Lab 6.1 – What is the relationship between temperature, salinity, and density?

Fundamental concept: Identify and describe relationships between temperature, salinity, and density that relate to layering (stratification) of the water column
Data skills preparation: Lab 3.4 – Station profiles 
Estimated time to complete: 30-60 minutes
Materials needed: None

Water stratification is when water masses with different properties form layers that act as barriers to water mixing. These layers are arranged according to density, with the less dense water masses sitting above the more dense layers. As you learned in the introduction to this lab, salinity and temperature determine the density of seawater. Stratification describes the layering of water properties (density, temperature, and/or salinity) relative to depth.

While density increases with depth, it does not necessarily do so at a constant rate. Layers where properties are changing rapidly with depth are called “clines”. If a variable like temperature changes over a small depth range, this would be a “rapid change” with depth. On a station profile, this results in a large change in the x-axis value, over a small change in the y-axis (depth) value.  Where temperature changes quickly is the thermocline, where salinity changes fast is the halocline, and where density changes rapidly is the pycnocline.

Oftentimes, there are regions where there is no change in water properties with depth, and these are called mixed layers. Mixed layers are isothermal, isohaline, and therefore also isopycnal (“iso” means equal).  Water can move vertically in a mixed layer. In a stable water column, the density increases with depth. When stable, it takes a lot of energy to mix water between any two layers, just like you have to vigorously shake a bottle of dressing to mix the layers of oil and vinegar. Essentially, the “clines” act as barriers to mixing and make the structure of the water column stable or hard to change.

In this activity you will analyze the relationship between temperature, salinity, and density in the Eastern Pacific. Consider revisiting or reviewing how to read station profiles in Lab 3.4 before starting this activity.

Below you are able to see a dataset of temperature and salinity at the Coastal Endurance Array in April. You can interact with the data by:

  • Clicking the profile and hovering your mouse over the profile to view temperature (left) and salinity (center) variables with depth
  • Predicting the corresponding density profile (far right) by clicking on the small blue circles on the dashed line and dragging them left or right to change the density to what you believe it should be based on the temperature and salinity at that depth
  • Check your predictions by clicking the button at the left of “Show Density” below the density profile.

Answer the Quick Check Questions below before making your prediction and comparing it to the calculated density profile.


 

Orientation Questions:

    1. What variable is shown on the vertical (y) axis?
    2. What variable is shown on the horizontal (x) axis in the left graph panel?
    3. What variable is shown on the horizontal (x) axis in the middle graph panel?
    4. What is the range of depths shown on these graphs (give units in your answer)?
    5. What are the units of density in the right graph panel?

Quick Check Questions:

[open in new window]

Interpretation Questions:

    1. Make your density profile prediction by clicking on the small blue circles on the dashed line and dragging them left or right to change the density to what you believe it should be. Describe the approach or steps you took in predicting the shape of the density profile you “drew”. How did you decide whether density should be high or low?
    2. Click on “Show Density” to check your prediction. How does the density profiles you drew differ from the calculated density profile?
    3. Why do you think your prediction did or did not match the calculated density profile?
    4. What is the depth range (in meters) of the pycnocline?
    5. How does the pycnocline relate to the thermocline and the halocline?

 

⇐ Previous LAB 6.0
Next LAB 6.2 ⇒

Table of contents