At the bottom of the world lies the vast and understudied Southern Ocean. On board the R/V Investigator, a research vessel that set sail to there this week, scientists will be studying surface water chemistry and the Southern Ocean’s clouds, but oceanographer Steve Rintoul will be focused further down.
The six-week expedition, a collaboration between Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) and several other institutions, will help Rintoul and his team better understand how the deep ocean surrounding Antarctica is changing and what that might tell us about global climate change and sea level rise.
Rintoul, a CSIRO researcher, will be deploying 11 automated monitors, known as Argo floats. Each float can sink down to 9,800ft (5,000m) and collect information on the temperature and chemistry of the deep waters before bobbing up to the surface to transfer its information via satellite. Then the float sinks back down to do it all over again. A network of 3,800 similar Argo floats already monitor the world’s oceans, but until now no Argo floats in the Southern Ocean have been able to go below 2,000m, which left researchers largely blind to what was happening there.
Oceans Deeply spoke with Rintoul about the expedition and the implications for a changing deep Southern Ocean.
Oceans Deeply: Why do we know so little about the Southern Ocean?
Steve Rintoul: It is, on average, the windiest part of the world ocean. The large waves make doing science out there challenging and uncomfortable at times. That is part of the reason. Historically, it’s also a very poorly measured area because there’s no reason to go there most of the time. Commercial shipping doesn’t need to be there, and it’s best avoided if you can because it can be pretty nasty.
For a long time, we didn’t realize how important the Southern Ocean was to the planet and to global ocean circulation patterns. I did my PhD in the late 1980s, and we learned about the Antarctic circumpolar current, and we knew it was the biggest current in the entire world ocean, but we really didn’t know how it worked. The ideas that allowed us to understand the currents at lower latitudes, like the Gulf Stream, didn’t work in the Southern Ocean, where there’s no land boundaries. It turns out that makes a big difference to the physics of how the current works.
Oceans Deeply: What is new about how you will be studying the Southern Ocean, and how does that compare to what researchers did in the past?
Rintoul: In the early 1990s, it was the first time we had satellites that could measure ocean currents directly from space. That was a major step forward, and the next real revolution was in the early 2000s, when we started deploying Argo floats. Just like the satellites gave us the first global view of the surface ocean, we have the first global, year-round measurements of what was happening in the upper 2 kilometers of the ocean. That still left the deep part of the ocean unmeasured, except by ships.
All our information to date about what happens in the deepest half of the ocean is based on ship observations. This will be the first time these deep Argo floats have been deployed in the waters near Antarctica, and that’s important because it’s that part of the deep ocean that’s changing most rapidly. That’s what the ship-based observations tell us, but the information we have so far is pretty piecemeal.
These floats can bob up and down underneath the ice. They can’t talk to us through the ice, but once the ice melts back in summer they can surface and tell us what they measured during the previous winter.
It’s a time-saver, but it’s even more important than that because the oceans and other parts of Earth’s system change all the time, whether it’s the seasons, El Niño cycles or climate change. If you’re only measuring something every five years or so, it can be tough to unravel short-term variability from longer-term trends.
Oceans Deeply: What do you expect to find once you send these instruments down into the deep?
Rintoul: The work we’ve done already shows that the Southern Ocean is warming, it’s freshening, it’s becoming more acidic and has more carbon dioxide stored in it than it used to. I expect those trends will probably continue. The real question is: by how much? How rapidly is this region changing?
Part of why we’re focused on the deep ocean is that it’s not really in contact with the atmosphere. If the bottom layer warms, we wouldn’t know about it on land. But the ocean currents that carry that dense water down into the deepest part of the ocean are the main way that oxygen reaches the deep ocean.
Changes in that current system would make a big difference to deep marine life, but it also does carry carbon and heat into the ocean as well. By detecting changes in temperature and salinity, it tells us what’s happening to that circulation system. That’s the real reason that we’re focused on the deep ocean.
Oceans Deeply: Is there anything else that you want to say?
Rintoul: We’re going to East Antarctica. People talk about the western half of Antarctica and the eastern half of Antarctica, and there’s been a lot of attention paid to the possibility that Antarctica may lose ice and that may cause sea-level rise.
We know that [part of] West Antarctica is changing rapidly now. The amount of ice that’s being lost has accelerated, at least over the last couple of decades. But we thought that East Antarctica was pretty stable, because we thought it was far removed from warm air currents and warm ocean currents. That was reassuring because of the 58 meters of sea-level rise [that could be caused by] Antarctica [melting], 53 of them are in East Antarctica. West Antarctica has five. Five is still a lot, but it’s better than 53 or 58.
Some work we did a couple of years ago went to an area called the Totten Glacier. Some satellite data had shown that it was thinning, and that the grounding line – the place where the ice starts to float as it reaches the sea – was retreating. That was kind of a surprise. That’s what we see in West Antarctica, but it wasn’t supposed to happen in East Antarctica.
That’s a bit of a warning sign that we can’t take East Antarctica for granted, and it may also respond to changes in this ocean. Our measurements are also aimed at sorting out just how vulnerable Antarctica is to changes in the ocean, and what the implications might be for future sea level.