Salty Staircase in the Atlantic Provides Clues to Ocean Mixing
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 today’s 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 five meters (about two 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.
Salt fingers form because heat and salt diffuse at vastly different rates, hence the term double diffusion. Salt molecules diffuse 100 times slower than heat; the faster conduction of heat allows the unstable salt distribution to fall out on a small scale. These staircase layers have existed for decades, laid down like geologic strata over a region larger than California and Texas combined, despite the active stirring of oceanic eddies. Water mass transformations apparent within the layers can only be explained by salt fingers, but until now their strength had not been measured.
In the ocean, density increases with depth, and in combination with gravity, makes for a generally stable stratification that is difficult to mix vertically. The large-scale circulation system requires that significant vertical mixing occur somewhere in the ocean, but this mixing has been difficult to find and measure. Since oceanic mixing occurs on very small scales, from less than an inch, and on rapid time scales of a few seconds, and climate models only resolve space and time in scales of tens of miles to hours and days, it is a challenge for scientists to resolve ocean mixing rates and improve the climate models.
Because the computer power to resolve such small scales in global models is perhaps centuries away, the mixing processes will be parameterized for the foreseeable future. Careful measurements of oceanic microstructure and turbulence are leading to improved parameterizations, but there had been no way to calibrate the measurements and models until the development of the tracer release technique by WHOI Senior Scientist Jim Ledwell of the Applied Ocean Physics and Engineering Department.
Raymond Schmitt, lead author of the study and a senior scientist in the WHOI Physical Oceanography Department, has studied salt fingers for years and sought Ledwell’s help to get a direct measure of the strength of mixing in the thermohaline staircase. He and colleagues conducted microstructure surveys in the western tropical North Atlantic Ocean with a sophisticated free-fall turbulence profiler while Ledwell used an injection sled to release the non-toxic tracer, sulfur hexafluoride, into the ocean in the middle of a staircase layer.
The tracer patch was sampled again nine months later to find that the tracer had mixed strongly vertically while being spread both east and west by the ocean currents. The microstructure data indicated that there was not enough ordinary turbulence to account for this dispersion, but the salt finger models worked well to explain it. The heat mixed half as much as the salt, the result expected for salt fingers. Thermohaline staircases are formed by salt fingers in laboratory experiments, but there had never been a direct measurement of their mixing strength in the ocean. Schmitt says this experimental result is the culmination of 45 years of speculation on the importance of salt fingers in oceanic mixing.
“The role of the small-scale double-diffusive processes in the ocean has always been controversial, with some modelers preferring to do all the mixing with larger-scale turbulence,” Schmitt said. “Measurements show that turbulence is rather weak in most of the stratified ocean, so that double-diffusion has a chance to play a significant role. This site is one of several where the salt fingering is expected to be particularly strong, and we can see the effects of the mixing further downstream as the salinity is transferred vertically from upper to intermediate waters. That small-scale salt fingers can have such large-scale effects on the ocean structure will come as a surprise to many.”
Schmitt and colleagues report that salt fingers transform the temperature and salinity structure of the surface waters entering the Caribbean Sea, increasing the salinity and density of the Antarctic Intermediate Water and preconditioning it for sinking at higher latitudes. “The tracer results are a strong confirmation of the strength of the salt fingering process. It is time that the climate modelers took double diffusion into account. While it is a more complicated mixing scenario, adding double diffusion will help to explain certain characteristics of the temperature-salinity structure of the ocean that are now artificially introduced into the models.”
Salt fingers are intense and easy to study in this region of the ocean but they can occur at many places around the world. Schmitt and colleagues hope to learn enough from their data to generalize to other areas, since the pace of ocean exploration is slow. Uncertainties in oceanic mixing are one of the major impediments to understanding how the oceans absorb heat and carbon dioxide. Since the ocean contains 99.9% of the heat capacity of the climate system, improved estimates of oceanic mixing are critical for anticipating the future of Earth’s climate.