- Salt fingering
Salt fingering is a mixing process that occurs when warm salty water overlies cold fresh water. It is driven by the fact that the molecular diffusion coefficient for salt is much smaller than that for heat. A small parcel of warm, salty water moving downwards into a cold fresh region will thus lose its heat before losing its salt, thus becoming denser than the water around it and sinking further. Likewise a small parcel of cold fresh water displaced upwards will gain heat by diffusion from the surrounding waters, which will then make it lighter than the surrounding waters, and cause it to rise further. Thus the fact that salinity diffuses much less efficiently than temperature paradoxically results in a turbulent process that mixes salinity much more efficiently than temperature.
Salt fingering was first described mathematically by Prof.
Melvin Stern ofFlorida State University in 1960 and important field measurements of the process have been made by Raymond Schmitt of theWoods Hole Oceanographic Institution and Mike Gregg and Eric Kunze of theUniversity of Washington, Seattle . A particularly interesting area for salt fingering is found in the Caribbean Sea, where it is responsible for producing a "staircase" of well-mixed layers a few meters in thickness that extend for hundreds of kilometers.A classic paper by the American oceanographer
Henry Stommel (actually predating the work of Stern) discussed the creation of a large-scale salt finger in which a column of water would be surrounded by a membrane that would allow diffusion of temperature, but not salinity. Once primed by moving cold fresher intermediate upwards, such a "perpetual salt fountain" would be able to draw energy from the local ocean stratification.References
Gregg, M.C., (1988). Mixing in the thermohaline staircase east of Barbados. In Small Scale Turbulence and Mixing in the Ocean, eds. J.C.J. Nihoul and B.M. 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.
Schmitt, Raymond W. The Ocean's Salt Fingers. Scientific American, May 1995, pp. 70-75.
Stern, Melvin E., (1960). The ”salt-fountain” and thermohaline convection. Tellus, 12,172-175.
Stommel, H., Arons, A.B., & Blanchard, D. (1956). An oceanographic curiosity: theperpetual salt fountain. Deep-Sea Research, 3,152-153.
Links
http://www.planetwater.ca/research/oceanmixing/saltfingers.html
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