Ocean Currents: Modeling the 'Global Conveyor Belt' in Your Kitchen
AbstractOcean currents have profound effects on the climates of the continents, especially those regions bordering on the ocean. For example, the Gulf Stream (a warm current that goes around the North Atlantic Ocean) is thought to make northwest Europe much warmer than it would otherwise be. Similarly, the California Current is thought to keep Hawaii cooler than other land masses at the same latitude as it. In this ocean science fair project, you will model the behavior of these "rivers" of hot and cold water within the ocean to find out how temperature affects the speed and direction of the currents.
David Whyte, PhD, Science Buddies
Braile, L.W. (2000). Thermal Convection and Viscosity of a Fluid. Retrieved October 15, 2008, from http://web.ics.purdue.edu/~braile/edumod/convect/convect.htm
- Sterno® is a registered trademark of The Sterno Group LLC.
- Pyrex® is a registered trademark of Corning Incorporated.
To make a model of ocean currents and measure how the heat input affects velocity of the currents.
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 is 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. This movement within a fluid created by hotter (and therefore less dense) materials rising, and colder (and therefore denser) materials sinking, is called convection.
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. A simplified version of the global conveyor belt is shown in Figure 1, below. The global conveyor belt moves water slowly, 10 centimeters (cm) per second (sec) at most, so it can take 1,000 years for water from the North Atlantic to find its way into the North Pacific! But the global conveyor belt moves a lot of water — around 100 times the amount of water that is in the Amazon River is transported by this huge, slow circulation pattern.
The global conveyor belt is simplified in this image and shows ocean currents that move in a cycle. As the Sun warms the northeastern section of the Pacific ocean the currents carry the heat over Australia and under Africa until it reaches the northern Atlantic. There the water cools down and travels south past the tip of South America, along the coast of Antartica and back up towards the northern Pacific.
Figure 1. This is a simplified version of the global conveyor belt. (Image credits: US Global Change Research Program, Thomas Splettstoesser)
The water moves mainly because of differences in relative density, which you will explore in this science fair project. In this science project, you will model ocean currents, with particular focus on the role of heat in the currents' direction and velocity.
Terms and Concepts
- Deep-ocean currents
- Global conveyor belt
- Convection cells
- What types of currents are present in the ocean?
- How do ocean currents affect the weather?
- What is the role of ocean currents in distributing nutrients?
- From where does the energy that drives ocean currents come?
Materials and Equipment
- Pyrex® baking dish, 2-qt. or 1.5-qt. in size. See the Procedure for an example.
- Alternatively, other similarly-sized, heat-resistant glass baking ware that can safely withstand the hot temperatures of being placed just above a flame for several minutes could be used instead.
- Thyme, dried (2 tsp.)
- Alternatively, any other dried leaf spice could be used.
- Vegetable oil (about 4 cups)
- Measuring cup
- Ceramic coffee mugs (2). These should be equal in height.
- Small candles or cans of Sterno® (4). These 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
- Paper for sketching
- Thermometer, like the one available from Carolina Biological catalog #745390.
- Adult helper
- Lab notebook
- Graph paper
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- On a surface that is safe to spill some vegetable oil, fill the baking dish about half to three-quarters full with vegetable oil. (Depending on the exact size of baking dish you are using, this may be about 4 cups of vegetable oil or less.)
- Mix the 2 teaspoons of thyme in with the vegetable oil in the baking dish. Stir thoroughly to distribute the flakes of thyme. The flakes of thyme will flow with the liquid, showing the direction and velocity of any fluid flow.
- Place the baking dish on top of the two ceramic mugs.
- Observe the oil and spice mixture. With no heat (energy) being added to the system, there should be little or no movement of the liquid, once it has settled.
- Place a candle underneath the baking dish, directly in the middle. Make sure the mugs still stably support the baking dish.
Light the candle and let the liquid heat up for at least one minute. Your setup should look like the one in Figure 2, below.
- You can also use a can of Sterno instead of a candle. Sterno cans produce more heat than candles do.
- The convection should start soon after the heat is applied.
Figure 2. Your ocean currents model should look similar to this one right after you have lit the candle.
As the oil heats and begins to flow, observe the pattern of fluid flow (circulation) by noting the location of individual flakes of thyme over time. Write down all of your observations in your lab notebook.
- This type of energy movement is called thermal convection, because added heat causes the fluid flow (circulation by convection) by lowering the density of the liquid.
- Be sure to view the model several times during the experiment, both from above the dish and from the side of the dish.
- Draw a sketch of the model in your lab notebook.
- Draw a sketch of the circulation in your lab notebook. Note the shape of the convection cell(s).
- Is the pattern approximately symmetric on the two sides of the heated area? Where do you observe upward flow? Where do you see downward flow? Where do you observe horizontal flow?
Measure the temperature of the oil in different parts of the model.
- What is the biggest temperature difference you can find?
- Record the temperature in your lab notebook.
Measure the horizontal velocity of the convective flow near the surface of the liquid.
- Place the ruler on the top of the container (oriented horizontally and lengthwise) to measure the distance a flake of thyme travels.
- Use a stopwatch to measure the time it takes a single flake to move a certain distance (such as the distance from where it reaches the surface to where it starts to fall).
- Divide the distance by the time to get velocity (in centimeters per second).
- For example, if the flake moves 3 cm in 4 sec, its velocity is .75 cm/sec.
- Make another sketch of the model with arrows showing the direction and velocity of the currents.
- Measure the velocity of the flakes in different parts of the model, some parts right above or next to the flame, and some parts much farther away from the flame. Record your results in your lab notebook, making sure to include how far the flake was from the flame when you started measuring the flake’s velocity, as well as the temperature of that part of the vegetable oil.
- Are all of the measurements approximately the same? Where are the velocities the largest? Where are they the smallest?
- What could explain variations in velocity?
- Are the directions of flow "away from" the heated central area of the container?
- Add another lit candle and repeat steps 11–14 to measure the temperature of the oil and the velocity of the flakes now.
- How does increasing the heat affect the velocity?
- Repeat step 15 two more times so that you add a third and then a fourth candle (if they safely fit beneath the baking dish, between the mugs) and repeat taking your measurements of temperature and velocity. Do your best to get all of the candles between the mugs and evenly heating the oil.
- Make a graph of temperature of the oil versus the velocity of the current.
- Do you see a correlation between the temperature of the oil and the velocity of the current?
- Can you relate your results to the global conveyor belt? (Hint: You may want to try re-reading the Introduction in the Background tab to answer this question.)
Ask an Expert
- Add an "island" made from a can or other object. Sketch the current flow around the island.
- Can you think of a way to add a "polar region" to the model, perhaps using a bag of ice? How does the current change with the polar region in place?
- Caution: Do not add the polar region to a heated setup, as the rapid change in temperature could be dangerous. Instead, add the polar region to a room-temperature setup (i.e., room-temperature vegetable oil prepared with thyme flakes in the dish) and then heat up the setup using the candles as described in the Procedure.
- Try a larger heat-resistant glass dish, like a lasagna pan. Place the candles in different regions and sketch the current flow.
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