Antarctic sea ice plays a critical role in regulating Earth’s climate. It reflects sunlight back into space, insulates the ocean from the atmosphere and supports unique polar ecosystems.

The Antarctic marginal ice zone (MIZ) is the outer edge of the sea ice cover, forming a nearly 200-kilometre-wide ring of ice that extends into the extremely energetic Southern Ocean.

Iceberg calved from ocean waves — Southern Ocean waves can penetrate ice cover, breaking it into floes, which melt during spring and summer. Picture: Pete Harmsen

The MIZ is one of the most dynamic parts of the sea ice cover. Here, powerful Southern Ocean waves penetrate the ice cover, breaking it into smaller pieces known as floes, which rapidly melt during spring and summer.

Despite its importance, studying the Antarctic MIZ is challenging due to its vast and remote location.

While satellites can routinely observe sea ice concentration, detecting wave activity within sea ice is much harder. As a result, the MIZ has often been identified using sea ice concentration alone.

The first continent-wide record of the Antarctic MIZ

Our study, published in Nature Communications, found that current methods often fail to identify the size and location of the MIZ, particularly where ocean waves are penetrating the ice cover.

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Instead, working with colleagues from the Institute for Marine and Antarctic Studies at the University of Tasmania, we revisited a little-used technique from the 1980s.

Using a radar altimeter – which uses radio waves to measure how long they take to reflect off the ground – on the SARAL-Altika satellite, we used the data to detect wave activity within sea ice.

This helped us construct the first continent-wide record of the Antarctic MIZ.

The data from the radar altimeter was collected over a 12-year period between 2013 and 2024.

For the kind of research we do, we found radar altimeters more useful than laser altimeters because they can ‘see’ through cloud cover to measure the heights of terrain or water directly beneath them.

SARAL satellite over Earth — A radar altimeter on the SARAL-Altika satellite allows researchers to detect waves entering the sea ice. Picture: NASA

Our research showed that the wave-affected MIZ is around 35 to to 180 kilometres wide on average, but that depends heavily on the time of year and the longitude.

Winter and early spring are the times of widest MIZ, because the ice edge is so far north that it abuts the most intense wave region of the Southern Ocean.

Previous estimates suggested that the MIZ is widest during summer and narrowest during winter.

Detecting ocean waves travelling through sea ice

Analysing a decade of satellite observations revealed that the area of Antarctic sea ice affected by ocean waves is much larger than we previously thought.

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By detecting ocean waves travelling through the sea ice-covered ocean, we find that the size of this critical region also changes dramatically throughout the year and varies from place to place around Antarctica.

Our study offers a new way to monitor one of the most dynamic parts of the Southern Ocean.

In some regions, waves penetrate hundreds of kilometres into ice-covered waters.

This is far beyond what estimates based on sea ice concentration alone would suggest.

In fact, around 16 per cent of the Antarctic sea-ice zone is wave-affected – and the inner limit of wave penetration is unrelated to sea-ice concentration.

Graphice of Antarctica sea height and ice concentration 2013-2024 — Mean Southern Ocean wave height during summer (February, left) and winter (September, right) from 2023. Sea ice is shown in white and the wave-detecting satellite tracks shown as yellow lines. Graphic: Supplied

It's important because when sea ice isn't affected by waves, it forms a more complete ‘cap’ on the ocean.

This limits the exchange of heat, moisture and gases, including carbon dioxide, with the atmosphere.

The MIZ is important for shielding inner-pack ice, landfast ice and ice shelves from waves.

It also helps sustain marine life, as meltwater at the retreating ice edge supports strong phytoplankton blooms that feed krill and, in turn, penguins, seals and whales.

Recent record-breaking lows in Antarctic sea ice coverage have highlighted just how rapidly the polar environment can change.

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Many of the largest changes in Antarctic sea ice have occurred during winter, when Southern Ocean waves are strongest and the MIZ is at its widest.

Antarctic sea ice continues to change

Changes in this wave-affected region can influence how exposed coastal sea ice, landfast ice and ice shelves are to ocean waves.

Understanding these interactions is likely to become increasingly important as Antarctic sea ice continues to change, particularly following the dramatic crash in Antarctic sea ice that began around 2016.

It makes it more important than ever to understand which regions are most at risk from break up and melting.

In order to better understand the processes affecting the MIZ, our team also developed computer simulations to predict seasonal variations in MIZ widths.

Emperor penguins on ice — The MIZ helps to shield inner ice packs from waves – sustaining marine life like penguins, seals and whales. Picture: Getty Images

Encouragingly, these simulations predict the same patterns we found in the satellite data.

This suggests that large-scale changes in the MIZ are strongly linked to wave conditions in the open ocean – giving us confidence that simulations like ours can be used to project how Antarctic sea ice will respond to future environmental change.

Together, our observations and simulations provide a new way to monitor one of the most dynamic parts of the Southern Ocean and improve our understanding of Antarctica’s changing sea ice cover.

This article was first published on Pursuit. Read the original article.

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Noah Day

Research Fellow in SEA Ice Modelling

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