In the mix. Warm salt fingers (light-colored) dribble down as fresh water (darker) rises.

Salty Fingers Do the Mixing

Small bands of salty water called "salt fingers" play a major role in stirring tropical waters, according to a new study. Understanding this process is crucial for determining how ocean mixing affects weather and distributes heat around the globe.

From a ship, the tropical mid-Atlantic can look deceivingly uniform. But oceans are hardly homogenous. They tend to be layered like a fancy cocktail, with layers of warm water capping colder, denser layers. And, just like a cocktail, the ocean can be stirred.

When a warm, salty layer of water contacts a colder and relatively fresher layer, it starts to cool. If a jigger of this salty water loses enough heat, it sinks down into the colder, fresher water, lengthening into a finger of salty water. The finger loses heat faster than it loses salt. So that means the salt finger will continue to sink (salty water is denser than fresh water of the same temperature), losing more heat and displacing the colder water around it, which rises up and mixes into the warm salty layer above.

Researchers first discovered salt fingers in 1960, and laboratory models predicted they were important for ocean mixing. But their true significance was hard to measure. In today's issue of Science, a group of oceanographic researchers describe how they measured salt finger mixing directly, using sulfur hexaflouride as a tracer. Sulfur hexaflouride diffuses through water at the same rate as salt, making it ideal for infiltrating salt fingers. The team injected the tracer into a patch of ocean east of the Caribbean and then tracked the chemical with water samples over a period of months as it spread over 1 million square kilometers.

The results showed that the models were right. Not only is salt fingering definitely at work in the tropical Atlantic, it is twice as effective at mixing salt than at mixing heat. In addition, it exacerbates differences in density between the surface ocean and the depths, unlike mechanical mixing from ocean waves or turbulence.

Previously, “there were some leaps of faith and a lot of room for disbelief,” on the role of salt fingers, says Bill Merryfield, of the Canadian Centre for Climate Modelling and Analysis in Victoria, British Columbia, but this study confirms the models.

Related sites
Bill Merryfield's salt finger tutorial
The researchers' Deep Ocean Mixing site