Seasonal low sea-ice cover in the Arctic
25 May 2020
Following the lowest-ever sea-ice extent for the month of April in 2019, this year the extent is only 204,000 km² larger, putting it in second place on the list of monthly mean values since 1979 (Figure 1). Particularly in the Bering Sea, but also in the Sea of Okhotsk and eastern Barents Sea, the amount of sea ice was below the long-term average for 2003 – 2014. In contrast, the ice margin was above the long-term average in the northeast Greenland Sea and western Barents Sea (Figure 2). In April, the speed of ice loss was ca. 36,100 km² per day. Compared to the month of March, the Arctic sea-ice extent declined by ca. 1.1 million km², an area roughly twice the size of France.
Air temperatures at 925 hPa pressure elevation (ca. 750 m) were 2 – 5 C° above the long-term average in the majority of the East Siberian Sea, Arctic Ocean and Bering Sea, with the exception of Svalbard and the Barents Sea, where temperatures were very close to the average (Figure 3, left). In Baffin Bay, northern Greenland, and the Greenland Sea, air temperatures were 6 – 8 C° above average, and ca. 8 C° above average in Siberia. Below-average air temperatures for the time of year continued to be observed in Canada (Figure 3, right). This pattern in the air temperatures is due to above-average barometric pressures at sea level over Alaska and Siberia, together with low pressures over the central Arctic Ocean, which help to transport warm air from the south across the Bering and Kara Seas. Unusually low barometric pressures over the Central Arctic produced a cyclonal (counter-clockwise spinning in the Northern Hemisphere) air current, in which warm air masses from northern Greenland were blown eastward to the north of Svalbard, producing unseasonably warm temperatures in some areas. For example, the MOSAiC Camp recorded its highest temperature to date, -0.2°C, on 19 April. Generally speaking, these warm air temperatures caused the snow to melt there and led to melting-and-freezing cycles, which significantly changed the snow’s characteristics. The weather conditions also affected the sea-ice conditions in the vicinity of the camp: leads and channels increasingly formed, and in some cases the ice concentration dropped below 90 % (Figure 4). As a result, some of the outlying monitoring stations could no longer be reached on foot, only by helicopter.
Sea-ice development in the Antarctic shows typical progression
In the Antarctic, the trend observed in the previous month continued, and the sea-ice extent developed in keeping with the long-term average (Figure 5). At the end of April the sea-ice extent was 8.26 million km²: just below the average value for the years 1981 – 2010 and well within the limit of two standard deviations from the mean for the reference period (Figure 6). There continued to be too little sea ice for the time of year, especially in the Weddell Sea and the marginal ice zone from the Bellingshausen Sea to the Ross Sea, while there was more ice than the long-term average in the Davis Sea, East Antarctica. As a result, although the trend of particularly low sea-ice extents in April for the past three year was broken and the value bounced back to an average level, nonetheless a long-term trend of 1.5 % loss in sea-ice extent per decade can be observed for the month of April in the timeframe 1979 – 2020 (Figure 7). Over the ice sheet, surface temperatures were predominantly too high in April. In broad areas of East Antarctica, air temperatures at 925 hPa were more than 6°C above the long-term mean for the years 1972 – 2000, whereas 1 – 3°C warmer temperatures were dominant from West Antarctica to the Antarctic Peninsula (Figure 8, left). Ocean surface temperatures continued to be influenced by the summer and were slightly above the long-term average for 1980 – 2000 (Figure 8, right).
As Dr Holger Schmithüsen, director of the meteorological observatory at the German overwintering station Neumayer III, reports on the summer season 2019 / 2020, near the station the air temperatures at 2 m were within the normal range throughout the southern summer. The mean monthly values for the months November to January were all slightly above the long-term average (November: 0.7°C, December: 1.3°C and January: 1.2°C). In contrast, recorded temperatures for February 2020 were very close to average (Figure 9). The lowest temperature (-26°C) was reached in November, while temperatures climbing up to the freezing point were frequently observed in December and January. After a fairly quiet November, December was windier than average. January and February were, typically, characterised by less wind (in terms of wind speed). Further, the year 2020 began with excellent working conditions, with whiteout only 5 % or 6 % of the time. So far there has only been one, fairly brief storm event, on 18 and 19 January.