The Cook Strait laboratory exploring rising sea temperatures
1 December 2023
A deep dive into what is happening in the Southern Ocean.
It may be one of the world’s most energetic stretches of water, and a pain when ferries are delayed, but the Cook Strait has become a natural laboratory for Professor Craig Stevens – and a window into the world of climate change.
As a member of the University of Auckland’s Joint Graduate School in Coastal and Marine Science based at NIWA in Wellington, Craig has spent the past two decades studying the Strait’s turbulent waters to better understand how the planet is dealing with rising sea temperatures.
“Ocean heat content is probably one of the big challenges facing humanity,” says Craig, who points out that while oceans currently capture around 90 percent of the heat from greenhouse gases, “we can only get away with that for so long."
Through Marsden-funded projects like CookieMonster (Cook Strait Internal Energetics MONitoring and SynThEsis Research) and TEddy-BaarE (Turbulence EDDY BAthymetRy Experiments), Craig and his team have been investigating how ocean heat is mixed in the circular currents known as eddies.
Ocean heat content is probably one of the big challenges facing humanity... we can only get away with that for so long.
“The ocean is full of eddies! We’re looking at the relationship between these eddies and turbulence to see how the energy cascades from a one-kilometre horizontal eddy into three-dimensional mixing at a molecular scale.
”Using the deep-water research vessel Tangaroa, whose running costs are being funded by MBIE to the tune of almost one million dollars, researchers have deployed a range of technology including an ocean glider and – for the first time in 2023 –an array of drifting turbulence sensors.
“What’s remarkable about that technology is its acoustic sampling of turbulence,” says Craig. “Hopefully, it will enable us to capture the life cycle of an eddy.”
The ocean turbulence research is part of a suite of ocean-climate projects that Craig leads. One of these is part of MBIE’s Antarctic Science Platform that’s studying Antarctic waters, where Craig says a warming ocean “comes into contact with the largest chunk of ice on the planet and is melting it at a greater rate than it’s done in the past.”
“Due to its remoteness, there is much to learn about the Southern Ocean and the Antarctic circumpolar current,” he says, “and it’s quite exciting being able to do oceanography in a part of the planet’s oceans that have never been sampled before.”
Some of this work samples how eddies behave around the coast of Antarctica where oxygen is absorbed into the ocean and then works its way around the planet, in what scientists describe as a big ‘conveyor belt’, before it emerges several hundred years later. “It’s what’s oxygenating the oceans, it’s the planet’s lifeblood if you like.”
The concern is that warmer oceans will interfere with the conveyor belt circulation or mixing process in deep ocean currents.
“The melting ice is reducing the amount of oxygen that gets into the global ocean and, in a hundred years’ time, we’re going to see some significant changes in marine ecosystems and coastal ecosystems. We will be in for some surprises,” says Craig.
Among the many unknowns is how deeply the upper ocean heat penetrates and how it is mixed and stored, which makes it difficult to feed parameters into long-term climate models that are looking out beyond this century.
“What’s not in question is the basic heat budget and heat balance. We’re trapping this amount of heat and it’s going to melt this amount of ice. What’s less certain is, is it going to be 500 years for a particular outcome or 200 years – and how will the impacts play out in different parts of the planet?”
Of immediate concern are rising sea temperatures which topped a record 36 degrees off the coast of Florida in mid-July, well above the 27 degrees, which can typically kick-start tropical cyclones.
“If that really warm layer extends down 50 metres or more, then that can keep driving these increases in tropical cyclones,” says Craig, “and that’s where we come full circle back to understanding ocean mixing and how to get those parameters and processes into long-term climate models.”
While US states like Florida have millions of dollars’ worth of measurement infrastructure in the water to provide forecasts about extreme weather events, Craig says that New Zealand has almost none of that infrastructure because there are “very few taxpayers, a lot of ocean, and not enough awareness of what the ocean does for us all.”
But if we’re going to see more cyclones like Gabrielle, he says there’s going to be a need for more storm prediction and more emergency response which the U.S. has done for decades.
“It will be something that we will be needing to take much more seriously here.”
Along those lines, Craig also has an interest in how to capture renewable energy from the ocean as “it is very clear that we have to do everything possible to reduce our greenhouse gas emissions –and soon!”
Another challenge for scientists is how to communicate the seriousness of the situation to people who are prone to switching off about an issue that’s so far into the future. Craig says that being quite nuanced, especially with younger audiences, and striking a balance “about the vibe” is important.
“You need to be very clear to audiences that it’s going to happen, we just don’t quite know when.”
Having a te ao Māori worldview about how oceans work also helps in this regard as it recognises the connectedness of systems, and he says that “we have to be very clear that everything we do to reduce emissions will improve outcomes.”
On a personal level, Craig says that he’s “in a privileged position” to be able to work at NIWA and be part of Auckland’s Joint Graduate School because it allows him to directly connect with Crown Research Institutes through basic and applied science.
“There’s lots of benefits having those two sides of the table come together.”