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The Effect of Rotation on the Sinking of Cold Water

Materials:
Record turntable
Two deep cylindrical containers with water, one on the turntable and one off
Cold, colored water and a dropper

Procedures:
1)Add cold water with the dropper to the center of the non-rotating container. The cold water will sink to the bottom and spread outward. You may notice that some of the drops form small rings as they sink in the water. These are called vortex rings and are like smoke rings.
2) Add cold water to the center of the rotating container. Instead of all sinking to the bottom and spreading, the cold, colored water forms a vertical column extending from the water surface to the bottom and stays near the center. Some swirling patterns can be observed from above.

This illustrates how strongly rotation can affect the behavior of fluids. Although hurricanes form differently, this demonstration can help understand how an intensely rotating storm like a hurricane can develope as a strong vertical structure.   Hurricanes are very organized systems that need the Coriolis force for their existence. Hurricanes cannot form right at the equator because the Coriolis effect is zero there. Hurricanes can only form within a few degrees of the equator where ocean water is still warm and there is some Coriolis effect.

Materials:

Record turntables
Pizza/Pie storage containers with circular tray to support them
Cold, colored water and a dropper

Procedures:

The Coriolis effect also influences the directions of ocean currents. One example that can be illustrated are deep boundary currents.

1) Using one of the pizza/pie slice containers that is 1/3  filled with room temperature water and is not rotating, add some cold, colored water with a dropper to the narrow corner. The dye should spread fairly evenly away from the corner with a front that crosses the whole width of the container.
2) Do a similar addition to a rotating pizza/pie slice container. For the best effect be sure to make several additions of the cold, colored water. The colored water should move quickly along the left wall of the container (when facing out from the center of rotation).

The rotating case is analogous to what happens to cold deep currents in the ocean. Cold water that is found in the deep ocean is formed in the North Atlantic Ocean and in regions near Antarctica due to cooling of surface water in those regions.
As the water sinks and moves away from the region of origin, the Coriolis effect causes the water to move in a current along the western boundary of the ocean. The turntable example is analogous to what happens in the deep Pacific Ocean in the Southern Hemisphere. The coldest waters in the deep South Pacific are observed along the western boundary (the left side when observed from the South Pole toward the Equator). In the Northern Hemisphere the current of cold water from the north Atlantic also flows along the western boundary of the ocean (the right side when observed from the North Pole toward the Equator). The formation of such currents is the result of both the rotation of the Earth (Coriolis effect) and the presence of north-south boundaries along which the flow can occur. All other ocean currents including the surface currents that are driven by winds are affected by the Coriolis effect.

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