Following the annual meeting for the Nansen Legacy project, which the Nansen Environmental and Remote Sensing Center is a partner of, we want to present our research contributing to the project. NERSC provides a sea ice model to the project, which has important implications.

“What we at the Nansen Center do in the frame of the Nansen Legacy project is developing and improving our sea ice model – neXtSIM. Sea ice plays an important role in the Arctic and the climate system and understanding how sea ice behaves helps us in many ways. We think that our new sea ice model is a really exciting tool that’s helping us learning new things about sea ice and atmosphere-ocean-ice interactions all the time.”, says Einar Ólason, who is leading Sea Ice Modelling research at NERSC and is a collaborator in Nansen Legacy.

Fractures in sea ice in the Arctic Ocean: Credit: NASA/Sinead Farrell; https://www.nasa.gov/topics/earth/features/arctic-seaice-study.html

Sea ice in the Arctic

Let us start out simple and make our way to the Arctic: As we are heading north, we go from open ocean into the marginal ice zone, where ice floes loosely drift in the ocean. The further north we go, the thicker the ice gets, and the ice floes freeze together into one compact “lid” of ice floating on the Arctic Ocean, which is constantly shifting, breaking, and refreezing. So, sea ice is very dynamic, and it drifts. Both the atmosphere and the ocean influence where and how sea ice forms, changes, and moves. Wind, precipitation, and air temperature, as well as water temperature and currents influence its behaviour. One particularly important feature in sea ice are fractures breaking up the ice cover; fractures that can be up to hundreds of kilometers long.

Fractures in sea ice

These fractures in sea ice, also called leads, actually resemble cracks in crustal rocks quite closely, both in where they form and how they propagate. Earthquakes generally happen in weakened zones where earthquakes have been occurring before, such as the San Andreas Fault in California. Surrounding a large crack in crust caused by a strong earthquake, you find smaller fractures occurring triggered by milder earthquakes nearby. Sea ice behaves just like that. Sea ice cracks open in zones that experienced fracturing earlier, since the ice is less robust there. A fracture that opens, keeps spreading further and further through the ice, and causes smaller fractures that weaken the sea ice nearby.

Effects of sea ice fractures

Fractures opening up in sea ice do not just affect the sea ice itself, as they have a considerable effect on the air above the sea ice and the ocean below. The air in the Arctic can be as cold as -40 °C above the sea ice, while the ocean temperature right below the sea ice is about -2 °C. This difference is thanks to the sea ice’s excellent insulating property. With sea ice cracking open, these two vastly different regimes come into contact: “Warm” freshwater rapidly evaporates from the ocean into the air, leaving the salt behind. The ocean loses heat and moisture to the air and is left behind with saltier water. Saltier water is heavier than water with lower salt concentrations, and it sinks down. These downwelling plumes of saltier water impact the ecology and oceanographic processes in the Arctic locally, and likely also regionally. Being able to model this improves our understanding of this unique system. And this is possible thanks to the sea-ice modelling group at NERSC, as well as cooperating scientists at other institutions.

Simulating sea ice break-up in the Beaufort Sea: In 2013 a large break-up of the winter sea ice cover occurred in the Beaufort Sea. For the first time, this event is simulated using the state-of-the-art sea ice model neXtSIM. In the figure the formation of extensive fractures and leads are clearly visible and is associated with a large increase in the ocean-atmosphere heat flux (Wm^(-2)). Heat from the ocean entered the atmosphere above fractures and is indicated in warm colours. Such break-up events may have widespread implications including Arctic sea ice volume and ocean circulation. Figure produced by Jonathan Rheinlænder, NERSC

Sea ice modelling

neXtSIM, NERSC’s sea ice model, is the only model to date that simulates fracture propagation well. What makes the model special is that it utilizes how similar fractures in crustal rocks and in sea ice are. The sea ice rheology used in neXtSIM – how sea ice deforms – is based on rock mechanics, and the results are very promising! neXtSIM forecasts are part of the Copernicus Marine Service for the Arctic, and the model is constantly being tested and improved to provide the best possible sea ice forecast.

Importance for the Nansen Legacy project

A sea ice forecast is relevant on short time scales. When we can model occurring fractures in the sea ice reliably, we know how the atmosphere in the Arctic is impacted directly; heat and moisture increase locally over opening fractures. This increase has a regional influence that we need to understand to assess Arctic atmospheric processes – like the weather – on a short time scale.

The effect of fractures on the ocean is relevant for ecological observations, as well as climate studies. The drastic salinity change in the ocean below an opening fracture affects ocean currents, which has an influence on both the Arctic ecological system and on climate in the long run. Modelling the sea ice on a longer time scale helps us to better understand how the Arctic ocean, its ecosystem, and the regional climate will respond to changes in the future.

Both the short-term sea ice forecasting possible with neXtSIM and long-term influence of sea ice behaviour on climate and ecosystem changes are important to the Nansen Legacy project. By accurately modelling sea ice we can vastly improve our understanding of this unique area, the Arctic.

If you want to hear more about the neXtSIM model and its applications, check out this podcast with Einar Ólason, produced by the Bjerknes Centre for Climate Research: https://bjerknessenteret.podbean.com/e/nextsim-the-next-generation-of-ice-forecast/

Facts on the Nansen Legacy project: