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Q: What is the dead water phenomenon?

Krishna: Dead water is the nautical term for a phenomenon which can occur when there is strong vertical density stratification due to salinity or temperature or both. It is common where a layer of fresh or brackish water rests on top of denser salt water, without the two layers mixing (1).

Dead water

Image source: Oceanography: waves

The phenomenon is frequently, but not exclusively, observed in fjords where glacier  runoff flows into salt water without much mixing (2).
What makes ships mysteriously slow down or even stop as they travel, even though their engines are working properly? This was first observed in 1893 and was described experimentally in 1904 without all the secrets of this 'dead water' being understood. An interdisciplinary team from the CNRS and the University of Poitiers has explained this phenomenon for the first time: the speed changes in ships trapped in dead water are due to waves that act like an undulating conveyor belt on which the boats move back and forth. This work was published in PNAS on July 6, 2020.
In 1893, the Norwegian explorer Fridtjof Nansen experienced a strange phenomenon when he was traveling north of Siberia: his ship was slowed by a mysterious force and he could barely maneuver, let alone pick up normal speed. In 1904, the Swedish physicist and oceanographer Vagn Walfrid Ekman showed in a laboratory that waves formed under the surface at the interface between the salt water and freshwater layers that form the upper portion of this area of the Arctic Ocean interact with the ship, generating drag.
This phenomenon, called dead water, is seen in all seas and oceans where waters of different densities (because of salinity or temperature) mix. It denotes two drag phenomena observed by scientists. The first, Nansen wave-making drag, causes a constant, abnormally low speed. The second, Ekman wave-making drag, is characterized by speed oscillations in the trapped boat. The cause of this was unknown. Physicists, fluid mechanics experts, and mathematicians at the CNRS' Institut Prime and the Laboratoire de Mathématiques et Applications (CNRS/Université de Poitiers) have attempted to solve this mystery. They used a mathematical classification of different internal waves and analysis of experimental images at the sub-pixel scale, a first.
This work showed that these speed variations are due to the generation of specific waves that act as an undulating conveyor belt on which the ship moves back and forth. The scientists have also reconciled the observations of both Nansen and Ekman. They have shown that the Ekman oscillating regime is only temporary: the ship ends up escaping and reaches the constant Nansen speed (3).
2.  Hambling, David (31 Aug 2018). "Weatherwatch: 'dead water' makes ships seem dead in the water"The Guardian. Retrieved 18 September 2020.
3.  Johan Fourdrinoy el al., "The dual nature of the dead-water phenomenology: Nansen versus Ekman wave-making drags," PNAS (2020).

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