In 1893, Norwegian explorer Fridtjof Nansen and his ship Fram were victims of a strange phenomenon as he sailed past the Nordenskiöld Archipelago, north of Siberia.
Nansen wrote afterwards: "Fram appeared to be held back, as if by some mysterious force, and she did not always answer the helm … We made loops in our course, turned sometimes right around, tried all sorts of antics to get clear of it, but to very little purpose."
Nansen called the effect "dead water", reporting that it slowed Fram to a quarter of her normal speed.
Research has already shown that dead water occurs when an area of water consists of two or more layers of water with different salinity, and hence density – for example, when fresh water from a melting glacier forms a relatively thin layer on top of denser seawater. Waves that form in the hidden layer can slow the boat with no visible trace.
Now French scientists recreating that scenario in a lab tank have revealed new detail of the phenomenon and even captured the effect on video. The work will help scientists to better understand dead water and the behaviour of stratified sea patches.
Physicist Thierry Dauxois and colleagues from the University of Lyon found that a hidden wave at the interface of the layers invisibly chases and slows a boat (see video, top right).
The toy boat is pulled across the 300-centimetre tank with a constant force by a cable. The water is separated into two layers of different saltiness and hence density, labelled with dye.
Just as described by people who have experienced dead water in the real world, the water's surface is smooth, but the boat suddenly slows as the concealed wave makes contact.
"It creates a depression below the boat that prevents it from moving," team member Matthieu Mercier told New Scientist.
It is the boat itself that initiates the wave – water from the layers below is dragged upwards to fill in the gulf its wake. That sets up an oscillation in the boundary between the layers, which gradually grows as the boat moves forward.
The wave gains size and speed until it, and the trough in front of it, eventually catch up with the boat and sapping its energy before the wave breaks against its side, Mercier says.
Although previous work on dead water considered two layers of water, the real ocean naturally separates into many different layers of slightly varying salinity. When the researchers added a third layer of water to their experiments, hidden waves appeared at both boundaries, slowing the boat by about the same amount.
Studying the way these "interfacial waves" build and develop across the different layers could help scientists to understand real ocean dynamics – for example, how pollutants mix and percolate down to the depths of the ocean, says Dauxois.
Leo Mass, a physical oceanographer at Utrecht University, was the first to study dead water in detail. He says the same effect may also explain how strong swimmers can experience unexpected difficulties in the ocean.