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In 2009, the oceanographic community decided to adopt the Thermodynamic Equation Of Seawater - 2010 (www.teos-10.org) as the standard to calculate the properties of seawater. As part of that, they changed from the conductivity-based salinity (Practical Salinity, PSS-78, as part of the EOS-80) to Absolute Salinity (mass fraction of salt in seawater). What are the main differences between the two standards? Where in the world ocean should we expect to see the biggest differences between the two?

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  • $\begingroup$ Is this a homework question? If so, could you mark it as such? Also, you might want to combine your temperature question with this one, I think the answers will be related. Looking into it more now. $\endgroup$ – Jareth Holt Jul 8 '15 at 21:15
  • $\begingroup$ It is not a homework question! It is quite relevant to a lot of the research that I used to do and the differences in specific environments can be substantial. I am interested in seeing the answers the community gives with the hope of providing a good point of access to the information. The temperature question, while related does not fit with this one. $\endgroup$ – arkaia Jul 8 '15 at 21:40
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From my reading of the primer and manual, I would summarize the difference as follows. Practical salinity is calculated directly from the conductivity of seawater (which is what's most often measured by buoys, floats, and other autonomous systems) whereas absolute salinity is based on the density of the seawater. Practical salinity is essentially measuring the distribution of ions in the seawater. Absolute salinity is based on the mass of all non-H2O material, whether it's ions, gases, or solids. Relating salinity to mass, rather than conductivity, makes writing conservation equations much more straightforward; a dissolution of HCO$_3^-$ into H$^+$ and CO$_3^{2-}$ changes the practical salinity but not the absolute salinity. Basing salinity on mass also makes the thermodynamics much simpler, but I'll address that in your other question.

Differences between the practical and absolute salinities crop up where there are big differences in the composition of seawater, especially in its dissolved but non-ionic components. Page 14 of the manual (Fig 2) has some maps of what this looks like: Large deviations in the lower-salinity Pacific, and large deviations at depth where density and conductivity aren't so closely related. Absolute salinity can handle measurements of seawater composition in a way that practical salinity doesn't.

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