Salt Fingering


Salt fingers were discovered around 1857 in laboratory experiments by W. Stanley Jevons, an English scientist working in Australia. Jevons did not understand the cause of the phenomenon, and a physical explanation had to await a 1960 article by Melvin E. Stern, now at Florida State University. Stern realized that because heat diffuses about 100 times more rapidly than salt, rising and sinking tendrils of water will develop wherever warm, salty water lies atop cooler, fresher water, as explained in more detail below.

The modern-day study of salt fingers in the laboratory began in the mid-1960's with work of J. S. Turner. Turner noted a tendency for salt fingers to form sequences of layers separated by vigorously-mixed slabs. He speculated that this phenomenon might be linked to staircase-like profiles of temperature and salinity observed beneath warm, salty masses of water such as the Mediterranean outflow. While such a connection remains speculative, undersea measurements have since provided abundant evidence for salt fingering, ranging from direct images of salt fingers to detection of alternating blobs of warmer and cooler water about 1 centimeter across.


Why is Salt Fingering Important?


Salt fingering contributes to vertical mixing in the oceans. Such mixing helps regulate the gradual overturning circulation of the ocean, which strongly affects climate.


What Causes Salt Fingering?







In fact, a blob need not lose all its heat, just enough to cause it to sink. (Also, the water beneath the blob needs to be only slightly fresher than that in the blob.)

In the ocean, these sinking `fingers' are about 1 centimeter across.


Where Does Salt Fingering Occur?


Salt fingering occurs where warm salty water lies over cooler, fresher water (red, orange, and yellow in the maps below).

300 meters depth


In temperate latitudes, evaporation exceeds precipitation. Because evaporation ``leaves behind'' salt, surface waters tend to be saltier than waters beneath. Surface waters also are warmer than waters beneath, and so conditions favoring salt fingering are realized throughout much of the upper temperate ocean.

In this map, blue indicates where cool fresh water lies over warmer saltier water. This happens primarily near the poles, where melting of sea ice leads to very cold, fresh surface waters. Such a situation gives rise to diffusive convection, a phenomenon related to salt fingering. Salt fingering and diffusive convection together are known as double-diffusive instabilities because water is set in motion as a result of the much greater diffusion of heat than salt.

2400 meters depth


At 2400 meters depth, salt fingering occurs beneath the warm, salty water which flows from the Mediterranean into much of the deep eastern Atlantic. This water gradually flows southward, and is eventually swept eastward (south of Africa and Australia) by the swift Circumpolar Current which circles Antarctica.

Conditions favorable to salt fingering also occur in the Gulf of Arabia, beneath warm and salty waters emanating from the Red Sea and Persian Gulf.


Computer Simulations of Salt Fingering


To aid in determining how effectively salt fingers mix temperature and salt, the fluid dynamical equations describing salt fingering were solved using a DEC Alpha workstation. The following images are stills from animations which depict how the salinity evolves in time. In these images, warmer, saltier water lies above cooler, fresher water. Lighter colours indicate higher concentrations of salt, and dark colours lower concentrations of salt. Sinking salt fingers thus appear bright, and rising fresh fingers appear dark:



The figures below shows examples ranging from disorganized saltfingers in a weakly stable stratification to organized saltfingers in a more stable stratification:

weakly stratified
 
 
strongly stratified

 


Further reading:

The Ocean's Salt Fingers. Raymond W. Schmitt, Jr. in Scientific American, May 1995, pp. 70-75.

Do-it-yourself salt fingering experiment: http://taylor.math.ualberta.ca/~eifl/teaching/saltfingers/index.html

Acknowledgement This page was contributed by Bill Merryfield with funding provided in part by the Office of Naval Research.


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