BACKGROUND:

The oceans are always moving. The movement is graceful and subject to the principles of fluid motion. Fluid motion refers to the response of liquid to forces of wind, density, and rotation of the Earth in space. Locally you must include factors such as topography and tidal forces.

The waters of the oceans are not homogeneous. There are ocean masses that move as units, called currents. The ocean’s movement is predictable based on mathematical modeling. The model takes into account many factors that control currents not only on the surface but with depth

Coriolis does not alone explain movements, but other factors not related to movement such as density of water, wind and local submarine topography need to be included. 

Wind also moves water around, especially the upper portions of the water column. However, wind is generally moving in the same direction caused by the rotation of the Earth, so they "add" forces. In areas where land and water meet, the direction of the wind is controlled by temperature differences of the land and water. Strong winds can influence the movement of coastal waters.

Wind is actually a bizarre phenomenon. It can change quickly depending on the balance of heat. A mountain can stop wind and add to the chaos of wind motion. Wind produces energy throughout the water column in a predictable pattern called the Ekman spiral. The transfer of wind energy from the atmosphere to the waters can cause motion far from where the energy first entered the system

Temperature and density can define a water mass. Warmer water can hold more salt, and colder water holds less salt. Salt water is more dense than fresh water of the same temperature. The salt water will layer itself below the fresh water. Warm water is less dense than cold water. So cold water will layer under warm water. Add the combination of different amounts of salt and different temperature and you have a layering effect in the water column. The world’s oceans are a three-dimensional nightmare of layers of different water masses that can move in different directions.

Bottom topography can act as a barrier to water masses already in motion. If a cold saline water mass is moving along the bottom and "hits" a mountain it would be forced upwards. This is one form of upwelling. This would displace the water masses above it, causing lots of movement.

All these reasons cause the movement of the oceans to spin into smaller eddies. Please keep in mind that although this motion looks chaotic, it really is responding to natural forces, which we can mathematically model.

Seawater density varies from place to place because it is affected by salinity and temperature. High salinity makes water denser. This is because there is more salt packed into the water.

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PROCEDURE:

INTRO:  Ask students what have you learned about that causes climate patterns so far (just get a few such as  the water cycle, wind patterns, coriolis effect, different levels of solar radiation by latitude, land vs. sea cooling and heating, high and low pressure air masses).   Today we talk about ocean currents and the impact of ocean currents on global climate.

Go over powerpoint presentation on Ocean Movements (slides 1 to 8)

1.    First slide review Coriolis effect (earth rotation) covered last time

2.    Second slide.  Remind the students about how the barriers to ping pong balls from prior lab.   Did the ping pong balls just stop or move to the side or bounce back?  Underwater has barriers too that influence ocean currents.  Ask them to name some.  (example, under water ridges, the continents, ice sheets, etc.)

3.    Slides 3 to 7  Differences in the density of water produces water currents.  Cold, high saline water is very dense. When icebergs form,  that means fresh water is taken out of the system, concentrating the salt in the water. This water is so dense it drops to the bottom and remains there for hundreds of years, as it travels on the bottom. The diagram shows "new" water starts in the North Atlantic and moves along the bottom. The cold water warms up slowly and emerges in the Pacific Ocean only to start circulating through different routes.

4.  Watch the NASA lab on the water currents and note how the land masses change the direction of the currents.  https://pmm.nasa.gov/education/videos/thermohaline-circulation-great-ocean-conveyor-belt

But there is another aspect that is affecting the currents and that is water density.

5.  Watch video https://www.youtube.com/watch?v=UuGrBhK2c7U covers thermohaline conveyor

After video what does thermohaline mean (thermo means heat and haline means salt)

Now look at another video of how changes in our conveyor belt can affect our climate. (Al Gore short video) https://www.youtube.com/watch?v=kSCT1UAfuzo

6.  To recap, dense water sinks below less dense water. In the ocean the two main drivers are saltiness and temperature.  This is the principle that drives the deep ocean currents that circulate around the world. A combination of high salinity and low temperature near the surface makes seawater dense enough to sink into the deep ocean and flow along the bottom of the basins.

FIRST ACTIVITY—density timers (5 to 10 minutes)

1.    Give students a density timer with a warning not to shake the timer.  Shaking ruins them.  Ask them to look at the 3 different liquids and see if they can figure out which is denser than the other.  (The first color liquid to drop is the most dense--heavier.)

2.    Put the density timer on its side and see if you can get the bottom layer to come upward by gently rocking it back and forth.  The sides of the timer are analogous to dense ocean currents hitting underwater barriers forcing the bottom water upward (upwelling).  Ocean currents are driven by variable density layers in the oceans, wind at the surface, and bathymetry (underwater barriers).

SECOND ACTIVITY  (20 minutes)  Making a water salinity density column

PREPARATION:  Place sets of large test tubes at the tables, containers of water, measuring spoons, a dish of salt and a discard bucket.

1.     Students will work in teams of two.  Each team gets two large test tubes one that says fresh on top and one that says salt.  They should fill both test tubes with fresh water to the second from the bottom line on the test tube. 

2.    Ask them to swirl the test tubes to see if they can produce a “tornado”.  Take the salt test tube and ask them to use the measuring spoons and add 1 ml of salt.  They should reapply the cap tightly and then shake  the test tube around until all the salt is dissolved.  When it is all dissolved, ask them to swirl the test tube and look at the “tornado”.  Is it easier to form?  Ask them to compare it to the freshwater one.

3.    Ask them to add another 1 ml of salt and repeat all steps.

4.    Ask them to keep adding salt until they have added 5 ml total.

5.    Ask students which is denser, the water with salt or without (salt is denser)   Denser water behaves a bit differently.

6.    Docent should then go around and add different food coloring colors to salt and fresh water tubes. Just 1 or 2 drops of the color will do.  It works best if the colors are contrasting (see photo).

Students should note the difference in how the food coloring reacts to the two tubes.  In the fresh water, the food coloring sinks and starts to spread out.  In the salt water the food coloring just sits on the top.  Ask the students why they think there is a difference (the food coloring is more dense than fresh water and sinks in fresh water and less dense than the salty water so stays on the surface).

7.    Students should now swirl the tubes to mix the food coloring evenly.  Ask students to predict what will happen when there is a salt water layer and a fresh water layer are combined (salt water layer should be at the bottom.)

8.  To make layered salt water, students will take the fresh water tube and hold it at an angle.  Fill the pipette with the salt mixture, place the pipette in the fresh water tube so the tip is near the bottom and SLOWLY add the salt water to the fresh. (Since the test tubes have round bottoms, if they pour the salt water or squeeze the pipette too fast the salt water will upwell quickly and the layers will mix.)  They should keep doing this until they get about half of the salt water in the fresh water tube.  If they do this correctly they will get two distinct layers that do not mix even when you move the tube gently back and forth.  

You should demonstrate this technique to them before they start.

If they go too fast and mix their layers, let them dump out their freshwater test tube and try again.   

9.    If they have time and want to try to make a third layer, they can make a highly salty cold water mixture which is denser than the other salty water.   To make super salty cold water put in the regular amount of fresh water, add 10 ml of salt and one or two ice cubes.  They should shake it vigorously for a minute or so the mixture gets quite cold.  They can leave this cold mixture clear or add a contrasting food color and add it carefully to the bottom of the test tube with the salt and fresh layers using the pipette.  This layer will remain at the bottom. 

At the end, students can shake their layered test tubes.  It will all mix.  If there is enough turbulence, the layers will mix and not separate.

10  Review.  What other layers do they think they could form?  Where would warm fresh water go?  How about cold fresh water? 

9. Review at the end that ocean currents start by cold salty water sinking to the bottom.  These sinking water masses help mix the entire ocean and take gases from the atmosphere to the deep ocean.

(For more info you can go to https://timescavengers.blog/climate-change/ocean-circulation-stratification/)

10.  CLEAN UP -- At the end, students should dump out their test tubes and rinse them with water only (NO SOAP) to be ready for the next class.