- The Arctic: Pronounced high- and low-pressure systems intensified the Beaufort Gyre and accelerated sea-ice export in Fram Strait. In the Laptev Sea, the combination of these factors led to an early breakup of the ice cover.
- AWI aerial campaigns: Due to the mild winter in the Canadian Arctic, the newly formed fast ice on the inner waters of the Canadian Arctic Archipelago was up to 30 centimetres thinner than in past years.
- In the Antarctic, the sea-ice extent was well below the long-term trend for a fourth consecutive month in April.
Arctic: Sea-ice drift and extent shaped by pronounced high- and low-pressure systems
This April, a pronounced high-pressure centre dominated the weather in the Central Arctic, while a low-pressure cell persisted over the Kara and Barents Seas (Figure 1). The combination of these two pressure systems most likely had a major influence on the movement and extent of sea ice in the Arctic.
The high-pressure system may have amplified the Beaufort Gyre, which transported the pack ice around the system in a clockwise motion. At the same time, winds generated in connection with the low-pressure zone over the Kara and Barents Seas may have accelerated ice transport toward Fram Strait, leading to a level of ice export in the strait that was far above average (Figure 2).
In the Laptev Sea, this constellation of pressure systems resulted in substantial offshore ice drift. In other words, offshore winds drove the pack ice away from the coast. At the same time, warm air masses flowed in from eastern Siberia (Figure 3), producing extensive patches of open water in the Laptev Sea from early in the year (polynyas, Figure 4).
Figure 1: Mean air pressure anomalies in the Arctic in April 2025, measured at sea level. The orange-coloured area in the middle shows the positive pressure anomaly, while the blue-green area in the right half of the image indicates the negative pressure anomaly over the Barents, Kara and Laptev Seas, which produced intense offshore winds. In addition to the pressure anomalies, the image includes wind vectors showing the wind direction and speed – the arrows show the direction, while their length is proportional to the windspeed.
Figure 2: Drift speed anomalies and mean sea-ice drift on the Arctic Ocean in April 2025 compared to the long-term April mean for 2010 – 2025. Clearly recognisable: the Beaufort Gyre and the pronounced transport of sea ice toward Fram Strait north of the Russian marginal seas. Source: Thomas Krumpen/Alfred Wegener Institute
Figure 3: Air temperature anomalies in April 2025 compared to the long-term mean temperatures for April in the period 1971 – 2000. The map also shows a warmer zone over eastern Siberia, with offshoots extending into the Laptev Sea.
When these patches of open water form early in the year, it can accelerate the summertime sea-ice melting in the Laptev Sea. “Then the sequence of processes is relatively simple: In spring, the surface water warms faster in the ice-free regions than in ice-covered regions. When winds then drive the young, thin sea ice of the Laptev Sea back into those regions with warmer surface water, the pack ice melts much faster than it would if they had kept their ice cover well into spring,” explains AWI sea-ice expert Thomas Krumpen. He and a colleague had previously described this atmosphere-ocean-ice dynamic in a journal article in 2017.
Figure 4: This NASA satellite image from 4 May 2025 clearly shows the extensive ice-free areas in Russia’s Laptev Sea. Source: NASA Worldview
Effects of the mild winter: Thinner sea ice in the Canadian Arctic Archipelago
In March 2025, during this year’s aerial sea-ice surveying campaign in northern Greenland and North America, sea-ice physicists from the Alfred Wegener Institute could see first-hand the short-term effects on the sea ice when the winter in a given Arctic region is unusually mild. Particularly in the inner waters of the Canadian Arctic Archipelago, the fast ice that formed in winter 2024/2025 was 30 centimetres thinner than in past years (Figures 5 & 6).
At this time of year, the ice thickness normally ranges from 1.7 to 1.8 metres. During the latest observations in 2025, the sea-ice thickness sensor EM-Bird measured a mean thickness of 1.3 to 1.5 metres. “We can directly link this loss of ice thickness to the comparatively warm winter temperatures in the Canadian Arctic. Between the islands of the Canadian Arctic Archipelago, the only sea ice is that which formed over the winter. So, when this ice is much thinner in spring than in previous years, it has to be due to unusual air or water temperatures,” says Prof Christian Haas, Head of the Sea Ice Physics Section at the AWI and a participant in this year’s IceBird surveying campaign.
Last winter, the mean air temperature over the Canadian Arctic Archipelago was consistently 1.5 to 6 degrees Celsius higher than in the reference period 1971 to 2000, as can also be seen in the time-lapse animation of air temperature anomalies in the Arctic (Figure 7).
Figure 5: In the spring of 2025, the sea-ice cover was significantly thinner and narrower than in past years in e.g. Eclipse Sound, an inner area of ocean in the Canadian Arctic Archipelago that the AWI research aeroplane Polar 6 is flying over in the photograph. The declining sea-ice thickness is reducing the stability of the ice cover – making it more dangerous for indigenous people living on the coast to hunt on the ice. Photo: Christian Haas/Alfred Wegener Institute
Figure 6: During the flight to a monitoring area, the researchers have a bit of time to look out the window and enjoy the view of the polar ice world. Once the ice-thickness or snow-thickness sensor is up and running, all eyes are on the monitors and gauges. Photo: Esther Horvath/Alfred Wegener Institute
Figure 7: Time-lapse animation of mean air temperature anomalies from October 2024 to April 2025 compared to the long-term mean temperatures for the respective months in the period 1971 – 2000. The anomaly maps reflect the prolonged, high temperature deviations in the Central and Canadian Arctic.
Spring is here to stay
In the monthly sea-ice balance for April, the areas of open water in the Laptev Sea and the thin ice in the Canadian Arctic Archipelago didn’t yet have any material influence on the overall sea-ice extent in the Arctic – which, at a monthly mean value of 13.9 million square kilometres, came in at ninth place in the monthly statistic (Figure 8). There was more pack ice compared to the long-term mean in e.g. the eastern Bering Sea and in Fram Strait. “In this regard, the higher sea-ice extent in Fram Strait can be attributed to the increased sea-ice export. For the past few weeks, a markedly high level of pack ice has drifted from the Arctic Ocean to the North Atlantic; as a result, the sea-ice extent in Fram Strait has automatically increased,” says Thomas Krumpen. In contrast, satellites detected less sea ice particularly in the eastern Barents Sea, western Bering Sea, and in the Sea of Okhotsk (Figure 9).
The map of daily sea-ice extents in the Arctic shows that the line for the year 2025 was full of twists and turns in the first half of April, and in two cases the sea-ice extent rebounded significantly. But since the 16th of April, spring is here to stay in the Arctic and, aside from minor daily fluctuations, the sea-ice extent is steadily declining – at a mean melting rate of ca. 40,000 square metres per day.
Figure 8: Development of mean sea-ice extent in the Arctic for the month of April. The light blue line represents the long-term trend.
Figure 9: Difference in the mean position of the ice margin in April 2025, compared to the long-term mean for the years 2003 – 2014. Regions marked in blue had more Arctic sea ice than the reference period; those marked in red had less.
Antarctic: Air masses over West Antarctica are far too warm
Conversely, in the Antarctic the sea-ice extent has continually risen since the beginning of winter. The gains in April 2025 were once more fairly substantial, producing a steep upturn in the daily ice-extent curve in April and yielding extents that were well within the span of minima and maxima for the period 1981 to 2010 toward the end of the month (Figure 10). Nevertheless, the monthly sea-ice extent in April 2025, at 6.4 million square kilometres, was comparatively low and below the trend line (Figure 11).
There was markedly less sea ice compared to the long-term mean for the period 2003 to 2014 in e.g. the western Weddell Sea, in the Bellingshausen Sea, in the Ross Sea and off the coast of Queen Maud Land (Figure 12).
More detailed analyses will be needed in order to determine to which extent the unusually high air temperatures over West Antarctica slowed the formation of new sea ice in the western Weddell Sea and in the Bellingshausen and Amundsen Seas. In broad expanses of the Antarctic Peninsula and West Antarctica, the deviations in mean air temperatures were up to 6 degrees Celsius above the long-term mean (Figure 13) in April.
Figure 13: Air temperature anomalies in April 2025 compared to the long-term mean temperatures for April in the period 1971 – 2000. The map shows substantially higher air temperatures over the Antarctic Peninsula and parts of West Antarctica. Based on daily mean temperatures measured at an altitude of ca. 760 metres.
Good news: Sentinel 1C satellite delivers new sea-ice data from the polar regions
Last but not least, some good news: Since being launched by its carrier rocket on 4 December 2024, the ESA satellite Sentinel 1C has now reached its standard orbit and gotten down to business. Since mid-April it has been transmitting e.g. radar data on sea-ice cover from the polar regions. The satellite’s radar allows it to penetrate cloud cover and make its observations night and day, offering researchers daily updated data on the sea-ice situation – data that is needed for aerial surveying campaigns and ship-based expeditions alike. On the former, the radar data helps them identify regions with especially thick or thin sea ice and plan their flight routes accordingly. On expeditions with the research icebreaker Polarstern, Sentinel data is essential to identifying channels (“leads”) in the ice, which ensures the ship can proceed with minimum resistance.
Figure 14: Sea-ice experts from the Alfred Wegener Institute use radar images from the Sentinel satellites to plan the flight routes for their aerial sea-ice thickness surveys. The data shows e.g. where the pack ice is especially thick or thin. Photo: Esther Horvath/Alfred Wegener Institute
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