Climate Research in the Southern Ocean

Polarstern can undertake expeditions despite the pandemic – researchers will gather important long-term data for climate forecasts

Polarstern in Weddell Sea (Photo: Mario Hoppmann)

The Alfred Wegener Institute has risen to the challenge of undertaking an expedition to the Weddell Sea in the Antarctic under pandemic conditions. Consequently, the participants in this year’s first Polarstern expedition will be able to resume their long-term measurements in the Southern Ocean, gathering data that forms the basis for our understanding of polar processes and for urgently needed climate forecasts.

They have painstakingly prepared for the expedition, and now, at last, it can begin: despite the pandemic, after a fortnight in quarantine and taking several corona tests, an international team of scientists will finally set off for the Antarctic. On 31 January, a chartered Lufthansa A350-900 will fly them – in stringent isolation – to Port Stanley on the Falkland Islands. From there, two days later they will set sail for the Weddell Sea in the Antarctic, starting their two-month expedition on board the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) research icebreaker Polarstern. The target region lies just off the Filchner-Ronne Ice Shelf, in the far south of the Atlantic sector of the Southern Ocean. The aim of the more than 50 researchers involved is to gain insights into the role of interactions and changes in the ocean-ice-biology system with regard to climate change, and to better predict their consequences. “These processes influence not only sea-level rise, but also the global carbon cycle, and with it the ability of the oceans to absorb carbon dioxide (CO₂) from the atmosphere and to store it long-term,” explains Dr Hartmut Hellmer, a physical oceanographer at the Alfred Wegener Institute and head of the expedition.

[Translate to English:] Ronne-Schelfeis — Ronne Ice Shelf (Photo: J. Fruntke)

On the continental slope north of the Filchner-Ronne Ice Shelf, the water depth rapidly plunges from a few hundred metres to over 3,000 metres. Here, large volumes of cold water from the ice shelf and briny shelf water collide and mix with relatively warm deep water from the north. This deep-water formation is a major component of the global ocean circulation, by means of which oxygen-rich and nutrient-rich water flows from the high latitudes toward the Equator, and in return, warm water reaches the polar regions. Thanks to the mixing of water masses, modified warm deep water flows toward the ice shelf – in other words, the part of the glacier that floats on the sea – and can melt it from below. “Our data from the years 2014 to 2018 and the measurements from our Norwegian and French colleagues show that in 2017, warm deep water spread more intensively and further toward the ice shelf than in the reference years. Therefore, we’re interested to see what the measurements since 2018 tell us,” reports Hellmer. According to the AWI oceanographer: “Permanent warming would affect the ocean circulation beneath the entire Filchner-Ronne Ice Shelf. Our models show that by roughly the middle of this century, the ice shelf could melt more markedly from below, which could accelerate the flow of inland ice as a result. The additional influx of freshwater would lead to a rise in sea level and changes to the ocean circulation and sea-ice formation, with far-reaching consequences for the biology of the upper water column.”

Mooring work on board the German research icebreaker Polarstern near Filchner Ronne Ice Shelf, Weddell Sea, Antarctica. — Mooring work (Photo: Alfred-Wegener-Institut)

Since the last Polarstern expedition to the region, in 2018, measuring devices anchored to the seabed have been recording the temperature and salinity, as well as the strength and direction of the currents, at various depths. To access the data, the devices now have to be retrieved. Then, outfitted with new batteries and memory cards, they will be returned to the ocean floor to resume their long-term recording of oceanographic parameters.

The target region is not only vital for global deep-water formation, but is also one characterised by intensive algal blooms, especially in the summer months. “Accordingly, during the expedition we will be investigating how much of the atmospheric carbon is taken up by phytoplankton in surface waters, what fraction of the algae bloom later sinks down from the surface, and how much additional carbon is exported from the continental shelf to the deep sea as a result of deep-water formation,” explains Dr Moritz Holtappels, a biogeochemist at the Alfred Wegener Institute. The transport rates of this bio-physical carbon pump depend not just on the sea-ice cover, but also on the availability of nutrients and trace metals, which determine the growth and composition of the algal communities and their predators. The carbon pump delivers nutrients to the depths, making it an essential lifeline for the unique fauna of the Southern Ocean sea floor. In addition, the research conducted on the carbon cycle will seek to identify how the pump influences the distribution and diversity of the fauna, and to assess the importance of these ocean regions and the life in them as a sink for atmospheric carbon.

[Translate to English:] Besenderte Weddellrobbe — Tagged Weddell Seal (Photo: C. Oosthuizen (MRI))

Soon, the experts will be supported in their work – by seals: up to twelve Weddell seals will be fitted with sensors that measure seawater salinity, temperature and depth. Biologists will affix the transmitters to the seals’ heads with adhesives, and they will be shed along with their old coats at the next annual moult. Until then, the transmitters will send the data collected underwater to their home institutions via satellite whenever the animals surface. Their diving patterns beneath the ice will also show where there are likely to be large numbers of organisms to feed on, since these are the only places where the seals will spend long periods of time.

Just how long Polarstern will be able to remain in the southern Weddell Sea will depend on local sea-ice conditions: by the end of the southern summer in March, when the days become shorter and temperatures drop, it will be time to head back north. “At the moment, the sea-ice conditions look promising on the satellite maps. We’re confident that we will be able to complete all our planned work in the southern Weddell Sea. But of course we also have a backup plan for a region further north, in case the ice conditions put an end to our activities in the Filchner Trough,” says Polarstern’s captain, Stefan Schwarze. By the second half of March at the latest, the research icebreaker will reach Atka Bay. There she will pick up the outgoing overwintering team from the Neumayer Station III, along with the technical team and the researchers who worked at the station during the summer season. After a short journey back to Port Stanley, most of the passengers will fly back home, while Polarstern, with a small group of researchers, will begin the return voyage to Bremerhaven, where she is expected to arrive in late April.