Ocean Currents and Global Climate

Transcription

1 7 LESSON Ocean Currents and Global Climate In the second half of the 18th century, American statesman and scientist Benjamin Franklin made some of the earliest studies of ocean surface currents. NOAA/DEPARTMENT OF COMMERCE INTRODUCTION Using temperature sensors and buoys that float at fixed depths, scientists have found that water in the oceans is on the move, even in the deepest trenches. The sun powers deep ocean currents. Wind guides surface currents. Ocean currents are essential to life in the sea. They dissolve oxygen from the atmosphere and bring minerals up to the surface, where tiny floating plants can use them. And like the air in the earth s atmosphere, ocean currents help move heat around the globe. You now know that temperature differences on the earth occur because the sun heats the earth unevenly. The sun is a major force that causes convection currents in the air and currents in the oceans. Temperature and wind are major forces that drive ocean currents. Other factors also affect ocean currents. These include the earth s rotation and the water s density. For example, salt dissolved in ocean water will cause the water to be heavy and to sink. In this lesson, you will investigate the effects of temperature and wind on ocean currents. You will also investigate how these currents affect the climate throughout the entire world. OBJECTIVES FOR THIS LESSON Analyze why the temperatures at the equator and poles differ. Investigate the effect of water temperature on the way water moves. Investigate the effect of wind on surface currents. Locate some of the major ocean currents on a map and a globe. 80 STC/MS C ATASTROPHIC E VENTS Analyze the effects of ocean currents on global climate.

2 Getting Started 1. Discuss the results from your homework, Student Sheet 6.2: Tracking Hurricane Andrew, with your teacher. Review your answers to the following questions, which are also listed on your student sheet: MATERIALS FOR GETTING STARTED For you 1 completed copy of Student Sheet 6.2: Tracking Hurricane Andrew A. Where did Tropical Storm Andrew start? Why do you think it started there? B. At what point (longitude and latitude) did the tropical storm become a hurricane? C. In what direction did the storm move? D. Look back to the reading selection Why Does the Wind Blow? in Lesson 5. What do you think caused Hurricane Andrew to move along this path? E. Where did Hurricane Andrew lose its energy and turn back into a tropical storm? Why do you think it happened in that location? F. If you had been working at the National Hurricane Center when Hurricane Andrew struck, which cities or areas would you have evacuated? What day would you have requested the evacuation? Why? STC/MS C ATASTROPHIC E VENTS 81

3 2. 3. Look at the class globe and the clamp lamp. Make a prediction: How do you think the light from the lamp will affect the temperatures of different parts of the globe? Discuss your predictions with the class. Then watch as one student records the starting temperatures. While the light shines on the globe for a few minutes, look at Figure 7.1. In a class discussion, answer the following questions: A. Examine the equator and the poles. How are they alike? How are they different? B. How do you think these factors affect the temperature of air and water at the equator? How do you think they affect temperatures at the poles? One student will go to the class globe to record the final temperatures at the equator, middle latitudes, and poles. What do the results tell you about global heating of the earth? On the basis of this activity, how do you think the sun s energy causes convection currents in both the atmosphere and the oceans on the earth? Brainstorm what you know and want to know about ocean currents. Then answer these questions: A. You know what causes convection currents in the air. What do you think causes currents in the ocean? B. How do you think ocean currents affect global weather? (B) Light to the pole North Pole Middle Latitudes Equator (A) Light to the equator Figure 7.1 Modeling uneven global heating 82 STC/MS C ATASTROPHIC E VENTS

4 Inquiry 7.1 Investigating the Effect of Temperature on Ocean Currents PROCEDURE Read Ocean Currents, on page 84. How would you define a current? In this lab, you will investigate the effect of temperature on how water moves, and you will apply your observations to ocean currents. Collect one copy of Student Sheet 7.1a: Investigating the Effect of Temperature on Ocean Currents. Read the question in the first box: How does the temperature of water affect the way water moves? Look at one set of materials. Share with your group and the class any ideas you might have for using the materials to investigate the question. Then discuss Procedure Steps 7 through 14. (You may have to complete all but the last box of Student Sheet 7.1a for homework.) Look at Figure 7.2 and at the tote tray of water your teacher has set up for this inquiry. This is a model of a deep ocean current. The crystals will show how the MATERIALS FOR INQUIRY 7.1 For you 1 copy of Student Sheet 7.1a: Investigating the Effect of Temperature on Ocean Currents 1 copy of Student Sheet 7.1b: Storms Review 1 pair of safety goggles For your group 1 tote tray 2 metal digital thermometers 2 beakers of roomtemperature water 1 dropper bottle of chilled water (with purple dye) 1 dropper bottle of heated water (with purple dye) 2 index cards 1 flexible straw Cold water and ice Potassium permanganate crystals Hot water Figure 7.2 Deep ocean current model. Do not disturb the water. View the model from the sides. STC/MS C ATASTROPHIC E VENTS 83

5 OCEAN CURRENTS The waters of the ocean move in streams called currents. A current results when a fluid, either gas or a liquid, moves in a definite direction. Ocean currents form in response to many factors: heat from the sun, wind, salinity (saltiness), land masses acting as barriers, and the rotation of the earth. Some currents are strong enough to affect the speed and direction of ships. Others bring drastic weather changes to faraway lands. In the 18th century, Benjamin Franklin made some of the earliest studies of ocean currents. From first-hand experience, he knew that ships crossing the North Atlantic from America were either helped or hindered by a current flowing in a northeasterly direction toward Europe. Franklin investigated the temperature of the current. He found that it was warmer than the water around it. Instead of mixing with the surrounding water, the current moved along like a river within the ocean. The boundary between the warm and the cold water was sharp. Franklin discovered how sailors could use thermometers to guide their ships into the current or out of it. Why do you think the warm water moved in a northeasterly direction? What might have caused the sharp boundary between the warm water and cold water? Think back to earlier lessons on how air moves. Try to apply some of the things you have learned about air currents to what you will learn in this lesson about ocean currents water in the pan moves. Make some predictions: How do you think the crystals will move near the cold water? How do you think they will move near the hot water? If you haven t already, complete all boxes on the student sheet except the last one. You will describe the materials and procedures you will use, how you will control all variables except the one you are testing, and what you will look out for and measure. As you work, your teacher may select one or more groups to observe the movement of the crystals in the tote tray of water. Review the Safety Tips on page 85 with your class. Pick up your materials. Use the metal digital thermometers to measure the temperature of the water in both beakers. Record these data on Student Sheet 7.1a. To make this a fair test, the temperatures should be about the same. Collect one hot dropper bottle and one cold dropper bottle from the water baths. Record the temperature of each bottle s solution. 84 STC/MS C ATASTROPHIC E VENTS

6 Set up your investigation. As an option, you can place the cold bottle in a beaker of ice water and the hot bottle in a beaker of hot water, as shown in Figure 7.3. Label one index card Hot and the other one Cold. Have a group member hold the index cards behind the beakers. This will make it easier to observe the results. Fill the dropper from each bottle, one cold and one hot. Have one group member slowly release 10 drops of water, 1 drop at a time, from each dropper, into its beaker of water. Drop cold purple solution into the beaker labeled Cold and hot purple solution into the beaker labeled Hot. To ensure a fair test, have the same person do both droppers because he or she will squeeze the droppers with the same force and at the same time. What do you observe? Discuss your observations with your group. In the last box of your student sheet, record what happened in the beakers. Remember to explain why you think this happened and give evidence to support your answer. SAFETY TIPS Be careful when you are handling the hot dropper bottle. The water bath and the water in the bottle are hot. Wear your safety goggles when you are filling or emptying the droppers. Do not get the purple solution on your skin or clothing; it will cause stains. Only one member of your group should use the straw. Do not share it. Cold dropper bottle Hot dropper bottle Figure 7.3 Setting up the temperature lab STC/MS C ATASTROPHIC E VENTS 85

7 Have a member of your group use the straw to gently blow across both beakers of water, as shown in Figure 7.4. The other members should watch the purple solution very carefully. What happened to the solution when air blew across the water s surface? Discuss your observations. Clean up by following these steps: A. Prepare for the next class by setting up the materials exactly as they were when you started. C. Pour the water in the beakers into a sink (or bucket). D. Fill the beakers with 200 ml of clean room temperature water. E. Place a thermometer next to each beaker. Turn off the thermometers. F. Throw away the straw. Set out a new one for the next group. B. Return the hot and cold dropper bottles to their water baths. Figure 7.4 Blow across the water s surface using a straw. 86 STC/MS C ATASTROPHIC E VENTS

8 REFLECTING ON WHAT YOU VE DONE Answer the following questions. Then talk with your class about how temperature affects the density and movement of water. A. How does temperature affect the way water moves? Which is more dense: hot water or cold water? What evidence do you have to support your answer? B. Think of your globe and how light reached it. How might the uneven heating of the earth cause ocean currents? C. What happened to the cold water (with purple dye) when you blew across the beaker? Why do you think this happened? Read How Trade Winds Cause Upwelling. Analyze your observations of the tote tray of water. How do you think it models deep ocean currents on the earth? Pick up one copy of Student Sheet 7.1b. You will use this to prepare for the Storms assessment in Lesson 8. HOW TRADE WINDS CAUSE UPWELLING When you blew across the surface of the water in the beaker in Inquiry 7.1, your breath pushed the surface water to one side. The cold, purple water at the bottom rose to the top. This is how the strong winds that blow near the equator work. These winds, known as trade winds, usually blow from east to west across the tropical Pacific Ocean. Just as your breath blew water to one side in the experiment, trade winds push warmer water toward the west, where it accumulates around Indonesia. Then cold water rises from deep areas of the ocean along North and South America. The rising water brings with it rich nutrients from the ocean bottom that feed fish and other marine life. This rising of cold, deep water is known as upwelling. Surface Wind water movement Movement of cold, deep, nutrient-rich water How surface winds cause upwelling Sometimes the trade winds weaken, and an El Niño warming takes place. El Niño is an unusually warm flow of surface water. During an El Niño warming, there is no wind to push the warm water westward, and the cold water near the Americas cannot rise to the surface. The nutrients from the bottom of the ocean stay at the bottom. Sea organisms that depend on those nutrients die. STC/MS C ATASTROPHIC E VENTS 87

9 MATERIALS FOR INQUIRY 7.2 For your group 1 tote tray 1 petri dish base 1 small cup, with lid, containing talcum powder 1 beaker containing room-temperature water 4 flexible straws 5 removable yellow dots 2 erasable transparency markers (blue and red) Catastrophic Events World Map Inquiry 7.2 Investigating Surface Currents PROCEDURE Read Surface Currents, on page 89. You will investigate how wind creates surface currents. Then you will apply your observations to ocean currents on the earth. Consider the following question: How do winds affect ocean waters? Discuss your ideas with the class. Review Procedure Steps 4 through 8 with your teacher. With your teacher s permission, adapt the procedures to match your group s ideas as needed. Then pick up your materials Pour water into your petri dish until it is half full. Use the tip of your finger to place a pinch of talcum powder on the surface of the water in your petri dish. Use only a pinch. More will interfere with your results. 88 STC/MS C ATASTROPHIC E VENTS

10 SURFACE CURRENTS The atmosphere and oceans on the earth are closely linked. Wind blowing over the oceans produces surface ocean currents. These currents, along with the wind, transfer heat from tropical areas near the equator, where there is a lot of heat energy, to polar regions, where it is colder. Because ocean currents move heat from one region to another, they have a strong effect on global climate. Flowing northward along the East Coast of the United States is a tremendous warm-water current called the Gulf Stream. The Gulf Stream carries huge amounts of warm tropical water into areas farther north. Winds steer the Gulf Stream away from the coast of North America and move it eastward toward Europe. Gradually, the Gulf Stream widens and slows as it merges with the broader North Atlantic Drift (see the illustration), another ocean current. The Gulf Stream then divides into two currents as it approaches Europe. Meanwhile, the Labrador Current brings cold water southward along the Atlantic coast of North America, as far south as Massachusetts in summer and North Carolina in winter. Where the Labrador and Gulf Stream currents flow side by side, there is a sharp temperature change between them. When warm Gulf Stream air blows over the cold Labrador Current water, fog forms. In this inquiry, you will discover where each of these currents is on the world map. Surface ocean currents have a major effect on global climate. Warm currents carry heat from the tropics and spread it to cooler areas. Without the North Atlantic Drift, the sea around Scandinavia and northern Britain would freeze in the winter. Cold currents have the opposite effect. As they flow from the poles to the equator, they lower the temperature of regions that are typically warmer. Greenland Westerlies North America North Atlantic Drift Europe Gulf Stream Atlantic Ocean Africa South America Trade winds Surface currents, such as the North Atlantic Drift, affect global weather. Equator STC/MS C ATASTROPHIC E VENTS 89

11 6. 7. Use a straw to blow gently across the water s surface. (See Figure 7.5.) Ask other members of your group to watch the water carefully. Using a new straw each time, allow each group member to try this. What do you observe? Discuss your observations with your group. Clean up by doing the following: Put the lid on the talcum powder. Do not throw it away. Pour out the water in the petri dish and wipe the dish dry. Do not throw away the beaker of water. The next class will use it. Throw away your straws. Replace them with four new straws. Return all equipment to the tote tray for the next class. 8. Take out your group Catastrophic Events World Map. Then do the following: A. Read the descriptions in Table 7.1. Each row describes an ocean surface current caused by global winds in a certain region of the world. B. Using the transparency markers, draw each current on your group map. Use a red pen for warm currents and a blue pen for cold ones. C. Label the currents with the appropriate letter (A, B, C, D, or E) from the table by following these steps: Write the letter on a yellow dot. Stick the dot on the map. Use a political map in your classroom, if needed, to help you locate the regions. Figure 7.5 Modeling surface currents 90 STC/MS C ATASTROPHIC E VENTS

12 Table 7.1 Some Surface Currents on the Earth Nearest Landmass Temperature Current Hemisphere or Ocean Direction of Flow of Water North American A. Gulf Stream Northern East Coast North; clockwise Warm B. Humboldt Current South American (or Peru Current) Southern West Coast North; counterclockwise Cold C. Antarctic Circumpolar Current Southern Antarctic Ocean West to east; straight Cold D. California Current Northern U.S. West Coast South; clockwise Cold Greenland and South; meets E. Labrador Current Northern Eastern Canada Gulf Stream Cold REFLECTING ON WHAT YOU VE DONE 1. Answer the following questions, and then discuss with your group what this inquiry tells about surface currents. A. What happened to the water when you blew across it? B. How did the direction of the wind affect the direction of the surface current? How do you think this is similar to or different from the earth s surface currents? C. Recall what you learned about upwelling during Inquiry 7.1. How can surface currents affect deep ocean currents? Look at the currents marked on your world map. Answer these questions: A. What patterns do you notice in the movement of surface currents? B. How are the paths of the ocean currents you plotted on the map similar to the paths of global winds (trade winds, westerlies, easterlies)? C. How do you think ocean currents affect weather globally? Watch as groups share the ocean currents they plotted on their world map. STC/MS C ATASTROPHIC E VENTS 91

13 For the penguins living on the Galapagos Islands off the coast of Ecuador, life is usually good. Trade winds cause a deep ocean current to rise to the surface of the Pacific Ocean at the edge of South America. This current carries nutrients that nourish tiny forms of life, which are eaten by fish. These fish, in turn, are a feast for the penguins. Every 3 to 7 years or so, the trade winds weaken. The Pacific Ocean becomes very warm there. Cold water stays at the bottom of the El Niño Stirs Up the WORLD S WEATHER ocean. The fish near the surface cannot live in the warm water, and they leave to search for food elsewhere. The penguins go hungry! For centuries, Peruvian fishers have noticed that their fish catch declined every 3 to 7 years. At the same time, the waters grew warmer. Christian monks named this warming El Niño, for the boy Jesus, because the warm water came at Christmastime and brought with it the gifts of abundant vegetation. These six globes represent the appearance and disappearance of El Niño s warm water in the eastern Pacific Ocean in NASA/GODDARD SPACE FLIGHT CENTER AND SCIENTIFIC VISUALIZATION STUDIO 92 STC/MS C ATASTROPHIC E VENTS

14 BILL BACHMAN/PHOTO RESEARCHERS, INC. LESSON 7 El Niño affects far more than penguins and fishers in South America. The warming causes rains and flooding on the west coasts of both North and South America. In other parts of the world, El Niño causes dry weather. Southern Africa, Indonesia, and parts of Australia have severe droughts during these episodes. Such big changes in the weather can be disastrous. In 1982 and 1983, an unusually strong OCEAN CURRENTS AND G L O B A L C L I M AT E El Niño caused 1000 deaths and billions of dollars in damage around the globe. In Eastern Australia, the worst drought in history killed livestock and nearly caused a famine. Many people in Indonesia died of starvation. On the other side of the globe, southern California had too much rain, causing floods and mudslides. Five hurricanes hit Tahiti in one year and left 25,000 people homeless. Above: The 1983 El Niño caused drought in Australia. No rain meant little food for this cow and other animals. Below: In 1998, this family was forced to leave their home north of Lima, Peru, to live in a refugee camp. AP/WIDE WORLD PHOTOS Heavy flooding caused by El Niño destroyed most of the area. STC/MS C ATA S T R O P H I C E V E N T S 93

15 OCEAN CURRENTS AND G L O B A L C L I M AT E AP/WIDE WORLD PHOTOS A. RAMEY/WOODFIN CAMP & ASSOCIATES AP/WIDE WORLD PHOTOS LESSON 7 Top: Girls play in a dry riverbed near Kota, Bahru, which borders Thailand. Scientists believe El Niño caused drought in this area. Bottom left: Excessive rains during El Niño caused landslides in the United States in Bottom right: The Amazon forest burns in northern Brazil in A record drought caused by El Niño caused fires in more than 1.5 million acres of forest. The El Niño of was also very strong. Ocean temperatures climbed to several degrees above average. The waters were the warmest they had been in 150 years. Without 94 STC/MS C ATA S T R O P H I C E V E N T S plentiful rains, Indonesian forests burned out of control. An airliner crashed in the thick smoke from the fires, and hundreds of people were killed.

16 Fewer storms than usual Dryer than usual Wetter and stormier than usual Warmer than usual The impact of El Niño in 1997 was felt worldwide. This map shows its effects during the months of June through September. In the mid-1980s, scientists began the Tropical Ocean and Global Atmosphere (TOGA) program to learn more about El Niño. For nearly 10 years, TOGA scientists measured air and ocean temperatures in the tropical Pacific Ocean at 70 weather buoys. The readings were sent by satellite to weather centers. This information helped forecasters build computer models to predict how El Niño might change weather patterns. In 1997 and 1998, the predictions helped farmers in Peru, Brazil, and Australia know what kind of weather was ahead. When they knew it would be wet, they planted grains that would grow well in rainy conditions. Something Has Changed There may be good news for those hungry penguins of the Galapagos Islands. In the spring of 1998, the temperature of a large area of the Pacific Ocean along the equator dropped 9 ºC. This cooling, called La Niña, follows some El Niño events. In La Niña years, some El Niño weather patterns are reversed. The northwest United States experiences a severe, wet winter. The southeastern United States suffers through dry weather and parched crops. More hurricanes brew in the Atlantic. And, luckily for the penguins, La Niña brings cold water and more fish to the coast of South America. STC/MS C ATASTROPHIC E VENTS 95