Sea-ice Development in the Arctic Remains at a Historically Low Level
30 October 2020
In September 2020 the Arctic sea ice reached its second-lowest extent since 1979 and is now in the phase of new ice formation. At 4.0 million km², the sea-ice extent at mid-month was roughly 530,000 km² above the record low in 2012, and ca. 440,000 km² below the figure for the same month in 2019, the year with the third-lowest minimum ever recorded (Fig. 1). Especially in the European Arctic (Barents and Kara Seas), there was significantly less ice than in the previous year (Fig. 2). This year, the new ice formation is progressing much more slowly due to the marked temperature anomaly over the coastal waters of the Siberian Arctic Ocean, which was not present in the past four decades. Since 14 October, there has been less ice in the Arctic than on any comparable day in October in the last forty-two years (Fig. 3). Prior to 14 October, the lowest October sea-ice extent was in 2012, followed by 2019. From 27 October, the year with the lowest recorded ice extent in the Arctic to date was 2016, a situation that continued until well into December 2016.
Following the absolute sea-ice minimum for the summer, on 14 September 2020, sea-ice formation began to increase, initially in the Beaufort Sea and the Canadian Basin as well as the Central Arctic, Nansen Basin and Greenland Sea. In the Kara and Barents Seas, ice growth slowly began in early October, while in the Laptev Sea it didn’t begin until mid-October.
The reason for this significantly delayed process was a marked high-pressure system over Siberia, combined with a low-pressure system over Svalbard. This pressure distribution favoured air currents from the Siberian Arctic to the Arctic Ocean, which manifested as air temperatures that were 6° to 8° C above the long-term average throughout the Central Arctic (Fig. 4, centre and left). As a result, the sea’s surface temperature has not cooled and is more than 3 °C above the long-term average, making it much too warm for this time of year and hindering sea-ice formation (Fig. 4, right). Particularly the Siberian marginal seas and west Greenland’s Baffin Bay and Davis Strait are characterised by unseasonably high surface temperatures. The current anomalous wind conditions from the land to the ocean promoted a retreat of the ice edge in the Central Arctic and as a result also prevented ice growth in the Laptev Sea.
October brought additional warmth and moisture (‘latent heat’) to the Siberian coastal waters via the Bering Strait (see Fig. 5, animation of the integrated water transport). This especially slowed the formation of ice in the Laptev Sea, which at the time covered less than 10 % of the area of the previous minimum in 2012 (see Fig. 6). Dr Monica Ionita, a climatologist at the Alfred Wegener Institute, describes the situation as follows: “The unusually warm summer in the Arctic, together with the now established atmospheric pressure conditions, are hindering the exchange of air masses in the Arctic and as such producing an accumulation of heat, which can subside only gradually. The situation was exacerbated by the inflow of warm air masses from the Siberian Arctic. But ocean currents from the Atlantic and over the Bering Strait also bring warmth to the Arctic Ocean. All these factors increase the heat content of the Arctic, leading to delayed and reduced ice growth, in other words, to the situation we’re currently seeing.” The outlook is alarming, since the later ice growth begins, the thinner the ice that can form over the winter is, and the more easily it can melt again in the coming spring. The Laptev Sea, with its unseasonably low ice cover, was also the birthplace of the MOSAiC floe, which a year ago today was selected for the start of the one-year-long ice drift with RV Polarstern. No-one can say whether this year’s ice conditions would have been sufficient to produce a comparable floe. And we will have to wait and see what the ice development for the coming year will be like. One thing is certain: “Without decisive climate protection measures, within a few decades the Arctic will be ice-free in summer,” concludes Ionita in her assessment of the current situation.
Antarctic reaches winter maximum sea-ice extent
While the world focuses on the alarmingly low ice extent in the Arctic, in the Antarctic the winter has reached its zenith: on 26 September, the ice extent reached its absolute maximum for this year, with an area of 19.14 million km² (Fig. 7). At mid-month the ice extent was roughly 18.9 million km², the eighth-highest maximum ice extent in the Antarctic since 1979 (Fig. 8). This relatively high ice extent compared to the past few years reflects the extreme year-to-year variability that can be seen in the Antarctic, in contrast to the Arctic. While the ice extent over the course of the year was generally similar to the average for the years 1981 to 2010, since mid-August the ice extent has been significantly above the long-term average and in the last two months reached values that were close to the upper limit of two standard deviations from the long-term average (Fig. 9). In terms of the long-term trend, the Antarctic is showing a rise in monthly mean values for winter sea-ice extent of ca. 0.6 % per decade. However, significant fluctuations have been observed in the past ten years, including the maximum extent in 2014 and the third-lowest in 2017 (Fig. 8).