Sea-ice Development in Both Polar Regions Largely Normal
15 February 2021
Sea-ice development in the Arctic
Following 2020, a year that managed to break a number of records in the Arctic, the new year has begun less spectacularly, as a closer examination of the months December and January shows. The monthly average ice extent in December 2020 was 11.94 million km², making it the third lowest extent since 1979, continuing the long-term trend of roughly 3.3% decline per decade. The sea-ice extent in December was ca. 0.97 million km2 below the December average for the period 1981 to 2010, but 0.5 million km² above the record low in 2016. Especially in the Bering and Barents Seas, the ice extent was below the long-term average. On 31 January 2021 the sea-ice extent in the Arctic was 14.29 million km² (Fig. 1), roughly the same level as in the previous two years. In terms of the long-term trend for the month, January 2021 comes in at 7th place, with 13.63 million km² (Fig. 2). Here, too, a long-term comparison shows a decline of ca. 2.8% per decade. Only the spatial distribution differs from that of 2020, since there is now less ice in the Barents Sea and north of Svalbard, while in the Greenland Sea, Bering Sea and Sea of Okhotsk, the sea ice has penetrated further southwards (Fig. 3). In January the sea-ice grew at a rate of 47,518 km² per day. This represents a daily growth almost exactly the size of Lower Saxony.
The current winter season in the Arctic is particularly influenced by the temperature anomaly, which in December produced temperatures up to 5°C above the long-term average for 1971 to 2000 in the Laptev Sea and Central Arctic, and over East Greenland and Northern Canada (Fig. 4, top left). In contrast, in Siberia temperatures were on average ca. 2 °C colder than usual, with a local record low of -49 °C in Tomsk. This trend continued and intensified in January. Over the Central Arctic, North Greenland, Northern Canada and Alaska, there was a temperature anomaly of 3 °C to 6 °C above the long-term average. At the same time, temperatures in Siberia were 6 to 8 degrees below the long-term average. The atmospheric circulation linked to this temperature pattern in January was dominated by high pressure over Siberia and low pressure over the northern North Atlantic and North Pacific. The map showing the atmospheric pressure anomaly at ground level (Deviation from the average atmospheric pressure at sea level compared to the climatological reference period 1971 – 2000) shows extensive areas of low atmospheric pressure over the North Atlantic and North Pacific (negative atmospheric pressure anomaly), while an area of high atmospheric pressure (positive anomaly) extends from Greenland over the Central Arctic and as far as Russia (Fig. 4, bottom right).
This typical circulation pattern – also known as Arctic Oscillation (AO) – describes the changes in the atmospheric pressure differences near the ground between the Arctic and the middle latitudes in the Northern Hemisphere, and is created by substantial temperature differences between the two regions. Whereas the AO was strongly positive for much of November, it was negative for virtually all of December. As a result of this change in AO phases, in December an atmospheric pressure pattern characterised by high pressure over the Arctic Ocean, very high pressure over the Beaufort Sea, and low pressure over the Atlantic and the Subarctic Pacific developed. Generally speaking, there is strong coupling between the Arctic Oscillation in the lower troposphere and the intensity of the polar vortex in the stratosphere / upper troposphere. A positive AO correlates to a strengthening of the polar vortex.
The polar jet stream and the weather in Europe
When the AO is positive, the polar jet stream is more pronounced, and in winter the circulation pattern drives warm Atlantic air to Northern Europe and Siberia, as well as to the polar regions, through strong westerly winds (Fig. 5). In contrast, in the negative AO phase, the polar vortex is weakened, and cold polar air can push southwards. As a result, cooler Arctic temperatures can penetrate Europe.
This atmospheric circulation pattern has now settled in, and is partly responsible for the cold temperatures and wintry conditions over Northern Germany. But the intrusions of cold air over North America, which have brought snowfall and extremely low temperatures as far as New York, are also due to the weakened polar vortex.
Our planet has so-called polar vortices – extremely large-scale low-pressure systems in the middle and upper troposphere – over the North and South Poles. These cold-air zones are caused by the poles’ negative radiation balance. When a polar vortex collapses, it is accompanied by a sudden warming in the stratosphere (at an altitude of roughly 15 to 50 kilometres). In the atmospheric layer below it lies the jet stream, which blows from west to east. The two systems influence one another and our weather. A weak polar vortex affects the jet stream; on the ground, it increases the likelihood of icy polar air from the Arctic spreading south. Extreme events in the stratosphere only last for a short time, but they can influence our weather over periods lasting several months. Whether or not the polar vortex will continue to falter in the coming weeks and therefore affect our weather remains to be seen.
Sea-ice development in the Antarctic
As in the Arctic, the sea-ice extent in the Antarctic in December and January can be described as average. After four years of minimal ice extent, in December the area covered by sea ice matched the long-term trend of a ca. 1.1 % decline per decade over the last 41 years (December average: 10.0 million km²) (Fig. 6). December is the month of the year with the most intense melting. In December 2020 it was roughly 239,287 km² per day, an area roughly the size of Romania, and took place mainly in the eastern Weddell Sea and the Lazarev Sea, as well as the southern Ross Sea.
In a recent study, Handcock and Raphael (2020) found that in the Antarctic most of the deviation from the average sea ice extent is dependent on the time that the ice loss occurs. Assuming that the ice extent decreases considerably each day, the timing of the onset and end of ice loss, or ice growth, has a significant effect, since deviations of just a few days can lead to comparatively large deviations from the average extent. Accordingly, the causes of an earlier or later onset of ice loss, such as the influence of weather or the ocean on the cyclical annual trend, can produce prolonged effects if they change the phases of the cycle in this manner (source: NSIDC).
In January 2021, the ice retreat in the Antarctic slowed to an average of 92,247 km² per day, resulting in a remaining extent of 3.4 million km² at the end of the month (Fig. 7). Particularly in the eastern Weddell Sea, a polynya formed along the coast from Queen Maud Land to the southern Filchner Ice Shelf, which could aid the research icebreaker Polarstern’s progress towards the Filchner-Ronne Ice Shelf during her next cruise. It remains to be seen whether these favourable ice conditions will hold in the coming weeks. Compared to last year, there is significantly less ice in the northwest Weddell Sea and eastern Ross Sea, while along the coast of Eastern Antarctica, in the Amundsen Sea and in the eastern Weddell Sea, there are even larger areas of sea ice (Fig. 8). You can find RV Polarstern’s current position and follow her route during the Antarctic expedition here.
Handcock, M. S. and M. N. Raphael. 2020. Modeling the annual cycle of daily Antarctic sea ice extent. The Cryosphere. doi:10.5194/tc-14-2159-2020.
Dr. Klaus Grosfeld (AWI)
Dr. Renate Treffeisen (AWI)