Moderate Ice Conditions in the Arctic and Antarctic
February / March and September are the months of the year in which Arctic and Antarctic sea ice reaches its minimum and maximum extent, respectively. While the summer minimum was reached in the Antarctic on 19 February 2020 (at 2.68 million km²), in the Arctic the extent has been growing steadily since the end of February, and hasn’t yet reached its winter maximum.
Accordingly, the Arctic sea-ice extent was ca. 14.41 million km² in February 2020 (see Figure 1), roughly 799,000 km² below the long-term average for February and 474,000 km² above the record low from 2018 (see Figure 2). At the end of February, the extent was below the long-term average in the Barents and Kara Sea regions, as well as the eastern Greenland Sea (see Figure 3). In the course of February, the sea-ice cover in the Arctic showed a comparable development to the past few years, and the extent on 9 March (ca. 14.74 million km² ) lies within the double standard deviation from the range for the years 1981 – 2010. The extent is now growing nearer the middle curve for these years (see Figure 4). If we include 2020, the linear decline in sea-ice extent for February is now ca. 2.7% per decade. This equates to 41,100 km² per year, or roughly twice the area of Sachsen-Anhalt. Over the last 42 years, since the beginning of satellite observation, the amount of sea ice in the Arctic in February has declined by 1.56 million km², an area roughly equivalent to that of Mongolia.
The air temperature over the Arctic Ocean at 925 hPa pressure altitude (circa 762 m) was between 1 and 7°C above the long-term average. In contrast, temperatures were as much as 6 °C below the long-term average over northern Alaska and the Chukchi and Beaufort Seas (see Figure 5, left).
The air pressure at sea level was similar to that in January. A low-pressure cell made its way from the northern North Atlantic to the Kara Sea, while a high-pressure system spread from Eastern Eurasia to Alaska and northern Canada. The air pressure over the Barents and Kara Seas was more than 9 hPa below the historical average (see Figure 5, right).
This pattern is above all connected to a positive Arctic Oscillation, which has been dominant since December. The index, which was very high (4-6) at some points in February, was connected with warm air and storm weather from Northern Europe, which penetrated to the Barents Sea (see Figure 6). This may help to explain the sea-ice retreat in the Barents Sea, as well as the one-week-long pause in sea-ice growth across the Arctic in mid-February. From 22 February we began seeing a resumption of overall sea-ice growth in the Arctic, thanks to substantial growth in the Bering Sea and moderate growth in the Barents Sea (see also NSIDC ). We surmise that a positive Arctic Oscillation in the winter tends to produce low sea-ice extent in the following September. Winds transport old and thick ice from the Arctic Ocean through Fram Strait and foster ice formation on the coasts of Eurasia, which is highly susceptible to melting in summer. However, the effects of the current positive Arctic Oscillation on ice formation in the coming summer remain unclear: they won’t manifest until September.
Summer sea-ice minimum reached in the Antarctic
While the winter is slowly drawing to an end in the Northern Hemisphere and the days are gradually growing longer, in the Southern Hemisphere the summer is over and autumn is in the air: in the Antarctic, the days are now getting shorter and shorter and the amount of daylight is decreasing markedly. As a result, sea-ice melting has also reached its maximum, and the minimum sea-ice extent was reached on 19 February (2.68 million km², see Figs. 7 and 8). The mean sea-ice extent in February was 2.90 million km², only ca. 200,000 km² below the long-term average for the years 1981 – 2010 (Figure 9). Although the sea-ice extent was slightly above the long-term average in the coastal region of Eastern Antarctica and the Bellingshausen Sea, especially in the western Weddell Sea, Ross and Amundsen Seas, there was substantially less sea ice (Figure 10). In the Ross Sea, we even discovered an extensive, isolated and free-floating expanse of sea ice: a remnant of the melting season that didn’t manage to drift out of the Ross Sea with the natural circulation (Figure 8).
In February, the Amundsen – Bellingshausen – Ross Sea sector of the Antarctic was characterised by a pronounced dipolar field in the near-surface air pressure anomaly, which transports warm air masses from the South Pacific to the Antarctic. This produced temperatures of up to 4° C above the long-term average, which reached to central Marie Byrd Land south of the Amundsen Sea (see Figure 11, left). Eastern Antarctica was largely dominated by offshore winds, which tended to produce temperatures generally lower than the long-term average. This could be one of the causes of the slightly larger pack-ice belt observed.
All in all, in terms of sea-ice cover, it was an average summer in the Antarctic. The long-term trend for sea-ice extent in February continues to be slightly positive, even though February 2020 continued the trend seen in the past five years, namely, of lower sea-ice extent in comparison to the reference period 1981 – 2010. An animation of the sea-ice minima since 2003 is provided in Figure 12.