The sea-ice extent in the Antarctic is slowly but surely nearing its seasonal minimum, which we expect to see roughly a month from now. With 3.36 million km² of pack ice covering the Antarctic continent, the current extent is slightly below the long-term average for the years 1981 – 2010, but above the extremely low extent from last year (see Figures 1 and 2). In comparison to the previous year, particularly the northeast Weddell Sea and the Ross Sea have more ice, whereas very low sea-ice concentrations have been observed in the Amundsen Sea.
The amount of pack ice surrounding the southernmost continent depends to a great extent on the prevailing atmospheric and oceanic circulation systems, which determine sea-ice drift. Only a small portion of the sea ice remains unaffected: the fast ice, i.e., sea ice that has frozen onto the (ice shelf) coasts of Antarctica, or onto icebergs in shallow areas of the ocean. Fast ice is frequently found in small inlets along the coastline – including a roughly 25-km-long (east to west) and 18-km-wide (south to north) one in the Ekström Ice Shelf at ca. 70°30’S and 8°W, also known as Atka Bay. Located ca. 8 km from the coast, the bay is home to the German research station Neumayer III, which rests atop the ice shelf (Figure 3). Here, the fast ice is not only an important component of the climate system and offers insights into interactions between the atmosphere, ocean and adjacent ice shelf; it is equally important for the ecosystem: every (southern) autumn, as soon as the ice on the bay becomes thick enough to support them, a huge colony of Emperor Penguins comes to live there (Figure 4). In addition, the ice offers a stable habitat for countless microorganisms and algae.
The fast-ice monitoring programme in Atka Bay
For more than ten years now, regular measurements have been taken of the fast ice, which normally breaks up during the southern summer, before drifting out of the bay into the Weddell Sea. These routine readings in Atka Bay are taken by the overwintering team at the Neumayer III station, starting as soon as the sea ice is safe to walk on (usually June), and ending when the ice starts breaking up in January or February. The team chiefly measures the thickness of the snow, fast ice and platelet ice along a 24-km-long west-to-east transect that spans Atka Bay (see Figures 5a and b). For the past three years, these measurements have been coordinated by sea-ice physicist Dr Stefanie Arndt from the Alfred Wegener Institute, and actively contribute to the Antarctic Fast Ice Network (AFIN), which was founded in the context of the International Polar Year (IPY) 2007/08. “The idea behind AFIN is to gather readings all across the Antarctic continent using a standardised protocol, which will allow us to better understand the fast ice and its role in the climate system on a larger scale,” Arndt explains. “In terms of the readings’ continuity, not to mention the length of the time series in the Atka Bay, they’re an outstanding achievement for those of us in the sea ice section, and for the AWI as a whole, offering a model for many of our international partners in the AFIN project.” That being said, there’s actually no dedicated team of ‘sea ice overwinterers’ that gathers the readings. “We’ve only been able to pursue our sea ice programme with this level of continuity and professionalism because, from the outset, we’ve received such excellent support from AWI Logistics, and from the other overwinterers on site. As a rule, the station meteorologist is our main contact – and who coordinates the readings. On measurement-gathering runs, they are supported by volunteers from the team,” says Arndt.
The continuous time series began in 2010, spearheaded by AWI sea-ice physicist Dr Marcel Nicolaus. Then, from 2011 to 2015, as part of the research project ‘Sea Ice Mass Balances influenced by Ice Shelves’ (SIMBIS) in the framework of the German Research Foundation’s Antarctic Priority Programme, Mario Hoppmann not only wrote his dissertation on fast ice and its importance for the climate system, but also assumed responsibility for coordinating the measurement work at the station – and was on site himself for three months. “But the very first readings taken on fast ice in Atka Bay are actually a good deal older than I am,” adds Stefanie Arndt with a laugh. During the very first overwinterings at the Georg-von-Neumayer Station in the early 1980s, Sepp Kipfstuhl recognised the significance of fast ice and repeatedly took readings in the bay. If we compare his numbers with recent data, we can clearly see: climate change hasn’t yet reached Atka Bay. Today, just as back then, the mean seasonal sea-ice thickness is ca. two metres, with ca. 70 cm of snow cover. Below the sea ice, a layer of platelet ice with an average thickness of four metres accumulates – depending on whether or not the sea ice is transported out of the bay at the end of the season. In 2012, for example, the large iceberg B15G blocked the mouth of the bay, preventing the fast ice from breaking off and drifting out – so that the bay remained covered with ice throughout the year, producing multiyear ice. As Arndt relates: “In the following Antarctic summer, which is to say, after two years, the platelet ice layer was roughly eight metres thick.”
In her latest publication, just released in the journal ‘The Cryosphere Discussions’, the young researcher summarises her findings on the readings from the bay. “In the paper, we show that the fast ice, the snow cover above it, and the platelet ice below it have remained unchanged over the monitoring period. In addition, it can very clearly be seen that the prevailing easterly winds in the bay are what dictate the spatial distribution of the snow cover.” Accordingly, east-to-west observational profiles of the bay reveal a distinctive gradient in the snow thickness, with very light snow cover in the eastern part of the bay, and at its centre. Katabatic winds, e.g. drainage winds blowing out to sea, intensify this effect at the easternmost monitoring point (ATKA24), while also producing comparatively low snow thicknesses at the western monitoring point (ATKA03, see Figure 5b). At the same time, snow cover is essential to the additional growth in fast-ice thickness from above: in some cases, the thick mass of snow (compared to the thickness of the ice below it) pushes the snow / ice interface below the surface of the water. “If we look at sea ice with no snow cover, it’s just like an iceberg: 90% of the ice is below the water’s surface, and only 10% is visible above it. But when we add a layer of snow cover, this balance no longer applies, and the mass of snow can become so heavy that it literally pushes the sea ice underwater,” explains Arndt, whose research chiefly focuses on snow cover and Antarctic sea ice. “When water reaches the snow/ice interface, the low temperatures cause it to quickly freeze – even in summer. Thanks to the newly formed snow-ice, the fast ice grows from above, a process that we’ve also frequently observed in Antarctic pack ice.” Further, as the platelet-ice layer continues to grow, it gains buoyancy, and some of the platelet ice grows into the fast ice. Consequently, as explained in the new study, the bay’s fast ice grows year-round – both from above and below, and not solely due to ‘natural’ thermodynamic processes between the ocean and atmosphere.
On 29 December 2019, the last readings for the 10th-anniversary year were taken in Atka Bay (Figure 7) – and what a remarkable year it was. Though the fast ice broke up in early 2019, only the ice in the eastern section drifted out of the bay. The entire western section experienced moderate compression before refreezing. According to Arndt: “We suspect that this behaviour was due to small icebergs that froze into the fast ice in early 2018, and which prevented it from drifting.” As a result, the ice in the western section was uneven and very difficult to drive on, which posed problems for the overwintering team, especially at the beginning of the season. “Working together with the overwintering team and AWI Logistics, we had to consult satellite imagery to keep finding new routes through this ‘ice debris field’. But our patience paid off: ultimately, drifting snow made the surface a bit more even, so that we could cross it more easily.” Now the 10th year of monitoring has successfully been completed. ‘I’m proud to see how the sea-ice programme has evolved over the ten years, becoming an established institution at the Neumayer III station. Working in such a professional setting, and being able to see the dedication with which each overwintering team carries out the sea-ice monitoring work, despite the wind and bitter cold, is truly unprecedented.” The last measurement-gathering run in December involved more than just collecting routine readings; it also gave the ‘old’ overwintering team the chance to show the ‘new’ team the ropes. As Arndt relates, “I look forward to a few months from now, when we’ll begin preparing for the next season once again.” After all, the monitoring programme will of course be continued. “For the future, my hope is that we can learn even more about the interrelations between the atmosphere, ocean, fast ice and adjacent ice shelves in Atka Bay. Since the effects of climate change aren’t yet as pronounced here as elsewhere in the polar regions, we have the opportunity to describe the initial conditions – which will in turn help us understand and explain future changes in the local climate system.”
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