Measuring wave heights in sea ice reveals highly variable ice properties in the Southern Ocean

This study by the LOPS "wave in ice team" has just appeared in the Proceedings of the U.S. National Academy of Science.

Southern Ocean sea ice plays a key role in regulating the uptake of carbon and heat by the global ocean. It is known that waves impact the sea ice, including ice break-up and pancake-ice formation. These processes explain large differences in sea ice properties between the Arctic and Antarctic. Ocean waves also impact sea ice by exerting a force in their propagation direction that compacts the ice. To date, this external forcing has not been quantifiable due to lack of large-scale observations.

This uses a method for measuring wave motions in ice-covered SAR images developed over the last 3 years, and validated with in situ data in the Arctic. These two SAR images show examples of a very strong attenuation (left) and a weaker attenuation (right). Both are taken in 100% ice cover, 200 km from the ice edge.

Since 1978, satellite-based SAR imagery has been used to study wave–ice interaction starting with Seasat and with the European remote sensing satellite ERS2. The new, higher resolution Sentinel-1 synthetic aperture radar (SAR) images motivated further analysis of wave in ice patterns, leading to a quantitative estimation method for the directional wave spectrum, from which wave heights, periods and directions can be derived.

The European Union’s Copernicus Sentinel-1 satellite operates over most of the oceans in a 4 m resolution “wave mode”, designed to monitor ocean waves. The wave mode provides 20 × 20 km images every 100 km along the satellite orbit, revealing stunning details of air–sea interaction processes including waves propagating underneath sea ice, as shown in the images.

Soon after launch in 2014, the Sentinel-1A acquisition cycle was modified to extend wave-mode coverage over Antarctic Sea ice. In 2016, Sentinel-1A was followed by its twin Sentinel-1B.

The Copernicus Sentinel-1A and -1B satellites combined with the new method to extract wave information from the imagery, make it possible for remote sensing and wave-mechanic experts to extract thousands of observations across the entire Antarctic marginal ice zone and for all seasons.

This dataset of wave conditions in sea ice is larger than all other field experiments combined and is expected to contain a wide range of sea ice conditions demonstrating the advantage of using satellites to study remote polar regions.

Experts find that the wave decay can be much faster than previously reported. More importantly, such decay is highly variable spanning three orders of magnitude. Particular radar images show drastic wave decay much larger than previously observed.

Therefore, the off-ice wave conditions do not determine the wave attenuation for the entire journey of a wave packet in sea ice. This means the same wave train can experience a weak attenuation over hundreds of kilometres and suddenly disappear over just a few kilometres.