Beyond satellite observations of sea ice

The floor vibrates and a crushing sound fills the bridge as the Norwegian research icebreaker Kronprins Haakon breaks through the sea ice. At 87°N and 17°W, north of Greenland and a mere 300 kilometres from the North Pole, the ship is struggling with a type of sea ice that is now almost lost.

Standing next to the ship’s captain, expedition leaders Agneta Fransson from the Norwegian Polar Institute and Bodil Bluhm from UiT The Arctic University of Norway follow the ship’s progress through the ice-packed sea. They almost feel they have gone back in time: back to when sea ice survived more than one summer; back to when the surface of the Arctic Ocean held wide expanses of thick, ridged sea ice that could be several years old.

Photo: Adam Steer / Norwegian Polar Institute

Old sea ice – an almost extinct habitat

Old, multi-year sea ice was once found throughout almost the entire Arctic Ocean. During the legendary Fram expedition almost 130 years ago, the Norwegian polar explorer and researcher Fridtjof Nansen beautifully sketched the shapes of this type of ice, and his meticulous measurements of sea ice thickness bear witness to how thick the Arctic sea ice once was. Since 1979, satellites have provided a consistent and continuous record of Arctic sea ice and documented its vast geographical retreat.

While the extent of Arctic sea ice in summer has decreased at a rate of almost 13% per decade since the satellite record began in 1979, the oldest and thickest ice in the Arctic has declined by 94%. These days, most of the sea ice is much younger and thinner, and the remaining multi-year sea ice is confined to a small girdle north of Canada and Greenland.

These changes affect the local Arctic environment but also the Earth system as a dynamic whole. Therefore, research efforts by both Norwegian and international science teams are focused on investigating sea ice and its interaction with its environment. That is also why Agneta and Bodil are leading a 35-person strong team of scientists deep into the Arctic Ocean as part of the Norwegian Nansen Legacy project.

Where sea ice cracks and leads of open water form, heat and gas are exchanged between the ocean and the atmosphere. This picture, taken during a Nansen Legacy research cruise in the northern Barents Sea in March 2021, shows how heat and vapour rise from the relatively warm ocean water openings in between sea ice. Photo: Sebastian Gerland / Norwegian Polar Institute

Beyond satellite observations

Onboard RV Kronprins Haakon, Adam Steer and Anca Cristea (both from the Norwegian Polar Institute) are getting ready for a new day out on the ice. Here, not far from the Pole, they will use the coming days to construct a fine-scale picture of the ice floe where the icebreaker has docked.

Sea ice is a complex composite assembled from different types of ice at different stages of growth and decay; air, water, slushy ice filling in voids; snow in complex layers of its own. These details are hard to see using satellites, yet critical to understanding both physical and ecological sea ice processes. To map this complex environment, Adam and Anca use a range of different measuring techniques. They will walk across the ice towing an electromagnetic induction sounder, measuring snow and ice thickness along the path. They will calibrate these measurements by drilling holes through the ice, and manually measuring the thickness of the ice and snow. High-resolution drone images of the ice floe will allow the team to map their tracking records of ice and snow thickness in context with the anatomy of the entire floe.

These data will all be assembled – along with similar observations from earlier Nansen Legacy expeditions and larger scale datasets – to construct a far more detailed picture of the ice than what can be retrieved from satellite imagery for better understanding the ice, for improving weather forecast models and for validating satellite remote sensing products.

Adam Steer and Anca Cristea investigate the structure of a large ice floe by pulling behind them an electromagnetic induction probe during the Nansen Legacy Arctic Ocean cruise in September 2021. Photo: Christine Lockwood-Ireland / UiT The Arctic University of Norway

Cracks in the cookpot lid

Creating a detailed description of sea ice anatomy is an important first step in understanding the impact the sea ice has on its environment. Especially during the colder seasons of the year, sea ice acts like a cookpot lid, insulating the relatively warm ocean water from the colder atmosphere above, allowing the ocean to retain heat and moisture while keeping the air cold and dry.

However, where cracks and leads between ice floes expose open water to the atmosphere, enhanced heat and gas exchange takes place. While Adam and Anca are mapping the ice floe, other scientists on RV Kronprins Haakon are taking detailed measurements of the heat and gas exchange through the adjacent leads.

Their measurements will complement data collected during the Nansen Legacy winter and spring field campaigns, and give important ground truth information to sea ice and atmosphere scientists back on land who are investigating the role of Arctic sea ice on its environment through mathematical models.

Details that improve weather forecasts

Malte Müller and his colleagues at MET Norway are working on improving physical models used in weather forecasting. For them, it is challenging to directly simulate the heat escaping from the ocean through sea ice and leads into the atmosphere. It is only by combining on-site measurements from ship campaigns with satellite observations that they can gradually improve the forecast models.

Sea ice is not a simple white layer but a complex composite as indicated in this cross-section through an ice floe. Sketch: Adam Steer / Norwegian Polar Institute

A set of model-based Arctic weather forecasting experiments illustrated the importance of a realistic, field-validated representation of leads and ice and snow in their models. These experiments showed that a realistic representation of sea ice leads in the forecast model improves the quality of weather forecasts even hundreds of kilometres away from the sea ice edge.

Malte and co-workers also compared the existing weather forecast models with field-based measurements of  sea ice surface temperatures. As it turned out, all existing weather forecast models overestimated temperatures over sea ice by 5 to 10°C because none of the models simulated snow on top of the sea ice. (Snow is a powerful insulator.)

Implementing snow cover into weather forecast models greatly improved the simulation of atmospheric temperatures and consequently the accuracy of the Norwegian daily weather forecast.

Local site map (top) and 3D model (bottom) generated from drone photography. Flights, map and image production: Adam Steer / Norwegian Polar Institute

This example from applied weather modelling illustrates the need for environmental observations on multiple scales, from large-scale observations by satellites to detailed field-based measurements like those conducted by Adam and his colleagues on RV Kronprins Haakon.

The ship returned to Tromsø in September 2021 with hundreds of samples and terabytes of data from the Central Arctic Ocean. The expedition connected also to other important projects, for example by being a Norwegian contribution to the international research initiative Synoptic Arctic Survey, which aims at generating a comprehensive pan-Arctic baseline, and by providing sea ice data for satellite remote sensing validation in the Norwegian Centre for Integrated Remote Sensing and Forecasting for Arctic Operations (CIRFA). The observations from the expedition will allow us to track climate change and its impacts as they unfold in the Arctic over the coming years, decades, and centuries.

This article was first published at FramForum 2022:

Beyond satellite observations of sea ice – Framsenteret