Key concepts
Ocean currents
Fluids
Density
Salinity
Movement
Heat
 
Introduction
Under the waves the world’s oceans have currents that can move massive amounts of warmer or cooler water from off the coast of one continent to that of another. These currents have profound effects on the continental climates, especially those regions bordering on the ocean. For example, the Gulf Stream (a warm current that flows up from the Gulf of Mexico into the North Atlantic Ocean) likely makes northwestern Europe much warmer than it would otherwise likely be. Have you ever wondered what causes these important currents? In this science activity you will model the behavior of these “rivers” of warm and cold water within the ocean to find out how temperature affects the direction and speed of the currents.
 
Background
Ocean currents profoundly affect weather and climate, marine transportation and the cycling of nutrients. Deep-ocean currents are driven by differences in the water's density, which is controlled by temperature (cold water is denser than warm water) and salinity (salty water is denser than freshwater).
 
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 these regions becomes cold, which increases its density. As ice forms 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 its lower temperature and the additional salt, so it sinks toward the ocean bottom. Surface water then moves in to replace the sinking water, creating a current.
 
A global “conveyor belt” is set in motion when dense 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—it is a huge circulation pattern that transports about 100 times more water than the Amazon River!
 
Although ocean currents depend on temperature and salinity to move, in this activity you will see how temperature alone can create currents in a liquid.
 
Materials

  • Pyrex baking dish, approximately two quarts in size
  • Dried thyme or other dried leaf spices
  • Teaspoon measuring spoon
  • Measuring cup
  • About three cups of vegetable oil
  • Spoon or whisk for stirring
  • Two sturdy ceramic coffee mugs, equal in height
  • Small candle (This should be much shorter than the coffee mugs.)
  • Lighter or matches (Have an adult assist handling the lit candle and lighter or matches.)
  • Surface that can handle oily spills, such as the kitchen counter—or towel to protect surface
 
Preparation
  • On a surface that can handle 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 oil).
  • 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 oil?
  • Place the baking dish on top of the two ceramic mugs.
  • Place a candle underneath the baking dish, directly in the middle (between the two mugs). Before you light the candle, make sure the mugs stably support the baking dish and that the candle, when lit, will not be too close to the bottom of the dish.
 
Procedure
  • Have an adult help light the candle and place it back underneath the baking dish, between the two mugs.
  • 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.)
  • 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?
  • 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?
  • Overall, how does it look like the fluid is flowing right around the candle, and then farther 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 with 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: Being careful to not upset the balance of your baking dish, 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 warmer fluid is less dense than cooler fluid, in this activity the fluid heated just above the flame should be less dense than that farther away from the flame. Consequently, the less-dense 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 flows farther away from the flame and cools, the now-denser fluid slows 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. Although 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! But effects from changes in the conveyor belt's circulation can be felt much more rapidly.
 
Cleanup
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
Thermal Convection and Viscosity of a Fluid, from L. W. Braile at Purdue University
Ocean Currents: Modeling the “Global Conveyor Belt” in Your Kitchen, from Science Buddies
Currents: The Global Conveyor Belt, from the National Oceanic and Atmospheric Administration
Science Activities for All Ages!, from Science Buddies
 

This activity brought to you in partnership with Science Buddies

Science Buddies