You have no doubt heard about “albedo feedback”, a loop of connections that warm our planet. The bright sea ice cover in the Arctic melts, exposing a dark ocean surface that absorbs more solar energy and accelerates global warming. But are there other Arctic feedback loops that can change our climate?
By: Polona Itkin // UiT The Arctic University of Norway
During winter the thin layer of sea ice between the atmosphere and ocean is cold and brittle. The force of the wind can easily fracture it. As the wind continues to blow, it can tear the shattered ice plates apart to expose the ocean water. Such wide fractures are known as leads. When fractures are pushed together and the ice piles up along them, ice ridges form. Multiple leads and ridges can aggregate into joined-up systems that can stretch across the ocean for hundreds of kilometres. In any given place, some wind directions are more common than others, and the same holds for the Arctic.
These prevailing winds drive sea ice currents – large, slow rivers of ice that can transport the fractured ice plates from one side of the ocean to the other. Taken together, this is a complex process: it starts with fracturing at the millimetre scale within fractions of a second, but may ultimately result in pan-Arctic features that have consequences lasting the entire winter. What kinds of climate-relevant feedbacks are hiding in that process?
Sea ice deformation, fracturing, and motion (kinematics) are connected to transfer of motion, heat, and light between the atmosphere and the ocean. Sea ice deformation generates leads and pressure ridges –rough features that further enhance the motion coupling. Deformation also diversifies the sea ice thickness and increases the winter sea ice volume. At the same time, leads are open windows that let the heat out from the ocean into the atmosphere and let light into it.
The ultimate impact of fracturing on sea ice resilience to the summer melt and climate change is unknown, as the effects are mediated by the albedo feedback, melt water accumulation, sea ice strength, and other processes. Leads and ridges are also crucial in ship navigation, where the former are desirable (often described as “highways to the Arctic”), while the latter are hurdles to be avoided.
From October 2019 to October 2020, interdisciplinary teams totalling more than a hundred scientists from many countries drifted on board the research ice breaker Polarstern with the ice current, from the northern Laptev Sea into the Fram Strait. The expedition known by its acronym MOSAiC (for Multidisciplinary drifting Observatory for the Study of Arctic Climate) started from Tromsø in northern Norway and collected a lot of data also about sea ice fractures, measured their motion, and observed lead and ridge formation.
At the same time, numerous radar images were taken from space satellites, recording the large scale fractures. We are now facing the exciting challenge to implement our observations into numerical models that simulate the weather and climate. The challenging task of combining on-site measurements, satellite observations, and numerical models will answer the big question: Do sea ice fractures accelerate or slow down climate change?
Most of what we know today about Arctic sea ice fractures and subsequent deformation is based on satellite remote sensing and drifting buoy trajectories. Ice fracture processes have also been explored in laboratory and floe-scale experiments. Furthermore, the engineering community has collected a lot of data on pack ice driving forces that should become part of the geophysical sea ice deformation puzzle. MOSAiC was designed to help implement this knowledge into the climate models. Three years after the end of the expedition, much of the sea ice deformation data has been analysed, but about the same remains to be examined and interpreted.
This contribution was written as an outcome of the “Sea ice deformation workshop” organised in Tromsø, Norway from 26 to 28 November 2023 and funded by the Research Council of Norway, via the project SIDRiFT (287871). Workshop participants were: Nik Aksamit, Angela Bliss, Mirjam Bourgett, Dmitry Divine, Wenkai Guo, Jari Haapala, Jennifer Hutchings, Nils Hutter, Polona Itkin, Jack Landy, Sönke Maus, Chris Polashenski, Pierre Rampal, Robert Ricker, David Clemens Sewall, Catherine Taelmann, Matias Uusinoka, Luisa von Albedyll and Daniel M Watkins
We suggest the following actions to be done in the coming years:
- MOSAiC observations need to be offered on standard grids. Most of the raw data are freely available to date, but also the sea ice kinematics, deformation, lead, and roughness products need to be added in formats that are referential in space and in time. Only in such a way can they be picked up by the numerical modelling community.
- We need to use the radar satellite remote sensing images to create an updated pan-Arctic sea ice deformation product, fused with the sea ice thickness products from altimetry. This will enable research of the feedback between sea ice deformation and thickness distribution.
- While none of direct measurements or remote sensing observations can measure fracturing properties or consequences at all spatial and temporal scales, the same is true for numerical model simulations. Several models need to have alternative rheologies (representations of ice kinematics) implemented, so that they can be compared more efficiently. Especially the newly developed brittle-class rheologies seem to be promising for modelling large-scale sea ice deformation.
- At smaller scales, likely below a kilometre, the assumption of the sea ice having negligible thickness compared to the extent of individual floes breaks down. In this regime, discrete element models developed in engineering can be used instead of continuum models (addressed in the previous point). We need efforts to bridge the methodological gaps between both types of numerical models.
- At even smaller scales, new technologies like sensors on drones and micro-tomography scans of fractures in ice can greatly increase the volume of collected data. Remote sensing radar data are already collected in overwhelming quantities. The Artificial Intelligence technology in data analysis can be used to process the data and finally also optimise parameter tuning in numerical models.*
- We need to better understand the scaling of sea ice fractures from laboratory experiments (1 m and 1 s and smaller) to the geophysical scale (10 m and 1 minute and larger). The data from MOSAiC and other campaigns should be integrated into an observational framework of fractures, their driving forces, and ice conditions at different scales and during all seasons, and especially during freeze-up and spring melt. The next large international interdisciplinary polar activity – the International Polar Year in 2032 – should be used as the ultimate target to finalise this.