Researchers from the Nansen Center and colleagues in France, Germany, and Mali have improved how to simulate sea-ice movement in the Arctic. Their sea-ice model neXtSIM with a new and unique rheology is very good at producing accurate sea-ice forecasts and can possibly enhance climate predictions.
Are you wondering now what “rheology” means? Don’t worry, we’ll get to that. But first, let’s start with sea-ice modelling and what it is good for. Researchers use computer models to investigate how sea ice in nature behaves, both in the past, the present, and the future. To do that, they use mathematical formulas to recreate processes as close to reality as feasible. This way they can for example forecast sea-ice conditions such as thickness and extent for the upcoming days (short time scales) or predict how fast the ice will melt with a warming climate (long time scales). Since the Arctic is warming much faster than the rest of the planet, scientists are extremely interested in what goes on there, both right now, and in the future.
Sea-ice conditions are important for human activities in the Arctic, such as shipping, fishing, tourism, and research. The position of the sea-ice edge also has important implications for life in the ocean – along the ice edge, animals find a lot of food that is otherwise scarce. And more reliable climate predictions for the coming years are necessary to develop efforts for effectively adapting to climate change. To be able to forecast how the sea ice will behave on short and longer timescales is therefore relevant on different levels.
The sea-ice model neXtSIM has been developed over the past years by researchers at NERSC in collaboration with researchers from France, to produce the best sea-ice forecasts and to improve climate predictions involving sea ice. It is functioning really well now, and its latest modifications are introduced in the recent article “A new brittle rheology and numerical framework for large-scale sea-ice models”. neXtSIM is already producing forecasts available to everyone with the new rheology 10 days ahead of time, benefitting people operating in the Arctic Ocean. But how do researchers generally get a model to simulate the ice movement correctly?
In the Arctic winds and ocean currents act on sea ice, and the ice reacts to these forces. The study of how materials deform and move in response to a force is called rheology, the word originates from Greek and translates to “study of flow”. In sea-ice models, the rheology is a description with mathematical formulas of how sea ice responds to winds and ocean currents.
Different sea-ice models use different formulas (rheologies) to define the sea-ice response to winds and ocean currents. Many simulate sea ice like something soft, but we know that it cracks in a brittle way when responding to force. All mathematical descriptions of sea-ice rheology have some kind of flaw, and the researchers at the Nansen Center and other institutions involved in developing the sea-ice model neXtSIM have been using the so-called Maxwell-Elasto-Brittle rheology in the past years. It performed well on short time scales, but they were not satisfied with its performance when predicting changes over multiple years. So, they investigated how to modify this specific rheology to make neXtSIM produce even better short- and long-term forecasts.
Interestingly, rocks and sea ice break in a very similar way, and researchers in France developed a sea-ice rheology based on rock mechanics. The sea-ice modelling team at the Nansen Center implemented it in neXtSIM, and the team around Einar Ólason recently further improved that rheology. Extensive testing has gone well: neXtSIM-produced sea-ice forecasts and actual satellite images of the Arctic show the same features. The model is now able to simulate nature as closely as currently possible.
Ólason and his colleagues have recently published a paper on the new rheology, which they call the brittle Bingham-Maxwell rheology. This new rheology is the last step in the chain from rock mechanics models to a full-scale sea-ice model. It includes a way to include damage propagation: When a crack forms in sea ice, it freezes over and is pushed shut again, but this former line is more prone to breaking again, because the ice is weaker along it. Including formulas for this allows our researchers to represent deformation happening to the Arctic sea ice on small and large scales, from hundreds of kilometers down to kilometers, or even meters. This means that cracks in the sea ice can be simulated reliably, and the forecasts produced with neXtSIM are generally better than forecasts produced with other sea-ice models. Better simulation of the cracks gives a better representation of atmosphere-ocean-ice interaction in the model, which will hopefully lead to more reliable climate predictions.
