The ocean to our south is the world's windiest and wildest. It's arguably also the most important, writes Jamie Morton.
They roll across the Southern Ocean like towering monsters.
Fuelled by persistent westerly winds, and a virtually unlimited area to build, the giant waves rise to the height of eight-storey apartment blocks, crashing down upon the bows of large ships with terrifying force.
When the HMNZS Otago met some stretching more than 20m high, the 1900-tonne offshore patrol vessel came close to capsizing, with 75 people onboard.
Discussing the December 2017 near-tragedy, Defence Minister Ron Mark blamed climate change.
"It's happening - the consequences are being seen and our Defence Force people are in the front line."
It was a dramatic call – and a correct one.
Changes in the height and strength of these beasts of the deep south have been some of the most clearly observed impacts of the climate crisis in our part of the world.
One recently published study, drawing on four billion satellite observations, showed extreme waves had increased by 30cm - or 5 per cent - in just the past three decades.
Similarly, the Southern Ocean had grown stormier, and even windier - it's extreme winds strengthening by 1.5m a second.
Antarctica, keeping frozen the equivalent of 60m of global sea level rise, has often been looked upon as the great white bogeyman of climate change projections.
But the ocean that encircles it – and, partly, those mammoth waves that roar across it - will arguably prove to shape the future of our planet.
The deep south
Explorers ventured to the fourth largest of the world's great oceans in search of a Terra Australis – a land mass thought to have existed since the time of the Roman Empire.
Captain James Cook never found this mythical continent – but he did prove that waters encompassed the southern latitudes.
Two hundred and fifty years since Cook's expedition, geographers have disagreed over where the Southern Ocean's northern boundary lies, or even if the ocean exists at all.
To those early adventurers, the ocean must have struck them as a bleak and hostile corner of the Earth.
Below latitude 40 south - deep into the "roaring 40s", "furious 50s" and "screaming 60s" - blew the strongest average winds found anywhere.
At the peak of winter, sea ice stretched to span half of the distance between Antarctica and New Zealand – equivalent to the length of the country itself.
And then there were those waves.
In May last year, the largest wave ever recorded in the Southern Hemisphere – a 23.8m giant that formed in the thick of a huge and deep storm – was measured by a buoy moored off Campbell Island.
During the depths of winter, these waves were enormous, averaging more than 5m, regularly exceeding 10m - and sometimes likely reaching more than 25m.
That's the height of 16 cars stacked on top of each other.
Anything more than 20m high was highly hazardous to vessels – waves that climbed to 14m forced the HMNZS Wellington to turn around partway to the subantarctic islands in 2014 – and ships tended to negotiate heavy seas by sailing head-on into the direction the waves were coming from.
But, in the hellish Southern Ocean, this could prove challenging when vessels faced long, large swells from one side and shorter, steeper seas from another.
Fortunately, there was little shipping traffic in the ocean; what vessels were operating ranged from icebreakers and research boats to fishing vessels and small cruise liners.
The wave buoys, deployed by science-based consultancy MetOcean Solutions, aimed to help researchers build an unprecedented picture of waves and air-sea interactions in extreme conditions.
"There are other ways of measuring these waves, such as through satellites, but that won't give you the detail you get from these deployments," MetOcean Solutions general manager Dr Brett Beamsley said.
"And because waves are generally defined based on Northern Hemisphere measuring campaigns, understanding the physics of the waves down there could also give us a better understanding of ocean dynamics."
The wave data added to our wider knowledge of what was a critically important part of the global climate system.
An ocean apart
The Southern Ocean is unlike any other.
Bridging New Zealand and Antarctica, it spans all the way from Antarctica to the Subtropical Front, the boundary where water from the tropics meets water from the polar region.
At this front, there's a sharp change in temperature and salinity.
The Subtropical Front flows around the bottom of the South Island, from Fiordland, around the Otago coastline and north to Banks Peninsula, where the Chatham Rise steers it back into the Pacific Ocean.
The relatively cold Southern Ocean waters keep the waters around southern New Zealand and our subantarctic islands cold.
As people in windy, exposed places like Wellington would know only too well, this gives southerly winds coming from Antarctica no chance to warm before they hit us.
As the only band of water on Earth that could complete a full longitudinal circuit, the ocean also acted as a roundabout for the world's largest oceans, linking up the Atlantic, Indian and Pacific.
When the surface of the ocean around Antarctica froze, cold and salty water dropped down to the ocean floor, sending a huge annual signal that kept the heartbeat of the world's ocean pumping.
It's also where the heaviest waters in the global ocean are made, and where there is massive absorption of atmospheric carbon dioxide (CO2) into the ocean.
This made it a key place for understanding climate change, as the dense waters and high uptake of CO2 help bury atmospheric CO2 into the ocean.
Scientists studying the ocean can observe and measure these changes in the climate and, using climate models, can make predictions about the future.
Because it wasn't surrounded by continents, strong westerly winds kept speeding and stirring the ocean, making it tough for scientists to get a detailed picture of what was happening to it.
It was hugely complex in its nature, because of the huge number of mesoscale eddies, or tiny whirls of water, that formed its Antarctic Circumpolar Current, which endlessly looped the base of the globe.
To Niwa ocean modeller Dr Erik Behrens, these small-scale processes were the little jigsaw pieces in a bigger puzzle.
Nowhere did we most need to understand their influence than around Antarctica, where bottom water in the ocean was formed through the complex interplay of strong winds and the formation of sea ice.
"Understanding the nature of this bottom water is hugely important because it is like a time capsule of past climate events and ventilates the abyssal ocean," Behrens said.
Using a new, specially built model that he compared to Google Maps, Behrens could zoom into specific eddies to trace how the bottom water connected Antarctica with the global oceans.
So far, the results have been surprising.
"We can now see that the spreading occurs twice as fast - and along more pathways than previously known," he said.
"This helps us to understand how quickly changes in the formation of this water mass would be traceable in other regions of the world ocean."
His colleague, Niwa physical oceanographer Dr Craig Stevens, has also been investigating the knock-on impacts of large-scale changes in Antarctica.
The Antarctic Science Platform project he was leading focused on how the ocean around the continent was responding to shifting patterns in wind and ice.
His team eventually planned to deploy instruments within the Antarctic Circumpolar Current, to reveal more of the extraordinary amount of energy and mixing packed within it.
"The importance of this mixing can't be overstated because most of the heat being captured by the planet is stored in the ocean," Stevens said.
Without this little-known buffer, atmospheric temperatures would be too hot for any of us to survive.
To put it another way: the ocean had been saving us from ourselves.
"Where and how this storage takes place is so central to our global future that it is a research priority for many science teams around the world working on the climate challenge," Stevens said.
"It makes a difference if the heat all stays at the surface - or if it is mixed or subducted down deep."
Our deep blue sink
By current estimates, the Southern Ocean sucked up around a third of the CO2 we produce.
At the same time, research has shown how the ocean has been growing warmer, with waters between 500m and 2000m heating by about 0.0020C every year.
As that water was eventually moved around the world by global ocean conveyor, all of that absorbed heat would slowly be released back into the atmosphere, until the climate would reach a new equilibrium state.
That meant that, even if our soaring global carbon emissions were to somehow cease today, we had already committed ourselves to a long future of warming due to the heat already taken up by oceans.
In the Southern Ocean, the same westerly winds that played a critical role in regulating its storing capacity were now threatening its future as a giant CO2 bank, by bringing deep carbon-rich waters up to the surface.
One new international study suggested that in the past, strong westerlies have been linked to higher levels of atmospheric CO2 because of their impact on the Southern Ocean carbon balance.
That meant stronger westerlies could actually speed up climate change if mankind continued to emit as much CO2 as it did today.
Niwa principal scientist Dr Cliff Law said the Southern Ocean was fast approaching a threshold as a result of ocean acidification, which would even make it challenging for organisms such as plankton and molluscs to maintain their shells.
Law said part of the ocean's function as a CO2 sink was the uptake of phytoplankton into the deep ocean when they died – and future projections suggested a global drop in these vital microscopic organisms.
"Add to this an increase in outgassing of CO2 in from deep waters in the Southern Ocean due to increased winds, and things don't look good for the future."
Untangling the complexities of the ocean's sea ice was a job just as urgent for scientist.
The seasonal swelling and shrinking of sea ice in the Southern Hemisphere, breathing in and out like a gargantuan pair of lungs, was the largest single change to occur annually on the entire planet.
The ice ballooned outward from Antarctica to blanket an area 50 times the size of New Zealand.
If this process seemed a little far removed from our daily lives, the extent that this frozen expanse reached directly affected all of us.
That was because the sea ice edge determined where and how the storm tracks that circulated in the Southern Ocean arose, which in turn controlled the weather patterns we constantly see coming up from the south.
Sea ice also acted like a giant mirror on the surface of the ocean.
Whereas dark ocean absorbed heat, the bright white of the sea ice instead reflected that energy back out to space, helping to keep the ocean cool.
"What we've seen happening in the Arctic is that as less sea ice forms, and more of the dark ocean is left exposed, more heat is absorbed making it less likely that sea ice will form, and the cycle intensifies," Niwa marine physicist Dr Natalie Robinson said.
"We call this a 'positive feedback mechanism', which refers to the fact that it's self-perpetuating, rather than whether it's a 'positive' or good outcome for the climate."
Robinson described Antarctic sea ice as the pulse of the global ocean – not just for ocean and atmospheric circulation, but also for the ecology of the Southern Ocean itself, with major flow-on effects for the planet's ecosystem and carbon uptake.
Since satellites began collecting data in the late 1970s, there had been a trend towards sea ice covering a slightly larger area of the Southern Ocean.
That flew in the face of what models have been predicting, in what was a gradually warming ocean.
"So, getting the underlying direction wrong means there's some fundamental physics missing from our understanding," Robinson explained.
"And this makes forecasting future sea ice conditions under future climate scenarios problematic – and represents a huge area of uncertainty when you consider the role that sea ice plays in the overall climate system."
In a warming world, it might sound like a comforting thing that sea ice had been expanding over time.
But, quite to the contrary, it was possible that this might be due to increased melting of the Antarctic ice sheet making its way into the ocean.
On one hand, this would be contributing to accelerated sea level rise - on the other, it would be generating cold fresh water that formed sea ice more easily.
"If this is true it implies that we're swapping the permanent ice for seasonal ice, which we would not consider a win for the climate system at all."
The mystery took another turn in 2016, when the trend reversed, and scientists recorded dramatic drops in summer sea ice that carried on through 2017 and 2018.
Another Niwa scientist working on the conundrum, Dr Sam Dean, said it was possible that either this new pattern of decline was here to stay – or that we had entered an era of "swings and roundabouts", with wildly fluctuating sea ice.
"But, for scientists like me, this still raises interesting questions about what might be missing from our climate models," Dean said.
"Why couldn't we predict the nature of that steady increase, followed by the recent dramatic drops?"
He'd always believed that a key clue lay in the pattern of change – a small overall increase that was really made up of big increase in sea ice in the Ross Sea, and correspondingly big decreases in the Bellingshausen Sea.
"This dipole pattern matches up well with changes that have occurred in the winds over the Southern Ocean, as well the sea surface temperatures we observe."
Yet, the part of his research that most fascinated him was how sea ice was coming into direct contact with those giant waves, in an area known as the marginal ice zone.
In the Pacific sector of the Southern Ocean in particular, waves had increased more than anywhere else, as storm tracks had shifted poleward and the westerly winds had increased in strength.
This change was an expected response to increased heating of the tropics, thanks to increasing greenhouse gases, as well as stratospheric cooling due to ozone depletion.
"Our climate models have not been able to consider the impact of these changes in ocean waves – since they simply aren't included," Dean said.
"Neither could we understand the effect on sea ice, since sea ice models have not previously allowed waves to break sheets of ice into the irregular floes that sea ice is made of in many places."
Traditional sea ice models considered only the change in thickness that occurred from exchanging heat with the atmosphere and ocean.
A Niwa-led project attempted to change all of this, by developing the first model capable of modelling floes and their interaction with waves based on what was understood about the physics involved.
"Unfortunately, when we plugged this new model into records of wave behaviour since 1979 we were not able to resolve the sea ice mystery," Dean said.
"But it was still well worth doing. We now have a much more realistic way of looking at sea ice, and one that has already provided a number of insights into the way sea ice melts and freezes.
"We can now explore what changing waves in the Southern Ocean might mean for Antarctica."
A race against time
By its very nature, Antarctica, the coldest, windiest, driest and most remote continent on Earth, was difficult to solicit answers from.
It wasn't easy to get to, was too cold for independent survival, and the land and its surrounding ocean were locked under thick layers of ice.
But its disproportionate role on our climate system meant it was increasingly urgent for scientists to understand.
"It's a place of fundamental discovery, where new processes, or drivers of processes, are still being uncovered," Robinson said.
"Hence, we rely on numerical simulations that represent what's happening there, how that affects the rest of the world and how it all might change under future climate scenarios."
These had come a long way in the last few decades, and there was now a range of incredibly sophisticated models, focused on different aspects of the climate system.
But Robinson added that we needed new and ongoing observational data to check that the models were representing something close to reality, and to reveal those new processes that the models don't even know about yet.
Thanks to climate change, we were entering a world that had never existed in the same way before – and where our observations of the past and present alone couldn't tell us what might what happen next.
"Climate models represent the ultimate expression of our understanding of the Earth System and have been proven repeatedly to be accurate in predicting changes in the rainfall and temperature for places like New Zealand," Dean said.
"But we should also care deeply about what all these human emissions of greenhouse gases will mean for our planet's iconic polar environment, home to unique ecosystems."
The world needed to know what a planet several degrees warmer might mean for the Southern Ocean and Antarctica, as nations like ours began to enact hard decisions to drive down emissions.
"Our time to save one of the last great wilderness areas may be short."
• Today is World Oceans Day. To find out more, visit www.worldoceansday.org .