Key Concepts
Ocean currents, fluids, density, salinity, movement, heat

Introduction

Have you ever wondered what causes ocean currents? Ocean currents have profound effects on the climates of the continents, especially those regions bordering on the ocean. For example, it’s thought that the Gulf Stream (a warm current around the North Atlantic Ocean) makes northwest Europe much warmer than it would otherwise likely be. In this science activity, you will model the behavior of these “rivers” of hot and cold water within the ocean to find out how temperature affects the direction and speed of the currents.

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

Ocean currents profoundly affect the weather, marine transportation and the cycling of nutrients. Deep-ocean currents are driven by differences in the water’s density. The water’s density is controlled by temperature (cold water is denser than warm water) and salinity (salty water is denser than fresh water). 

How does the varying density of the ocean’s waters create the global currents? To understand the deep-ocean currents, it’s easiest to look first at Earth’s polar regions. Water flowing into the polar regions becomes cold, which increases its density. As ice is formed when the water freezes, freshwater is removed from the ocean (it has turned into ice), making the ocean water saltier. The cold water is now denser, due to the added salts, thus it sinks toward the ocean bottom. Surface water then moves in to replace the sinking water, thus creating a current. 

A global “conveyor belt” is set in motion when deep water forms in the North Atlantic, sinks, moves south, circulates around Antarctica and then moves northward to the Indian, Pacific and Atlantic basins. This conveyor belt moves lots of water – around 100 times the amount of water that’s in the Amazon River is transported by this huge circulation pattern!

Materials

  • Pyrex baking dish, approximately two quarts in size
  • Dried thyme or other dried leaf spices
  • Teaspoon measuring spoon
  • Measuring cup
  • Three cups of vegetable oil
  • Two ceramic coffee mugs, equal in height
  • Small candle. This should be much shorter than the coffee mugs so that the flame does not get too close to the bottom of the glass baking dish.
  • Lighter or matches

Preparation

  1. On a surface that is safe to spill some vegetable oil, fill the Pyrex baking dish about half full with vegetable oil. (Depending on the exact size of baking dish you are using, this may be a little more or less than three cups of vegetable oil).
  2. Mix two teaspoons of dried thyme (or other dried leaf spices) in with the vegetable oil in the baking dish. Stir thoroughly to distribute the flakes of thyme. When you are done stirring the flakes, what do they do in the vegetable oil?
  3. Place the baking dish on top of the two ceramic mugs.
  4. Place a candle underneath the baking dish, directly in the middle (between the two mugs). Make sure the mugs still stably support the baking dish.

Procedure

  1. Have an adult help light the candle and place it back underneath the baking dish, between the two mugs.
  2. Watch the vegetable oil in the baking dish for at least one minute. The flakes of thyme will flow with the liquid, showing the direction and speed of any fluid flow. What is the pattern of fluid flow that you see? (If you do not see any movement yet, keep watching for a couple more minutes until you do.)
  3. Watch the vegetable oil and flakes of thyme in the baking dish from the top. How are the thyme flakes moving when viewed from the top? Where are they moving fastest, and where are they slowest? Is the fluid flow pattern symmetric?
  4. Watch the vegetable oil and flakes of thyme in the baking dish from the side. Where do you see upward flow? Where do you see downward flow? Is the fluid flow pattern symmetric? 
  5. Overall, how does it look like the fluid is flowing right around the candle, and then further away from it? How do you think your observations are related to what happens with the global conveyor belt of currents in the oceans?

Extra: In this science activity you should have seen the thyme flakes moving faster in some areas compared to others. You can quantify the speed of the flakes by using a ruler and stopwatch and timing how long it takes one specific flake to move from one point to another. Just how much faster are some flakes traveling than others? How does their distance from the flame affect their speed?

Extra: If there is enough space, try adding one more candle (or more) to the area beneath the baking dish, next to the first candle. How does adding more candles change the fluid flow in the dish?

Extra: Add an “island” made from a can or other object to your model. How does adding an island change the fluid flow?

Observations and Results

Did you see the dried thyme leaves move quickly upward when above the flame, then travel horizontally out to the side and slowly fall back down?

Because hotter fluid is less dense than colder fluid, in this activity the fluid just above the flame should be less dense than fluid farther away from the flame. Consequently, fluid just above the flame should move upward. When it reaches the surface, it should then move horizontally outward (in all directions, symmetrically). As it becomes farther away from the flame and cools, the fluid moves slower and sinks back down. At the same time, while the fluid just above the flame moves upward, nearby fluid at the bottom of the dish moves inward, toward the flame, to replace the rising fluid, and thus a current is created. This is the pattern you should have observed in this activity by watching the movement of the dried thyme leaves. 

This movement within a fluid created by less dense matter rising and denser matter sinking is called convection, and is the basis for the global “conveyor belt” of ocean currents. While lots of water around the world is moved this way, the global conveyor belt moves water slowly, at ten centimeters per second at most, so it can take about 1,000 years for water from the North Atlantic to find its way into the North Pacific!

Cleanup

  1. If you made any oily messes, try cleaning them up with warm water and soap. The vegetable oil and dried thyme leaves mixture made in this activity can be composted or disposed of in the trash (but should not be poured down a drain).

More to Explore

Credits

Teisha Rowland, PhD, Science Buddies

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Key Concepts
Ocean currents, fluids, density, salinity, movement, heat
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