Salt fingers are a phenomenon of paradoxical mixing of sea water in a situation where the distribution of salinity and temperature in depth is gravitationally stable, and the Rayleigh - Taylor instability cannot develop. The instability leading to mixing arises due to the fact that salt diffusion is slower compared to thermal conductivity . This phenomenon is a special case of double diffusion .
Content
Education Mechanism
Consider a situation in which warmer and more salty water is located above colder and more fresh water, and the density of water above is lower than below. At first glance, this condition is gravitationally stable. However, let a small volume of warm salt water move down to a colder and more fresh area. The temperature of this volume due to heat exchange will decrease faster than the concentration of salt in it - due to diffusion. Thus, it will become denser than the surrounding water and continue to sink further. Similarly, a small volume of cold fresh water, moved up, will receive heat due to heat transfer faster than the salt diffuses into it, which will make it easier, and it will continue to float.
One can describe the development of this instability in another way. Heat spreads from the upper layers of water to the lower ones much faster than the salt diffuses from the lower layers to the upper ones. As a result of heating the lower cold layers, their density decreases to such an extent that it becomes less than the density of the upper warm layers (which are also cooled, which increases their density). So there is an inverse density distribution, which leads to the development of Rayleigh - Taylor instability .
Discovery History
The theory of salt finger formation was proposed in 1960 by Melvin Stern [1] . Field measurements were taken by Raymond Schmitt [2] from the Woods Institute of Oceanology, Mike Gregg [3] and Eric Kunze [4] from the University of Washington, Seattle.
An interesting area in which salt fingers are observed is in the Caribbean Sea , where a “stepped” vertical water structure is formed in which well-mixed layers alternating several meters thick, extending horizontally for hundreds of kilometers, alternate.
Notes
- ↑ Stern, Melvin E., (1960). The "salt-fountain" and thermohaline convection. Tellus, 12,172-175
- ↑ Schmitt, Raymond W. The Ocean's Salt Fingers. Scientific American, May 1995, pp. 70-75.
- ↑ Gregg, MC, (1988). Mixing in the thermohaline staircase east of Barbados. In Small Scale Turbulence and Mixing in the Ocean, eds. JCJ Nihoul and BM Jamart, Elsevier Oceanograohy Ser., 46, 453-470.
- ↑ Kunze, Eric, (1987). Limits on growing, finite-length salt fingers: A Richrdson number constraint. Journal of Marine Research., 45, 533-556.
Literature
- Robert Stewart Introduction to physical oceanography. Chapter 8. The equation of motion with friction . - 2005. Archived on January 3, 2011. Archived January 3, 2011 on Wayback Machine
- Kirillov S.A. Thermohaline intrusions in the northern part of the Laptev Sea and their contribution to the processes of vertical heat and salt exchange . - St. Petersburg, 2007.
- Lagovsky V. “The Big Spoon of Nature”. Science and Life, 1978, No. 4.