What causes ocean currents?
The students will be able to explain and to demonstrate ocean currents.
Large rectangular pan, Pencil shavings or Cheerios, Water, World Map, Fan, Food coloring, Colored ice cubes
Explain to the students that ocean currents are produced and maintained by the rotation of the earth, the winds, and differences in water density. In addition, the depth of the water, location of the land,the underwater topography all affect the ocean’s circulation.The stress of wind blowing across the ocean causes the surface layer of water to move. This motion then causes the underneath layer to move. But the internal friction with the water decreases with the depth.This current is called a wind current.
Do a simple demonstration with the students. Put the water in the pan, and turn the fan on (facing the water). Have the students observe how the air blowing over and on the water puts frictional stress on the water surface.Ask what is happening to the water that was motionless before? (The water is circulating around the pan.) Drop some of the shavings onto the circulating water and observe what is happening. Use dialogue with the students instead of having them write their observations. If you choose to do this using food coloring, DO NOT USE MORE THAN 1-2 DROPS OF THE FOOD COLORING, or the coloring will spread before you can follow it.
Have the students return to their seats, pull down the world map, and continue explaining about the cause of water currents while pointing out locations. Ocean water near the equator is warmer then ocean water near the North and South Poles. Warm water is lighter than cold water, so warm water rises to the surface. When this happens in the ocean, cold water moves in under the warm water. In the ocean, the warm water becomes a current that flows along the surface. It moves toward the cooler water ahead of it.
As the warm water moves away from the equator, it meets cooler air and it loses some heat to the cooler air. The ocean water in this current becomes cold and gets heavy. Being heavy, it sinks deep into the ocean and moves along under the surface current. This deeper cold water current flows in the opposite direction from the surface current. It flows back toward the equator. A current of water flowing in this way forms a huge wheel of water in the ocean.
Have the students observe the following experiment, writing their observations on the Experiment Form.
1. Using warm water, place the water in the large pan.
2. Place some colored ice cubes at one end.
3. Place Cheerios on top. Observe what is happening.
The children will see the Cheerios move toward the ice cubes as the cubes melt. Ask what happened to the water around the ice? Why? What happened to the Cheerios?
Another cause of movement in the ocean is the different amounts of salt in the water. Because the water that has more salt is heavier than the water with less salt, it will sink in the ocean. As it does this other water moves in over the top. This gives rise to a current.
Ask the students to list the causes for water currents. ( Wind stress, heating and cooling of the water, salt in the water, water depth and the location of the land.)