Swell: the key to Antarctic sea-ice breakup − Observation of sea ice outflow in Lützow-Holm Bay gives insights into climate change and navigation assistance for icebreaker Shirase (by the University of Tokyo and Kitami Institute of Technology)

A research group led by Professor Takuji Waseda (members include Lecturer Tsubasa Kodaira, and Project Assistant Professor Takehiko Nose of the Graduate School of Frontier Sciences at the University of Tokyo, Associate Professor Kazutaka Tateyama of the Kitami Institute of Technology, and Associate Professor Takeshi Tamura of the National Institute of Polar Research) has developed a wave-ice buoy in collaboration with Jean Rabault of Met Norway and Mario Hoppman of Alfred Wegener Institute that measures sea ice movement. By deploying wave buoys over a wide area in Lützow-Holm Bay, they successfully captured the sea ice outflow and the coincident heave motion of sea ice that they conjecture could be a precursor to sea ice breakups. Such observation is understood to be the first. The group successfully captured a major breakup in the first year of their 6-year observation campaign "Understanding the variations of the marginal ice zone, drift ice zone, and land fast ice for the optimum routing of Shirase", as part of the Phase X Japanese Antarctic Research Expedition. In the coming years, the sea ice is expected to thicken until it becomes less prone to breakup and cause navigation difficulty for Shirase. This study will reveal the impacts of climate change and climate variation on sea ice in the Antarctic Ocean. The outcome will be used to assist the navigation of the icebreaker Shirase, which transports more than 1,000 tons of essential supplies to Syowa Station every year.

Lützow-Holm Bay, where Syowa Station is located, is enclosed by thick sea ice, even in the summer. The icebreaker Shirase navigates through this thick sea ice to transport supplies to the station. Analysis of the past 30-year data has revealed that the number of ramming operations in the land-fast ice was small in the years when sea ice frequently broke-up in Lützow-Holm Bay. By contrast, the number of ramming operations increased in years when the number of sea ice breakup events was small. (Note: a ramming operation refers to a ship maneuver in which a ship first backs up 200-300 m, then the ship makes way at full speed to drive onto and break the ice under the weight of the ship)

Sea ice grows thermodynamically at the bottom of the ice floe. Once its thickness reaches a certain value, the sea ice acts as an insulator between the atmosphere and the seawater preventing further thermodynamic ice growth. In Antarctica, however, massive amounts of snowfall accumulate on top of sea ice, freezes as the seawater floods, and subsequently thickens the ice from above, which is known as the snow-ice. As the portion of snow-ice increases during summertime, sea ice weakens. Although it has been conjectured that such relatively fragile sea ice may be subject to breakups by waves, this fact has not been verified through observation.

Typically, drift ice zone (DIZ) forms to the north of fast ice, and further to the north, a marginal ice zone (MIZ) forms where waves actively interact with sea ice. The MIZ and DIZ serves as a buffer zone for incoming waves. Waves attenuate rapidly and lose energy beneath the sea ice. However, long waves are less likely to attenuate and therefore enter fast ice as swells. The key to revealing the mechanism of fast ice breakup is understanding the incoming waves propagate from the MIZ, through the DIZ, and into the fast ice as swell.

The research group deployed wave sensors on both fast ice and drift ice to capture the waves entering fast ice from the MIZ and the DIZ, as well as the horizontal movement of the disintegrated drifting ice floes. Additionally, wave buoys were deployed in the open water between ice in the DIZ and MIZ. The wave sensors were developed by the research group in collaboration with Norway and Germany. A durable floating platform was also developed in-house to deploy the sensors on ice. Owing to the successful production of small, low-cost wave-ice buoys, a total of 33 buoys were deployed.

Fifteen of them were deployed on fast ice, 8 on drift ice, and 10 in open water. Data were transmitted every hour. The buoys were deployed between December 2022 and February 2023, using cranes and helicopters on Shirase with support from the Japan Maritime Self-Defense Force.

The researchers also landed on fast ice using a helicopter and drilled holes in sea ice near the deployed buoys to directly measure the ice thickness. The ice thickness was successfully measured at 13 locations. This was the first attempt at measuring the fast ice thickness in the center of Lützow-Holm Bay. The researchers found that the ice thickness gradually increased from 1 meter to 2 meters from north to south. The thickness exceeded 3 meters at the southern located buoys, which drifted away after a major breakup in April 2022. Sea ice thickness between 1- and 2-meter is a first-year ice, whereas ice over 3-meter thickness is considered as a multi-year ice.

The researchers hypothesize that a major outflow of fast ice occurs in autumn in the Southern Hemisphere when drift ice on the northern side melts and permits swells to enter more easily. The fast ice wave-ice buoys, which remained stationary until mid-March, began to move from the north at the end of March. On April 1, all but three buoys on thick ice began to move, and satellite images clearly showed that fast ice had cracked wide open and begun to drift. Furthermore, on April 9, measurements revealed that sea ice of 3-meter thickness had cracked and begun to drift away.

Through conducting similar measurements over the next five expeditions until the 69th Japanese Antarctic Research Expedition, the researchers aim to reveal the mechanism of long-term sea ice growth. The findings will not only support the navigation of Shirase but also lead to better understanding of wave-ice interactions and improve the predictability of MIZ forecast, contributing to the sustainable development of the Northern Sea Routes. These studies will also provide valuable data that contribute to the fundamental research on the relationship between wave-induced sea ice breakup and the floe size distribution, and on the identification of the physics that directly causes sea ice breakup.