Unravelling Antarctica's sea ice puzzle

Melbourne: Sea ice around the Antarctic has shrunk and is responding to the atmosphere differently. The challenge is to work out why.

Throughout 2023, the area of the ocean around Antarctica covered by sea ice was so far below the norm that scientists have struggled to communicate their shock.

This month, as the sea ice shrinks to its smallest point of the year, it is once again tracking well below its previous levels.

Research released in September 2023 shows that ocean warming was a key contributor to the dramatic change in sea ice.

The question is where the heat comes from.

A new satellite launched recently may provide the key to understanding how the ocean transports heat to Antarctica’s margins where it has a devastating impact on sea ice and ice shelves.

Sea ice insulates the ocean, reflects heat, drives currents, supports ecosystems and protects ice shelves.

Every year, its annual cycle of freezing and melting around Antarctica has been extremely reliable. Until recently.

Now we have a preliminary indication that since 2016 Antarctic sea ice coverage has shrunk.

Changes in the relationship between the ocean and sea ice suggest that the current low sea ice state may represent a new “regime” for Antarctic sea ice.

After years of relative stability, Antarctica's sea ice appears to have shrunk since 2016. Sea ice forms a thin layer between the ocean and the atmosphere and is affected by both.

Lately, sea ice seems to be responding to atmospheric drivers differently than it did in the past, suggesting a stronger influence from the slowly varying ocean. Parts of the ocean 100–200m below the surface began to warm in 2015, and those same regions lost substantial sea ice in 2016.

Since then, the warm subsurface ocean seems to have maintained the low sea-ice coverage. The record-breaking low sea ice of 2023 may be the new abnormal, the beginning of the inevitable decline in Antarctic sea ice, long projected by climate models.

For millions of years, the icy continent has been ring-fenced by the Antarctic Circumpolar Current, separating the warm northern waters from the cold polar ocean. Flowing clockwise around Antarctica and driven by westerly winds, the current is the world’s strongest, with a flow 100 times stronger than all rivers combined.

The Antarctic Circumpolar Current flows around Antarctica, keeping warm water out — but eddies can let heat through.

The current ‘feel’ the seafloor and the mountains in its path. Where it encounters barriers like ridges or seamounts, ‘wiggles’ are created in the water flow that form eddies.

Ocean eddies are the weather systems of the seas, and they play a key role in transporting heat through the circumpolar current to the ocean around Antarctica. But they’re small and hard for satellites to see.

Broad-scale ocean mapping identifies at least five major ‘heat flux gates’ or eddy hotspots in the circumpolar current. One is south of Australia, about halfway between Tasmania and Antarctica.

To understand the ocean dynamics happening now and how these may change in the future, we need much higher-resolution data to see smaller-scale features like the eddy hotspots.

Enter the Surface Water and Ocean Topography (SWOT) satellite. Jointly developed by NASA and the French space agency Centre National d’Études Spatiales (CNES), the SWOT satellite measures differences in the height of the ocean within a few centimetres from an orbit of more than 890 km above the surface.

The advanced radar altimeters on the two-tonne satellite detect surface water features with 10 times better resolution than previous technologies.

Oceanographers say it’s like a short-sighted person looking at a tree in the distance, and then putting on glasses to reveal all the leaves.

As SWOT passes over the Southern Ocean, the high-resolution topography it records of the shape of the ocean surface shows the fine streams of current to capture the eddy hotspots spinning off the Antarctic Circumpolar Current.

This means scientists can monitor these smaller-scale circulation features thought to be responsible for transporting most of the heat and carbon from the upper ocean to deeper layers – a critical buffer against global warming.

For the first time, we can see them on the surface in detail – but we still need to work out what’s happening beneath the waves.

In November 2023, scientists were able to validate the SWOT satellite data from an eddy hotspot in the Southern Ocean in an ambitious voyage on a CSIRO research vessel (RV) Investigator.

The five-week FOCUS voyage travelled 850 nautical miles south of Hobart to the Macquarie meander, one of the five eddy hotspots.

A meander may sound gentle and slow, but in fact, it’s where the world’s strongest current races through a series of hairpin bends, steered by mountains on the seafloor.

As the satellite passed overhead, the team led by CSIRO and the Australian Antarctic Programme Partnership deployed a variety of high-tech observational equipment.

Researchers and crew anchored a tall mooring 3.6 km high at the centre of the survey area, carrying over 54 instruments on a cable stretching from the seafloor to near the surface.

They also released free-floating autonomous instruments like floats, drifters and gliders into the eddies, while more than a hundred CTDs – conductivity, temperature, and depth sensors – plumbed the depths and a Triaxus was towed behind the ship through the satellite’s path.

Researchers use a variety of instruments to understand the ocean. Some float along the surface, some dive deep in the water, and some follow directed paths using motors.

The wealth of information gathered by all these instruments ‘ground-truths’ validates the satellite data from the surface.

The Antarctic is rapidly changing, and with further disruptions to the sea-ice cycle on the cards, there's a race to understand why.

Strong winds over the Southern Ocean have been increasing for decades and are likely to continue. It’s expected this will send more heat southward through leaky meanders, accelerating ice shelf melting in Antarctica and sea level rise.

Ultimately, this research aims to turn daily maps of ocean sea surface height from satellites into daily maps of the movement of heat in the Southern Ocean toward Antarctica.

This is vital information in a climate crisis. It will help governments plan how to respond to ocean warming and rising sea levels and how quickly action is needed.

At the same time, as the transition to a net-zero world gathers momentum and carbon levels in the atmosphere start to level out, we need to be able to track the response of the Southern Ocean and the global climate system. (360info.org)