Cook Strait boasts some of the planet's most turbulent waters, with currents twice that of walking speed. Photo / File
Cook Strait boasts some of the planet's most turbulent waters, with currents twice that of walking speed. Photo / File
Scientists are about to send underwater drones to the floor of wild Cook Strait in a new $900,000 study dubbed "Project Cookie Monster".
Despite its humorous name, the three-year project should provide a vital contribution to global models showing how our oceans are warming due to climate change.
Ninety per cent of the solar heat captured by the planet is stored in the oceans, and understanding the processes that transport and mix this heat is vital for predicting its transfer around the globe - and thus our climate.
While efficiency of ocean mixing has now been shown for areas of Earth with low-to-moderate turbulence, scientists are yet to understand and describe what happens in those places with extreme turbulence.
"We know that heat starts off in surface waters - but we need to know how it gets down deeper, and our current computer models are based on some pretty basic understandings," said Niwa and University of Auckland physical oceanographer Associate Professor Craig Stevens, who is leading the new Marsden Fund-supported study.
"They don't really take into account the fast-flowing or high energy areas of the ocean, so we are directly targeting those kinds of processes to build a better way for models to represent that mixing."
As it turned out, Cook Strait, which is right on the doorstep of Niwa's Wellington offices, offered the perfect natural laboratory to investigate them.
Regular users of the Interislander ferry might not be surprised to know the strait packs some of planet's most turbulent currents, which reached speeds of around 10km/h, or twice that of normal walking speed.
The gliders travel at a speed of about 20cm per second. Photo / Dave Allen
Stevens noted how the narrow and treacherous French Pass that separated D'Urville Island and the South Island - and which could stun fish with its currents - had a much more infamous reputation for turbulence.
But while Cook Strait flowed slightly slower, it was 10 to 15 times deeper, and much wider than the pass.
The gliders are usually deployed for a month at a time in but will be the Cook Strait for stretches of only days. Photo / Dave Allen
"So the eddies that are the turbulent stirring get to be much, much bigger, which makes for a lot more turbulent energy in the Cook Strait."
To measure that turbulence, Stevens' Niwa colleague Dr Joe O'Callaghan and researchers from Australia and US will use the latest technology - including a pair of autonomous underwater gliders.
These torpedo-like drones will travel around extreme areas of the strait, at speeds of only around 20cm a second, capturing data such as flow speed, temperature and water salinity.
The instruments will help researchers better determine turbulence rates in Cook Strait. Photo / Craig Stevens
"It zig-zags between the seabed and the surface and every few hours it transmits its recordings back to a ground station where Joe O'Callaghan can make sure it's doing the right thing, check the data and adjust where it's going to go next," Stevens said.
"We typically send these out for a month but in this particular experiment, we don't have them in the water for much longer than a couple of days because they get spat out of the areas we're most interested in."
Other information would be gathered by sampling from vessels, and drifting instrument platforms.
Later in the project, drifting turbulence-measuring robots will be released into the massive Antarctic Circumpolar Current to look at how this understanding holds up in the largest ocean current on the planet.
In demystifying the processes involved in ocean mixing in such high-turbulent areas, Project Cookie Monster will help to quantify a fundamental parameter in modelling ocean fluids - and ultimately boost our ability to estimate the distribution of heat around the globe.
Niwa and University of Auckland physical oceanographer Craig Stevens. Photo / Aitana Forcen Vazquez
"What we'll get won't be a new model as such, but kind of like a smart carburettor on a car - it's the critical piece that makes everything work better."
New Zealand and climate change
• Under present projections, the sea level around New Zealand is expected to rise between 30cm and 100cm this century. Temperatures could also increase by several degrees by 2100. • Climate change would bring more floods; worsen freshwater problems and put more pressure on rivers and lakes; acidify our oceans; put even more species at risk and bring problems from the rest of the world. • Climate change is also expected to result in more large storms compounding the effects of sea-level rise. • New Zealand, which reported a 23 per cent increase in greenhouse gas emissions between 1990 and 2014, has pledged to slash its greenhouse gas emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels by 2030. • The new coalition Government has promised greater action from New Zealand, with a proposed new independent Climate Commission and Zero Carbon Act and goals for a carbon-neutral economy by 2050 and 100 per cent renewable energy by 2035.
