22 October 2008

"Dead water" explained


"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… 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."

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