- During this year’s southern winter, the sea ice in the Antarctic covered a maximum area of 17.34 million square kilometres – only 180,000 square kilometres more than the all-time low in 2023.
- There are substantial regional differences in sea-ice development. Determining their causes is difficult, as on-site observational data on the respective regions is lacking.
- In early 2024, experts from the AWI’s Sea-ice Physics section will deploy a range of measuring devices in the Weddell Sea to take a closer look at local developments during the next southern winter.
On 11 September 2024, satellites recorded a maximum winter sea-ice extent of 17.34 million square kilometres – 1.17 million square kilometres below the long-term mean for the years 1981 to 2010, an area three times the size of Germany.
However, in comparison to the record low winter in 2023, at the end of this southern winter the sea ice covered an area 180,000 square kilometres larger. Accordingly, this year’s sea-ice maximum came in just behind last year’s figure, making it the second-lowest winter sea-ice extent since the beginning of continuous satellite observations in 1979. The same is true for the mean monthly sea-ice extent: this September, the area was 17.22 million square kilometres (Figure 1).
Figure 1: Overview of mean September sea-ice extent in the Antarctic. The mean for September 2024 is marked in red.
“Many experts had hoped that the extremely low winter sea-ice extent last September was a major exception. In fact, the data on the current sea-ice extent confirms that what we thought was an exception has now been repeated. This is cause for a growing concern that fundamental shifts have now begun in the Antarctic’s climate system,” says Dr Renate Treffeisen, editor of the Sea Ice Portal.
Sluggish sea-ice formation: Substantial regional differences
The sea ice in the polar regions follows a pronounced seasonal cycle. Every year, roughly 15 million square kilometres of sea ice are formed and melt in the Antarctic. In the winter months of 2024, ice growth was sluggish, particularly in the waters off the coast of Dronning Maud Land, off the coast of East Antarctica, in the eastern Ross Sea, and in the western Weddell Sea. In September 2024, there was markedly less sea ice than the long-term mean in all four regions. There was more pack ice than the long-term mean in the western Ross Sea, off the coast of Adélie Land, and in the Bellingshausen Sea (Figure 2).
Figure 2: Difference in the mean position of the ice margin in the Antarctic in September 2024 in comparison to the long-term mean for 2003 – 2014. Regions marked in blue have more sea ice than the reference period; those marked in red have less.
If we compare this year’s September sea-ice extent with last year’s, there is less sea ice in the western Weddell Sea, in the Amundsen Sea, and off the coast of Dronning Maud Land. Conversely, there is more ice in the eastern Weddell Sea and in the Ross Sea (Figure 3).
Figure 3: Difference in the mean position of the ice margin in the Antarctic in September 2024 in comparison to September 2023. Regions marked in blue have more sea ice than the reference period; those marked in red have less.
“These two comparison maps underscore the fact that there are significant regional differences in sea-ice development in the Southern Ocean. In the western Weddell Sea, for example, we can observe sea-ice loss, both in comparison to the long-term mean and to last year. In contrast, there is more ice in parts of the Ross Sea,” says Prof Stefanie Arndt, a sea-ice physicist at the Alfred Wegener Institute and expert on the physical characteristics of Antarctic sea ice.
In her view, especially this year’s developments in the Weddell Sea could once again be the result of macro-scale atmospheric processes. In a study released just a few months ago, AWI climatologist Dr Monica Ionita showed that, in the southern winter of 2023, warm and moist air from the north penetrated to near the ice margin in the Weddell and Ross Seas. There, it combined with the unusually warm surface water of the Southern Ocean to delay the formation of new ice.
Wind is another important factor. “In the southern winter of 2023, our sea-ice monitoring stations near the research station Neumayer III recorded a comparatively high number of storms. These were subsequently discussed as a potential cause of the rapid sea-ice retreat, since powerful winds can drive floes before them or densely compress the pack ice. But in this year’s southern winter there weren’t as many storms as last year, at least not near Neumayer III,” says Stefanie Arndt.
An armada of autonomous measuring devices soon to be deployed
For more precise analyses, the sea-ice physicist needs temperature, water, radiation and sea-ice data directly from the winter pack ice of the Weddell Sea – but no such data is available. That’s why she’s looking forward to the upcoming Antarctic expedition on board the research icebreaker Polarstern. “The journey to the Weddell Sea, which will begin at Christmas, promises to be exciting from a scientific standpoint, for three main reasons: firstly, experts from the AWI’s Oceanography section will retrieve a number of what are called moorings; their onboard measuring equipment has been documenting the conditions in the water column for the past two to three years. The data allows us to draw conclusions on which ocean conditions could have delayed winter sea-ice growth in the northwest part of the Weddell Sea, or what could have led to more sea ice in the eastern Weddell Sea in the southern winter of 2024,” says Stefanie Arndt.
Secondly, she and her team will collect samples from the remaining summer pack ice in the southeast Weddell Sea and analyse its snow and ice cover, along with the sea-ice structure. The percentage of the pack ice’s surface that was once snow, for instance, tells the researchers what air temperatures were like over the ice in the southern winter. In addition, they can measure the surface features and coarseness to determine the extent to which the sea ice was compacted by storms.
However, the third and most important task for the sea-ice physics team will be to deploy an extensive armada of autonomous monitoring systems in the eastern Weddell Sea, including snow and ice-thickness buoys, weather and radiation stations, and oceanographic instruments for the surface water. “All the devices will drift through the Weddell Sea with the pack ice for a year. With regard to the southern winter of 2025, we’ll hopefully have enough autonomous observation systems in place to understand what’s happening in the region and why the Weddell Sea is developing the way it is,” says Stefanie Arndt.
Arctic sea ice: The downward trend continues
After reaching its summer minimum on 7 September (4.39 million square kilometres), the sea ice in the Arctic Ocean began growing again, reaching a monthly mean value of 4.52 million square kilometres. That makes it the sixth-lowest September sea-ice extent since the beginning of satellite observations in 1979, continuing the downward trend in Arctic sea-ice extent development (Figure 4).
Figure 4: Monthly mean sea-ice extent in the Arctic Ocean since the beginning of satellite observations in 1979. The mean for September 2024 is marked in red.
In September, sea-ice levels were particularly low in the Kara Sea, Laptev Sea and eastern Beaufort Sea. As we can see on the ice-concentration maps, the waterway through the Canadian Arctic Archipelago, or Northwest Passage, was ice-free. With a single exception, the Northeast Passage along Siberia’s Arctic coast was also ice-free, making it navigable for shipping (Figures 5 and 6). This summer, the experts at Norway’s Center for High North Logistics recorded a total of 79 transit voyages through the Northeast Passage.
At Wrangel Island, the crews of eastbound ships were in for an eyeful: a large field of ice became trapped between the island and the eastern coast of Siberia. According to data from the US-based NSIDC portal, the field consists of seasonal sea ice that was compacted by powerful ice motion in the early summer and was first observed in June. Thanks to its thickness, the compact ice survived the summer melting and can be clearly seen as a small blue area on our comparison map for the month of September (Figure 5).
For a detailed analysis of sea-ice development in the Arctic Ocean in the summer of 2024, please check out our news item “Arctic sea ice reaches summer minimum”.
Figure 5: Difference in the mean position of the ice margin in the Arctic in September 2024 in comparison to the long-term mean for 2003 – 2014. Regions marked in blue have more sea ice than the reference period; those marked in red have less.
Figure 6: Mean monthly value for the sea-ice concentration in the Arctic Ocean for September 2024. The green line indicates the mean September sea-ice extent from 1981 to 2010.
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