Practical Salinity Scale 1978

Practical Salinity Scale 1978
Name	Practical Salinity Scale 1978
Abbreviation	PSS-78
Introduced	1978
Developed by	International Association for the Physical Sciences of the Oceans
Units	dimensionless
Based on	conductivity ratio
Superseded by	TEOS-10 (absolute salinity)

Practical Salinity Scale 1978 The Practical Salinity Scale 1978 is an empirical scale for describing seawater salinity based on the electrical conductivity ratio of a seawater sample relative to a potassium chloride standard. Developed in the late 1970s, it provided a standardized, reproducible descriptor for salinity that is widely cited in oceanographic literature and incorporated into international datasets and programs.

Introduction

PSS-78 was promulgated by the International Association for the Physical Sciences of the Oceans and integrated into recommendations from the Intergovernmental Oceanographic Commission and the World Meteorological Organization, influencing work at institutions such as the Scripps Institution of Oceanography, the Woods Hole Oceanographic Institution, and the Lamont–Doherty Earth Observatory. Its adoption affected measurement campaigns by agencies including the National Oceanic and Atmospheric Administration and the British Antarctic Survey, and it appears in datasets produced by the Global Ocean Data Analysis Project and the World Ocean Circulation Experiment.

Background and Development

PSS-78 emerged from earlier efforts to quantify seawater salinity following chemical titration methods attributed to investigators at the Royal Society of London and laboratories connected to the United States Geological Survey and the National Physical Laboratory (United Kingdom). Debates among scientists from the International Association for the Physical Sciences of the Oceans, representatives of the Intergovernmental Oceanographic Commission, and technical committees of the International Union of Geodesy and Geophysics led to consensus on an electrical method. Key contributors included researchers affiliated with the Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, and the University of Washington.

Definition and Calculation

PSS-78 defines salinity, S, as a function of the conductivity ratio R = K_15(S,t,P)/K_15(NaCl,15,0), where standard conductivity K_15 refers to conductivity at 15 °C and 1 atmosphere. The practical salinity is computed using polynomial expressions that were developed and published under the auspices of the International Association for the Physical Sciences of the Oceans and implemented in software libraries used by projects like the World Ocean Circulation Experiment and the Argo program. Implementation details are used in routines at the National Centers for Environmental Prediction and in numerical models maintained by institutions such as the Met Office and the Naval Research Laboratory.

Instrumentation and Measurement Techniques

Measurement of the conductivity ratio for PSS-78 relies on salinometers and conductivity cells produced by manufacturers and validated by reference laboratories including the National Physical Laboratory (United Kingdom), the National Institute of Standards and Technology, and university labs at Massachusetts Institute of Technology. Field platforms include CTD systems deployed from ships operated by the Scripps Institution of Oceanography, autonomous profilers from the Argo program, and moorings maintained by the Alfred Wegener Institute. Calibration uses standard KCl solutions prepared following protocols from the World Meteorological Organization and checked against intercomparison cruises coordinated by the Intergovernmental Oceanographic Commission.

Accuracy, Uncertainty, and Limitations

PSS-78 yields practical salinity as a dimensionless quantity with stated uncertainties arising from instrument calibration, temperature control, and pressure effects; these issues were addressed in intercomparison studies with participation from the National Oceanic and Atmospheric Administration, the Scripps Institution of Oceanography, and the British Antarctic Survey. Limitations include its empirical basis tied to classical seawater composition, which motivated subsequent developments such as the thermodynamic formulations advanced by the Intergovernmental Oceanographic Commission and the International Association for the Physical Sciences of the Oceans. Users in projects like the Global Ocean Data Analysis Project and the World Ocean Circulation Experiment must account for PSS-78 limitations when merging historical chemical titration data from archives maintained by the National Oceanographic Data Center.

Adoption and Usage in Oceanography

PSS-78 became the de facto standard for international oceanographic data exchange, adopted by programs and institutions such as the Argo program, the World Ocean Circulation Experiment, the Global Ocean Observing System, and national hydrographic offices including the National Oceanic and Atmospheric Administration and the British Antarctic Survey. It is embedded in ocean models developed by the National Oceanography Centre (United Kingdom), the Met Office, and the Naval Research Laboratory, and is used in climatologies produced by the International Council for the Exploration of the Sea and the Intergovernmental Oceanographic Commission.

Relationship to Other Salinity Scales

PSS-78 is related to earlier chemical salinity scales based on titration methods developed in laboratories associated with the Royal Society of London and the United States National Bureau of Standards and to later thermodynamic approaches such as the Absolute Salinity formulations in the Thermodynamic Equation of Seawater 2010 developed by the International Association for the Physical Sciences of the Oceans and endorsed by the Intergovernmental Oceanographic Commission. Conversion between PSS-78 and absolute salinity used in TEOS-10 is implemented in software libraries maintained by organizations like the International Association for the Physical Sciences of the Oceans and the Intergovernmental Oceanographic Commission for use by modeling centers such as the Met Office and the Naval Research Laboratory.

Category:Oceanography