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Salty Ocean Circulation

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Author: Randall Frost, Ph.D.

The earth’s climate produces large scales difference in temperature and salinity in the oceans. Water circulation patterns in the ocean are driven by temperature and salinity differences, which translate to water density differences. Water density decreases when the water molecules move apart due to increases in temperature. Water density increases when salt molecules take up places between water molecules. 

As the salt concentration in surface waters changes, the density of the water changes. Fresh water is less dense than salt water and floats on the surface. In warmer, fresher waters, the density is low.

In a process known as thermohaline circulation, cold salty water sinks at high latitudes, pulling in warmer water from lower latitudes in to replace it. The cold deep high salinity water then flows from the North Atlantic to the Southern Atlantic and from there east to the Indian and Pacific Oceans, where it returns to the surface through a process known as upwelling. There the water is heated by the tropical sun. Evaporation causes the water left behind to become saltier.

With global warming, high latitude regions are expected to experience more warming than other areas. The melting of polar ice caps, increased precipitation, and other inflows of fresh water into the North Atlantic should make the water in the North Atlantic less dense.

Problem:

This project models the effect of global warming on thermohaline circulation.

Materials:

  • Wide mouth quart jar
  • Kitchen spoon
  • Salt
  • Food coloring
  • Drinking glasses
  • Food thermometer

Procedure:

  1. Read about global warming and thermohaline circulation.
  2. Formulate a hypothesis to explain how global warming might affect thermohaline circulation patterns through temperature and salinity changes.
  3. Have ready a 1 quart wide mouth (clear glass) jar.
  4. Mix 4 teaspoons of salt in an 8 oz. glass of refrigerated tap water. Measure the temperature of the water. This sample will represent cold polar water.
  5. Place the cold salt water in a wide-mouth jar.
  6. Mix 4 teaspoons of salt in a second 8 oz. glass of room temperature tap water. Measure the temperature of the water. This sample will represent the warm, salty water flowing into the polar regions.
  7. Add several drops of food coloring to the room temperature water and stir.
  8. Carefully pour the room temperature water into a kitchen spoon placed over the jar, and allow the water to slowly spill over from the spoon on the cold water in the jar. The measuring spoon should cushion the transfer of water.
  9. Observe the layering effects as the cold bottom layer is forced down.
  10. Repeat these steps using different temperature and salinities for the lower layer.
  11. Compare the layering effects with what you observed in the first case.
  12. Give some thought to how the presence or absence of layering might affect the generation of the cold water currents essential to thermohaline circulation.
  13. Evaluate your hypothesis. Make revisions if necessary, and propose additional experiments.
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