Thermohaline Circulation - Instructor's Guide
Published December 2, 2019
This exercise is a case study that takes students through the entire learning cycle from invitation to reflection as applied to the development of North Atlantic Deep Water in the Irminger Sea. Several types of data are presented and students are asked to draw some conclusions (exploration). Students are asked to speculate on the relationship between the wind and different water properties and to identify the seasonal cycle in water temperature (concept invention). Students are challenged to apply what they have learned to another situation (application). This case study requires students to think about the interactions among wind, circulation, and water properties, so at the conclusion students are asked to think about what they have learned about this complexity (reflection).
After engaging with this Data Exploration a student will be able to:
Describe patterns in individual graphs and correlations between the different graphs presented.
Interpret the provided data.
Explain the relationship between wind speed forcing and ocean water temperature in the near surface.
Explain the relationship between ocean water temperature and density.
Identify the seasonal temperature cycle in ocean water and explain how it varies with water depth.
Describe the formation of NADW and its contribution to the thermohaline circulation.
Context for Use
This exercise could be used for either lower level or upper level students. For an introductory course, the activity could be used upon completion of the water properties ocean circulation modules to reinforce concepts covered. For upper level courses such as a Physical Oceanography class, it might be a more stand-alone activity used to stimulate questions and review understanding prior to the introduction of the concept of deep convection.
For an overview of this activity and additional tips, you can also check out the webinar recording.
Students will need to be familiar with concepts from earlier in the semester including: water properties (temperature and density) and basics of the thermohaline circulation. Advanced students will benefit from a knowledge of latent heat exchange across the air sea interface.
At the lower level, this activity requires synthesis of a variety of concepts (temperature, density, ocean circulation, surface winds) that may not have been considered together previously. At the upper level students can begin to visualize the process of deep convection.
While the hope is to guide students to a more accurate understanding of the concept, the broader learning goal is in making these connections. After the activity is over, discussion could center around how climate change affects the ocean, and the potential for disruption of the global thermohaline circulation.
Invitation: As an introductory activity, you can:
Show a clip of high sea state in the North Atlantic. Invite the students to describe what they observe such as waves and whitecaps. Ask them to speculate what may be happening at the air-sea interface in terms of winds and the temperatures of both the air and seawater.
Think back to concepts we've already covered in class that might have something to do with air-sea interactions. Discuss the following prompt with the person next to you: In what ways do you think the atmosphere influences the ocean? What concepts did you discuss that might be in play here?
Show a T-S diagram with identified water masses to illustrate that water density varies with other water properties. Remind students they previously learned about water properties such as temperature and salinity. Ask them which of the properties, salinity or temperature, seems to have the greatest impact on seawater density. What evidence are they basing their answer on?
Students use the widget to examine the ocean and air temperature plots in relation to the seasonal cycle. During this exploration, allow students to explore with a partner, make predictions, and try to explain their reasoning.
Have students turn to the person next to them and describe any pattern(s) they observe in how the ocean surface temperature changes as the seasons change and what might cause this pattern. [Students should be able to identify seasonal changes in the temperature along with the time lag associated with this warming or cooling, as well as the impacts on temperature of periods of sustained high winds.]
Students make predictions about how other physical factors (sea surface temperature and density) will change in response to changes in wind forcing.
Have students share with a partner what they think the graph of those physical factors will look like over time, and a rationale for each of their predictions.
Have students make a quick sketch of their predictions with a few notes about the discussion they had with their partner about their rationales so that they can look back at their ideas later.
Students use the widget to examine a stacked set of temperature plots as well as a stacked set of density plots. During this exploration, allow students to explore with a partner, make predictions, and try to explain their reasoning. During the exercise, have students respond to the following prompts:
What patterns do you observe? How do the patterns compare to the predictions you made in Exploration #1? In what ways were they similar/different? What additional explanations might explain the results?
Describe how temperature and density change as you go deeper. Which depths show the greatest amount of seasonal change in temperature? Which depths show the least seasonal change in temperature?
Students use the widget to compare the temperature and density plots at each specific depth to see the relationship between temperature and density. During the exercise, have students discuss the following prompts with a partner or small group:
What happens to the density as the water gets colder?
How do these identified changes in the seasonal near-surface temperature and density contribute to the development of thermohaline circulation?
In what season of the year is this temperature/density relationship strongest? How can you explain that?
What would happen to the development of the thermohaline circulation if the wintertime near-surface temperatures were to become warmer.
Have students discuss in small groups and then lead a whole group discussion with the following prompt: Is there anything they are wondering about having studied the data that they might not yet be able to explain? What more information do they want or need?
Lead a whole group discussion about what students have observed so far in the two exploration widgets. Encourage them to share their explanations of the plots and observations, and what questions they have. Help students come to a more accurate scientific explanations of the concepts covered by these widgets.
Students use the widget to examine the surface wind speed plot and compare it to the near-surface temperature plot to investigate the relationship between wind speed and winter cooling. Have students discuss the following prompts, first with a partner/small group, and then whole group:
What happens to the near-surface temperature as the wind speed increases in the winter? Why does this occur? What concepts are at play here?
Where does the lost heat go?
Students use the widget to examine and describe the relationship between wind speed, near-surface water temperature and water density. Have students discuss the following prompts, first with a partner/small group, and then whole group:
What correlations do you observe between these variables?
How does what happens at the surface with these water properties produce the cold, high density water found at depth?
Students examine the potential density plots for all depths. Are they similar? What does this imply about the vertical movement of the water?
Describe in your own words how the wintertime conditions in the Irminger Sea produce the deep water which forms part of the thermohaline circulation.
Students use the widget to examine and describe the attenuation with depth of the seasonal cycle of water temperature. Have students discuss the following prompt, first with a partner/small group, and then whole group:
Why does the seasonal change of temperature with depth contribute little to the formation of deep water?
Ask students to identify another area in the world where deep water forms. Ask: Are processes similar to what occurs in the Irminger Sea also occurring in that area? How can you tell?
Compare the Irminger Sea to the ocean area surrounding Antarctica. Identify similarities between the two areas which could lead to the Antarctic area being a source region for deep water.
Compare the Irminger Sea to the Gulf of Mexico. Why are there no areas of deep water formation in the Gulf.
Have the students speculate why the deepest layer in the Irminger Sea data has no seasonal signal.
What might happen to the production of deep water if average winter temperatures in this area were to increase as a result of climate change?
Consider the implications for the global climate if the thermohaline circulation was to considerably slow or even cease.
Reflection: You could...
Have students reflect on the entirety of the case with a question like: Although we often study separately the different pieces of the ocean system, this case study nicely shows the interaction of the atmosphere (wind), water properties (temperature and density), and the thermohaline ocean circulation. As you tied together all of these pieces you learned something new about the ocean. What did you learn?
Ask the students to consider how they learned and how much they learned and applied their learning through one or more of the following:
What new skills did you learn that helped you to figure out how the deep water was forming?
What concepts did you need to learn more about?
What new connections between concepts did you make?
In what ways did these connections help you to understand the concepts better?
What was the most difficult part of this activity/unit/challenge for you? Why? What helped you to figure it out?
Subject / Topics
Ocean thermohaline circulation
Water properties: temperature and density
Ocean dynamics: formation of North Atlantic Deep Water
This exercise could be used to ask students to apply previously learned concepts, such as wind speed, ocean temperature, and density to explain Deep Water Formation. If these topics have not yet been covered, this example can provide the impetus to learn about them. It requires taking information in one form, mostly graphs, and conceptualizing about the Irminger Sea. In introductory classes, instructors may want to have students grapple with one or a couple of the concepts and fill in the details for them on the rest.
In more advanced classes students should be challenged to recall previously learned information and apply it to this problem. Instructors should be ready to jog their memories about the learned information.
Undergraduate students in Introduction to Oceanography courses (for either marine science majors or non-science majors)
This exercise can also be a case study in upper level Physical Oceanography course, where students apply what they have learned about water properties and deep convection.
Our scope in exploring ways to use professionally-collected data in our teaching:
Using professionally-collected data in teaching of concepts
Visualizing data in a user-friendly and authentic way
Enabling students to interactively engage with data in their learning to see the patterns as they are learning the why
Most of the quantitative reasoning in this exercise revolves around reading and interpreting graphs.
Students must articulate the seasonal pattern in the data presented as well as its attenuation with depth.
With the help of the widget students will examine time series of ocean temperature, ocean density, and wind speed. They should see that there is a strong correlation among all three of these variables.
Students will be able to connect the formation of dense water to the sinking of this water to drive the thermohaline circulation.