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Temperature, salinity, and seawater density

Change the water temperature and salinity of the two tanks and observe the density difference and flow through the connection door.

    E15 · Temperature, salinity, and seawater densityReady
    You can also experiment with the numbers and controls below the scene.

    Earth Science · Ocean · Climate · Water Cycle / Principles and Exploration

    Temperature, salinity, and seawater density — principles and investigation

    Water temperature and salinity together change the density of seawater, so it is difficult to determine which water will sink based on just one condition. Adjust the temperature and salinity of the two sea waters and compare the density and temperature-salinity diagrams. Find out from the isodensity line that even seawater under different conditions can have the same density.

    How does it work?

    Cold or salty water is heavier. Densities are the same on the same line on the T–S diagram.

    Key relationship

    ρ ≈ 1027·[1 − 1.7×10⁻⁴(T−10) + 7.6×10⁻⁴(S−35)] kg/m³

    Try this investigation

    1. Press the open connection door to observe the movement of colored ink. Blue is water that started from A, and pink is water that started from B. Look at the flow arrows and density numbers together.
    2. Run and record conditions where there is a difference in water temperature and conditions where the water temperature and salinity are all the same.
    3. This time, set the water temperature to 10 °C. Change the salinity of the two waters and compare whether the heavier one changes.

    Interpreting your results

    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.

    Assumptions and limitations

    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.