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FRINATEK-Fri prosj.st. mat.,naturv.,tek

FRActal properties of Sea Ice Leads and their impact on the Arctic physical and biological environments

Alternative title: Havis-råkers fraktale egenskaper og deres innvirkning på det fysiske og biologiske miljøet i Arktis

Awarded: NOK 6.3 mill.

The Arctic has proven very sensitive to increased global temperatures, warming substantially faster than the rest of the globe. This has resulted in thinning and reduction in sea ice cover, which is fracturing more frequently than in the past. When the ice cover is fracturing new channels of open water, also called leads, are formed. In these leads, the exchange of energy (e.g., heat) between the atmosphere and the ocean are much stronger than when the ice cover is present and is acting as a shield between the two. These energy exchanges in the leads and their influence on climate are actively debated, partly because current numerical models have shortcomings in representing them. By using a new generation of modelling tools, FRASIL will enable a new perspective on the changes endured by the Arctic and their consequences on marine life. Our main achievement in the project is to have developed a fully coupled ice-ocean model using the next generation sea-ice model, neXtSIM. This development was challenging for both technical reasons and reasons related to the model physics. The neXtSIM model is unique in that it uses both a moving computational mesh and innovative physics formulas to describe the ice motion. In FRASIL, we have overcome the technical challenges of coupling the moving mesh of neXtSIM to ocean models using traditional stationary grids. More importantly, though, we have improved how the model simulates the sea-ice physics related to drift, ridging, and the opening of leads. Previous versions of the model used an approach that gives good short-term results but proved unusable for the long simulations we do in FRASIL, covering two decades. The new and improved model physics is a breakthrough, which allows us to propose a wide range of applications for both neXtSIM and the new physics. The latest result of this work is the newly funded MuSIC project, also funded by NFR. In FRASIL, we have worked hard to take advantage of the newly developed physics and coupled models. We are currently putting the finishing touches on two scientific papers in which we use neXtSIM coupled to the widely used NEMO ocean model. In the first of these, we use the model to study the mass balance of Arctic sea ice. We use the model to investigate how much ice forms, melts, and drifts out of the Arctic. We also study ice growth in leads because the new model captures this better than previous models. The model tells us that between 20 and 30% of the Arctic sea ice is formed in leads. It also tells us that this fraction has increased over the last two decades. Another study we have undertaken with the new model focuses on the age of the ice. We can generally partition sea ice in the Arctic into two categories: first-year ice and multi-year ice. The first-year ice has yet to survive a summer melt, while multi-year ice may have survived many summers. First-year ice and multi-year ice have several different physical characteristics. This makes it possible to distinguish between the two from satellite images. But satellite only sees the ice surface while the model can tell us the ice thickness in addition. We, therefore, use the model to investigate years of extreme sea-ice loss to get a better idea of the fate of the ice. Our investigations show very different processes at play in the two years when we have the greatest sea-ice loss, 2007 and 2012. In 2012 ice was lost because it melted, most likely due to warm conditions and thin ice. In 2007, on the other hand, ice was not really lost but rather compressed against the Canadian and Greenland coast. A process not readily visible from satellite. Finally, we have studied the impact of leads on fluxes between the ocean and atmosphere through leads in the ice. In one paper, we investigate some mathematical properties of leads. These results help us better understand the processes involved and can help improve their representation in climate models. The other avenue of investigation has been the role of leads for light and biological activity. For this, we have used the ocean model HYCOM coupled to the biogeochemistry model ECOSMO. We've used the neXtSIM-HYCOM-ECOSMO system to investigate the impact of leads on both biology and oceanography in the Arctic. The model tool developed here can be further used to understand how sea ice is important for other aspects of the marine ecosystem. This includes the effect of leads on export production, which is important for both carbon sequestration and the influx of food to deep and benthic ecosystems. Furthermore, the leads open a window to the atmosphere, so by including carbon chemistry with the ecosystem model we can quantify the air-sea CO2 flux through the leads. Our work in FRASIL has also opened avenues to more research and more discoveries on the interplay between ice, atmosphere, ocean, and biology. Research that we hope and expect will improve our understanding of Arctic climate and biology in years to come.

The most important outcome of FRASIL is that we demonstrated a successful coupling between an ocean model and a sea-ice model using a brittle rheology. The rheology is at the heart of a sea-ice model, dictating how the movement of the ice changes with changing winds and ocean currents. Despite this central role, alternative rheologies are few and far between. To date, every CMIP model and all geophysical-scale forecasting, reanalysis, and scientific sea-ice models use a rheology whose physics have not changed since the late 1970s. In FRASIL, we have demonstrated that a credible alternative to the classical approach exists and can be used to deliver new and exciting scientific results. Our progress in FRASIL has allowed us to attract further funding for our research, both national, European, and American. As such, FRASIL has been a cornerstone for sea-ice research at the Nansen Center, eventually impacting the global sea-ice modelling community.

The Arctic has proven very sensitive to increased global temperatures, warming substantially faster than the rest of the globe. This has resulted in thinning and reduction in sea ice cover leading to a new dynamical regime in which sea ice fracturing and ridging are more frequent. The fractal properties of the ice cover and its extreme variability strongly influence the atmosphere-ocean interactions and the dynamics of the Arctic marine ecosystems. These interactions and their influence on climate are actively debated, although the supporting model premises are presently weak. FRASIL will enable a new perspective on the changes endured by the Arctic and their consequences for marine life. In this project, we will assess the role of sea ice dynamics on the upper part of the Arctic Ocean energy budget and on primary production using for the first time a Lagrangian sea ice model, neXtSIM, coupled to an ocean-marine ecosystem model. The neXtSIM model is currently being developed at the Nansen Environmental and Remote Sensing Center, and is unique among sea ice models owing to its rheological framework that is based on solid mechanics and allowing to reproduce the multifractal scaling invariance of sea ice deformation with an unprecedented realism. Leads opening in the ice will change the fluxes of heat and light penetration through the sea surface and the lower trophic levels of the marine ecosystem. Sea ice deformations also impact melting and freezing in leads, ridging and sea ice circulation, which are key players in determining sea ice mass balance and age, and freshwater mass distribution in the Arctic Ocean. Advancing the knowledge on the effects of sea ice deformations on upper ocean stratification and ecosystem will have profound implications on our ability to forecast ongoing changes in Arctic Ocean. The new coupled system will represent a major contribution towards giving the Norwegian research community a leading role in studying the Arctic climate system.

Publications from Cristin

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FRINATEK-Fri prosj.st. mat.,naturv.,tek