Climate Research in the Southern Ocean

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.”

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.