THE Southern Ocean punches above its weight when it comes to fighting climate change. But until now, researchers had applied a broad brush to understanding how the vast region influences climate, sea-level rise and marine life. This has all changed with new research published in Nature, which proposes a new conceptual model highlighting the importance of local ‘hotspots’.
“We’ve discovered a lot about the Southern Ocean over the past decade or two, and realised the central role it plays in climate and the global ocean system,” says Dr Steve Rintoul, a CSIRO Fellow who leads several projects for CSIRO’s Centre for Southern Hemisphere Oceans Research, the Antarctic Climate and Ecosystems CRC and the National Environmental Science Program’s Earth System and Climate Change Hub.
Researchers have known for some time that the Southern Ocean is important when it comes to putting the brakes on climate change. It makes up just under a third of the global ocean surface area, but accounts for three-quarters of anthropogenic heat uptake by the world’s oceans. By taking up heat, the Southern Ocean slows the pace of climate change. The Southern Ocean helps limit climate change in another way, by absorbing more of the carbon dioxide released by human activities than any other part of the ocean.
An ocean bridge
The Southern Ocean extends over a broad band of latitudes between about 40 and 65 degrees south, some of it circling the hemisphere without any land barriers. It reaches up into all of the world’s oceans, providing a bridge between ocean basins, and connecting surface water with the ocean depths.
Dr Rintoul says most of the world’s oceans are like a layer cake, with light layers on top of heavier layers: “The Southern Ocean is different. Here the cake gets tipped on its side, and the heavy layers reach the surface, where they interact directly with the atmosphere and with lighter surface waters. It’s the only place in the world where the deep and surface layers are connected in that way. That’s why it has such an influential role on the world’s climate.”
The Southern Ocean also supports much of the biological productivity in the global oceans.
“Sinking of dead organisms and poop carries nutrients to the deep ocean. Upwelling of deep waters in the Southern Ocean returns nutrients to the surface layer, where they can support more productivity. Without the Southern Ocean providing this pathway for nutrients to reach the surface, global ocean productivity would plummet,” says Dr Rintoul.
“We’ve known that upwelling is important, but we didn’t know where it took place or what controlled it. We now know that local topography controls the dynamics of the Southern Ocean.”
Dr Rintoul says there are hotspots of localised upwelling that occur downstream of mountain ranges on the sea floor that sit in the pathway of the Antarctic Circumpolar Current.
“What happens in local areas determines how the currents work and how they respond to change – whether human or natural.
“Topography also influences turbulence, with eddies spawned downstream of mountain ranges. These are the mediators in the ocean system; the mechanism by which heat and carbon gets transferred from one place to another.”
The thermohaline circulation plays a major role in the Earth’s climate. It comprises massive deep-ocean currents powered by differences in water density, controlled by temperature and salinity. The Southern Ocean is central to the thermohaline circulation because it links the upper and lower levels of the ocean.
“How the Southern Ocean circulation responds to changes in the atmosphere is a key question for projections of future climate. We need to know if the Southern Ocean will continue to take up heat and carbon dioxide. It’s like a battle between what the winds are doing and what the eddies are doing – we need to know who wins the battle, to improve climate projections.”
The Southern Ocean holds the fate of Antarctica, transporting warm water to the base of ice shelves.
“Ice on the Antarctic continent flows down to the ocean and then floats. These floating ice shelves are important as they help hold the thick ice sheet on the continent. But because they’re floating, they’re exposed to the ocean. If warm water gets beneath the shelves and melts them from below, more ice will flow into ocean, raising sea level.”
A surprise arising from the new conceptual model is that carbon and heat move from the surface to the deep ocean in particular locations.
“Recent work has shown carbon is being pumped into the ocean at local hotspots, and this uptake of carbon varies over time more than anticipated. A decade ago, researchers were concerned that the ocean sink was becoming saturated. That would have meant more carbon dioxide in the atmosphere, which would accelerate climate change. While that was true for a while, new work has shown the carbon sink has recovered; the Southern Ocean carbon dioxide sponge is back on track.
“This underscores the importance of changes in winds, changes in upwelling and downwelling, changes in circulation and climate on the sensitivity of the carbon sink. It’s more dynamic than we thought, and future changes may cause the uptake of carbon to decline again.”
Dr Rintoul says recent developments in knowledge have come from improved and more complete observations, from advances in computer models that better simulate the ocean system, and from increases in theoretical understanding.
“All of these have contributed to a paradigm shift – we now recognise that local ocean processes have large-scale, even global consequences.
“The older, two-dimensional view of the Southern Ocean was useful, but not enough to understand how things might change. We have a new understanding of the full three-dimensional structure of the Southern Ocean, with local hotspots where most of the important upwelling, downwelling and circulation is taking place.
“This new dynamical understanding provides clues on how the Southern Ocean will change in future. We expect it will continue to provide this service of picking up heat and carbon dioxide, transferring to ocean and regulating climate change. What we don’t yet know is whether the region will take up heat and carbon dioxide at the same rate.”
Read the Nature Insight special on Antartica and Steve Rintoul's article "The global influence of localized dynamics in the Southern Ocean", Nature, vol 558, issue 7709, 14 June 2018.