159 years ago today, a legend was born: Fridtjof Nansen, a man who was not afraid to risk his life to prove that sea ice in the Arctic Ocean is drifting. His legacy lives on today, for example through the work we do at the Nansen Center in the Sea Ice Modelling group, which is connected to the Nansen Legacy project. I, Guillaume Boutin, want to take you on a journey to the Arctic and tell you about how Fridtjof Nansen’s research still inspires our research today.

Two polar bears in the Arctic in August 2020: Photo: Espen Storheim, NERSC - KV Svalbard research cruise, CAATEX 2020Imagine being on the wooden ship “Fram” in the Arctic Ocean, frozen into sea ice, around 127 years ago. Onboard is the very determined Fridtjof Nansen leading this expedition to prove the theory that there is an east-west current in the Arctic Ocean. This current, the transpolar drift, is supposed to transport you and your frozen ship across the North Pole. If Nansen is wrong, all the crew will likely never see home again. But the men trust their charismatic leader, and they will carry on their journey until the end - without him though. He decided to take the mission further to try to reach the North Pole on skis together with Hjalmar Johansen, being fed up with the unpredictable and slow progression of the ship, and well aware of polar bears posing a real threat. What a character. 

Sea ice is drifting in the Arctic

Nansen was right about the transpolar drift: sea ice in the Arctic moves! Up to a few tens of kilometres a day, not so bad without any sail. Actually, the main driver of sea ice is the same as for the “Fram”: the wind. The sea ice surface is not smooth: when the wind blows, the irregularities at the surface of the ice cover can “feel” the pressure exerted by wind. This pressure pushes the ice floes: the drift starts. Nansen understood that, he even estimated that the ice drift velocity was about 2% of the wind velocity. But what about the direction of this drift: if the wind is pushing the ice, then it should drift in the same direction as the wind, shouldn’t it? Not exactly. As Nansen had also very smartly noticed, the ice often drifts with an angle of about 30 degrees to the right of the wind direction. It might have been a simple rule of thumb, but ice drift prediction was born.

Sea ice in the Arctic in August 2020: Photo: Espen Storheim, NERSC - KV Svalbard research cruise, CAATEX 2020

The drift is very complex

Sadly there are limitations to the rules established by Nansen. They apply pretty well in lots of conditions though, particularly in the summer or close to the ice edge, where sea ice is not too compact. But when sea ice is very compact, or if the sea surface currents are very strong, then it becomes a little more complicated, making navigation in sea ice a real challenge, even for the toughest icebreakers! Fortunately, nowadays we have techniques to predict how the sea ice moves in a more accurate way, as we understand better what is driving the sea ice. There is the ocean below the ice of course. Very strong currents can be the main driver of the ice, but most often the ocean surface only poses resistance to the ice drift due to the friction under the ice. The ratio between the push from the wind compared to the resistance of the ocean can be estimated, giving a theoretical ice drift speed of about… 2% of the wind speed! There is also the Coriolis Force, which is responsible for the turning angle of about 30 degrees. And finally, there is the ice itself which is solid, and solids have a behaviour that is quite hard to represent. Especially for an oceanographer like me, we are more used to liquids!

Guillaume Boutin onboard Pourquoi Pas? in the Arctic in August 2017 with the sea ice edge in the background

We are improving Sea Ice Modelling at the Nansen Center

My work as part of Nansen Legacy is to improve the behaviour of the sea ice as a solid floating on the ocean. Sea ice breaks, then cements, then breaks again, then collides, piles up, diverges, melts, freezes… making it a nightmare for the numerical models we use to predict sea ice drift. At NERSC where I work, we have developed a numerical model, neXtSIM, that represents all these behaviours of sea ice very well. The development of neXtSIM has been a long and tough challenge led initially by Pierre Rampal and now by Einar Olason, but now it is working well, and we can start using it to better understand sea ice. I have tried to make neXtSIM work in connection with an ocean model, to have a complete representation of the Arctic Ocean. I spent a lot of time tuning this model to get the best ice drift possible compared to satellite observations, and I am quite proud of my results! It means that our model represents the sea ice dynamics well, which will help us to better understand the way sea ice is formed, transported and deformed in the Arctic. 159 years after Nansen was born, we have learned a lot about sea ice, but there is still so much to discover!

Nansen Legacy Project website: https://arvenetternansen.com/

Model image, compiled by Guillaume Boutin: The percentage of the sea surface covered by sea ice (Ice fraction) superimposed on a map of the sea surface temperature (SST) given by the coupled model we develop. The date shown here is October 10th 2011, Nansen’s 150th birthday. The data used to force the model (for example wind and temperature) come from the ERA5 dataset distributed by the European Centre for Medium-Range Weather Forecasts (ECMWF). The development of the coupled model is a result of a collaboration between NERSC, Ifremer (Brest, France) and OceanNext (Grenoble, France).

Observational image, compiled by Guillaume Boutin: Sea ice fraction for the same date as estimated from a satellite using passive microwave sensor (SSMI). Product distributed by CERSAT/IFREMER (http://cersat.ifremer.fr). The black hole at the pole is due to missing data.