index occurs when there is less difference in atmospheric pressure between the Arctic and the temperate latitudes which encourage the arrival of warm air over the Arctic. Consistently, 2013 was a remarkably warm winter in the central Arctic. Furthermore, it seems that a thinner ice cover at the end of summer, by encouraging the warming of the lower layers of the atmosphere overlying the ice-free ocean, can cause an atmospheric shift towards a state of negative Arctic Oscillation. We can therefore discern a complex link between some atmospheric regimes and Arctic sea ice which we are still far from understanding and which nonetheless constitutes one of the essential links in the internal variability of the Arctic climate.
Marie-Noëlle Houssais, June 2013
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