Large-scale currents are ocean conveyor belts that transport water and nutrients and control Earth’s climate. Surface currents are relatively easy to measure and monitor. But what lies deep in the ocean remains mostly a mystery. Now a new study has been published natural geology, He published the largest set of data to date on the speed and direction of currents flowing near the seafloor, and it wasn’t what scientists expected.

In the region near the Mozambique coast studied by the authors, bottom currents were previously thought to be permanent and flow from south to north. But the results showed that deep-sea currents are much more dynamic than previously known. The findings suggest that current modeling used to monitor deep-sea rainfall and fluxes of pollutants and reconstruct ancient ocean conditions needs to be updated.

“These conveyor belts of currents passing around our planet will be much more complex than textbook models suggest,” said Mike Clare, a sedimentologist at the National Oceanography Center and senior author of the study. “They really require a very thorough investigation.”

Measurement of complex currents

Scientists can measure deep-sea currents using sensors called acoustic Doppler current profilers (ADCPs) fixed to the seafloor. However, because these mooring sites are complex and expensive to deploy and manage, many studies have used them sparingly for short periods of time.

Fortunately, an Italian oil and gas company called Eni has deployed an unprecedented group of 34 ADCPs for industrial purposes in an area of ​​approximately 2,500 square kilometers (965 square miles) in the Mozambique Channel just off the coast. By sharing the data, the company gave scientists a unique and detailed view of the seafloor. The devices measured the speed and direction of the currents every 10 minutes for 4 years. “What’s unique about this study is the long-term series of bottom currents,” said physical oceanographer Jacob Wenegrat of the University of Maryland, who was not involved in the study.

When Lewis Bailey, now a geologist at the University of Calgary, began analyzing the mountain of data from the ADCP, the results looked so different from the expected trend of persistent northern currents that he wondered if he had made a mistake. “The first thing I thought was, ‘This can’t be true,'” he said.

But after analyzing all the numbers, researchers found that ocean currents often speed up, slow down, and even change direction. “We were very surprised to see how variable all the currents were, even between anchorages that were quite close together,” Bailey said.

“All the geologists involved in this project were absolutely amazed by the variability,” Claire said.

Researchers investigated what might cause the variation. “It was almost like detective work,” Bailey said. Currents varied depending on season and tidal cycles. ADCP and cleared seafloor models have shown that currents on exposed seafloor slopes generally flow northward on average. However, in underwater canyons running roughly east to west, the current often changes direction, flowing up or down their length.

Scientists suggest that tides and topography of the seabed are largely responsible for the nature of the currents.

Well-studied surface currents often vary in speed and direction, but Venegrat said there has been a recent increase in interest among physical oceanographers in studying waters near the seafloor. “A lot of the things that happen on the surface of the ocean are also happening below,” he said. “It’s exciting to see good data on all the temporal and spatial variability,” Venegrat said of the new study.

Clair noted that limited studies of deep-sea currents sometimes contradict each other, but the differences likely depend on when and where measurements were made. “I think it’s okay for these different camps to disagree with each other,” he said.

Where do sediments settle?

Scientists rely on modeling of ocean currents and limited seafloor core samples to study the transport and accumulation of sediments and pollutants such as microplastics and how these may affect deep-sea ecosystems. They also use similar techniques to reconstruct ancient ocean conditions.

The authors of the new study suggested that these simulations may be too simple. Given the variability of currents between study areas, a single core sample may be too limited to characterize sediments in the region, Clare said. “It made me realize that we need to think very carefully about the placement of instruments and not just assume that it’s a continuous, one-way flow.”

The researchers acknowledged that the study was conducted in only one region of the ocean and that more data in other areas would be invaluable to building better models.