28 July 2009

What is seawater?

Not just sweaty-tasting water, seawater is a highly variable cocktail. Its exact composition can substantially impact marine life from corals to currents, and affect ocean models and climate projections.This instrument deployed from a research vessel measures temperature and salinity in the ocean. Variations in salinity and heat influence ocean currents and measuring those variations is central to quantifying the ocean’s role in climate change. (Credit: CSIRO)

Seawater Gets a New Definition
In hopes of better modeling salinity and currents, oceanographers have adopted a new international standard for gauging seawater
Katherine Harmon, Scientific American 27 Jul 09
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.

Science adopts a new definition of seawater
CSIRO 7 Jul 09;
The world's peak ocean science body has adopted a new definition of seawater developed by Australian, German and US scientists to make climate projections more accurate.

In Paris late last month the General Assembly of UNESCO's Intergovernmental Oceanographic Commission (IOC) accepted the case for the introduction of a new international thermodynamic description of seawater, cast in terms of a new salinity variable called Absolute Salinity.

Hobart-based CSIRO Wealth from Oceans Flagship scientist, Dr Trevor McDougall, made the case during his presentation of the Bruun Memorial Lecture to the Paris meeting.

"Scientists will now have an accurate measure of the heat content of seawater for inclusion in ocean models and climate projections," Dr McDougall says.

"Variations in salinity and heat influence ocean currents and measuring those variations are central to quantifying the ocean's role in climate change. The new values for salinity, density and heat content should be in widespread use within 18 months."

Marine scientists have been searching for the 'magic formula' for measuring salinity – which varies from ocean to ocean and between tropical, temperate and polar regions – for more than 150 years.

"These variations in salinity and temperature are responsible for driving deep ocean currents and the major vertical overturning circulations of the world's oceans, which transfer ocean heat towards the Arctic and Antarctic regions," Dr McDougall says.

Unchanged since the last assessment 30 years ago, the case to review ocean thermodynamic measurements began in 2005 when the Scientific Committee on Oceanic Research (SCOR) established a working group, chaired by Dr McDougall. Supporting him were Dr Rainer Feistel from the Leibniz-Institut für Ostseeforschung in Warnemünde (Germany), Dr Frank Millero, from the Rosenstiel School of Marine and Atmospheric Science at the University of Miami in Florida, Dr Dan Wright of the Bedford Institute of Oceanography, Canada and Dr David Jackett of CSIRO.

Salinity, comprising the salts washed from rocks, is measured using the conductivity of seawater – a technique which assumes that the composition of salt in seawater is the same in all the world's oceans.

"The new approach, involving Absolute Salinity, takes into account the changes in the composition of seasalt between different ocean basins which, while small, are a factor of about 10 larger than the accuracy with which scientists can measure salinity at sea," Dr McDougall says.

Until the new description of seawater is widely adopted, ocean models will continue to assume that the heat content of seawater is proportional to a particular temperature variable called "potential temperature".

"The new description allows scientists to calculate the errors involved by using this approximation while also presenting a much more accurate measure of the heat content of seawater, namely Conservative Temperature," Dr McDougall says.

"The difference is mostly less than 1ºC at the sea surface, but it is important to correct for these biases in ocean models."

More links

Why is the Ocean Salty?
by Herbert Swenson, US Geological Survey Publication
Why Is The Ocean Salty? on Science Daily 14 Oct 07

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