The Great Ocean Conveyor Belt - Teacher
This experiment is simple enough that we trust you'll do this with your students. We have included a series of images on this page (along with a movie) showing the results that we got when we tried this in our lab.
Explain
North Atlantic Deep Water (NADW to oceanographers) flows south along the bottom of the North Atlantic, eventually spreading into most of the world's oceans. The densest water in the world is actually created in very salty semi-enclosed seas, such as the Red Sea. But the densest water in the open ocean is cold salty water that can be found at the bottom of the ocean.
Looking at a map, there are only a few places in the world's oceans that such water could possibly be created. It takes a source of warm salty water delivered into polar regions where the atmosphere is very cold. The Antarctic is totally surrounded by a circulating ocean which prevents salty surface water from reaching southern-most latitudes near the continent. The Kurishio current off of Japan delivers warm salty water northward, but the Bering Strait prevents most of it from reaching the Arctic Ocean. Thus the best place in the open ocean to make large volumes of dense water is the Norwegian Sea.
Our demonstration is not exactly what occurs in the real world, but it is closely related. The NADW is not created from melting glaciers, but instead from the cooling of surface waters by the frigid atmosphere.
It is interesting to note that the creation of bottom water is not continuous, but is episodic in nature. Most bottom water is created during years with intensely cold winters.
Oceanographers track deep waters by measuring the characteristics of the water, most typically temperature and salinity. Salinity is a conservative property in the absence of evaporation or fresh water inputs. Thus salinity can be used to track waters, although the weak mixing that occurs in the deep oceans still must be taken into account.
In the questions at the end of the activity, we ask about the final state of the tank, after the ice cube melts. The flow from the ice cube moves extremely slow, as shown in the pictures below. The flow is slow enough that it does not trigger the onset of turbulence. Such flows are called laminar flows. The interesting point of all this is that this is a very effective way of delivering cold water to the bottom of a tank filled with warm water. If you try to just pour the cold water into the tank (or try to pour warm water on top of cold) the water will invariably mix because of turbulence. Dye some water and try it! All this shows that conduction of heat is not a very strong process in water unless there is turbulence present to mix the water. The same thing is true for the transfer of salt and even momentum in the ocean. These properties are not exchanged easily unless the flows become turbulent. Another way to say this is that turbulent flows are effective at mixing properties of water.
Extend
Upwelling
The ocean is layered with less dense water lying on top of denser waters. When the organisms living in the warmer top layers die, their remains fall down to lower layers of the ocean. Their remains contain nutrients that can be used by other living things. This "rain" of nutrients enriches the deep ocean waters. Deep ocean circulation moves this enriched layer around the ocean basins.
In some places the cold nutrient-rich water rises to the surface. This is called upwelling. The Earth's rotation and strong seasonal winds push surface water away from some coasts, so deeper water rises on the edges of continents to replace it. Marine life thrives in these nutrient-rich waters. Upwelling supports about half of the world's fisheries, although these cool waters account for only 10 percent of the surface area of the global ocean.
http://www.pmel.noaa.gov/toga-tao/el-nino/
Demonstration Results
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The cube starts to melt.
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The cold melt water continues to sink off the ice cube, and the beginning of bottom flow is visible creeping across the tank. |
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Note that the flow falling from the ice cube is extremely smooth and without turbulence. This is called laminar flow, which occurs when a flow is slow enough to not spontaneously break into turbulence and unperturbed by external forces. Another good example of laminar flow is a candle's flame in an extremely still room, which looks like an upside down version of the ice cube's laminar flow. |
The process continues.
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Note that if the water is not allowed to settle, or the ice cube is thrown into the container, then the flows will be turbulent and lots of mixing will occur. This can mask the bottom flow, which is the point of the demonstration. |
You can download an MPEG movie which shows the results of our Conveyor Belt experiment. You will need an MPEG player application to see this movie. The movie is 1.3 Mb in size, so it will likely take a few minutes to download at 56 kbaud.
Download results movie (1.3 megabytes)
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Return to Ocean conveyor belt activity page |
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