Recent studies reveal surprising changes in the Ross Sea region, a choke-point in ocean circulation. David Williams reports
As the three-masted British ship Erebus sailed south in Antarctic waters with the slightly smaller Terror in January 1841, commander James Clark Ross, the world’s most experienced polar explorer, saw a low white line extending as far as the eye could see.
“It presented an extraordinary appearance,” he wrote, “gradually increasing in height, as we got nearer to it, and proving at length to be a perpendicular cliff of ice, between one hundred and fifty feet and two hundred feet above the level of the sea, perfectly flat and level at the top, and without any fissures or promontories on its even seaward face.”
To Ross and his crew, the ice shelf looming over them must have seemed like the end of the Earth. Indeed, the 1841 expedition took humans the farthest south they’d ever been.
But there was no way through the monstrous mass of ice – which Ross initially named the Victoria Barrier, after the young British queen, but was eventually re-named after him. After sailing east, along the ice shelf face, for 320km, the ships abandoned their search for an entrance and set sail for Tasmania.
“We might with equal chance of success try to sail through the cliffs of Dover, as to penetrate such a mass,” the Erebus commander quipped.
The story of Ross has been told many times. What’s only just becoming clearer, however, is what happens beneath the wind-blasted, ice-choked waters traversed by Erebus and Terror in what is now known as the Ross Sea.
Rather than the end of the Earth, this wild, forbidding place is actually a hub for the world’s oceans, a key stop on a 1000-year, global conveyor belt supplying the lower limb of an overturning circulation. Plumes of dense water, created by Antarctica’s freezing winds, plunge into the deep ocean, pumping “ventilation” or oxygen into the abyss, and are replaced by warmer currents emanating from the lower latitudes.
“The Antarctic is where the global ocean connects,” says oceanographer Dr Melissa Bowen, of the University of Auckland. “That’s really where a lot of water changes its temperatures and salinities and moves around. Around the Antarctic is where all of the oceans are able to connect with one another and where that water gets exchanged.”
To underline its global reach and the long timescales at play, the cold, dense water that slid down Antarctica’s continental shelf during Ross’s 1841 voyage is thought to have been propelled south of Australia, rounded the east side of New Zealand, past the Campbell Plateau, and northward past Samoa. Associate Professor Craig Stevens, an oceanographer at the NIWA/University of Auckland Joint Graduate School in Marine Science, says it’s probably starting to appear at the surface in the centre of the Pacific Ocean about now.
For humans, this system helps to regulate the world’s climate by soaking up excess heat and carbon dioxide from the atmosphere. Unsurprisingly, this human-caused change is having an effect, by modifying the water entering the Antarctic system from the wider ocean.
“That’s the water that we know we’re warming,” Stevens says. “Or we’re pushing it somewhere that it didn’t used to go, because we’ve changed how the winds operate.”
More work is being done to determine how much of that warmer water might be reaching the Ross Ice Shelf, which, right now, is thought of as being quite stable.
Warmer water will be a death knell for ice shelves, by weakening and melting them from below. A big focus of Antarctic science is to identify how changes to the ice sheet because of melting feed into sea level rise predictions generated by computer models. Until scientists properly understand the below-surface processes – like the movement and prevalence of warmer water, and the conditions for creating the dense water cascading into the deeper ocean – the picture won’t be complete.
Scientists are working with urgency, producing new research at an increasing rate.
A journal paper from last December from Australian researchers points to anomalous atmospheric conditions for an unexpected rebound in what’s known as Antarctic bottom water, which has been reducing in density, salinity and volume for 50 years. In January, another paper, whose lead author was the University of Auckland’s Bowen, suggested the flow of this cold, salty bottom water, created when sea ice forms, escapes into the broader ocean because of weak tides.
Given the complexity of local interactions, variable regional warming, and a lack of long-term data, scientists are trying to discern how these processes connect Antarctica to New Zealand and the rest of the world.
“We’re trapping a lot of heat, and so we are changing the way the oceans operate,” Stevens, of NIWA/University of Auckland, says. “There’ll be spinoff impacts: one will be sea level rise, through a melting Antarctica, but the other, that will be a lot closer to home, will be the changes in the ocean around us that will affect our daily lives and our daily economics.”
Some changes, such as sea level rise from a melting Western Antarctic Ice Sheet, are already locked in. “It’s more a matter of when rather than if,” Stevens says. “And so we’re narrowing down the ‘when’.”
At the centre of the two recent Antarctic papers are polynya, semi-permanent areas of open water in sea ice. Dr Natalie Robinson, a specialist in polar oceanography for Crown research institute NIWA (National Institute of Water and Atmospheric Research), says polynyas in the south-western Ross Sea are driven by the winds flowing off the continent, pushing ice away as it’s forming, allowing more open ocean in which to form new sea ice.
Sea ice formation is crucial for generating Antarctic bottom water – which is thought to occupy up to 40 percent of the total volume of the global ocean, and is a massive reservoir of heat and carbon, storing it in the oceanic abyss for centuries.
When sea ice forms from salty water the salt has to go somewhere, Robinson says, as it can’t be incorporated in the ice at the top of the water column.
“That’s what drops down to the bottom of the sea floor. You get this brine, it’s a more salty version of ocean water and it’s cold because it’s just been at the surface forming sea ice.
“So, it drops right down, and it’s dense – it flows away and that’s one end of the global conveyor belt. This dense water that gets formed each winter around Antarctica and it pushes, on one end, away from Antarctica.”
(Brace for a flood of acronyms. This salt-rich brine known as high salinity shelf water, HSSW, is a precursor to Ross Sea bottom water, RSBW, which mixes with warmer circumpolar deep water, CDW, from the broader ocean as it descends the continental slope, to eventually become Antarctic bottom water, AABW.)
About a quarter of Antarctic bottom water comes from the western Ross Sea, but over the past 50 years it has become fresher, lighter and thinner. There are competing theories as to why, but increased meltwater from the Amundsen Sea – where the well-known Thwaites and Pine Island glaciers are melting – has often been blamed. (Not as much was known, then, about meltwater emerging from the Ross Ice Shelf.)
Robinson says sea ice has other important roles. It forms a substrate for algae to form, providing a food source for the rest of the food chain – “you can think of them like the grass meadows of the ocean”. It controls light needed for the algae to grow, it’s a barrier to the exchange of oxygen and other gases, as well as a barrier to heat exchange. Atmospheric temperatures above the ice can be colder than minus 20°C. “The sea ice being there acts like a blanket.”
Dr Alessandro Silvano, a research fellow in physical oceanography at England’s University of Southampton who completed his PhD in Australia, says a reduction in Antarctic bottom water can limit the ocean’s ability to mitigate warming, with lasting consequences.
Silvano was the lead author of a paper published in the journal Nature Geoscience in December, observing a surprising boost in the formation of Antarctic bottom water, with properties similar to those last seen in the 1990s. The Australian team found it was anomalous atmospheric conditions – extreme El Niño in 2015 and 2016 combined with stronger westerly winds over the Southern Ocean, that drove the rebound. It shows the strong connection of the global climate, and the sensitivity of the Ross Sea regime.
(During an El Niño event, the east-to-west trade winds weaken or reverse, blowing warm water from the western Pacific to the east, as far as the coast of South America. For New Zealand, that means stronger or more frequent winds from the west in summer, leading to more rain in the west and less rain in the east.)
Near Cape Adare, just downstream of the main outflow of deep shelf water from the western Ross Sea, the Antarctic bottom water layer in 2018 was 300m-500m thicker than observed in 2011. The increase in salinity was three times faster than the previously observed decline before 2011 – or five times faster near the seafloor.
What does all this mean? Previous studies suggested increased melting of the Antarctic Ice Sheet because of a warming climate will decrease the volume of Antarctic bottom water.
“Our work shows that climate extremes (such as strong El Niño) can counteract this tendency,” Silvano says via email. “And these extremes are projected to become more common if greenhouse gas emissions due to human activities continue at current rates.
“What the interplay between these two phenomena – i.e. ice sheet melting versus climate extremes – will be in the next decades and centuries remain unclear, and more work is required to understand the ‘fate’ of Antarctic bottom water.”
Robinson, of NIWA, says changes to waters around Antarctica, and the effects that will filter through to the rest of the world’s oceans, aren’t well appreciated by the general public or policymakers.
“People are talking a lot about sea level rise, and that’s fair enough because we are actively trying to pin down exactly how Antarctica is melting from underneath, so we can put better predictions on that. But there isn’t a lot of attention being paid to the heat uptake of the Southern Ocean and then its impacts for how that heat is distributed everywhere else.”
It’s fascinating how Antarctic bottom water moves, where it goes, and why.
The water follows the Drygalski and Glomar Challenger troughs to the outer edge of the Ross Sea, where the continental shelf drops off to the deeper ocean. “The water cascades off of the shelf, kind of like an underwater waterfall,” says Bowen, the University of Auckland oceanographer.
Water goes down the slope and turns to the left because of the Coriolis force. Bowen says the 18-year cycle of the moon, can also be detected in the tidal velocities.
The release of dense water is replaced with circumpolar deep water, bringing heat and nutrients to the Ross Sea, and some mixing occurs. (Bowen compares it to opening the bathroom door after a shower, with steam rushing out being replaced by cold air coming in.)
A scientific team from New Zealand dropped moorings on the slopes of the Ross Sea’s continental shelf to measure the flow of dense water off the shelf. Bowen describes the moorings as a big anchor – a weight on the bottom, then a wire with instruments on it. Floats hold it upright. Sitting in 1500m of water, they’re 500m tall – higher than Auckland’s Sky Tower, which is 328m.
They’re hardy things, as they have to withstand tidal velocities of more than a metre per second, at times.
The team’s study – funded by the Business Ministry, MBIE, through the Deep South National Science Challenge – was published in the open-access journal Scientific Reports in January. Bowen, the lead author, says they discovered water came off the shelves in two main pulses timed with the equinoxes. The measurements also confirmed the heightened salinity noted in Silvano’s paper.
“Previously it had been thought that the water was escaping off the shelves when the winds were allowing it to come off,” Bowen says. “Now we think that it may be the tides, and that means that it’s going to be a different way of simulating how that water is getting into the deep ocean for those computer models.”
The Scientific Reports article says at one mooring, labelled P3, distinct plumes passed with temperatures decreasing and salinities increasing over 20-to-60 minutes, returning to their previous values within the next four-to-10 hours. “These plumes bring dense water very rapidly (with a few hours) from the trough down the slope. We did not expect to see dense plumes at this mooring.”
Another set of moorings installed in 2019 – as part of the New Zealand Government-funded Antarctic Science Platform – have just been retrieved and technicians are going through quality control. “It looks like we’ve had a good return, so we’ll have another two years of data,” Bowen says. “I assume we’ll still see those two pulses.”
(New Zealand-based scientists have collaborated with Italian colleagues, who have an active mooring programme in the Ross Sea, and have drawn on data from the huge US project called AnSlope, which took field measurements between 2003 and 2005.)
The recent research measuring Antarctic bottom water coming off the Ross Sea slope has been extremely useful, Bowen says, but it’s still early days. Another set of moorings, going across the Drygalski Trough near the shelf edge, will measure how much of that warm circumpolar deep water is flowing into the Ross Sea. They’ll be retrieved in 2023.
“The longer the time series the more you discover,” she says. “My job at the moment is to figure out what processes are in control, and then I’ll be able to tell people what the likelihood is of things changing.”
Stevens, of NIWA/University of Auckland, agrees long-term data is key. “We should have been measuring some of these things for decades,” he told an Antarctic science conference in Christchurch in February.
Climate science is often like looking in three directions at once: direct observations now, measured against evidence from the past, and fed into computer models about the future.
The picture is complex and often not well understood.
Processes often aren’t clear, and regional effects can vary – sea ice might be contracting in some places but expanding in others. Furthermore, even if scientists do have a basic understanding, there might be sparse data, or it might take too much computational effort to add local details to global-scale climate models.
But details are important. Oceanic warming in some places might be more crucial for ice shelf melt than others, leading to sudden shifts.
Some scientific measurements are incredibly thin – like satellite imagery of sea surface temperatures or sea ice extent. To add depth, it’s important to understand what’s going on beneath the surface. That includes under the Ross Ice Shelf itself.
Research in this hidden ocean is something James Clark Ross would probably be proud of.
Sitting about 500km from the front of the Ross Ice Shelf, accessed by drilling through 600m of ice, the ocean is about 30m thick. Under what’s known as the Kamb Ice Stream, it’s about 5km from the grounding line – where the ice leaves the land and starts floating.
(The Ross Ice Shelf is the world’s largest floating body of ice, above an ocean cavity comparable in size to the North Sea.)
This tiny corner of the cavity, which has barely been sampled, is where the really exciting science is happening, Stevens says. It’s completely different to a normal ocean – no surface wind, no surface heating from the sun. The drivers are tides and melting.
“It’s where all the melting starts,” he says. “And the nature of the initiation of that melting controls how the meltwater flows out through the rest of the cavity.”
The technology used to take measurements is rugged and simple. Conventional instruments for sampling temperature and salinity, currents and pressure, are fed through the ice, and connected to a satellite transmitter on the surface. Stevens: “We get the data back pretty much the next day.”
What has the work uncovered? Surprisingly, two clear, distinct layers in the water column, says Robinson, the NIWA specialist in polar oceanography.
The lower layer is high salinity shelf water, the dense, saline water created by sea ice at the front of the Ross Ice Shelf, 500km away. (Not all of it flows into the abyss.)
Meanwhile, the top layer, about 10 metres thick, is the same water but with an added component of ice shelf melt. “It’s cooler and it’s fresher,” Robinson says.
The stratification is sharp, she says, with change occurring over only one or two metres, with surprisingly little mixed water in between.
Another reason scientists thought there would be more mixing is ice shelf “pumping”. Twice a day, with each tidal cycle, the solid ice on the ocean surface moves up and down by a metre or two. That’s a “big push”, Robinson says, adding: “It’s amazing how distinct these two layers are.”
The ice shelf cavity’s super-cooled top layer emerges into the sea ice regime – feeding back into the polynya. Weirdly, the water is lower than its own freezing temperature, so billions of ice crystals form in the water and float up against the base of the sea ice. It’s called platelet ice.
Algae, the base of the food chain, likes this unusual, dense habitat. And Antarctic silverfish, a keystone species for the Ross Sea, use this platelet layer as a nursery – they lay their eggs there.
But for how much longer? Might the winds of change blow?
Even natural variability in the Ross Sea can throw up huge, sporadic changes.
In 2000, a few years before Robinson started as a marine physics student, massive icebergs broke off the front of the Ross Ice Shelf, near Roosevelt Island. The biggest, known as B-15, was about 300km long (the distance from Christchurch to Dunedin) and 40km wide – one of the largest icebergs ever observed. The sea ice regime changed for about a decade, Robinson reckons.
Robinson: “There’s real potential that if atmospheric patterns and processes change – even not very much, actually – you’ll be affecting how these polynyas work and therefore how much of this deep, cold water is formed in that area.”
Right now, high salinity shelf water created by sea ice formation acts like a curtain at the front of the Ross Ice Shelf, Robinson says, preventing circumpolar deep water getting in.
If that brine disappeared for some reason, the path would be open for warmer water to flow deep into the Ross Ice Shelf cavity.
“If that were to happen, we would see really quite dramatic changes to the Ross Ice Shelf, which we currently think of as being quite a stable system. It has changed rapidly in the past – we can see that from the geological records.”
At February’s Antarctic science conference in Christchurch, Associate Professor of ice core paleoclimatology Nancy Bertler, of Victoria University of Wellington’s Antarctic Research Centre, said meltwater pulses from ice sheets have caused sea level rise increases of up to 4m per century.
The world’s climate today, about 1.2°C on average above pre-industrial levels, is similar to 125,000 years ago, during the last interglacial period – when sea levels were six-to-nine metres higher.
“There are some large questions about how stable those ice shelves are.” – Erik Behrens
Antarctica is a head-whirling maelstrom of complexity and connection.
In the Ross Sea alone, water of varying temperature and density flows on and off the continental shelf, strong winds can boost the sea ice that keeps warmer water from the ice shelf cavity, which, in turn, produces more super-cooled water ideal for producing more sea ice.
But there’s clear evidence the region’s quite sensitive – the huge icebergs changing the sea ice regime, and Silvano’s work showing the rebound in Antarctic bottom water.
Asked how much the Ross Ice Shelf is expected to contribute to sea level rise to 2100, Erik Behrens, an ocean modeller at NIWA, says: “I don’t know, and I think most people don’t really know. There are some large questions about how stable those ice shelves are.”
Large uncertainties exist about how much ice is going to melt, when, and where, Behrens says. “That is probably the largest uncertainty that exists in those future projections. At the moment they are ballpark figures.”
Computer models do a reasonable job of getting the “mean state” of the climate right, Behrens says. But the global models struggle to incorporate tides, because of “computational resources”. (Including that level of local detail might correct the models plus or minus 10 percent, he guesses.)
Behrens warns the Antarctic is a highly non-linear system. “Those shelves could become quite unstable very rapidly.”
If high salinity shelf water gets lighter – through a combination of warming and freshening – it won’t plummet as deep into the oceanic abyss. That might create a positive feedback of warming and melting of the Ross Ice Shelf. Conversely, Robinson, the NIWA polar oceanographer, says more under-shelf melting might actually produce more super-cooled water, and more platelet ice. But she admits it might go either way.
Stevens, doesn’t think the Ross Ice Shelf will disappear anytime soon. “It is just so big.”
While still remaining intact, however, it could generate a lot of meltwater with knock-on effects for elsewhere.
What was once seen as an impenetrable barrier – compared to the cliffs of Dover by James Clark Ross just 180 years ago – now seems vulnerable.
That’s not only because of the depth of research being undertaken around, and under, parts of Antarctica, but because of our better understanding of the effect human activity is having on the planet’s climate and ecosystems.
The warnings are stark.
In recent days, the International Energy Agency has warned global greenhouse gas emissions – dulled by worldwide coronavirus lockdowns – are set to surge this year, to levels above 2019. “We are on the verge of the abyss,” UN secretary general António Guterres said of rising temperatures on Monday.
What happens now is up to us. But if humans fail to sufficiently curb emissions – something the Paris Agreement promises, as they stand, will not do – that is done with the full knowledge heat and carbon will continue to be absorbed into the oceans, contributing to the warming of Antarctic waters, with all the expected flow-on effects for the rest of the world.
Imagine the world without the Ross Ice Shelf. Something that monumental would probably make one of the world’s most famous polar explorers turn in his grave.