MOSAiC was the largest Arctic research expedition in history. The goal was to spend a full year drifting with Arctic sea ice in order to improve understanding of climate change right in the epicentre of global warming, the central Arctic Ocean.
By: Benjamin A Lange, Dmitry V Divine and Mats A Granskog // Norwegian Polar Institute.
Polona Itkin and Jessie Gardner // UiT The Arctic University of Norway
The MOSAiC expedition lasted from September 2019, when the German icebreaker Polarstern left Tromsø until the ship returned to Bremerhaven, Germany, in October 2020. Polarstern reached the northern Laptev Sea by mid-October, just in time to start the drift at the beginning of polar night.
Polar darkness and the harsh winter climate are challenging enough on land, but sea ice research adds a dynamic component. The ice floe where we set up the central observatory was thin, only half as thick as the typical Arctic drifting stations built on old multiyear ice floes. Researchers working on the ice were often forced to retreat from opening cracks and emerging mountains of ice ridges.
In November, a fierce storm cracked our observatory floe into two pieces and slid them 600 m in opposite directions.
When weather calmed, the work still wasn’t easy. The combination of temperatures below ‑30°C, sharp crystals of ice (what we call diamond dust), strong winds and complete darkness is not just unpleasant, but can become fatal. Fiddling with small instrument components and tools can result not only in screws being lost in the snow, but also to frostbite and hypothermia. To prevent such outcomes, we had to work slowly and carefully. This made each measurement exceptionally precious (but not as precious as our hands and noses!). We meticulously planned every new measurement, practiced on board, walked only on established paths, and honoured the forces of nature by being flexible in our planning and scheduling shifts of watchful eyes from the ship’s bridge.
Observatories on two different ridges were slowly being filled with instruments. The data from those installations showed that most of the newly fallen snow was trapped by ridges, while the snow blanket on level ice remained meagre. Thin and patchy snow cover allows more heat to escape from the ocean through the ice to the atmosphere and promotes ice growth.
Nevertheless, the well-insulated ridges continued to gain ice. The water-filled voids in the ridges were freezing. Although the keels were not getting any deeper, ocean currents filled the rough underside of the deformed ice with platelet ice crystals growing in the supercooled ocean beneath. The complex labyrinth of ice blocks in the ridge keels also offered protection for juvenile cod who could hide there from predators like seals.
At the end of the workday (or night actually) ample breaks on the warm ship were simply essential, and our sacrifices were rewarded by wholesome German cuisine. The threat of running out of potatoes, cabbage and sausages only became real towards March, when the logistical support of the Russian icebreaker Kapitan Dranitsyn was much delayed by the power of the winter ice pack.
A few months later, when vital re-supplies were again required, and a rotation of participants and crew was scheduled, the pandemic had been declared around the world. The preparation for MOSAiC took over a decade and was an unprecedented logistical feat, but nobody planned for the coronavirus outbreak. Participants had to travel to Bremerhaven, Germany to undergo two weeks of isolation. Once confirmed coronavirus-free they sailed onboard the research vessels Maria S Merian and Sonne to meet Polarstern near Svalbard. But this required Polarstern to leave the ice floe temporarily.
When Polarstern returned to the floe, the ice was rapidly melting beneath the scientists’ feet. Light and (relative) warmth had returned to the Arctic Ocean, bringing with them teeming life. Due to the rapid melt, a lens of fresh water formed beneath the ice, and at the boundary between fresh and salty water, a bright green layer of algae was observed (the so-called Oreo layer – yummy!). To help capture the dynamic biological activity, sediment traps were hung below both level and ridged ice using a robotic vehicle. Interestingly, the samples taken under ridges indicated more flora and fauna than those taken under level ice. Additionally, within the ridges, pockets of isolated water were discovered to be rich in algae, a feature which was absent in level ice.
However, the voids within ridges disappeared at the start of July due to the infiltration of fresh meltwater, originating from melting ridge keels and snow, which subsequently refroze. The replacement of voids by fresher ice likely resulted in the loss of suitable habitat for inhabitants of ridge voids. It was unexpected to have such an important refreezing event during the peak of melt season but capturing the unexpected is always exciting!
MOSAiC was an unprecedented research expedition that will provide invaluable information to improve our understanding of the Arctic climate system for decades to come.
MOSAiC followed in the footsteps of the legendary explorer Fridtjof Nansen, whose revolutionary expedition in 1893-1896 froze the wooden sailing vessel Fram into the ice to drift across the Arctic Ocean. The conditions experienced during Fram’s sojourn in the ice are something the MOSAiC scientists can only imagine. Still, we endured challenges far outside our everyday comfort zone.
Our fingertips have now recovered from the cold and we are working hard on analysing the data. In the years to come our findings will be integrated into Earth System Models to improve global climate change predictions. This was – and is still – the ultimate goal of the whole interdisciplinary research community involved in MOSAiC.
Sea ice ridges are formed when level sheets of ice are crushed into mountains of ice blocks forming ridge “keels” under water, and ridge “sails” above the ice surface.
These topographical features provide traps for snow and enhance wind- and surface ocean-induced drag and therefore turbulent mixing, likely introducing higher nutrient fluxes and more favourable conditions for algal growth.
The submerged part of a ridge provides a unique habitat for a range of organisms, from microscopic algae to the larger polar cod, seals and polar bears. Apart from providing a large surface area for colonisation of algae and a refuge from predators, ridges modify the light regime and nutrient fluxes experienced by sea ice algae. First-year ice ridges may become a refuge for ice-associated organisms during summer melt. Thus, a projected shift to a more dynamic ice cover with more ridges is likely to be a key factor in terms of sea ice habitat availability and type. Despite their importance, ridges are understudied habitats in the Arctic due to the difficulty of sampling in such thick and complex ice structures.
The MOSAiC expedition
Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) spent a year drifting through the Arctic Ocean on the German research vessel Polarstern. The goal of the MOSAiC expedition was to gain better understanding of the Arctic climate system. Hundreds of researchers from over twenty countries were involved.
Projects undertaken within MOSAiC:
- HAVOC (Ridges – Safe HAVens for ice-associated Flora and Fauna in a Seasonally ice-covered Arctic OCean) was the largest Norwegian project to participate in MOSAiC, and was led by the Norwegian Polar Institute. The project, funded by the Research Council of Norway, allowed six researchers from Norway to participate in the expedition to study sea ice ridges and their role in the Arctic sea-ice system.
- PHOTA (Physical drivers of ice algal HOTspots in a changing Arctic Ocean) is a project funded by Fram Centre Arctic Ocean flagship. It investigates spatio-temporal variability of biophysical properties of different sea ice habitats.
- SIDRiFT (Sea Ice Deformation and Snow for an Arctic in Transition) and MiSNOW (MOSAiC snow) are funded by the Research Council of Norway and the National Science Foundation, and led by UiT and Colorado State University. Both projects will use MOSAiC and satellite data, and numerical models to estimate the role of ice ridges in the insulation of the ocean from atmosphere and in sea ice growth.