Potential density is a thermodynamic property of a fluid, most commonly used in oceanography to describe the density that a parcel of seawater would attain if it were moved adiabatically (without heat exchange) and without altering its composition to a specified reference pressure, typically the surface pressure or a standard pressure such as 0 decibars. By referencing density to a common pressure, potential density allows scientists to compare water masses that reside at different depths without the confounding influence of the compressibility of seawater.
Definition and Calculation
The potential density, often denoted by the Greek letter ρ θ (rho sub theta), is derived from the equation of state for seawater, ρ = ρ(S, T, p), where ρ is density, S is salinity, T is temperature, and p is pressure. The potential temperature, θ, is the temperature a water parcel would have if moved adiabatically to the reference pressure. Potential density is then calculated as:
$$ \rho_\theta = \rho\bigl(S,; \theta,; p_{\text{ref}}\bigr) $$
where $p_{\text{ref}}$ is the chosen reference pressure (e.g., 0 dbar). Modern oceanographic software packages (e.g., TEOS-10) provide algorithms to compute potential density from measured salinity, temperature, and pressure.
Importance in Oceanography
- Stratification analysis: Potential density surfaces (isopycnals) are used to assess the stability and stratification of the water column because they are less affected by pressure-induced density changes.
- Water mass identification: Oceanographers classify water masses based on characteristic ranges of potential density, salinity, and temperature, facilitating the study of circulation patterns.
- Mixing and transport: The conservation of potential density under adiabatic, inviscid flow makes it a convenient tracer for diagnosing horizontal and vertical mixing processes.
Related Concepts
- Potential temperature (θ): Temperature a water parcel would have at the reference pressure, essential for calculating potential density.
- Neutral density (γ n): An alternative density coordinate designed to follow surfaces of truly neutral buoyancy in the ocean; often used alongside potential density.
- Sigma (σ) notation: Historically, σ θ = ρ θ – 1000 kg m⁻³; this simplified form is still common in some literature.
Measurement and Data Products
Potential density is not measured directly; it is computed from in‑situ measurements of salinity, temperature, and pressure obtained via CTD (conductivity–temperature–depth) sensors. Global oceanographic databases, such as the World Ocean Atlas and Argo float data, routinely provide climatological fields of potential density.
Limitations
- Reference pressure choice: The selection of $p_{\text{ref}}$ influences the absolute value of potential density, though relative differences are often robust.
- Compressibility effects: While potential density accounts for adiabatic compression, it does not capture irreversible processes such as mixing with different composition, which may alter density in ways not reflected by potential density alone.
References
- International Thermodynamic Equation of Seawater – 2010 (TEOS-10), IOC, SCOR, and IAPSO (2010).
- Talley, L. D., Pickard, G. L., Emery, W. J., & Swift, J. H. (2011). Descriptive Physical Oceanography: An Introduction (6th ed.). Academic Press.
- UNESCO (1983). Algorithms for Computation of Fundamental Properties of Seawater. UNESCO Technical Papers in Marine Science, No. 44.