Lab 6.2 – What seawater property (temperature or salinity) is most important in determining density at different locations?

Fundamental concept: Predict whether temperature or salinity has a more controlling effect on density
Data skills preparation: Lab 2.1 – Latitude and regions,  Lab 3 – Introduction and Lab 3.1 – Time series
Estimated time to complete: 60-90 minutes
Materials needed: none

Seawater density varies by location and with depth because it is affected by both salinity and temperature (and pressure too, but the water molecule is nearly incompressible). Recall that temperature and density have an inverse relationship – as temperature decreases, density increases, while salinity and density have a direct relationship – as salinity increases, density also increases. 

Which variable, temperature or salinity has the more important effect on seawater density?

In this activity you will be comparing temperature, salinity, and density profiles at two different locations over a year period, and be able to isolate the influence of just one variable (temperature or salinity) on density by holding one of the variables (temperature or salinity) constant. 

Below are datasets of temperature, salinity and density data for two ocean locations. You can interact with the data by:

  • Zooming in and out of the data to look at different time scales that interest you by using the sliders (white rectangles with black lines) at the bottom of the graph back and forth. This will change the width of the highlighted section corresponding to different time intervals. Note when you first open the page, all of the available data are highlighted.
  • Showing calculated density by either holding temperature (in red) or salinity (in blue) constant.
  • Selecting which location to show

[open in new window]

[ADD explanation of what it means to “hold constant”]

Orientation Questions

    1. What time period is shown on these graphs?
    2. At what depth is the data from the Irminger Sea collected?
    3. At what depth is the data from the Coastal Pioneer collected?
    4. Review what you learned so far in Lab 6 by summarizing how changes in temperature affect density and how changes in salinity affect density.

Quick Check Questions:

     

    Interpretation Questions:

    1. Does temperature or salinity have more of a controlling effect on seawater density in the Irminger Sea (sub-polar)? Explain how you came to this conclusion. 
    2. Does temperature or salinity have more of a controlling effect on seawater density at the Coastal Pioneer mooring (temperate region)? Explain how you came to this conclusion.
    3. Does salinity have a larger influence on density at the Irminger Sea or the Coastal Pioneer location? Explain how you came to this conclusion.

     

    Application Questions:

    The density of seawater is controlled by temperature, salinity and pressure. The equation for density can be expressed as Density = Mass/ Volume.  Answer the following questions and explain your reasoning for each question.

    1. Use the density equation above to explain how a change in seawater temperature can increase or decrease seawater density. Do temperature and density have a direct or inverse relationship?
    2. Use the density equation above to explain how a change in seawater salinity can increase or decrease seawater density. Do salinity and density have a direct or inverse relationship?
    3. It turns out that pressure has little effect on the density of most seawater. Why do you think this is the case?
    4. Which of the three factors (temperature, salinity or pressure) is the primary control on the density of seawater in most of the surface ocean?
    5. The data in this lab activity come from the ocean surface. Do you think the same factor (temperature, salinity, or pressure) would control the density of seawater in the deep ocean? Why or why not?

     

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