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Seawater Gets a New Definition

In hopes of better modeling salinity and currents, oceanographers have adopted a new international standard for gauging seawater
Seawater measuring



CSIRO

Seawater is a lot more complex than a simple solution of water and sodium chloride. As researchers have been discovering for the past century, it is a highly variable cocktail, and its particular makeup can have a substantial impact everything from coral to currents.

To make sure monitoring and modeling of the globe's biggest bodies of water stay accurate, UNESCO's Intergovernmental Oceanographic Commission (IOC) has adopted a new official definition for seawater. A report [pdf], published earlier this year, details the updated methods for keeping tabs on the changing seas.

Historically, oceanographers have defined seawater based on its salinity, which they inferred by taking measurements of the water's electrical conductivity. The measurements, tracked on the PSS (practical salinity scale), helped provide standardized data sets for the world over, making possible a more nuanced level of mapping.

But in the 1970s researchers Peter Brewer and Al Bradshaw noticed that deep-sea waters (which have a higher concentration of salt) did not quite behave the same as did samples from standard North Atlantic surface waters (a 35 to 1,000 salt to water ratio).

So they enlisted the help of Frank Millero, a professor of marine and physical chemistry at the University of Miami's Rosenstiel School of Marine & Atmospheric Science, to find a better way to gauge the world's seawater. Millero and others, including the mathematician and physicist Rainer Feistel and Trevor McDougall at the Center for Australian Weather and Climate Research, set to work developing standard guidelines that would allow for more accurate assessment of salinity.

Samples will still be measured using conductivity (via carefully calibrated instruments) to maintain data consistency, says Gary Lagerloef, a senior scientist at the nonprofit Earth & Space Research (ERS). Only after the measurements are recorded will they be corrected using the new calculations, he explains.

And the new definition doesn't stop at corrected salinity levels. It also takes into account other crucial properties, such as heat capacity and enthalpy (a system's energy determined by its temperature, pressure and composition). Millero calls this new seawater equation a "black box," into which researchers can input salinity values, collection location and depth, and come out with more accurate information about the properties of the water in that area. "This is much more useful to modelers because everything is built into one equation," Millero says. It will allow them to paint a clearer picture of where ocean mixing happens and thus permit more accurate ocean current—and climate—models.

"It's going to be important in the long run," says Lagerloef, who is leading NASA's Aquarius satellite mission to measure ocean salinity from space. "It's a small correction," he says, but "the ocean responds to very subtle changes in density" triggered by its heat content and chemistry.

The new calculations have even proved to hold fast at high temperatures, and they have already been put to work in the industrial sector. The new guidelines are slated to take effect in the research world next year.

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