Layers of salty ocean water mix with layers of fresher water, creating a salty staircase or layering driven by small-scale convection known as salt fingers. Although scientists have known about salt fingers since 1960, when they were discovered at the Woods Hole Oceanographic Institution, they have not understood their role in ocean mixing and the ability of the ocean to absorb heat, carbon dioxide and pollutants from the atmosphere. Results of a new experiment, sponsored by the National Science Foundation and reported in todays issue of Science, indicate that salt fingers are vertically mixing ocean waters more than previously thought. The finding will improve understanding of how water masses in the ocean mix, leading to better climate prediction models.
Researchers from the Woods Hole Oceanographic Institution (WHOI) studied salt fingers by injecting a dye or tracer into the ocean, much like dyes are used in medical tests to trace bodily fluids. The tracer was released from an injection sled towed at a depth of approximately 400 meters (about 1,200 feet) from a ship in the tropical Atlantic near Barbados. Returning to the area nine months later, they found a significant vertical spread of the tracer indicating an enhanced mixing process, with salt and the tracer mixing twice as much as heat.
In this region, warm, high salinity subtropical water lies over cooler, fresher water flowing northward from Antarctica, creating a unique stratification with distinct layers of water. As many as ten to fifteen layers, each 10 to 30 meters thick (roughly 30 to 90 feet) with uniform temperature and salinity, are separated by interfaces with rapidly changing temperature and salinity, half a meter to 5 meters (about 2 feet to 10 feet) thick, to form a "thermohaline staircase" of sorts. The process known as salt fingers occurs at the interfaces and keeps the mixed layers uniform.
Shelley Dawicki | EurekAlert!
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