Measuring the temperature of the ocean

“It’s a bit rough tod–”.

Shane Keating suddenly turns around to look through the porthole behind him. “Woah! That was a big one!”

An Associate Professor in Oceanography at the University of New South Wales, Keating is talking to me from the CSIRO research vessel RV Investigator as it sails through heavy seas, roughly 700 kilometres east of Sydney. He brings his camera closer to the porthole for me to see outside. There’s a gloomy grey sky above large, white-crested peaks that are crashing against the 94-metre research vessel. When he refocuses the camera on his face, he asks if it is okay if he turns his video off.

“Staring at a screen isn’t ideal in conditions like this,” Keating says, laughing. “Yesterday was beautiful: glassy water, light breeze. But it’s really picked up overnight and we’ve actually got a front coming in tonight. So, it’s gonna get even bigger and more roly.”

The RV Investigator set sail from Sydney Harbour on October 9 on a 24-day voyage off Australia’s southeast coast, with more than 60 scientists, support staff, and crew on board. And although they’ve had some moments to admire the pods of migrating humpback whales and flocks of short-tailed shearwaters, the majority of their time so far has been spent in a much less leisurely manner.

“Most of us are working flat out, twelve hours every day.” Keating – who is on the 2:00AM to 2:00PM shift – explains. “There’s always people up and doing science.”

The ocean is not flat. It actually changes height as you move across it

Shane Keating

Much of the science that is happening on board – indeed, one of the main objectives of this particular voyage – is connected to science happening hundreds of kilometres above the ship.

In December 2022, NASA and Centre National D’Etudes, the French space agency, launched a satellite as part of the Surface Water and Ocean Topography (SWOT) mission. It is making the first global survey of Earth’s surface water and mapping, in unprecedented detail, the ocean’s topography, using an innovative instrument called the Ka-band radar interferometer.

“The ocean is not flat,” Keating explains. “It actually changes height as you move across it. It can be a few metres higher in one location than another. And the reason for that is low- and high-pressure systems, just like we have in the atmosphere.”

Oceanographers like Keating can then use the detailed topographical measurements gathered by the SWOT satellite to study the behaviour of ocean currents.

These vast rivers of seawater were once thought to exist in a state of relative permanence. As Rachel Carson writes in her 1951 classic, The Sea Around Us, although they have “undoubtedly” changed their courses many times since the beginning of the Earth, “so far as the brief period of human history is concerned…it is most unlikely that there has been any important change in the major patterns of oceanic circulation”.

But much has changed in the sixty-odd years that have passed since Carson wrote these words. In recent times, for example, scientists have detected ocean currents like the Gulf Stream and the East Australian Current becoming more energetic.

If you’re driving a ship and want to save a bit of fuel, you can go with the flow if you know where the currents are.

Shane Keating

Keating says that improving our knowledge of how ocean currents are changing is important for a number of reasons, including developing a better understanding of how ecosystems that contain commercially-important fisheries are responding to climate change, and how to make better use of ocean environments.

“For example, if you’re driving a ship and want to save a bit of fuel, you can go with the flow if you know where the currents are,” Keating says.

He also points out that the ocean has a “strong influence on the atmosphere and we suspect that regions in the ocean where there are strong temperature differences are associated with the formation of large-scale weather systems. And we need to understand that better.”

But in order to be able to properly interpret the data that is being collected by the SWOT mission and understand what ocean currents are doing, comparative data must also be collected simultaneously from the locations on Earth’s surface the satellite passes over.

This is the responsibility of a large consortium of international scientists, of which Keating is part – and it is exactly why he is out on the RV Investigator now.

“This cruise is the first to make measurements in the blue ocean around Australia,” he says.

But as he points out, rendezvousing with a satellite that is roughly 900 kilometres overhead and moving at very high speeds is “a tricky thing to get right.” Yet, despite the logistical challenge, “we’ve managed to do it several times already on this cruise and have a few more opportunities before we sail back into Sydney.”

The last flyover occurred a few days beforehand, on the 20^th and 21^st October, above a large, biologically-rich cold eddy about 500 kilometres offshore of Eden, on the New South Wales south coast. (“The way the satellite orbits, it actually samples some certain locations twice within a twelve-hour period,” Keating says.)

With the ship in position, Keating and his colleagues onboard first scanned the sky in vain to spot the satellite. Next, they deployed a microwave-shaped instrument known as a TRIAXUS, which is attached to the back of the ship and towed through the water to measure variables such as temperature and salinity.

“That helps us get a better understanding of what the pressure and density is doing in that region, which we can then compare with what the satellite sees,” Keating says.

One of the eddies we have been studying this month at sea is enormous and hot.

Prof. Moninya Roughan

But this voyage of the RV Investigator has other objectives aside from collecting data for the SWOT mission. Keating’s onboard colleagues have also been surveying seabird populations, which appear to be recovering from the past couple of years; measuring biological activity throughout the water column using a laser optical plankton counter that is fixed to the TRIAXUS; and tracking a serious marine heatwave that has recently developed in the region.

“One of the eddies we have been studying this month at sea is enormous and hot, with anomalously hot temperatures extending hundreds of metres down through the water column – more than 3 degrees Celsius above average,” RV Investigator voyage leader Professor Moninya Roughan says.

“The warm eddy is offshore, so not yet fully impacting coastal waters, so people haven’t noticed it yet. If it pushes down the coast or onshore it will bring the warm water with it likely earlier than normal. There is some evidence of warm filaments already spreading into coastal waters.”

The SWOT satellite will continue orbiting Earth and collecting data about its surface for three more years, before a controlled deorbit. Keating says that once the mission complete, it will “totally revolutionise our view of the ocean”.

“Just to put it in perspective, the spatial resolution that we’ll get from the satellite will be ten times what we’re able to see right now. I did a quick calculation, and that’s kind of like going from black and white television to high-definition television in one go.

“We currently view the ocean as a kind of slowly, smoothly evolving thing. In fact, it’s incredibly complex, turbulent and chaotic – and SWOT will reveal that to us.”