Researchers at the Nansen Center developed a method to track the evolution of multi-year ice more reliably in our sea-ice model neXtSIM.
First things first: What is multi-year ice? It is the sea ice that survived at least one summer in the Arctic Ocean, so one melt season. And from satellites we know that the sea ice in the past decades has been decreasing. They give us reliable information on the area of multi-year ice, so how large the area is that is covered with multi-year ice after the summer melt, but the volume is a different story. The volume takes the ice thickness into account, and it represents the actual amount of multi-year ice surviving the summer. Observations from satellites are not entirely reliable here. Understanding how much multi-year ice actually survives the summer melt is relevant, as climate models predict ice-free summers in the Arctic within this century. Such a change will lead to many changes in conditions in the Arctic Ocean, two of these being more shipping opportunities, and increased warming through the colour change from white to black.
Researchers from the Sea Ice Modelling group at the Nansen Center set out to find a way to assess the multi-year ice volume, and they succeeded. They recently published their findings. Under the lead of Heather Regan, they used the in-house sea-ice model neXtSIM to simulate both multi-year ice area and volume between 2000 and 2018. They also wanted to find out if neXtSIM could be used to differentiate between different processes that affect the multi-year ice.
Heather Regan and her colleagues found that the area and volume have both been declining in these 19 years, but interestingly this has not been constant. The largest declines in multiyear ice area in the model occurred in a few key years in the model. The processes that affect both the volume and area of multi-year ice are melting, export (ice floes drift south and leave the Arctic), and replenishment (new is ice formed during the previous winter and survives a melt season). One of the key years of large multi-year ice area decline was due to high melting and low replenishment, and also saw a large decline in multi-year ice volume. But Heather Regan and her colleagues quantified another important process to consider when looking at multi-year ice in the Arctic: ridging. Ice can be pushed on top of each other along weak zones, forming a ridge. And while the area is reduced by this process, the volume is not. In one of the key years of decline, this process caused a big reduction in area but not volume. This important finding highlights the benefit of using sea-ice models like neXtSIM in addition to satellite observations when interpreting how the multi-year ice evolves over time in the Arctic.
