Weather, Climate and  Change

 Density of different layers of ocean can cause movement.
SIXTH GRADE - OCEANS IN MOTION

OBJECTIVES:
·         Exploring and identifying water masses and currents.
·
Discovering how different water masses cause circulation throughout the oceans.

VOCABULARY:
·
currents
·
Ekman Spiral.
·
Water mass
·
salinity

MATERIALS:
·
density timers (with 3 colors)
·
big test tubes (two per pair of kids)
·         food coloring
·         salt
·         spoons
·
worksheet (optional)
·         powerpoint

https://msnucleus.org/membership/html/jh/earth/oceanography/lesson5/oceanography5a.html

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:

Go over powerpoint presentation on Ocean Movements

1.    First slide review Coriolis effect (earth rotation) covered last time (should take about 2 minutes max)

2.    Second slide.  Remind the students about the barriers to ping pong balls from last time.  What stopped them.  Did the pinp 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 6  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 tens 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.    Dense water sinks below less dense water. 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.    Give students a density timer with a warning not to shake the time violently.  That ruins the density difference if small bubbles arise.  Ask them to look at the 3 different liquids and see if they can figure out which is denser than the other.  (The slower color to fall is the least dense.)

6.    Put the density timer on its side and see if you can get the bottom layer to come upward.  If they rock it back and forth the bottom current, will “upwell.”   The sides of the timer are analogous to bottom denser water hitting to an underwater mountains and the water is forced upward. The rocking is basically a steady wind blowing in one direction which can bring up bottom water. Density in the oceans, wind, and bathymetry (underwater mountains) also help in making the motion in the oceans change.

SECOND ACTIVITY  how different densities of water cause ocean currents.

PREPARATION:  Fill all the test tubes up with fresh water to the line second from the top (the second one down) before the class.  Rinse out the test tubes and re-apply fresh water between classes.

1.     They will now work in teams of two.  Each team gets two large test tubes one that says fresh on top and one that says salt.  Ask them to swirl the test tubes to see if they can produce a “tornado” shape.

2.    Take the salt test tube and ask them to add ½ teaspoon (it doesn’t have to be exact if you don’t have measuring spoons).  They should reapply the cap tightly and then swirl 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 ½ teaspoon of salt and repeat all steps.

4.    Ask them to add another ½ teaspoon of salt and repeat all steps

5.    Ask students which is denser, the water with salt or without (salt is denser)

6.    Docent should then go around and add food coloring to the fresh water (about 4 to 8 drops (enough so it is a dark color).

7.    We are now going to combine the two.  Ask students to predict what will happen.  The students should tilt the fresh water to the side and SLOWLY add the salt water to the fresh by pouring it onto the inside of the tilted test tube.  (that is don’t pour it in the middle or it will get all mixed up.)   What happens?  The clear salty water should sink to the bottom.

8.    Tell them you are going to give them an ice cube to add to the test tube.  Ask them to predict what will happen.  Which is more dense, the ice cube or the water (the water).  As the water melts it is colder.  Which is more dense, the cold water or the colored water?  (the cold water) Add an ice cube on top (tell them to let it slip in slowly.  Don’t drop it in to not disturb the layers.

9.    Put the test tube back in the stand and watch.  What do they think will happen as the ice cube melts?

10. Discuss:  what happened with the melted ice?  What happened with the layers.  Why.  How does this help them understand the ocean currents better?