Modeling Salinity and Deep Ocean Currents

Overview

In this experiment, students make a claim about the cause of ocean currents and then develop a model to explain the role of salinity and density in deep ocean currents.  This lesson is modified from "Visit to an Ocean Planet" Caltech and NASA/Jet Propulsion Laboratory.

Materials Required

• 4 Baby food jars 
• 2 Laminated index cards 
• ½ cup Table salt
• 2 Colors of food coloring (red and blue)
• 1 Stir stick
• Dishpan (for spills)
• Towels
• Map of surface currents
• Map of surface salinities 
• Optional: aprons 
• Student Data Sheet

PREPARATION 

• It is important to do this activity before your students do it. This will give you a chance to see and work out any potential problems beforehand. Gather the supplies or send a supply list home with the students. Make sure that the students mark their names on anything they bring to class that will be returned home.

Procedure

Engage the Learner:

1. Display the maps of surface ocean currents and surface salinities of the oceans [Figs. 1 & 2].  

2. Describe the color bar; it shows the long-term average sea surface temperature, with dark green depicting less saline waters and white depicting more saline waters. 

3. Have the students look for relationships between salinity and the locations of warm and cold currents. 

4. Distribute the Student Sheet.  Direct students to write their observations down on Student Sheet in the Initial (Quantitative/Qualitative) Observations section.
    

   

Asking Questions:

1. Next, have students brainstorm questions that come to mind about these data and how they may be related.  Students write their questions in the Your Questions section.
2. Guide students through the process of generating a claim statement.  Ask the students to write a claim that addresses these relationships in the Claim Statement section.  Example., Surface currents control the water’s salinity.  

 Carrying Out the Investigation:

First model the experiment for the students before having them conduct the investigation with their group:

1. Fill both baby food jars with water of the same temperature. Note to students the parts of the experiment that you are holding constant:  jars, amount of water, the temperature of the water, etc. 
2. Dissolve ¼ salt in one of the jars and add blue food coloring. Make sure to mark the jar “Salt Water.” 
3. Add three drops of red food coloring to the other jar and label it “Fresh Water.” 
4. Place a 3 x 5 index card (laminated, preferred) on top of the saltwater and carefully & quickly invert it. Place the saltwater jar on top of the freshwater container and have someone carefully remove the card. 
5. Observe the results. 
6. Use the second set of jars to repeat the experiment. 
7. This time, invert the freshwater jar over the saltwater jar. Remove the card, and observe the results. 
8. Take both sets of jars, turn horizontally, remove the card and observe the results. 

Analyzing the Results:

1. Allow time for students to document their observations in the Evidence section of the dataset.
2. Students observations should include the following:

• When the saltwater is placed on top of the freshwater, the water in the two jars will mix and the saltwater will sink to the bottom. 
• When the freshwater is placed on top of the saltwater, the freshwater will sit on top of the salty water and no mixing will occur.

Interpreting Data: 

Provide the following questions to help students with their observations/evidence:

1. Describe how the salt and freshwater moved in the experiment?  Saline water sunk to the bottom, under fresh water.
2. Why does the salty water sink?  It is denser (has more molecules in the same amount of space).

Explain The Results:

Facilitate a discussion based on the following points:

1. The greater the amount of salt in the water,  the higher its density will be because there are more molecules found in the jar. The saltwater will be heavier than the freshwater so when salt water is placed on top of freshwater the two will mix and the saltwater will sink to the bottom. When freshwater is placed on top of saltwater the higher density water is already on the bottom and is held there by gravity so no mixing occurs. 
2. This model displays the movement of water based on its salinity, especially in the deep ocean.  When salty water sinks deeper through the water column because of its greater density, deep ocean currents are formed as the water moves horizontally and travels from areas of high salinity to those that are less salty.  The salinity of the water may increase due to evaporation or freezing of water into sea ice at the poles.
3. Queue the Sea Surface Temperature animation.  Explain the color bar used in the following animation shows the long term average sea surface salinity, where white regions have the highest salinity and dark regions the lowest. 
4. Students will observe the video and look for locations that show patterns of higher salinity regions vs low.  
5. Run the video of Sea Surface Salinity. 

1. Discuss the key findings. Students should observe the higher salinity values of the Atlantic than the Pacific.  This is due to the greater rainfall amounts in the Pacific, and the lower salinity at the mouths of major rivers.

2. Display the following image, drawing students' attention to the Atlantic Ocean basin.  

   

    

3. Ask what is happening here?  The Atlantic features a circulation pattern of warm surface currents, including the Gulf Stream, that brings warm surface waters from the tropics northward into the North Atlantic. There, in the seas surrounding Greenland, the water cools; sea ice forms removing freshwater from the water making it denser, sinking to great depths and changes direction. What was once warm surface water heading north turns into cold deep water going south. This overturning is one part of the vast conveyor belt of ocean currents that move heat around the globe.
4. Present the following question to the students:  What effect may increasing air temperatures have on this conveyor belt that distributes heat around the world?  Increasing air temperatures will cause an increase in the melting of land and sea ice in the polar regions.  This would release more freshwater into the polar regions and may affect the salinity-driven deep water circulation.

Constructing Explanations:

Engage students in a discussion around the following questions/tasks:  

1. What water is heavier: salt or freshwater?   Saltwater
2. Which has the greatest density (the amount of “stuff” in a given amount of space)?  Explain. Saltwater has a greater density because it has a greater amount of “stuff” (mass) in its volume. The saltwater has the water plus the dissolved salt, while freshwater does not contain the extra “stuff” (mass) of the dissolved salt. 
3. Where do you expect to find the most saline ocean water?  Deep ocean
4. What may explain why the surface waters at the poles have lower salinity values?   The more saline waters sink to the deep ocean.
5. What explains why the surface salinity values (Figure 2.) are so high at the equator and mid latitudes?  Students may say that the water is warmer; the sun’s radiation is more intense at the equator; there is greater evaporation in this area.  
6. Now have students return to their student sheet and write the scientific principles that affect the movement of water based on salinity.  They should connect scientific understanding to their observations in the Evidence section.  
   • How does our understanding of the science processes affecting the movement of water in the oceans based on their salinity values relate to their Claim Statement.  
   • Is your Claim supported by your evidence and backed by our scientific understanding of the ocean circulation?  Why or Why Not?
7. Have students share their Claim Statements with the class and how their deeper dive into salinity-driven currents have changed their understanding of the Hydrosphere.

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