The Causes of Ocean Circulation

Transcription

1 Activity 1 The Causes of Ocean Circulation Goals In this activity you will: Understand the effect of the wind on the movement of water at the ocean surface. Understand the effect of the Coriolis force on the movement of objects or materials when they are observed in a rotating system. Learn why the ocean is layered by temperature and density. Describe the meaning of pycnocline and thermocline in your own words. Think about It Ocean waters are constantly on the move. How they move influences climate and living conditions for plants and animals, even on land. Why does water move in the oceans? What do you think? Record your ideas about this question in your notebook. Be prepared to discuss your ideas with your small group and the class. F 4

2 Activity 1 The Causes of Ocean Circulation Investigate Part A: Wind on the Ocean Surface 1. Prior to starting the investigation, predict the effect that wind speed has on water circulation. a) Record your hypothesis in your notebook. 2. Place a fan on a tabletop. Set a pan on the tabletop so that its rim is level with the lower part of the fan, as shown in the diagram. Fill the pan almost full of water. Position the fan so that it blows across the water surface in the pan. 3. Turn the fan on the lowest speed, and let it blow across the water surface for a minute or two, long enough for the resulting water motion to settle into a steady state. 4. Put one drop of food coloring onto the water surface near the upwind edge of the pan. 5. Using a stopwatch, time how long it takes the mass of colored water to move from the edge of the pan to the center. Ignore the little waves generated by the wind on the water surface. a) Measure and record the distance traveled, and the time taken. b) Calculate the speed of the water by dividing the distance traveled by the time of travel. Record the speed in your notebook. c) In your notebook, describe the pattern of water motion in the pan that is revealed by the travel of the colored water. You can repeat the experiment by waiting until the coloring is uniformly mixed in the water, and then adding another drop of colored water at the surface. 6. Repeat Steps 3 5 for the medium speed of the fan, and then for the highest speed. a) Summarize your results. b) How do your results compare to the hypothesis that you developed at the start of the investigation? Explain. wind Be sure that the fan has an adequate guard around the blade so that fingers cannot be put near the blade. Do not put any items into the guard. Keep water away from the fan, especially the motor housing. Wipe up any spills immediately. Wear eye protection. Wash your hands when you are done. F 5

3 Part B: The Coriolis Effect 1. Read Steps 2 5 below. Before you do the investigation, predict the path that the pen will trace. a) Draw two rectangles on a sheet of paper. Indicate the direction of the posterboard rotation with an arrow. Record your predictions by drawing them in the appropriate rectangle. b) Explain your drawings. Why do you think that the pen will take the path that you predict? 2. At the chalkboard or a blank wall, one student holds a piece of posterboard against the wall with one thumb. The thumb should be pressed tightly enough against the center of the posterboard to prevent it from slipping down, but not so tightly that the posterboard cannot be rotated. 3. A second student stands next to one edge of the posterboard and rotates it slowly, and as nearly as possible at a constant speed, with a hand-overhand motion. Try for a rotation rate of about one full turn in five seconds. 4. A third student stands next to the other edge of the posterboard. With a marker pen, he or she slowly draws a straight line on the posterboard while it is rotating. Important: the trick is to try to ignore the motion of the posterboard, and move the pen downward in a straight line relative to the wall or chalkboard beneath. It would be good to practice with the tip of a finger before using the marker pen. a) In your notebook record the shape of the line and the direction of rotation. 5. Turn the posterboard over, and repeat Steps 2 4 while rotating the posterboard in the opposite sense. a) Record in your notebook the shape of the line and the direction of rotation. b) How do your results compare with your predictions? Explain. c) How do you think the shape of the line would vary with: the speed of rotation of the posterboard? the speed of travel of the pen point? d) Which sense of rotation of the posterboard corresponds to the Earth s surface in the Northern Hemisphere, and which sense corresponds to the Earth s surface in the Southern Hemisphere? e) If time permits, test your ideas from 5(c). F 6

4 Activity 1 The Causes of Ocean Circulation Reflecting on the Activity and the Challenge You observed that a wind blowing across a water surface creates a surface current that moves in the direction of the wind. You also observed that the wind-driven current affects only the surface layer. In the second part of the activity, you found that it was difficult to draw a straight line on a rotating sheet of posterboard. The Coriolis effect makes for some very strange phenomena on the Earth s surface, including ocean currents. Knowing this, would you feel comfortable predicting ocean circulation patterns just from the results of your investigations? Digging Deeper THE NATURE OF THE OCEANS Oceans of the World You are probably familiar with three large oceans on Earth: the Pacific Ocean, the Atlantic Ocean, and the Indian Ocean. Each of these three oceans is constricted at its northern end but wide open at its southern end, as shown in Figure 1. Each is connected at its southern end to a fourth major ocean, called the Southern Ocean or the Antarctic Ocean.The Southern Ocean is different from the other three because it is continuous all around the Earth instead of being bordered on the east and west by large continents. The open ocean is mostly very deep: the average deep ocean depth is four or five kilometers.the bottom of the deep ocean is a place of inky blackness, eternal cold, and water pressures that are unimaginably large. In another sense, however, even the deep ocean is shallow relative to the surface area covered by the oceans. Suppose that an ocean like the Atlantic or Pacific were the size of a football field or a soccer field.you would be wading around in an ocean that is no deeper than your ankles! Keep that fact in mind as you work through the activities in this chapter. The Warm Ocean and the Cold Ocean The ocean is layered by temperature.there is an upper layer of warm water and a much thicker deep layer of cold water.at low latitudes, the change between these two layers is at a depth of a few hundred meters.above that F 7

5 ATLANTIC OCEAN PACIFIC OCEAN PACIFIC OCEAN INDIAN OCEAN ANTARCTIC OCEAN ANTARCTIC OCEAN Figure 1 The Southern or the Antarctic Ocean is continuous all around the Earth. Geo Words thermocline: the zone of rapid change from warm water to cold water with increasing depth in the ocean. pycnocline: a layer of water in the ocean characterized by a rapid change of density with depth. depth the oceans are as warm as 25 C over large areas at low latitudes. Below that depth, the ocean is much colder. In the lowest parts of the ocean, the temperature is not much above 0 C, even at the Equator! Between about 200 m and 1000 m below the surface, the temperature of the water decreases sharply with depth.this zone of rapid change from warm water to cold water with increasing depth is called the thermocline. It is a larger-scale version of the difference in temperature that you may have felt when treading water in a lake.water around the upper part of your body might feel warm, while the water near your feet feels cooler. Because cold water is more dense than warm water, this transition zone is also called the pycnocline (pycno- means density).the reason that the pycnocline is also the thermocline is that the density of seawater depends mainly on the temperature of the water. F 8

6 Activity 1 The Causes of Ocean Circulation temperature ( C) surface layer 1000 tropics upper water Equator thermocline depth (meters) 2000 high latitude deep water Figure 2 Sample temperature depth profiles for waters at the Equator, tropics, and high latitudes (poles). density (g/cm 3 ) surface layer 1000 upper water Equator tropics high latitude pycnocline depth (meters) deep water Figure 3 Sample density depth profiles for water at the Equator, tropics, and high latitudes (poles). F 9

7 The Circulation of the Oceans Geo Words oceanographers: scientists who study the Earth s oceans. wind stress: the friction force exerted on the ocean surface by the wind. The warm, shallow part of the ocean, above the thermocline, is in a state of continuous movement, or circulation.the cold, deep part of the ocean is also in a state of continuous circulation.as you will see in later activities, the nature of these two circulations is very different. Oceanographers (scientists who study the Earth s oceans) have been describing the circulation of the ocean, and trying to develop theories to account for it, for over a hundred years.the circulation of the oceans is a very complicated problem, however, and there is still much to be learned. It is a major area of research in oceanography. The Wind Stress on the Surface of the Ocean The circulation of the upper layer of the ocean is caused mainly by the friction force exerted on the ocean surface by the wind.this friction force is called the wind stress.you saw in Part A of the investigation that a wind blowing across a water surface sets the water in motion.this should make good sense to you, because any material, whether it s a gas, a liquid, or a solid, exerts a friction force on some other mass of material when it moves, slides, or flows across that other material. Hold your hand horizontally in front of your mouth, palm down, and blow across it.you can feel the friction force of the wind on your hand. The wind blowing across a water surface also makes waves on the water surface.water waves are a very important feature of the ocean surface, partly because when they grow to be large they are a hazard to ships, and also because when they reach a shoreline they can cause coastal erosion.the presence of waves on a water surface allows the wind to exert a larger friction force on the surface, because the wind can push against the slowly moving waves. Figure 4 Surface winds on the ocean can produce waves that are hazardous to boats. F 10

8 Activity 1 The Causes of Ocean Circulation The Coriolis Effect One of the basic principles of physics is that a mass of material moves in a straight line at constant speed unless it is acted upon by some force.when water in the ocean is set into motion, it tends to move in a straight line. What makes this seemingly simple idea very complicated, however, is that the Earth is rotating under the water while it s moving in a straight line! You, the observers of the water movement, are rotating with the Earth, so it looks to you as though the water is moving in a curved path.this is what you saw in Part B of the investigation.the pen point was moving in a straight line, but to an ant rotating with the posterboard, it seemed to be moving along a curved path. It takes a force to make a mass of material move in a curved path instead of a straight line.the force that seems to make the water or the pen point move in a curved path is called the Coriolis force. It s not really a force, but to someone rotating with the Earth or the posterboard, it seems like a force. Scientists use the word counterintuitive for things like this: your intuition tells you one thing, but the real-world situation shows you something very different.the overall circulation of the oceans depends in a very fundamental way on the Coriolis effect.you will see this in more detail in later activities in this chapter. Understanding and Applying What You Have Learned 1. Based on the observations you made in Part A of the investigation, do you think that waves always travel in the same direction as currents? Explain. 2. How would you expect waves to change over time when: a) wind speed increases? b) wind blows over a part of the ocean for a long time? 3. Refer to Figures 2 and 3 (thermocline and pycnocline) to answer the following questions: a) What is the temperature of ocean surface water at the Equator, in the tropics, and at high latitudes? b) In what part of the ocean is the thermocline best developed? Least developed? c) In what part of the ocean is the pycnocline best developed? Least developed? d) Use your understanding of the relationship between temperature and density in the ocean to explain your answers to questions 3 (b) and (c). Geo Words Coriolis force: the apparent force caused by the Earth s rotation which serves to deflect a moving body on the surface of the Earth to the right in the Northern Hemisphere and to the left in the Southern. Check Your Understanding 1. Examine Figures 2 and 3 on page F9. Compare and contrast the thermocline and the pycnocline. How are they similar? How are they different? 2. How does the wind cause surface currents in the ocean? 3. Why are waves an important feature of the ocean surface? 4. Describe the Coriolis effect in your own words. F 11

9 Preparing for the Chapter Challenge Use the information you have gained from this activity to write an introduction that explains the forces that drive ocean circulation. Think about the ultimate energy sources for these forces and try to think of how a change in these sources might affect ocean circulation. Inquiring Further 1. Careers in oceanography Would you like to study the oceans for a career? To learn more about the different fields of oceanography and what a professional oceanographer does at work, visit the web site. 2. Harnessing wave power Energy produced by moving water can be used to generate electricity, as at hydroelectric power plants located along rivers. The energy created by ocean waves exceeds that produced by rivers, but difficulties lie in efficiently harnessing this energy. Research the benefits and difficulties associated with generating electricity using the energy of ocean waves. Where is the potential for using ocean wind-power systems the greatest? To get started, visit the web site. 3. Scale diagram of the ocean floor You were told in the Digging Deeper portion of this activity that oceans are shallow in comparison to their width. Investigate this claim by making a 1:1 scale profile of water depth across an ocean. A 1:1 scale profile is a cross section in which the horizontal scale is equal to the vertical scale. You will need a world map that indicates ocean bathymetry (water depth), a metric ruler, and graph paper. You may need to tape sheets of graph paper together. F 12