Scientists are 'gobsmacked' by strange reversals in deep-ocean currents

Large-scale currents are the conveyor belts of the ocean, transporting water and nutrients and controlling Earth's climate. Surface currents are relatively easy to measure and track. But those in the deep ocean are mostly a mystery. Now, a new study published in Nature Geoscience unveiled the biggest data set to date on the speed and direction of currents that flow near the seafloor, and it's nothing like what the scientists anticipated.

Previously, seafloor currents were believed to be steady and, in the region off the coast of Mozambique that the authors studied, to flow from south to north. However, the results revealed that deep-sea currents are far more dynamic than previously known. The findings suggest that current simulations used to track the flow of sediment and pollutants in the deep sea and reconstruct ancient ocean conditions need an update.

"These conveyor belts of currents that operate the whole way around our planet are going to be far more complicated than the textbook models suggest," said Mike Clare, a National Oceanography Centre sedimentologist and senior author of the study. "They really do warrant very careful investigation."

Related: Where did ocean currents come from?

Measuring complex currents

Scientists can measure deep-sea currents using sensors called acoustic Doppler current profilers (ADCPs) secured to the seafloor. But deploying and managing these moorings are challenging and expensive, so many studies have used them sparingly for short time periods.

Fortuitously, an Italian oil and gas company called Eni deployed an unprecedented array of 34 ADCPs for industrial purposes over roughly 2,500 square kilometers (965 square miles) in the Mozambique Channel, just off the coast. The company shared the data, giving scientists a unique and detailed view of the seafloor. The instruments measured the speed and direction of currents every 10 minutes for 4 years. "The thing that's unique about the study is the long time series that they have of near-bottom currents," said Jacob Wenegrat, a physical oceanographer at the University of Maryland who wasn't involved in the study.

When Lewis Bailey, a geoscientist now at the University of Calgary, started analyzing the mountain of data from the ADCPs, the results looked so different from the expected trend of consistent northward currents that he wondered whether he'd made a mistake. "The first thing I thought was, 'This can't be right,'" he said.