BBC Science Focus
Between Scotland, Greenland and the eastern coast of Canada, an array of underwater sensors stretches for more than 3,000 kilometres. Prof Penny Holliday from the UK’s National Oceanography Centre describes the array as a picket fence. She’s the UK principal investigator of OSNAP, the Overturning of the Subpolar North Atlantic Programme, which installed the 58 sensors in 2014. Each sensor consists of a giant, air-filled ball that sits near the ocean surface. The ball is fixed to the seafloor thousands of metres below via a mooring line. The ball’s buoyancy keeps the mooring line upright, and along the line’s length are various instruments that measure water temperature and salinity, as well as the speed and direction of currents flowing past. The aim is to monitor the subpolar gyre, which is a massive current of warm water that swirls anticlockwise across the North Atlantic. The gyre releases heat to the atmosphere, which flows over the UK and Europe. “That’s what keeps us warm,” says Holliday. We can see this effect by comparing temperatures of the UK or Europe with the same latitude in Canada. The difference is caused by the heat brought to us by that large-scale ocean circulation.
The Atlantic’s subpolar gyre is part of a global process called ‘overturning circulation’. This is the process by which warm, shallow seawater flows from the tropics towards the poles, where it gradually cools, becomes denser and sinks, then flows back towards the tropics again. It plays a key part in climate models, distributing heat and carbon around the planet, but in the North Atlantic it’s not well understood. Until the array was installed, scientists had no idea how strong the overturning circulation was at this latitude, or how it changed over time.
From the first few years of data, Holliday and her team have begun to get a handle on it. “Even just getting those [first] numbers feels like a huge step forwards,” she says. “The interesting thing we found is just how variable it is.” They also realised they’d been wrong in assuming that the most important place for the overturning circulation was between Canada and Greenland in the Labrador Sea. In fact, the centre of action lies between Greenland and Scotland. “That doesn’t sound very exciting, but it’s important for the way we interpret climate models and the predictions we make about changing climate,” she says.
The OSNAP array will stay in place until at least 2024, to continue monitoring the gyre and boost the confidence of future climate predictions. Holliday’s team are also bolting on new devices to measure oxygen levels. “One of the big questions in the world at the moment is an issue of whether the ocean and some shelf seas are losing oxygen,” she says. “We’re able to build on this array that was designed for another purpose to get extra information, which is really exciting.”