Antarctica’s heartbeat – podcast

Listen to the second episode of six in the Voice of the Sea Ice podcast series from RNZ science communicator Dr Claire Concannon.

Each year, a massive patch of ocean around Antarctica freezes and then melts again come summer – Antarctica’s heartbeat. In winter, the ice effectively more than doubles the size of this already massive continent, and it plays a huge role in controlling our planet’s climate and driving the Global Ocean Conveyor.

Step out on the sea ice just outside New Zealand’s Scott Base with physicist and senior research fellow at the University of Otago Associate Professor Inga Smith and PhD candidate Antonia Radlwimmer. They explain their research to better understand the physics of the sea ice annual cycle.

Jargon alert

Albedo effect: How much sunlight a surface reflects. Bright things like ice bounce sunlight back into space, while dark things like ocean water soak it up and get warmer.

Frazil ice: Ice made up of tiny, slushy ice crystals that form in super-cold water, kind of like a snow slushie in a river or the sea.

Platelet ice: Thin, flat ice crystals that float up from cold ocean water and stick together under sea ice.

Pressure ridges: Tall, jagged piles of sea ice that form when large pieces of ice crash into each other and get pushed up or down.

The long-form version of this podcast with an additional interview with Professor Wolfgang Rack is on the RNZ website here.

Transcript

Associate Professor Inga Smith

Scott Base, Scott Base, this is Inga.

Scott Base Operator

Inga, this is Scott Base, go ahead.

Associate Professor Inga Smith

Yeah, Scott Base, Claire and I are just going down onto the pressure ridges. We’ll call you back at 12, over.

Scott Base Operator

Copy that Inga, Scott Base listening, have a nice walk.

Dr Claire Concannon

Scott Base sits on a long point coming off Ross Island. Out and to the left is the transition where island ends and Ross Ice Shelf begins. Remember, the hundreds of metres thick ice shelves stick around all year.

But just out front is the ocean. And for most of the year, it’s frozen. And here, at this transition between a frozen ocean pushed by tides and currents and the Ross Ice Shelf on one side, Ross Island on the other, the ice gets squashed and pushed upwards, forming what’s called pressure ridges.

Beautiful structures of deep blue and white ice towering upwards and folding over like waves, as if the ocean were still moving, just in incredibly slow motion. Flags marking a safe route through the ridges flutter in the breeze as we step out onto the ice.

Associate Professor Inga Smith

Yeah, every day is different, every year is different, it’s very different.

Dr Claire Concannon

Inga points to some thicker-looking ice that’s been wedged up. It’s multi-year ice that has stuck around.

Associate Professor Inga Smith

2022, so the sea ice right in front of Scott Base didn’t break out the last couple of summers.

Dr Claire Concannon

Breaking out is the term for when the fast ice finally gives up its grip on the land, succumbing to the weakening of melting and getting pushed out to sea by wind and currents. It’s part of the cycle of the sea ice, and Inga and her team are keen to learn about the physics of it all – because the massive area sea ice covers, combined with its whiteness, saltiness and blanketness, makes it kind of a big deal for the planet’s climate.

This land-fast ice is covered with a layer of snow. And so as you look across the 60ish kilometres to the Antarctic continent coast, all you see is white.

Associate Professor Inga Smith

It’s got very high albedo, so albedo is reflectivity. So it’s reflecting 80% of the sunlight where it’s got a snow cover. And so that’s helping keep Earth cool.

Open oceans may be 7% reflective rather than the 80% for the sea ice with snow that we’re standing on.

Dr Claire Concannon

You get this. It’s the difference between melting your flip-flops on the scorching hot black sand at Raglan versus strolling in bare feet across the white sand beaches in Coromandel in the summer. Darker surfaces absorb sunlight, but the white snow reflects it back into space.

And the blanketness is kind of related to this.

Associate Professor Inga Smith

The sea ice and then the snow on top is insulating that heat transfer from the ocean to the atmosphere. And so if you just had open water here, that heat, if it’s from sunlight coming in, could go straight into the ocean and warm it up.

Dr Claire Concannon

This insulation ability is also linked to how sea ice forms, which connects to the saltiness. Because this is the thing that makes sea ice quite different to your regular freshwater ice.

Associate Professor Inga Smith

When you freeze seawater, and here it freezes about minus 1.9°C, the ice can’t actually fit the salt ions into the structure, so it pushes most of the salt out.

Here, it’s about probably minus 20 today, so the heat’s going from the warm – minus 1.9 is warm – to the cold minus 20 atmosphere, so that’s that freezing process. And that ice will get thicker from the bottom down as more and more ice freezes into the bottom.

Dr Claire Concannon

But remember, as it freezes, there’s no space for the salt. So you get this mix of salt-free ice molecules in super salty water, or brine.

Associate Professor Inga Smith

It’s rejecting this concentrated brine, and it’s very salty and very cold. So that will sink down to the bottom of the seafloor in places. And particularly if it’s a very active area of sea ice formation, you can form what’s called Antarctic bottom water, and that’s the cold salty water that connects all of the world’s oceans. Some people call it the conveyor belt, global conveyor belt, where all the oceans have cold deep waters going at the bottom and then warmer, fresher waters coming back at the top. So that’s how they’re all connected.

Dr Claire Concannon

Because it’s cold and dense, this Antarctic bottom water drops down and runs along the slope of the Antarctic continent and then spreads northwards to fill most of the ocean deeper than 4 kilometres. This is a lot of water – like, it makes up a third of the volume of the whole ocean.

Remember that description of the flux of Antarctic sea ice is like the heartbeat of the world? Well, this is why. Each year, the deep veins of the world’s ocean are replenished with cold, dense water flowing down as the sea ice forms.

To help study the heartbeat of sea ice here, Inga and her collaborators use a monitoring station, which gets placed into the ice by the Scott Base staff coming into winter.

Associate Professor Inga Smith

It’s a long probe – it’s around 2.5 metres long – and it’s got temperature sensors inside it.

And so when it’s put in through the ice into the ocean, you can see how thick the ice is and where the ocean is. And then as it freezes and grows, it includes more and more of those temperature sensors. And we can start to see the fluctuations in the temperature of the sea ice through the season. But we’re particularly interested in the growth rate – how quickly that ice is growing over the winter.

Dr Claire Concannon

When she visits Antarctica, Inga collects sea ice cores near where the monitoring station is placed. These get taken back to the University of Otago for analysis.

This lets them see the structure of the ice crystals. And when you do this, you realise that there are different kinds of sea ice in McMurdo Sound. There’s the top layer, formed when the heat moves from the ocean to the air. Inga calls this columnar ice, and its crystals are nice and orderly. And then, below this, the crystals are totally all over the place. This is what’s called platelet ice.

Associate Professor Inga Smith

That’s the type of ice that’s connected with what’s called super-cooled water.

Dr Claire Concannon

As if minus 1.9° wasn’t cold enough, stand by for super-cooled water.

Associate Professor Inga Smith

The freezing point of seawater depends on its salinity, but it also depends on the pressure that it’s at. And so if it’s deeper down, it will have a freezing point that’s much lower. So at some depths it might be minus 2°C, and then if it’s coming up to the surface where it’s minus 1.9°C, then it could actually be super-cooled. So it could actually be colder than the local freezing point.

Dr Claire Concannon

OK, so it’s not that much colder. But where is this minus 2° super-cooled water coming from?

Associate Professor Inga Smith

As brine is rejected from that sea ice as it’s forming, then you have very salty water sinking because it’s more dense. And if it comes in contact with the bottom of the ice shelf, which is a giant floating glacier – it’s made of compacted snow – then it can melt or dissolve that. That’s a normal process that happens all the time.

And that then slightly fresher water will be less dense, so it will rise up along the bottom of the ice shelf.

Dr Claire Concannon

So it’s not yet frozen, but it’s primed. They still don’t completely understand what happens next, says Inga, but something triggers the super-cooled water to snap freeze into small ice crystals called frazil ice.

Associate Professor Inga Smith

And that frazil ice floats up, attaches to the bottom of the columnar sea ice – so the ice that’s grown through heat conduction. And when it does that, it starts to grow in place and you have these giant crystals that can be the size of your hand or bigger, and they look like leaves – they’ve got veins and wiggly edges and they’re very, very cool – and then that heat conduction process will keep going and it will fill in the gaps.

So you’ll end up with solid ice. If you take a section through at the top where it’s just the columnar ice with the heat conduction, it will look a bit like carpet, a very ordered structure. And then when you get down into that incorporated platelet ice, it’s just chaos. It’s just like crystals pointing in every which direction but it’s still pretty solid to walk on.

And then if you go deeper, there’s what we call the sub-ice platelet layer, where there are those crystals that are all interwoven sitting at the bottom, and they haven’t been completely solidified with the water in between.

Dr Claire Concannon

Now, to get this platelet layer, you need that cold ice shelf water doing its rising to the surface thing, which means you don’t get platelet ice under all the sea ice in Antarctica.

Associate Professor Inga Smith

Here in McMurdo Sound, there is a lot of platelet ice. It’s very common and it makes the ice grow more quickly, and therefore it ends up being thicker.

Dr Claire Concannon

So that’s how the sea ice here grows and thickens. What about the other part of the cycle? The melting and breaking out. Well, they want to learn about that too. And to help with that, they’re using some sea ice trackers.

In one of the labs at Scott Base, Inga is opening a large cardboard box and lifting out one of the four trackers to be put out on the ice. It’s a 60 cm diameter wide hard plastic bowl. It takes measurements of GPS location, temperature and conductivity, which is basically a measure of saltiness.

PhD candidate Antonia Radlwimmer will study the data they send back in real time via satellite as the breakout happens.

Antonia Radlwimmer

The ice and the trackers start drifting northwards out of the region. And yeah, that drift is what we’re interested in. So when does it start? Where does it go? And what might be driving it? Is it the ocean? Is it the wind? Does it matter how much other ice is around or not?

Dr Claire Concannon

These trackers are designed to be OK on ice or in the water, so Antonia should be able to follow their journeys and recreate a picture of what’s going on for them. The batteries are designed to last for up to 2 years. But it’s a dangerous life for a sea ice tracker.

Antonia Radlwimmer

They could be crushed between ice floes or underneath ice tongues or in sea ice that is rafting on to each other.

Dr Claire Concannon

Breakout time isn’t so bad, says Antonia. So she expects they’ll last at least that interesting initial stage.

Antonia Radlwimmer

We’ll hopefully get a nice drift pattern from the breakout until it starts refreezing again in winter. And then the best case of course would be if they survived winter and went on next season as well.

Dr Claire Concannon

These four trackers were named by some school children around Aotearoa, who may also be keeping an eye on their progress. With the help of a helicopter, Inga and Antonia place them on different locations – one beside the sea ice monitoring station and the other three near the sea ice edge at crossing points of satellite lines. While there, they also took measurements of the snow thickness and the sea ice thickness. And as well as providing data for sea ice growth models, these snow and platelet layer measurements will help in figuring out what’s going on with the base of the food web that powers all life in Antarctica.

Dr Jacqui Stuart

Oh phytoplankton, who we should be thanking, making air OK to breathe …

Dr Claire Concannon

That’s next time on Voice of the Sea Ice.

Thanks to Dr Inga Smith and Antonia Radlwimmer of the University of Otago. Production help for this episode was from Ellen Rykers, editing by William Ray, sound design and engineering by William Saunders and additional sound design by Steve Burridge.

I’m Claire Concannon. Have a great week. Kia pai, te wiki.

Acknowledgements

This podcast is courtesy of RNZ. It is from the Voice of the Sea Ice series by Dr Claire Concannon. The series was made with travel support from the Antarctica New Zealand Community Engagement Programme.

Dr Inga Smith, University of Otago

Antonia Radlwimmer, University of Otago