Salinity, the measurement of dissolved salt content in water, is a fundamental property of oceans and inland waters. The concentration of salts directly controls the water’s density, driving global ocean circulation patterns. Salinity is also important for climate science, as it influences the exchange of heat and moisture between the ocean and the atmosphere. Furthermore, the salt balance determines the habitability for various forms of marine life.
Defining Salinity and Standardized Units
The earliest approach defined salinity by measuring the mass of dissolved solids in a kilogram of water, typically expressed in parts per thousand (ppt). This mass-based ratio was difficult to measure precisely and failed to account for subtle variations in the chemical composition of salts. To standardize measurements, the Practical Salinity Scale (PSS-78) was introduced. PSS-78 defines salinity as a dimensionless ratio derived from electrical conductivity, based on a conductivity ratio relative to a standardized potassium chloride solution.
The most modern standard is the Absolute Salinity (\(S_A\)) scale, adopted in the Thermodynamic Equation of Seawater 2010 (TEOS-10). Absolute Salinity is defined as the true mass fraction of dissolved material in seawater, expressed in grams of salt per kilogram of seawater (g/kg). This standard was necessary because PSS-78 does not perfectly account for small regional differences in the ion ratios of seawater, which affect precise calculations of density. While Practical Salinity remains the measured variable archived in national databases, Absolute Salinity is used to calculate all thermodynamic properties of seawater.
Determining Salinity Through Electrical Conductivity
The primary method used today leverages the fact that dissolved ions increase a solution’s ability to conduct electricity. Scientists use high-precision electronic instruments to measure this electrical conductivity. The standard instrument is the Conductivity, Temperature, and Depth (CTD) sensor, deployed from ships or autonomous platforms.
The CTD measures conductivity, temperature, and hydrostatic pressure (used to derive depth). Because conductivity is highly sensitive to temperature and pressure changes, these three parameters must be measured simultaneously for accurate salinity calculation. The conductivity measurement is converted into Practical Salinity units using a standardized mathematical algorithm that accounts for temperature and pressure effects. This method is favored for its speed, precision, and ability to provide continuous, high-resolution measurements throughout the water column.
Historical and Chemical Determination Methods
Before the widespread adoption of electronic salinometers, salinity was determined through chemical analysis based on Marcet’s Principle. This principle assumes that while the total salt concentration varies, the relative proportions of major dissolved ions remain constant across the open ocean. This constancy allowed scientists to calculate total salt content by measuring just one major component.
The most common chemical method was determining “Chlorinity,” which measures the total halide ion concentration (primarily chloride and bromide). The classic technique was the Knudsen Titration, involving reacting a seawater sample with a standardized silver nitrate solution. The amount of silver nitrate needed to precipitate the halides allowed calculation of Chlorinity, which was then converted to Salinity using an empirical formula.
Large-Scale Monitoring via Remote Sensing
While CTD instruments provide detailed vertical profiles, they cannot continuously cover the immense surface area of the global ocean. To monitor sea surface salinity (SSS) over vast regions, scientists use satellite-based remote sensing technology. These satellites use passive microwave radiometers operating at a specific L-band frequency (1.4 GHz).
This technique measures the microwave energy naturally emitted from the sea surface, known as brightness temperature. The intensity of this emission is influenced by the water’s physical temperature and its salt content. Specialized sensors on missions like SMOS and SMAP map SSS by detecting these slight differences in microwave radiation. Remote sensing only measures salinity in the top few centimeters of the ocean, providing a surface view that complements deep-water profiles gathered by in-situ instruments.