Seawater Properties and Thermohaline Structure

Temperature and salinity are the master variables of the ocean: together with pressure they fix seawater density through a nonlinear equation of state, and that density field organizes the water column into stably stratified layers.

Definition

Thermohaline structure refers to the vertical arrangement of seawater by temperature (thermo-) and salinity (-haline), and the density layering that results, which controls stability and the formation of distinct water masses.

Scope

This topic covers the definition and measurement of temperature, salinity, and pressure; the equation of state and the modern TEOS-10 thermodynamic standard; potential temperature, conservative temperature, and density variables such as sigma-t and neutral density; and the resulting vertical structure of the mixed layer, thermocline, halocline, and pycnocline.

Core questions

• How are temperature, salinity, and density defined and measured throughout the water column?
• Why is the equation of state of seawater nonlinear, and what phenomena does that nonlinearity produce?
• What sets the depth and strength of the mixed layer, thermocline, and pycnocline?
• How do temperature-salinity diagrams reveal the origins and mixing of water masses?

Key theories

Nonlinear equation of state

Seawater density depends nonlinearly on temperature, salinity, and pressure, so mixing two water parcels of equal density can produce denser water (cabbeling) and compressibility varies with temperature (thermobaricity).

Stratification and stability

A water column is stable when density increases with depth; the buoyancy (Brunt-Vaisala) frequency quantifies that stability and governs how readily vertical motions and internal waves are restored.

Mechanisms

Solar heating warms and freshwater fluxes (precipitation, evaporation, ice melt and formation) alter surface salinity, setting buoyancy at the sea surface; wind and convection mix the upper ocean into a near-uniform layer, beneath which temperature and salinity grade through the thermocline and halocline to the deep ocean. The density that results, computed from the equation of state, determines static stability and the depth to which surface water can sink.

Clinical relevance

Accurate seawater thermodynamics is essential for computing the ocean's heat and freshwater content, calibrating salinity and density from conductivity-temperature-depth (CTD) and Argo float sensors, and tracking the ocean's role in storing anthropogenic heat.

History

Early twentieth-century work by Knudsen standardized salinity determination by titration; the practical salinity scale based on conductivity followed in 1978, and the thermodynamically consistent TEOS-10 standard, adopted in 2010, replaced earlier formulations with absolute salinity and conservative temperature.

Key figures

• Martin Knudsen
• Bjorn Helland-Hansen

Seminal works

• talley2011
• iocTeos2010

Frequently asked questions

What is the thermocline?

The thermocline is the layer of the ocean where temperature decreases rapidly with depth, separating the warm, well-mixed surface waters from the cold deep ocean.

How is ocean salinity measured today?

Salinity is now derived chiefly from electrical conductivity, temperature, and pressure measured by CTD instruments and autonomous floats, rather than from older chemical titration methods.