Unit 3.3 What causes ocean stratification?

By the end of this unit, students will be able to:

• Observe and describe the ocean's stratified structure in order to explain how density varies within the ocean's stratification
• Practice calculating the density of various fluids in order to design a model the ocean of the ocean's stratified structure and model oceanic circulation processes
• Describe the physical processes that drive ocean circulation to explain the role of density within oceanic circulation
• Use mathematical representations to support a claim regarding the layered structure of the ocean

One of the prevalent patterns in our physical world is a layered structure that is at least partially controlled by material density. This module is designed such that students can use both real world data and their own experiment to examine the role of density in both the stratification of the ocean and major oceanic circulation processes.

The first activity is designed to facilitate students investigating the physical properties of seawater within our ocean's basins using real data, and then using these data as evidence to support hypotheses on seawater density and ocean structure. In the second activity, students will be focused on comparing density of fluids to both practice making these measurements and to set up their experimental design for modeling the structure of the ocean in the third activity. The oceans are vast, making it nearly impossible to observe circulation between layers or from pole to pole. However, a small-scale model with exaggerated layers can simulate surface and deep water circulation to observe these processes. This model allows students to experiment with contrasting density fluids and observe the process of upwelling as a result of surface winds - a typically unobservable phenomenon. This activity can also be used to discuss the effect of ENSO events.

These materials build a foundation for understanding the unit's motivating question, but also function well as a standalone module. The materials in this unit should take 145 minutes of class time, plus an additional 50-80 minutes if you complete the optional Extension Lab to Model Ocean Stratification and Circulation. Most of the lab exercises rely on small group work and are best suited to smaller classes or a lab meet-up outside of a traditional lecture room.

Teaching Materials:

All Slides: U3.3 All Slides.pptx (PowerPoint 2007 (.pptx) 23.2MB Jul10 24)

Cartesian Diver Class Demonstration (only one set-up needed): plastic water bottle, water, two paperclips, straw. Cartesian Diver youtube video courtesy Hungry SciANNtist.

Graphing Ocean Data Activity:

• Argo Buoy data access
• Activity Instructions: For Students U3.3 Graphing Ocean Data - Student.docx (Microsoft Word 2007 (.docx) 288kB Jul10 24), For Instructors
  U3.3 Grading Guide for Graphing Ocean Data - Instructor.docx -- educator-only file
  Hide
  U3.3 Grading Guide for Graphing Ocean Data - Instructor.docx

  
  This file is only accessible to verified educators. If you would like access to this file, please enter your email address below. If you are new to the site, you will be asked to complete a short request form. If you have already been verified by the project, you will be taken directly to the file download page.

  Email Adress Submit

Measuring Density of Liquids Activity materials:

• Liquids to measure, e.g. Salt, Sugar (syrup?), Water, Alcohol, Oil, Glycerine
• Volume measurement devices, e.g. Pipettes, Graduated cylinders, Beakers (and/or other similar)
• Scales, Kettle/microwave (For hot fluids), Ice (For cold fluids)

Modeling the Ocean Activity materials: Ice, Funnels, Clear plastic tubs (approx 8" long by 4" wide by 6" deep - e.g. clear food handling containers), Kettles, Salt, Food coloring, Plastic tubing, Hair dryer (optional - can also create wind by blowing on the water surface), Pitchers

Reflection Assignment: U3.3 Reflection.docx (Microsoft Word 2007 (.docx) 68kB Jul10 24)

Sample Student Reflections (this is Reflection 9): Reflection Examples Redacted.pdf (Acrobat (PDF) 1.8MB Jul8 24)

Argo Buoy Website Troubleshooting video (note also the site does not work well in Safari): U3.3 - WebsiteNavigation.mp4 (MP4 Video 5.2MB Jul10 24)

The Lab(s) is/are assessed as a Science Journal, as always. Science/Lab Journals General Instructions/Rubric (Microsoft Word 2007 (.docx) 2.9MB Aug30 24)

Pre-Class Assignment(s):

Read Density and watch Monty Python clip

• Map the concepts from the reading to the Monty Python clip.
• How is density of objects used for the "logical" argument?
• Can you create a concept map of the density-based logic argument?

In Class, Part 1: Visualizing the Ocean Through Data (~75 min)

In class (15 min): Introduction to the ocean

• Begin with a question: what is the ocean made of? (water, salt... sediment...)
• Use another question "where does the ocean's water come from?" to lead into the hydrologic cycle. Briefly introduce the sources of water to the ocean from the hydrologic cycle. Discuss that the same runoff bringing water into ocean basins also carries salts and sediments.
• Think-Pair-Share: Think about how parts of the ocean vary globally. Identify as many different physical properties as you can that might vary in seawater around the world. (i.e. temperature & dissolved solids). Discuss student ideas within the whole class.
• Showing a photo of sea ice and then a photo of an iceberg, ask:
  • What do we see in this photo? (sea ice)
  • Water is represented in this photo as clouds, ice bergs, and water. Only the sea is saline, water elsewhere is all fresh. Nudge to examine the iceberg in the foreground. Should be able to observe more iceberg visible under water.
  • What does this image make you think of? (iceberg)
  • Perhaps students will immediately go to climate change, polar bears, and melting sea ice and glaciers. Direct the discussion toward climate, that ocean is the "great climate regulator", and the application of understanding the physical properties of the ocean to improving climate models.
  • What interpretations can be made from this photo? (iceberg)
  • Observe that the iceberg is floating – interpret that the iceberg is less dense than the water. Observe that most of the iceberg is submerged – what interpretations can be made about density differences?
• This discussion leads into the first activity: compiling and interpreting ocean data

Lab exercise - graphing a cross section of ocean data (60 min)

• In this activity students will use real world ocean buoy data to examine how the properties of temperature and salinity vary around the world. They have already thought about these ideas and discussed it in pairs, now they will compile global data to illustrate variability from surface to ocean floor and pole to pole.
• Data from ARGO is already shown graphically from surface to depth, so students will transcribe buoy data graphs of their choosing to assemble a cross section.
• Once cross sections are completed, students will examine them for patterns and make interpretations. Finally, they will use these interpretations to predict where they expect to find the highest and lowest density seawater.
• End activity discussion questions:
  • How is the ocean stratified?
  • How does its stratification compare with that of the atmosphere?
  • How do the influential factors on ocean stratification vary with latitude?
  • Think-pair-share: Hypothesize how deep ocean water can circulate upward to the surface.

In Class, Part 2: Density of Liquids (70 min)

In class (10 min) What is density?

• In Activity 1, you hypothesized areas of higher and lower salinity seawater in the ocean and how deep ocean water might circulate to the surface. What information do we need to test these hypotheses? (lead in to class with this discussion -- the idea being that students will recognize that we need more density data)
• Before proceeding on with density data, now is a good time to really solidify what density is. Explanation of mass, volume, density, and the density formula triangle.
• Complete and optional demonstration of density: The Cartesian Diver. A pocket of air is trapped inside a portion of straw and placed inside a water bottle. When you squeeze the bottle, the air compresses and the straw sinks. When you release the bottle, the air expands and the straw floats.
• This leads into the second activity: measuring the density of liquids.

Lab exercise - measuring densities of liquids (60 min)

• Students will be working in groups to share equipment and will use pipettes, beakers, and/or graduated cylinders to make their liquid density calculations. Each group will have 1 scale and one of the three (or more) liquid volume measuring devices (pipettes, cylinders, etc.). Each group will be responsible for calculating the density for each of the fluids provided (e.g. water, oil, syrup, glycerine...). They should each make at least 3 trials and calculate an average (and std dev?) for their given equipment.
• Students will record their data (observations) in tables in their groups, then all data will be shared amongst the class for comparison. Based on the data comparison, discuss:
  • How do the average density values compare between method? If they vary, why do you think this is so?
  • Which method has the greatest standard deviation? What do you think that means?
• Based on this experience, students are finally tasked with designing a model of the ocean, with which to test their hypotheses from the previous activity. Students are asked to consider which fluids would be the most representative, how to maximize the density contrast between layers, how many layers are needed, etc. for their models.

In Class, Part III: Modeling Ocean Circulation (50-80 min)

In class (10 min): Accuracy, precision, and modeling

• In the last activity we compared methods for measuring the density of liquids. What is the meaning of "standard deviation"? Here we can briefly examine the concepts of accuracy and precision.
• Next, considering this discussion, transition the discussion of accuracy and precision to the ocean model design plans. We are modeling the ocean at a tiny scale! Discuss and evaluate representation with exaggeration vs accuracy and trying to observe the unobservable in the real world. Aim here is to encourage their own thinking, but lead them toward using high temperature and salinity contrasts of water to model the ocean to facilitate making observations despite the lack of representative accuracy, but that using water with extreme salinity and temperature is more representative than modeling with oil and water...

Modeling ocean stratification and circulation extension lab (30-60 min). Shorter time if done as a class demonstration.

• This activity has the students using their designs to build models of the ocean to observe ocean stratification due to density and try to test how upwelling is possible. Context discussion questions to begin the lab: Why is the ocean stratified? What is "upwelling"? How is this phenomenon possible? What causes ocean water to circulate?
• Now the students should be set free to make their models, observe, interpret, and experiment. Throughout the experimentations, they should document their observations and set themselves up to be able to make evidence-based interpretations.
• After modeling, bring the class focus to a map of sea surface temperature. Students are asked to make observations based on the patterns shown on the map, which do subtly show locations of upwelling. Direct the discussion toward upwelling, and how this process brings cold, nutrient-rich, water to the surface at many coastal regions. (this is a prelude to next class)
• Final lab discussion on thermohaline circulation:
  • Both heat and salt change water density, explain how
  • Explain where in the world you would expect to see inputs/outputs of warmer/cooler/fresher/saltier water and how these inputs might impact thermohaline circulation

In class (10 min): Discussion of applications of ocean circulation

• Small Group Discussion:
  • The ocean is the "great climate regulator", explain how this is possible given your experiments with ocean circulation.
  • Show the sea surface temperature map again: What evidence of oceanic circulation can you identify on the NOAA sea surface temperature map from June 2022?
• Prompt for think-pair-share and final whole class discussion. Application: economy. Given that upwelling brings cold water that is rich in nutrients up to the surface, why do you think this phenomenon is critical to the economy of many countries?

Measuring Density of Liquids Activity:

Prepare salt & sugar water in advance.

Students should work in groups. Each group should have a scale. Within the group they will collectively work together to gather their data and then compare the methods they used with other groups at the end of the lab activity. Provide different groups with different containers to measure the volume of their liquids (i.e. pipettes, beakers, and/or graduated cylinders) such that each group will have 1 scale and one of the three (or more) liquid volume measuring devices.

Modeling the Ocean Activity:

This activity works best with extremely exaggerated temperatures and salinities to enhance the contrast of density in order to facilitate observation of processes on a very small scale.

Setting up the saline water in advance is a big time-saver. It is far easier to dissolve lots of salt in hot water, then cool it down, than it is to try and dissolve salt in ice water (a challenge my students often attempt despite the instructions). If the saline water is prepared ahead of time and put on ice, this activity can be set up quite quickly and students can spend more time conducting experiments.

Additional experiments that make for great observations with this mini ocean model are to 1) toss in a couple of ice cubes, watch where (within the layers) they float, and consider what they would represent from the real world, and 2) model a black smoker using hot salty water added directly to the bottom of the basin (using the same funnel-tubing method), describing the behavior of that fluid within the established layers, and explaining the role of density in that circumstance.

The lab activities in this unit use water. Please exercise care and caution to prevent spills if computers are also used at desks or lab stations.

• A pre-class assignment (described above) is graded for completion only, not correctness. Administer using the same format throughout your course (through the LMS, turn in paper copies, guided discussion/participation in class, etc.). Consider setting the due date an hour or so before your class begins to give you time to summarize where your students sit with these concepts (this is a form of Just in Time Teaching).
• The Labs are assessed as Science Journals, as always. Science/Lab Journals General Instructions/Rubric (Microsoft Word 2007 (.docx) 2.9MB Aug30 24)
• The reflection assignment in this unit asks students to reflect on reflect upon their understanding of the structure of the ocean, thermohaline circulation, and climate. Reflections ask students to put their learning in their own words and also to apply their knowledge in a new and novel situation. As always, reflections should be about 500 words and they should both discuss content that reflects understanding and thoughtfully reflect on the materials. Unit3_Module3_reflection.docx (Microsoft Word 2007 (.docx) 161kB Jul27 22)
  • Read: The Planet's Thermostat, Ocean Circulation
  • Reflect upon your understanding of the structure of the ocean, thermohaline circulation, and climate (from the reading) after having completed the activities. Are you able to describe and explain the role of density in the layered structure of the ocean? Do you feel you are able to explain the similarities and differences and interconnected nature of our world's ocean and atmosphere? Explain using specific examples that showcase your understanding!

End-module readings are fact sheets from the Ocean & Climate Platform

• The Planet's Thermostat
• Ocean Circulation

ARGO buoy data is from Euro-Argo European Research Infrastructure Consortium for observing the ocean (European contribution to the Argo programme). Note: At the time of writing in 2023, this website does not work in Safari. Use a different browser.