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Thermohaline Circulation Demonstration

Cynthia Venn, Bloomsburg University of Pennsylvania
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Summary

This activity helps the students to visualize the effects of temperature and salinity on water density, and the resulting thermohaline circulation. Important processes visualized in this demonstration are upwelling, downwelling, and the formation of haloclines, thermoclines and pycnoclines. In addition, mixing by advection is clearly demonstrated.

Context

Audience

undergraduate introductory oceanography course of mostly non-majors

Skills and concepts that students must have mastered

Students should be able to understand the concept of conservative properties, as well as the effects of salinity and temperature on water density.

How the activity is situated in the course

This is a classroom demonstration/exercise usually performed after students have been introduced to the concepts of seawater density and its dependence on salinity and temperature, as well as the controls on temperature and salinity of seawater. Sometimes I will do the demonstration before I introduce the topic of thermohaline circulation in the deep ocean, and sometimes I do it afterwards.

Goals

Content/concepts goals for this activity

  1. salinity and temperature as conservative properties and ability to predict the salinity and temperature of a mixture if the proportions are known
  2. effect of salintiy and temperature on density of water
  3. upwelling and downwelling processes
  4. concepts of thermocline, halocline and pycnocline

Higher order thinking skills goals for this activity

  1. determination of salinity and temperature of a mixture
  2. prediction of behavior of waters of different salinities and temperatures

Other skills goals for this activity

Description and Teaching Materials

Materials needed for this demonstration include a small aquarium (or other flat-sided, non-leaking container, an underwater platform upon which a small bottle can rest on its side, a small, flat-sided bottle for the very salty water, another small vial for the boiling water, a 500 mL bottle of salty (50 ppt or so), a hot pot for boiling the water, a 250 or larger beaker for mixing the salty water and the hot water, and a pair of long forceps to use to handle the hot water vial. The detailed procedure and the data sheet to hand out to the class are included below.
Procedure for Class Demonstration of Thermohaline Circulation (Microsoft Word 25kB Jun8 13)
Student Worksheet for Thermohaline Circulation (Microsoft Word 32kB Jun8 13)
Picture of needed materials for Thermohaline Demonstration (Acrobat (PDF) 249kB Jun8 13)

Teaching Notes and Tips

I find it most useful to have the students make calculations and predictions along the way. One variable that always changes is the last step where the red mixture is added to the tank, because it is very hard to introduce that water without a vertical velocity component. Therefore, it usually goes to the bottom but then is pushed back up above the blue very salty water eventually. A lot of mixing occurs, but this can also be useful for the students to visualize how mixing can occur and the result on the salinity and temperature (the less blue, the less salty; the less intense the yellow, the less warm. I have only two tanks, so I do this as a demonstration with classroom participation. However, it could also be done as a student activity in a lab if there were enough tanks for students to work in small groups.

Assessment

Students hand in their sheets at the end of the class and I go over their answers and predictions to gauge their understanding of the concepts demonstrated. I also ask them one on one whether they are better able to visualize upwelling and downwelling after seeing the demonstration.

References and Resources

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