Meaning and mechanism
When comparing substances with the same volume, those with greater mass have a larger density. Within typical surface seawater ranges, cooling and an increase in salinity tend to increase density.
Different combinations of water temperature and salinity can create the same density. The tendency for denser water to sink provides a clue for understanding the stratification and circulation within the ocean.
Relationships and conditions
The isopleth lines on temperature-salinity charts are lines that connect points with the same density. We differentiate between experiments where only temperature is changed, and experiments where both temperature and salinity are changed.
The experiment represents a linear approximation within a limited temperature and salinity range. However, the actual seawater equation includes pressure and non-linear effects, and the changes resulting from mixing are complex.
What to observe in the simulation
Cold or salty water is heavier. Densities are the same on the same line on the T–S diagram.
Surface seawater with a water temperature of 0 to 30 °C and a salinity of 30 to 40 g/kg is approximated by a linear equation of state. Density changes due to pressure, nonlinearity, and mixing are omitted. The color ink path is a conceptual flow and does not predict flow rate or mixing time.
Key relationships and symbols
ρ ≈ 1027·[1 − 1.7×10⁻⁴(T−10) + 7.6×10⁻⁴(S−35)] kg/m³
| Symbol | Meaning | Unit |
|---|---|---|
| ρ | Seawater density | kg/m³ |
| T | water temperature | °C |
| S | Salinity by mass | g/kg |
Worked example
At the same temperature, what happens to seawater density as salinity increases?
In this linear approximation, the lower the water temperature or the higher the salinity, the greater the density. If you change two variables together, their effects may cancel out.
Common misconceptions
- Simply knowing that one body of water is warmer than the other is not sufficient to always determine the order of densities of the two bodies of water.