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An unusual year for Arctic sea ice
Sixth lowest minimum ice extent followed strange melting pattern in an August warmer than July, and anomalous September weather Alan Bailey Petroleum News
A review by the National Snow and Ice Data Center of this year’s Arctic sea ice melt season shows some unusual features in the weather patterns and the accompanying ice melt phenomena. As previously reported in Petroleum News, the sea ice minimum, at the end of the melt season, was tied with the 2008 minimum as the sixth lowest recorded since satellite observations began in 1979. And the minimum came unusually late, between Sept. 19 and 23.
The minimum extent remained consistent with a long-term trend of a 12.8 percent per decade decline in the minimum sea ice cover.
Started with near record low extent This year’s melt season began after record low winter extents in January and February, followed by the second lowest maximum ice extent in March. However, the ensuing ice melt began slowly over most of the western Arctic Ocean and the East Siberian Sea, the NSIDC reported. That happened despite June temperatures that were slightly above average. July then saw unusually low temperatures, ranking as the ninth coldest July since 1979.
But, despite these cool July conditions, the rate of ice melt was actually faster than the long-term average. NSIDC said that this apparent anomaly may reflect the roles of atmospheric processes in moving ice northward, and of the melting of ice as a result of ocean warmth.
Temperatures in August were higher than those in July, a phenomenon that has only been observed once in the past 70 years. Over the Laptev Sea, August air temperatures were up to 9 degrees F above average.
But, again, the ice melt defied logic by happening at a rate almost exactly corresponding to the long-term average, despite those elevated air temperatures.
September ice melt However, a large tongue of persistent ice in the East Siberian Sea did begin to melt in August. Air temperatures that remained above average into early September then helped the continued melt of ice that had persisted through the summer. Ice loss during the first half of September primarily occurred in the Easter Siberian, northern Laptev and northern Chukchi seas. NSIDC attributes this phenomenon to southerly winds that brought relatively warm air into these regions, inhibiting the southward movement or growth of the sea ice. However, the loss of ice in these regions was partly offset by the expansion of ice in the eastern Beaufort Sea and the northern Kara and Barents seas.
The NSIDC also reported that old ice that had persisted in the Beaufort Sea near Prudhoe Bay had almost melted out by the end of September.
A very pronounced high-pressure ridge that drifted eastward and became especially strong in late September contributed to the unusual warmth in September. And this phenomenon led to warm, dry conditions across much of Alaska, with Anchorage, Alaska, experiencing record high September temperatures - air tends to move downwards under a pressure ridge, thus inhibiting cloud formation and precipitation, the NSIDC said.
Loss of multiyear ice The NSIDC also commented on the dramatic long-term loss of multi-year sea ice in the Arctic. Multi-year ice tends to be thicker than young ice, and thus more resilient to the melting effects of warm weather and sea conditions. The thickness differential between younger and older ice also impacts the total volume of sea ice, as distinct from the ice’s aerial extent.
A plot of the multi-year Arctic sea ice extent for September ice minima demonstrates a dramatic fall in the extent since the mid-1980s, bottoming out at a very low level since around 2012. Ice that has persisted through at least four melt seasons now covers a total area of around 36,000 square miles. The extent of ice of this age used to be about 579,000 square miles, the NSIDC said.
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