3rd record low in the minimum extent of summer sea ice
Contrary to expectations, the minimum annual extent of Arctic sea ice recorded on 10 September 2016 is associated with a record value which places it, with the minimum for 2007, in second place behind the minimum for 2012, which remains unequalled in 37 years of microwave satellite measurements. This annual minimum therefore identifies 2016 as a new record year for its low levels of ice cover. On 10 September last, the extent of the Arctic sea ice reached 4.14 million km2, i.e. only 750,000 km2 more than the minimum record of 2012, but over 2 million km2 less than the average annual minimum calculated over the reference period (1981-2010).
Minimum summer Arctic sea ice extent on the 10 septembre 2016 - © Université de Bremen (Germany)
Large expanses of open water were then visible in the Siberian seas (Laptev and East Siberian Seas) and in the Pacific area of the Arctic (Chukchi and Beaufort Seas). The North West passages (Amundsen Route) and the Northern Route were open. The extent of the oldest multi-year ice (4 years or older) had also reduced: this old, and therefore thick ice, now only accounted for 3% of the summer covering (i.e. 100,000 km2) while it made up one third of it in the mid-1980s.
This minimum record on 10 September 2016 came as a surprise because the continued relatively extensive ice cover at the end of August suggested that the end of the summer would have relatively extensive ice cover. Conditions in July and August were relatively unsuited to the melting of ice: relatively cool atmospheric temperatures (0.5 to 2°C lower than the reference average in the centre of the Arctic Ocean) combined with cloudy conditions and frequent storms as well as persistent low pressure in the centre of the Arctic Ocean (encouraging the sea ice to expand due to the effect of cyclonic winds) helped maintain relatively extensive ice cover during the summer. The month of August 2016 ranks only 4th among the months of August with the least ice cover in the satellite period.
But this was without taking account of the effects of a particularly mild winter 2015-2016 which by March 2016 had resulted in Arctic sea ice which had a very low extent (close to the 2015 record) and in particular was very thin and therefore more vulnerable to atmospheric and oceanic forcings. Although it is recognized that summer atmospheric conditions play a key role in how sea ice changes during the melting season, thus preventing the emergence of a clear link between the maximum extent at the end of the winter and the minimum extent at the end of the following summer, it would appear that the sea ice’s state of preconditioning linked to its thickness at the end of winter could be a possible indicator for how it will change the following summer. Thinner sea ice breaks up more easily in the wind and is also more likely to melt due to heat from a relatively warm ocean, which has a particularly strong influence at the end of summer when the atmosphere is already cooler. We indeed observed an earlier warming of the ocean in the west of the Beaufort Sea and the East Siberian Sea in summer 2016. But we also noted the two major cyclones which crossed the Arctic Ocean between 14 and 23 August 2016, which without doubt led to the break-up of ice cover due to strong waves (5.9 metre-high waves were simulated in the Kara Sea in response to the first of these events) which encouraged a mix with warm ocean waters that caused the ice to melt. All these factors combined led to a record drop in the extent of sea ice in early September 2016. Over 34,000 km2 of ice melted in this way every day, compared to a level of below 20,000 km2 per day in the previous record year of 2012.
Compared Arctic sea ice extent since 2012 - © NSIDC
Despite this exceptional minimum extent on 10 September 2016, we observed that ice formed particularly rapidly from the second half of September, with the result that based on the average ice extent for the month of September, 2016 now ranks 5th among the months of September with the least ice cover in the satellite period. This ice formation was particularly prominent in the centre of the Arctic Ocean and in the Laptev Sea. The average monthly ice cover, however, remained abnormally low around the Beaufort, Chukchi and East Siberian Seas. The rate of multidecadal decline, which measures the decline in Arctic sea ice cover in the satellite period, now stands at 13.3% per decade.
The future of the Arctic sea ice over the coming winter months remains largely uncertain. Following the early and rapid ice formation at the end of September, October saw a slowdown, with particularly weak rates of ice growth, at levels only about a third of the average levels recorded at the same period in the reference years. This unusual situation seems to be linked to shifts in major atmospheric pressure systems. Low pressures in the Bering Sea on the Pacific side, and the presence of high pressure in northern Scandinavia on the Atlantic side, encouraged the arrival of hot air over the Arctic. However, it is highly likely that the particularly warm ocean surface temperatures recorded in the ice-free areas are the reason for October’s abnormally warm atmospheric conditions in the Arctic. The continuation of such influences will be a key factor in the formation of ice cover in the weeks ahead.
Marie-Noëlle Houssais, Decembre 2016
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