Do you know how sea ice grows during winter in the Arctic? This is what a recent study led by Guillaume Boutin at the Nansen Center investigates, focusing on how cracks in the sea-ice cover impact sea-ice growth, using our unique sea-ice model neXtSIM.
Formation and impacts of cracks in the sea-ice cover
Envision the Arctic sea-ice cover. Do you see it as a flat, white surface extending over thousands of kilometers on top of the Arctic Ocean? Well, almost! But it is not flat, instead it is full of ridges and cracks separating large pieces of ice. These cracks, called leads, are generally long (from ten to hundreds of kilometers) and narrow (up to a few kilometers). Leads are the results of sea ice breaking as it is moved around the Arctic by winds and ocean currents. In winter, when sea ice covers most of the Arctic, these leads are the only openings through which the ocean is directly exposed to the atmosphere. That means that within the leads, ocean water gets exposed to the extremely cold and dry air. New ice forms as a thin layer on the water surface, until the lead closes again (because of ice motion, or because the ice layer becomes thick enough to fill the lead). At the same time, heat and moisture escape into the air above the lead. This means that leads control how much exchange of heat happens between ocean and atmosphere. Therefore, climate scientists suspect that leads affect both weather and climate, but they have not managed yet to put a number on how important these features are.
Using models to study sea ice
Ideally, scientists would use observations from across the Arctic to answer this question, but fieldwork in the polar regions is expensive, logistically challenging, and dangerous. Therefore, the only option left is to use a numerical model, simulating the evolution of sea ice on computers. Scientists use sea-ice models for different applications, from short-term forecasts of ice conditions relevant for safe shipping in Arctic waters, to long-term climate projections. However, when it comes to leads, they face a challenge: Generally, sea-ice models are not good at reproducing these. This has been the motivation for developing neXtSIM, a sea-ice model developed at the Nansen Center in collaboration with colleagues in Grenoble, France. neXtSIM has special features making it unique among other sea-ice models out there, and these features make it particularly good at reproducing sea-ice breakup and leads in the ice cover.
New findings: about one third of new ice forms in leads during winter
As part of the Nansen Legacy project, Guillaume Boutin and his colleagues looked at the impact of leads on winter ice production for the years 2000-2018 by combining neXtSIM with an ocean model (NEMO). Over this 18-year period, they found that between 25 and 35 % of the new sea ice produced every year in the Arctic forms within the leads. These values are supported by the few observations available, but this is the first time such an estimation is performed over such a long period and for the whole Arctic. Another finding by the team is that this amount significantly increases over the period they studied: Ice growth in leads is 10% bigger in the recent years than in the early 2000s! Guillaume Boutin commented on this phenomenon: “This increase may be a symptom of climate change: As the ice gets thinner, it is easier to break it, which may produce more leads, causing more new ice to form within them.” He and his colleagues will try to confirm this hypothesis in their future work!
