Lab 5.1 – What is the relationship between temperature, salinity, and density?

Fundamental concept: Identify and describe relationships between temperature, salinity, and density that relate to stratification of the water column
Data skills preparation: Lab 2.4 – Station profiles 
Estimated time to complete:
30-60 minutes
Materials needed:
None

There is a complex relationship between temperature, salinity, and density. Most of the variability in seawater density is due to changes in salinity and temperature. As the salinity of seawater increases, the density increases, due to the change in mass of dissolved salts in a given volume of water. A change in temperature of seawater results in a change of volume for a given mass of water. An increase in the temperature of seawater causes the volume of a water parcel to increase and its density to decrease. The temperature and salinity of seawater can change dramatically with depth, or be pretty stable, depending on many different factors.

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. Stratification describes the layering of water properties 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”, so 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 with depth, and these are called mixed layers. 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. Essentially, the “clines” act as barriers to mixing in a stable water column, and could prevent nutrient-rich deep water from coming to the surface to support primary production.

If a change in temperature or salinity occurs that results in a layer of dense water being above less dense water, the water column is unstable and overturning is the result. This is when denser water sinks until it reaches a depth that is of the same density (called an isopycnal), and less dense water rises to replace it. Overturn is common in polar regions, due to the extremely cold temperatures and the formation of sea ice, which both increase the density of surface waters. An unstable water column in polar regions is the main driver of thermohaline circulation, which affects climate. Overturn in the water column caused by variations in density can affect timing, magnitude, and location of biological productivity.

In this activity you will analyze the relationship between temperature, salinity, and density during the months of April and October in the North Atlantic. Consider revisiting or reviewing Lab 2.4 before starting this activity.

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

  • Select the month to view from the “Select Month” menu below the graphs
  • 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.


 

Quick Check Questions:


 

Interpretation Questions:

  1. Make your density profile predictions for each month 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”.
  2. Click on “Show Density” to check your predictions. How do the density profiles you drew differ from the calculated density profile for each month?
  3. Why do you think your predictions did or did not match the calculated density profile?
  4. What is the depth range (in meters) of the pycnocline in October? How does the pycnocline relate to the thermocline and the halocline?