Record minimum for arctic sea extent in September 2013
As of 13 September 2013, the annual minimum extent of arctic sea ice has not reached the record minimum values of 19 September 2012. This year, the arctic ice pack still covered, at its minimum point, 5.1 million km² whereas in 2012 it covered 3.4 million km² only. Since 2007 which initiated a series of years with particularly low ice extent, 2013 is, like 2009, the year with the greatest extent of ice pack in September. Therefore, although the average decreasing trend of September sea ice extent since 1979 continues to be confirmed, this trend has for the first time corrected slightly: estimated at 14% (of the mean over the 1981-2000 period) in 2012, it is only 13.7% in 2013. However, what one could assume to be a reconstitution of the summer arctic sea ice pack may not be a persistent feature. The extent of the arctic ice pack, since 2007, has never returned to the levels observed in the period prior to 2007, remaining below the lowest value of that period (5.6 million km² recorded in 2005).
Minimum summer Arctic sea ice extension on the 13th November 2013 - © Polar View - Bremen University
Although characterized by similar overall extents, the late summer ice covers in 2009 and 2013 were actually relatively contrasted. In September 2009, the ice had retreated more markedly in the western Arctic, freeing in particular the east of the Chukchi Plateau and the south-western Canada Basin. By contrast, in 2013, the ice was closer to the climatological mean in these regions, while the retreat of the ice pack in the eastern Arctic was very marked, reaching, or even exceeding that of September 2012. The ice extent along the east coast of Greenland was also particularly low in 2013, and the Northern Sea Route along the Eurasian (Barents, Kara and Laptev) seas remained largely ice-free during the summer. The comparatively extensive ice cover in the western Arctic this summer was associated with the presence of an unusually compact ice pack in these regions. In parallel, a relatively compact ice pack was also preserved in the northern East Siberian Sea. These two anomalies contributed much to the significant difference in ice extent between 2012 and 2013.
The case of 2013, more than any other year, highlights the possibility of very marked contrasts in the ice cover from one year to another. Likewise, the difference in September extent between the two consecutive years 2012 and 2013 (1.72 million km²) is, along with that recorded between 1995 and 1996 (1.75 million km²) a record in the year-to-year variations of the late summer minimum extent since the beginning of the satellite observation era. However, while the contrast between 1996 and 1995 can be attributed to the exceptional nature of 1996, which saw an ice pack covering the whole of the central Arctic Ocean, including the marginal seas (apart from the Chukchi Sea), the contrast between 2013 and 2012 is more certainly a result of the exceptional nature of summer 2012.
Compared sea ice concentration annual variation since 2007 - © Polar View - Bremen University
2012 was a particularly warm year, but that is probably not the only reason for the exceptional ice retreat. The latter was surely also promoted by preconditioning of the ice cover, which has become increasingly thin and labile in recent decades. Recent studies suggest that these year-to-year contrasts in the ice area minima remain however difficult to predict. Compared to 2012, summer 2013 was cold, with certain regions experiencing temperatures even lower than the climatological mean. The presence of persistent atmospheric low pressures during the summer in the central Arctic may have reinforced the effect of the relatively low atmospheric temperatures by favouring wind driven spreading of the ice pack.
September sea ice extent minima appear to be closely linked to prevalent wind conditions in the Arctic during the summer. These wind conditions can in particular promote export of sea ice to the North Atlantic through the Fram Strait (the passage between the Svalbard archipelago and Greenland), or lead to anticyclonic wind anomalies in the central Arctic which can result in ice pack shrinking. The increased intensity or frequency of these wind regimes observed in recent years could also explain the general decreasing trend of the September minimum ice extent over these years. The most remarkable September minima also seem to be associated with an increase in the transport of humidity towards the Arctic, enhancing the greenhouse effect and the surface warming, or to an increased transport of warm air masses from the lower latitudes. The low atmospheric pressures which dominated the Arctic during the summer 2013 may have acted as a barrier to such heat transfers. It should however be noted that all these mechanisms need to be further assessed, in particular their robustness which cannot be established while the time series of observations remains limited.
Marie-Noëlle Houssais, September 2013
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