Piloting underwater gliders into the heart of Earth's strongest current

On board the RV Investigator, it's perfect glider weather.

Calm seas make launching – or deploying – an autonomous sea glider into the Southern Ocean less of a blast off and more of a splash.

But for oceanographers, today’s high tech autonomous vehicles still bring all the excitement of sending a rocket into space to explore the vast unknown.

We deployed three ocean gliders from our research vessel (RV) Investigator in 2023.

Their work starts around 55 degrees south – about halfway between Tasmania and the Antarctic. Here, the calm surface belies the strength of the water in the Antarctic Circumpolar Current they’ve been sent in to explore.

The science team on board RV Investigator was led by the Australian Antarctic Program Partnership and CSIRO.

Researchers wanted to paint a more detailed picture of these eddies and small-scale processes, to better understand the role they play in transporting heat across the current.

The voyage sought to validate, for the first time, data of the Southern Ocean taken by the new Surface Water and Ocean Topography (SWOT) satellite.

The gliders were part of an array of monitoring equipment deployed from the ship. Their task was to directly measure the properties of the ocean inside the current. The satellites indirectly measured the height of the ocean surface from space.

Radar information from the satellite is combined with a description of the ocean below the surface over exactly the same location. This allows scientists to create a 3D view of the current.

They move by adjusting buoyancy. Using hydrodynamics and small fins, they can travel up and down the water column up to 1000 metres deep. They can also move against and across the current.

Craig Hanstein is our Technical Officer. He says the buoyancy engine is fundamental to the glider's ability to navigate the currents.

Autonomous underwater gliders are increasingly sophisticated thanks to a payload of finely tuned instruments, or sensors, which measure the water column. These sensors measures things like temperature, salinity, pressure, and dissolved oxygen.

Dr Laura Herraiz Borreguero notes while they’re not new, the gliders are still not common.

“They became commercially available in 2013. Only a very few lucky scientists have actually been able to own or use them,” Laura says.

“A strong constraint on achieving a better understanding of the small-scale ocean processes has been the lack of suitable instruments to resolve the scales at play.”

Once deployed, the gliders were not alone. Back in Hobart, the mission control team was led by Laura and Dr Beatriz Peña-Molino.

Bea and Laura were joined by pilots in the United States, on the west coast at Caltech and on the east coast at Brown University. As part of our team, their job was to oversee the gliders 24/7.

“Because of the way the time zones work with the Australia and the US, there's always somebody with eyes on the glider,” Bea says.

Each time it surfaces, a glider establishes communications with the shore station over an Iridium satellite link. The glider then transmits data and receives piloting instructions.

This allows the pilots to monitor progress, check the state of critical systems such as battery level, and make changes to the mission course.

“It requires a lot of thinking and anticipating what the current is doing. We've been working closely with the satellite information, and we change the direction on the glider and try to counteract a little bit what the currents are doing,” Bea says.

“It's like a bit like a kayaker trying to cross a river and thinking, you know, where do I have to start paddling if I want to end up on the opposite side of the river?”

The beauty of the gliders is they can be piloted from shore and sent into the small-scale (10-100km) eddies of the Antarctic Circumpolar Current.

And when the gliders reached the surface to transmit data from the sensors, scientists were truly excited by what they saw.

These gliders had been cruising the polar front profiling the Antarctic Circumpolar Current for four months, transmitting data every four hours.

What’s revolutionary is the scale of data.

Laura describes the sea glider as a “step-changer in ocean observing capabilities”.

“In fact, the data collected has exceeded our expectations!” she says.

Co-pilot Bea says typically, on trips in the Southern Ocean, transects were 100km apart.

"But the glider has been giving us data often every two, three or four kilometres,” Bea says.

“The feature in particular I’m thinking of is like a little filament. It's something relatively narrow that you can track from the surface all the way down through time and space into the deep ocean," Bea says.

“We were able to trace one of these events where eddies are able to break these barriers in the current moving the heat southwards. We were super lucky that we were following one of these eddies when it broke off from the current. We made several sections across that eddy as it moved south and it basically was dissipating and releasing all that heat.

“One of the very interesting things that I'm keen to do is to look at how much heat was trapped in this area and then connect it with the satellite data and say, when we observe this at the surface, this is what we see underneath.

“We can then extrapolate this across the circumpolar current and review some of our understanding of how the heat from the subtropical ocean goes south.”

Ocean research is teamwork. Not only did we work with Caltech to deploy their additional gliders, and NASA and CNES on the SWOT satellite, we needed help getting our glider back.

Technical officer Craig Hanstein wasn’t on board when our glider was deployed, but he was on board for the recovery.

“We coordinated with Laura and Bea. The day we arrived on station to find out where the float was, they could tell the glider via satellite to send a GPS position back to them, which they sent to us. We would go to the office of the watch and sail the ship toward that position,” Craig says.

“We weren’t far away from it and the officer of the watch at the time spotted it.

“RV Investigator was close enough that we were able to make a rescue mission possible,” Elizabeth says.

"Really, the success is owed to the captain and the officers and the deck crew who made it look like an easy job of scooping this tiny little thing up in a specialised recovery net we use off the midship coring boom. Really exciting, lots of happy people on board yesterday, or the day before, when we were finally able to get it back.”

“We were jumping up and down when they finally picked the gliders up, because it’s been a long time,” says Bea.

“The most exciting part for us is this combination with a new satellite technology. Some of the features we’ve seen would have been invisible to us with traditional tools. But with SWOT and the gliders, it’s zoomed into something that we haven’t seen before.

“We’re really excited about putting the data and science in the bigger context and understanding what it means for the Southern Ocean as a whole.”

This research was supported by grants of sea time on RV Investigator from our Marine National Facility. To read more about the science deployed from the FOCUS voyage and highlights from life onboard, go to the voyage blog on our partner’s website Australian Antarctic Program Partnership.