chapter three Rivers of the Sea: Ocean Currents - RCAS Shroeder...chapter three Rivers of the Sea:...

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chapter three

Rivers of the Sea: Ocean Currents

It is virtually impossible to understand Earth without under-standing the oceans. These great bodies of water regulate the weather and climate, provide most of the oxygen in the atmo-sphere, and feed much of the human population.

If youve looked at the ocean from shore, you know that the seas are in constant motion. Waves crash against rocks and push sand and cobbles into ever-changing landforms. Hurricanes whip up storm surges that wash away whole coastal communities. It may seem that the motions of the ocean are chaotic, with the patterns of movement always changing. However, underneath all this hectic motion is an order that usually cant be observed from shore. You need to travel beyond where the waves crash and into the open ocean to find the more orderly motion of currents.

Some refer to ocean currents as the rivers of the sea. Varying in size and speed, they move massive quantities of water around the world, redistributing heat from the equator to the poles and making Earth habitable. They are the lifeblood of the marine biosphere, transporting plankton and fish, dispersing eggs and larvae, bringing up nutrients from the ocean floor, and conveying vital substances such as salts. Phytoplankton in ancient oceans produced much of the oxygen that is now in Earths atmosphere, and these organisms continue to help main-tain the steady balance of oxygen that you breathe in and out every day.

Ocean currents have also been intimately involved in human his-tory, as one man, Thor Heyerdahl, set out to prove. In this chapter, youll hear about his theory and the crazy idea he had about how to prove it. Youll gather knowledge about ocean currentswhere they are located and the forces that drive them. Youll then use the expertise you gain to help you decide whether you would have the courage to set adrift with him on a primitive raft across the largest ocean in the worldthe Pacific.

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EDC Earth SCiEnCE unit 1 hydrosphere: water in earths systems

Consider Investigate Process

Brainstorming discuss the following with your partner and be prepared to share your ideas with the class. Dont worry if you dont know all the answers at this point. You will explore many of these questions in thischapter.

1. Have you had any experiences with the ocean? For example, have you ever sat by the ocean and observed it for awhile? Have you traveled on the ocean in a boat, surfed, or swum in the ocean? If so, describe one of these experiences.

2. Describe all the ways water in the ocean moves, based on your experi-ences with and knowledge of the ocean. What do you think causes this motion?

3. Do you think that all ocean water is the same? How might the proper-ties of ocean water vary from one area to another, or at different depths?

4. Describe what you think would happen if you landed in the middle of the ocean on a raft and drifted. Would you move in any particular direc-tion? Do you think this is predictable?

taSK Ocean Quiz ShowMost of Earth is covered with ocean water, but humans are land-based crea-tures. Even though these oceans are vital to the workings of this world, people dont spend much time thinking about them. To help you immerse your mind in the watery part of the planet, youll draw on your competitive spirit and review some ocean basics.

Procedure1. The class will be divided into two teams. Arrange the chairs to form two

lines, beginning at the front of the classroom and extending toward the back. Face all chairs toward thefront.

2. A grid will be set up on the board with four categories listed at the topOcean Statistics, Water Cycle, Major Oceans, and Ocean Definitions. Underneath each category, youll see the point values100 through 500.

3. With direction from your teacher, the two people in the chairs closest to the board (one from each team) choose the question category and point value.


chapter 3 rivers of the sea: ocean currents

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4. After youve chosen, your teacher will project the question and read it out loud. After the question is read and your teacher gives the go-ahead, one of the students in the front chairs tries to answer the question. If the question is answered correctly, the team is awarded the questions point value. If not, the person at the head of the other team can try to answer the question to earn the points. Next, the two students at the front move to the back of the row of chairs, and everyone shifts forward.

5. This same process is repeated with each pair of students until all the points have been won. The points are then totaled and the winning team is declared.

Now that youve dusted off your brain cells and refreshed your basic knowledge of the ocean, youll read a true story about an adventurer who risked his life to prove a theory about how the ocean has influenced human history.

WHATS THE STORY?

A Crazy IdeaOnce in a while, you find yourself in an odd situation. You get into it by degrees and in the most natural way but, when you are right in the midst of it, you are suddenly astonished and ask yourself how in the world it all cameabout.

If, for example, you put to sea on a wooden raft with a parrot and five companions, it is inevitable that sooner or later you will wake up one morning out at sea, perhaps a little better rested than ordinarily, and begin to think about it.

One such morning I sat writing in a dew-drenched logbook:May 17. Norwegian Independence Day. Heavy sea. Fair wind. I am

cook today and found seven flying fish on deck, one squid on the cabin roof, and one unknown fish in Torsteins sleeping bag . . . 1

Thor Heyerdahl, Kon-Tiki: Across the Pacific by Raft

It is no wonder that Thor Heyerdahl had some second thoughts about the adventure he had begun. It was arguably a crazy idea to set adrift in a primitive balsa wood raft across the largest, deepest ocean in the world, but it was all to prove a theory. Heyerdahl had found evidence that more than 1,000 years ago people using simple tools, most likely made of stone, may have sailed with such rafts from the west coast of South America, led by a legendary hero, Kon-Tiki. He believed these ancient people had traveled thousands of miles across the Pacific and settled the Polynesian Islands (Figure 3.1). Most people to whom Heyerdahl espoused his theory at the time thought this was quite impossible. In fact, pretty much no one would even listen to his idea. To prove his critics wrong, he set out to duplicate the voyage.



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figure 3.2This is a photograph of the Kon-Tiki, now in the Kon Tiki Museum, Oslo, Norway. It floated on nine large balsa logs and had a small cabin constructed of bamboo.

Youre mad! A raft? exclaimed his friend Carl when Heyerdahl first shared his idea. As he set out to reproduce in detail the rafts used by the ancient South Americans, many people agreed setting adrift in the Pacific was a bad idea and warned him strongly not to attempt the trip. They said he would never make it across that vast expanse of ocean alive. Nevertheless, Heyerdahl was determined to prove his theory. He studied the ocean currents and prevailing winds, and calculated how to use these forces to convey him on his journey. He painstakingly researched the techniques used by these ancient sea-goers to build their boats, believing that the construction of the raft could be critical to its ability to move efficiently and safely across the ocean. He built the raft Kon-Tiki out of balsa wood and bamboo lashed together with hemp rope (Figure 3.2), fitted it with a crude square sail, and set sail on April 28, 1947. His survival depended on hisknowledge of the ocean currents and a raft that had no engine to power its movements, only the primitive sail and some wooden paddles to help withsteering.

Was Heyerdahl Heyerdahl crazy, or was he onto something? How could it be possible to travel in an unpowered raft across the vast Pacific Ocean and survive? What forces drive the ocean currents, and could these forces really carry him so far across the ocean? Could he really predict where he would end up, or would he just drift around aimlessly? In this chapter, youll look for the answers to these questions and make a personal decision about whether you would choose to set sail on the Kon-Tiki with Heyerdahl.

3861 EDPS Earth Science Student Book, Part 1Figure: 3861 EDPS EaSci SB03_01Cronos Pro Regular 8/9

Hawaii

Lima, Peru

Mexico

SOUTH PACIFIC OCEAN

POLYNESIAN ISLANDS

Australia

New Zealand

Samoa

Equator

Tahiti

NORTH PACIFIC OCEAN

0 960 Miles480

0 960 KM480

0 1,000 miles500

0 1,000 km500

figure 3.1Thor Heyerdahl planned to launch his raft from the coast near Lima, Peru, and travel to the Polynesian Islands.



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About the ReadingWrite your ideas about the following questions in your notebook. Be prepared to discuss your answers with the class.

1. What was Thor Heyerdahl trying to prove by setting off into the Pacific on a primitive raft of ancient design?

2. If he set sail off the west coast of South America, in which direction would you predict his raft would move?

3. Based on what you know about the ocean, what forces drive the ocean currents?

4. What is your initial reaction to Heyerdahls idea? Do you think its crazy? Do you think his survival will depend solely on chance, or is there some scientific validity to his prediction that he will make it?


ChaLLenge

What natural forces could propel you across the ocean from the coast of Peru to the Polynesian Islands?

many thought heyerdahl was out of his mind when he decided to push off the coast of south america on a primitive raft with no engine. trusting the ocean to carry him to a far-off destination, he was willing to risk the lives of all aboard the Kon tiki in what is essentially a part of the largest, deepest ocean in the world. Could he really predict in which direction he would go? What forces could propel him such a distance across the water? If he were about to set sail today, would you be willing to join him on his adventure?

gather KnowLedge

Thor Heyerdahl seemed quite confident that he would be able to ride on the ocean without the benefit of an engine and arrive safely in Polynesia. To figure out if you agree with him or not (and if you would be willing to join him), become more of an expert in the movements of ocean water. In the following investigations, gather information about the following questions. What are the driving forces that cause the ocean water to move and that

can affect the current patterns?

What are the predictable water patterns, especially off the coast of Peru, that will be used on the journey?

Begin by investigating how wind affects the movement of water in ocean currents.



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activity 1The Effect of Wind on Ocean CurrentsPre-Activity DiscussionIn preparation for this activity, discuss the following question with your classmates.

1. What do you know about wind? What is it? What causes it?

ProcedureRecord all observations and answers in your notebook as you work. 1. Fill your ocean basin with water, leaving some space at the top.

2. Let the water settle while you discuss the following with your group, recording your ideas in your notebook using words and sketches:

If you blew on the surface of your ocean basin in a consistent direction, how do you predict this would affect the water in your model?a. Would the water move in a certain direction?b. Would the water near the surface behave the same as the water near

the bottom?

3. Use the materials provided to design and carry out investigations to test your predictions. Record your observations using labeled sketches and words.

Hint: A sprinkling of pepper and a drop or two of food coloring may help you see what the water does.

4. Adjust the strength and direction of the wind so that you observe the following currents in ac below. Make labeled sketches to describe how hard and where the wind was blowing from and the observed motion of the water for each situation.a. a clockwise gyre (circular current)b. a counterclockwise gyrec. a matched pair of gyres, one clockwise and one counterclockwise

5. Rinse out your basin and fill it with clean water. As the water settles, discuss the following with your group. Use words and/or sketches to record your ideas.

How would the flow of the water be affected if you added islands and submerged features (such as hills on the ocean bottom that dont reach the surface) to your model ocean basin? Would the shape of these fea-tures make a difference? Draw sketches to record your predictions. Be sure to consider the water at both the surface and at depth.

materialsFor each group oF StudentS

1 ocean basin

4 straws

access to water and paper (or cloth) towels

4 short ocean features

2 long ocean features

2 pepper packets

1 bottle of red food coloring

1 bottle of blue food coloring



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6. Selecting from the materials provided, design and carry out investiga-tions to test your predictions about the movement of the water when islands and submerged features are present. Record how you set up your model and your observations in sketches and words, and then answer the Analysis questions that follow.

AnalysisComplete the following questions, and record your answers in your notebook. Be prepared to share your answers with the rest of the class.

1. How might what you observed with your model relate to the actual movement of water in ocean basins?

2. Study the map of global wind patterns in Figure 3.3 at right. Based on what you observed in this activity, predict the actual path of the Earths oceans surface currents. Record your prediction on a sketch map that shows the continents

Thor Heyerdahl knew that understanding the patterns in surface currents in the Pacific Ocean was crucial to making a successful journey in a raft across the Pacific. Now that youve made some pre-dictions about where these surface currents might go, youll study some maps to learn more about how they move.


geographical equator

trade winds

trade winds

figure 3.3Prevailing wind patterns over the Pacific Ocean.

Earths Rotation: The Coriolis Effect

The patterns of movement of the surface currents are af-fected not only by the position of the continents, but also by the Coriolis effect. The Coriolis effect on a moving body results from the fact that Earths surface is rotating eastward at greater speed near the equator than near the poles. For example, at the equator, an object has to cover 40,000 km in 24 hours (one revolution in one day), resulting in a speed of 1,670 km/h. at a 40-degree latitude, an object has to cover only 30,600 km, resulting in a velocity of only 1,275 km/h. What happens if an object (for example, a cannonball) is fired due south from a 40-degree latitude? The cannonball has an eastward speed of 1,280 km/h at launch. however, as it travels south toward the equator, it flies across areas that travel at a higher speed eastward. Consequently, the slower cannonball falls behind and appears to bend to the west in the reference frame of Earth.

Similarly, a body traveling from the equator toward either pole veers eastward (right in the direction of move-ment) because it retains the greater eastward rotational speed of the lower latitudes as it passes over the more slowly rotating Earth closer to the pole.

people sometimes refer to the Coriolis effect when they watch the direction in which water drains in the sink. However, the time this takes (and the distance covered) is too small for the Coriolis effect to become visible (the direction of the draining water is determined by the shape of the container, previous water movement, dirt in the sieve, and so on). to observe the Coriolis effect, you need to observe a larger movement over a longer period of time (for example, a Foucault pendulum like that displayed in many science museums).



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activity 2Natural PatternsEven though the technologies of people living along the Peruvian coast 1,000 years ago were primitive, they were skilled sailors and very in tune with nature. By studying the world around them, they had noted patterns in the night sky that allowed them to navigate without the use of compasses or the global positioning systems (GPS) available today. They recognized patterns in the direction of prevailing winds and in the direction of ocean currents. This allowed them to use the force of wind and moving water, instead of the engines used by ships today, to propel them on their journeys.

Most people today are quite comfortable with modern technologies but are not so in tune with nature. What are these patterns that were so familiar and useful to ancient coast dwellers? In this activity, youll investigate pat-terns in ocean currents. Scientists have already been collecting observations of the oceans for many decades. In addition to directly observing natural phenomena, theyve had the benefit of modern scientific technologies such as satellites and other remote-control instruments. As a consequence, scientists have a much more global view of the workings of the ocean than people did hundreds or thousands of years ago. Study some of the observations that have been gathered using these modern techniques and find the patterns.

Pre-Activity Discussion Discuss the following topic with your classmates to help you prepare for the investigation. Record your ideas in your notebook.

1. In Brainstorming, you talked about the different ways that water moves in the ocean. You may or may not know much about ocean currents. Describe in as much detail as you can, based on your knowledge and your experi-ments in Activity 1, how you think water moves in an ocean current. How is this different from the movement of waves? Sketch your ideas.

ProcedureRecord all observations and answers in your notebook as you work.

1. Study the map of surface ocean currents in Figure 3.4. Notice that the arrows show the direction of the currents, and the color indicates whether the current is warm or cold. Write as many statements as you can to describe patterns in how these currents move. For example, compare how currents move in the Northern and Southern Hemispheres or from one ocean to the next, and soon.

2. Find the current that is closest to where you live (if youre not near the ocean, just find the nearest coast). Describe how this current moves and whether it carries cold or warm water.



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3. Look off the coast of Peru. Do you see any currents that the Kon-Tiki might be able to ride to the Polynesian Islands? What are your initial ideas about the route the raft might take?

4. Write answers to the Analysis questions, and prepare to discuss your ideas with the class.

Analysis With your group, discuss the following questions and record your answers in your notebook. Be prepared to share your answers with the rest of the class.

1. You may have recorded some observations about how cold and warm currents move relative to each other. What do you think is the overall effect of warm- and cold-water movements on the ocean?

2. Based on the patterns youve observed in this map, and the patterns you observed in your experiments in Activity 1, does the position of the continents seem to affect circulation in the oceans? Provide some evi-dence for your answer.

3. How do you think the movement of these currents might affect the climate on the continents? Explain your thinking.

4. You may have noticed that currents flow consistently to the east in the ocean near the South Pole. Why dont you see this same smooth pattern near the North Pole, in the top portion of the map?

By studying Figure 3.4, you learned that surface currents in the worlds oceans move water in all directions. You may have observed certain general pat-terns in their movements. These patterns and other information about surface currents are discussed in the next reading. Take notes and think about how this information relates to the currents that Heyerdahl rode from Peru to Polynesia.


Canaries Current

North Atlantic Current

North Pacific Current

Alaska Current

Arctic Circle

EquatorNEC

ECC

Kamchatka Current

Tropic of Cancer

Tropic of Capricorn2326

SEC

KEYwarm currentscool currentsNorth Equatorial CurrentEquatorial CountercurrentSouth Equatorial Current

NECECCSEC

Guinea Current

West Wind Drift

Bengueta Current

Agulhas Current

Peru CurrentEast

Australia Current

West Wind Drift

NEC

SEC

NEC

SEC

Kuroshio Current

Lima, Peru

Polynesian Islands

Gulf Stream

Labrador Current

6633

2326

0

figure 3.4Long-term average location and movement of surface ocean currents. Warm currents are shown in red, and cool currents are shown in blue.



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readingPatterns in Surface Ocean CurrentsOcean GyresYou observed in Activity 2 that few of the currents seem to take a straight path. In fact, there is a circular pattern of movement that repeats itself in each of the major oceans. For example, Figure 3.5 shows the currents in the North Atlantic. The Gulf Stream flows generally northward just off the east coast of North America. Some of this current circles around and flows southward as the Canary Current along the west coast of Spain and Africa. The circle is com-pleted by the North Equatorial Current and the Antilles Current. These large subcircular current systems are called gyres. There are five major ocean gyrestwo each in the Pacific and Atlantic Oceans and one in the Indian Ocean.

If you look carefully at the direction of flow within these gyres in Figure 3.4, youll notice another interesting pattern. The gyres in the Northern Hemisphere flow in a clockwise direction. In the Southern Hemisphere, water moves the opposite wayin a counterclockwise direction.

Not all currents move in gyres. Some seem to take their own path. Some appear to have been channeled through narrow openings between landmasses and even at times deflected into small eddies, like those you might see in a stream. Some, such as those currents that are near the South Polewhere there are few land obstructionsand near the equator, take a more direct path (although keep in mind that you are looking at a flat map of a round Earth, so even these paths are curved).

figure 3.5The North Atlantic Subtropical Gyre.


60 N

55 N

50 N

45 N

40 N

35 N

30 N

25 N

20 N

15 N

10 N90 W 80 W 70 W 60 W 50 W 40 W 30 W 60 W 10 W 0 10 E

GULF STREAM

North Atlantic Ocean

North Equatorial Current

Canary Current


Irminger Current

Labrador Current

Africa

Canada

USA

Caribbean Islands

Halifax

Bordeaux



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Warm and Cool CurrentsOne of the most important patterns can only be observed if you pay attention to the flow of warm and cool currents. Generally, warm currents move water from the equator toward the poles, and cold currents move water from the poles toward the equator. The net effect of all this water movement is to redis-tribute heat from the warmer to the cooler areas of Earth. These warm and cool water masses have a profound effect on the weather and climate on the continents they flow past. Figure 3.6 shows surface water temperatures in the Northern Atlantic in April and July of 2011. Take some time to study these maps (because they may be different from maps youve looked at before), and answer the Think About It questions.

figure 3.6The average temperature of the ocean surface water during April and July of 2011. Yellow and orange indi-cate the warmest water, whereas blue and purple are the coolest. Note the temperatures of Halifax, Nova Scotia, and Bordeaux, France, which are close to the same latitude.2

3861 EDPS Earth Science Student Book, Part 1Figure: 3861 EDPS EaSci SB03_06aCronos Pro Regular 8/9

0 12 21 27241815963 30Temperature (degrees Celsius)

Africa

Canada

USA

Caribbean Islands

Halifax

Bordeaux

60 N

55 N

50 N

45 N

40 N

35 N

30 N

25 N

20 N

15 N

10 N90 W 80 W 70 W 60 W 50 W 40 W 30 W 60 W 10 W 0 10 E

3861 EDPS Earth Science Student Book, Part 1Figure: 3861 EDPS EaSci SB03_06bCronos Pro Regular 8/9

0 12 21 27241815963 30Temperature (degrees Celsius)

Africa

Canada

USA

Caribbean Islands

Halifax

Bordeaux

60 N

55 N

50 N

45 N

40 N

35 N

30 N

25 N

20 N

15 N

10 N90 W 80 W 70 W 60 W 50 W 40 W 30 W 60 W 10 W 0 10 E

July, 2011

april, 2011


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Think about it 1 The two maps in Figure 3.6 show the northern Atlantic Ocean, and the colors represent average sea-surface temperatures for the months of April and July 2011.

1. In general, what regions of the ocean had the coolest temperatures during April?

2. What regions of the ocean had the warmest temperatures during April?

3. Write a few sentences that describe how sea-surface temperatures changed between April and July.

Think about it 2 Now, think about why you think the temperature patterns in the Atlantic Ocean look the way they do.

1. Do the water temperatures seem to be similar at the same latitudes? Describe parts of the ocean in the two maps where the temperature trends are not what you would predict simply based on latitude.

2. Do you see any evidence that ocean currents might be redistributing heat in the Atlantic Ocean? Describe where you think this might be happening.

3. Look back at Figure 3.5 , which shows the ocean currents in the North Atlantic. Do any of the following ocean currents seem to be affecting the pattern of sea-surface temperature? Explain how.a. Gulf Streamb. Canary Currentc. Labrador Current

Think about it 3 Halifax, Nova Scotia, and Bordeaux, France, are both lo-cated near the Atlantic coast at a latitude of approximately 4450 N. If you go to weather websites, you can get statistics about average temperatures calcu-lated based on 30 years of data. The average temperature in Halifax in April is 4.2C or 39.6F. The average temperature in Bordeaux in April is 11C or 64F. Hypothesize about why the average temperature in Bordeaux is higher than in Halifax, even though they are at the same latitude.

how much water do Currents Carry? how fast do They go?Maps such as the ones in Figures 3.4 and 3.5 can only convey certain infor-mation about ocean currents. The slim arrows dont provide much of a sense of the volume of water that is moving or its speed. Although currents vary in speed and strength, the major currents carry massive quantities of water. For example, the Gulf Stream transports 90 million cubic meters per second, moving 100 times as much water as all the rivers on Earth. If the Gulf Stream was moved like a hose and used to fill the basin of Lake Superior, the lake would be full to the brim in just one and a half days!



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The fastest-flowing currents tend to be along the western edges of the ocean gyres and are called western boundary currents. These currents flow at a speed of 4 to 120 km/day. Although surface currents typically flow in the top 400meters of the ocean, western boundary currents usually extend much deeperup to 1,000 meters. Currents on the eastern portions of the gyres, eastern boundary currents are shallower and move more slowly approximately 3 to 7 km/day. However, the amount of water conveyed by the western and eastern portions of the gyres is very close to the same: the western boundary currents tend to be relatively narrow (no more than 5075km [3045 mi] across), whereas the eastern boundary currents are much wider and are often hundreds of kilometers in width.

About the ReadingWrite your responses to the following questions in your notebook. Be pre-pared to discuss your answers with the class.

1. Add the definitions of the new terms introduced in this reading to your notebook.

2. Explain in your own words how currents redistribute heat from the equator toward the poles. Use some specific names of currents in your explanation.

3. Summarize in your own words what you have learned about ocean cur-rents so far. Do they follow predictable patterns? How fast do they go, and how much water do they carry? What else have you learned?

4. At this point, what do you think about the type of current Heyerdahls raft would ride off the coast of Peru? a. In which direction does it flow? b. Is the water warm or cool? c. Can you hypothesize about its width and speed relative to other

currents? d. Is it part of a gyre? If so, what other currents are part of this gyre?

You now have a sense of the patterns of ocean currentsat least those that are near the surface. Youve even thought about the current you might ride from the coast of Peru to Polynesia. However, do you know enough to feel confident shoving off in your raft? To know whether these currents would be a reliable form of transportation, you need to understand all the forces that drive them. In the next activity, youll carry out more investigations to explore the effect of density on ocean currents.



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activity 3The Effect of Density on Ocean Currents3

Pre-Activity DiscussionDiscuss the following questions with your classmates to prepare for this activity.

1. The currents shown on the maps you have studied so far are those near the surface, and the arrows show their horizontal movement. Do you think there are also vertical movements within the ocean water and deeper ocean currents? Explain your thinking.

2. Describe your understanding of the term density. What are some factors that could affect the density of water in the ocean? Can you think of a way that variations in density could cause water to move? Your teacher may have you do some simple investigations of density to help you review what you know about this.

ProcedureRecord all observations and answers in your notebook as you work.

1. Add room temperature water to your ocean basin until the water level is about 5 cm from the top.

2. Fill one wide-mouthed bottle about full with hot water and the other about full with ice water.

3. Place the bottle of red food coloring into the hot water bottle and the bottle of blue food coloring into the ice water bottle.

4. Place the bottle containing the hot water and food coloring at one end of the basin and the bottle containing the ice water and food coloring at the opposite end of the basin.

5. Let the water in the basin settle for 1 minute then carefully remove the bottles of food coloring from their water baths and add four drops of the warm red food coloring to the water next to the hot water bottle and four drops of the cold blue food coloring to the water next to the ice water bottle.

6. Observe the food coloring for several minutes, watching carefully for both vertical and horizontal water movements. Observe what is hap-pening on the surface and at depth within the pan. Record your obser-vations using sketches and words.

7. Discuss the following with your group, and then record your ideas in your notebook.a. Explain what caused the water movements in the pan. Relate your

answer to the concept of density.

materials For each group oF StudentS

1 ocean basin

2 wide-mouthed bottles

1 bottle of red food coloring

1 bottle of blue food coloring

access to room- temperature water, hot water, and icewater

access to paper (orcloth) towels



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b. Describe some factors that could cause ocean water to be warm orcool.

c. Where would you find ocean water that is relatively warm and cool?d. If you left the pan model sitting without adding any hot water or ice,

would the water still be moving when you returned tomorrow? Whatcould cause the water movements to stop? Would you expect real ocean currents to stop? Why or why not?

8. The temperature of ocean water is not the only factor that affects its density. When salt dissolves in water, the volume of the water does not increase by as much as the mass. The more salt that is added, the more the density increases. Discuss the following with your group, and record your ideas in your notebook.a. Describe some processes occurring in the oceans that could cause

the water to become more or less salty (saline).b. Where would you find ocean water that is relatively salty versus

lesssalty?c. How do variations in salinity cause movements of ocean water?

Analysis Complete the following questions, and record your answers in your notebook. Be prepared to share your answers with the rest of the class.

1. Based on your observations during this activity, describe how variations in density can cause ocean water to move.

2. Describe some factors that can cause ocean water to vary in density.

3. What ideas do you have about why ocean water varies in temperature at different locations on the globe? What could cause it to vary in salinity?

The models you observed in Activity 3 showed that if you just look at surface currents you only see a simplified view of water movement in a basin. Currents actually move in three dimensions, and some travel at depth below the surface. The great oceans on this planet contain many currents moving at different levels, from the currents near the surface shown in Figure 3.3 to deep currents flowing along the bottom of the ocean basins, often at depths of many kilometers. These currents move over and under each other, sometimes mixing and sometimes deflecting each other. Their paths may seem indepen-dent at times, but they interact in complex ways.

Where does the energy come from that drives all this movement? What forces move the currents and determine their paths? Its easiest to analyze the forces if you divide currents into two typesthose flowing near the surface and those flowing at depth (although their movements are actually connected). Think about your observations of the models as you learn about wind-driven surface currents and density-driven deep currents in the reading that follows.



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readingStriving for Equilibrium: The Forces That Drive Ocean Currents

The Earth is surrounded by two great oceans: an ocean of air and an ocean of water. Both are in constant motion, driven by the energy of the Sun and the gravity of the Earth.4

An Introduction to the Worlds Oceans

Wind-Driven Surface CurrentsThe primary driving force of ocean currents is energy from the Sun. How does the Suns energy drive currents? In Activity 3 you observed the forma-tion of a circulating current of water between the cold and warm ends of the pan. The warm water rose and moved toward the cold end of the pan, and the cold water sunk and moved toward the warm end of the pan. The net effect was the redistribution of heat through the water in the pan. As in your pan model, the Sun delivers more radiant energy to Earths equator than the poles, and as a result, the air and water are heated unevenly.

Think about it 1 Can you explain why the region around the equator has the highest temperatures (air and water), and the coldest regions on Earth are at the poles? Figure3.7 might help you with your explanation. You can also try making a simple model with a soccer ball or basketball and a flashlight. Shine the flashlight directly at the equator and observe how the intensity of the light reflected from the surface changes as you move toward the poles.


equator

figure 3.7 Diagram of the Suns rays hitting Earth at different angles depending on the latitudes.



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For the moment, turn your attention from the ocean and think about the atmosphere. The uneven heating of Earths atmosphere causes circulating cur-rents of air to form in the atmosphere (Figure 3.8), which are similar to the currents you observed forming in your pan model. These circulating air cur-rents are called convection currents, and they transfer heat from the equator (the hot end of your model) toward the poles (the cold end of your model).

If it were not for Earths rotation, air circulation would be simple, with one circulating convection current extending from the equator to the poles. However, because of the rotation, this simple circulation pattern is broken into three circulating cells within the atmosphere in each hemisphere (Figure 3.9). In each cell, the pattern is the same, with rising warm air and cool sinking air. Earths rotation also deflects the path of the air within the three cells (the effect of Earths rotation on moving objects or fluids is called the Coriolis effect, as explained earlier in the chapter). The trade winds, westerlies, and easterlies are the portions of the cells that blow along Earths surface. As you can see in Figure 3.9, instead of moving air directly toward the poles, winds in the Northern Hemisphere are deflected by Earths rotation to the right of their path, and winds in the Southern Hemisphere are deflected to the left of their path.

These prevailing winds (winds that blow from one direction more fre-quently than any other) push on the surface of the water in the oceans, causing surface water currents to form, just as they formed when you blew on the water in your models in Activity 1. The combined effects of these winds, the deflection of the water by the Coriolis force, and the position of the conti-nents create the large-scale patterns you observed in surface ocean currents in Figure 3.4 during Activity 2.

The primary driving force of surface currents is wind, which sets the water in motion. Once the water is in motion, the Coriolis force deflects the paths of the ocean currents toward the right of the wind direction in the Northern Hemisphere and toward the left in the Southern Hemisphere. The prevailing wind directions and the Coriolis force, along with the land masses sur-rounding the ocean basins, cause the surface currents to circulate in gyres.


trade winds

polar easterlies

westerlies

trade winds

polar easterlies

60 N

30 N

equator

60 S

30 S

westerlies


rising warm air

air cools and sinks

ground

figure 3.8 A simplified illustration of a convection current in the atmosphere.

figure 3.9 Prevailing winds blowing along Earths surface are deflected by Earths rotation.



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Figure 3.10 shows the direction of prevailing winds over the North Atlantic, as well as the paths of the ocean currents in the North Atlantic Gyre.

think about it 2 Before moving on to the next part of the reading, take a few minutes to summarize in your own words what youve learned about the driving forces ofsurface currents.

a. Describe how radiant energy from the Sun provides the energy to set surface ocean cur-rents in motion. (Hint: Youll also need to include information about the atmosphere.)

b. The prevailing winds set the surface currents in motion. What other factors affect the direction in which they flow?

Density-Driven Deep CurrentsWind-driven currents are found in the upper few hundred meters of the oceans. Below that point, wind blowing on the surface doesnt have much direct effect. So what drives the movement of currents below this depth?

In Activity 3, you explored the effect of two other factors on the movement of water: temperature and salinity. You observed that when water is warmer than the surrounding water, it rises above it, and when it is cooler, it sinks. Variations in salinity have a similar effectwater containing more salt sinks, whereas water containing less salt rises. These vertical movements occur because of differences in the density of the water.

The force of gravity pulls dense materials down, and less dense materials rise above them. You observed this happening in Activity 3 when you added drops of food coloring to the cold and warm ends of the pan. The cooler water sank, and the warmer water rose to the surface. As water was displaced vertically, water flowed in horizontally to take its place, and this initiated a circulating current.

When water masses of different salinities interact, a similar phenomenon occurs. The salinity of ocean water is changed by processes that add or sub-tract water. Freshwater is added by streams and rivers along coastlines, and by precipitation. Water is removed from ocean water by evaporation and by the freezing of water, which increases ocean waters salinity. It might surprise you to know that icebergs floating in the ocean are actually made largely of fresh-water. When water freezes, it expels the salt. The salt that is expelled increases the salinity of the water just below the ice, making it denser.

When a mass of water is flowing horizontally and it encounters another mass of water that is less dense, it dives below the less dense water. You can see evidence of this occurring off the coast of Panama in Figure 3.11.

The temperature and salinity of water vary independently. The densest water in the ocean is cold and high in salinity. At certain specific locations in the oceans, near the poles, cold temperatures, ice formation, and high evaporation rates due to wind create very dense water. At these deep-water formation areas, this exceptionally dense water sinks to the bottom of the ocean, driving what is known as the global conveyor belt, a global pattern of circulation within



equator

westerlies

trade winds

Gulf Stream Canary

CurrentNorth Equatorial

Current

figure 3.10The currents of the North Atlantic Gyre (shown in blue) are driven by prevailing winds (shown in red).



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the oceans. The currents of the global conveyor belt, shown in Figure 3.12, are important because they connect the water of all the worlds ocean basins, transporting energy and matter, and significantly affecting the climate of Earth.

The deep, dense currents of water that originate in the deep water formation areas then flow along the bottom of the ocean basin, driven by gravity, much like a river on land. The topography of the ocean bottom controls the direction of flow, much like hills and valleys on the continents control the flow of streams and rivers.

When deep water forms in the North Atlantic, it sinks, moves south, circulates around Antarctica, and then moves northward to the Indian, Pacific, and Atlantic Ocean basins. These deep currents flow slowly (less than 1 cm/sec) and can spend as much as 1,000 years on the ocean bottom before returning to the surface. Although the global conveyor belt moves slowly, it transports a great deal of water. One hundred times the amount of water transported by the Amazon River is conveyed around Earth by this global circulation.

About the ReadingWrite your responses to the following questions in your notebook. Be prepared to discuss your answers with the class.

1. Add the meaning of the new terms introduced in this reading to your notebook.

2. Where does the energy that drives the motion of ocean currents comefrom? 3861 EDPS Earth Science Student Book, Part 1

Figure: 3861 EDPS EaSci SB03_11bCronos Pro Regular 8/9

debris collects

saltwater of ocean (denser)

freshwater flowing from land

(less dense)


deep water formation

deep water formation

surface current

deep currentfigure 3.12The surface and deep currents that circulate water connect all Earths oceans in the global conveyor belt. The red lines represent surface cur-rents, and the blue lines indicate deep currents.5

figure 3.11The floating debris in the photo marks the boundary line between masses of the denser salt water and the less dense freshwater off the coast of Panama, as shown in the diagram.



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3. How does the circulation of air in the atmosphere affect the motion ofsurface currents?

4. Describe how the Coriolis effect changes the direction of prevailing winds and ocean currents.

5. Describe how salinity and temperature differences drive the motion ofdeep currents. Relate your answer to the concept of density. Also include a description of what happens at deep water formation areas.

6. How would you expect the following to affect the salinity of ocean water?a. freezing of ocean water and formation of iceb. evaporationc. melting of iced. rain

7. Hypothesize about what could happen in deep water formation areas if there was more melting and less freezing occurring over a period of years near the poles.

8. What are the likely primary driving forces that affect the flow of the Peru Current off the coast of South America?

address the ChaLLenge

Youve gathered scientific knowledge about ocean currentstheir patterns of movement and the forces that drive them. Now its time to pull together what youve learned, along with any further research you feel you need to do, and make your best judgment about whether Thor Heyerdahl has a chance of sur-viving his journey.

Youll communicate what youve learned in a news feature article written at the time right before Heyerdahl set sail. As you can imagine, Heyerdahls idea has created quite a stir. Many people are interested in his theory and wonder what will become of him when he sets adrift in the vast Pacific Ocean. Your article should educate the public about his idea and the science behind it, and must address the following questions and concepts.

1. Are the movements of ocean currents predictable, or are they random? What are the basic patterns of their motion?

2. What are the forces that cause ocean currents to move across vast oceans? Explain both surface and deep currents.

3. Specifically, describe the currents that Heyerdahl could ride from Peru to the Polynesian islands.

a. Is there a current off the coast of Peru that could carry Heyerdahls raft? b. What are the forces that drive this current? c. Is it part of a gyre? d. Where does the current come from and where does it go? e. Will it take him all the way to Polynesia, or must he take more than

one current? f. What do you think are his chances of survival, and why?



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SHARE

In class, share your views about the following two questions:1. Did Thor Heyerdahl have a good idea or was it crazy?

2. Would you be willing to set sail with Heyerdahl on the Kon-Tiki?

In this Take-A-Stand Line-Up, you will stand along a line in the class-room. If you stand at one end of the line, it means (for example) that you feel strongly that Heyerdahls idea was achievable, and the raft would without a doubt make the journey successfully. If you stand at the other end of the line, it means you feel strongly that he was absolutely crazy. If your position is somewhere between these two points, you should stand in the position that best represents how you feel. During the Line-Up, listen carefully to your classmates explanations of their decisions. You may be asked to explain yours. You can change where you stand based on what you hear.

Your teacher will facilitate the discussion. Only one person should talk at atime.

disCuss

With the class, discuss the following questions.1. Where does the energy come from that drives ocean currents?

2. What are the factors that affect the direction in which ocean currents move?

3. Is it likely that any single ocean current could be driven by only wind or only density? Explain your thinking.

4. How might ocean currents affect where you live?

5. What would some of the consequences be if the flow direction of a major ocean current changed?

6. Describe some ways that ocean currents in the hydrosphere are con-nected to Earths other major systems. Specifically, discuss the following:a. How does the geosphere affect the flow of ocean currents?b. How does the atmosphere affect the movement of ocean currents? c. Describe some ways that ocean currents affect the biosphere.



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Extend

As the troughs of the sea gradually grew deeper, it became clear that we had moved into the swiftest part of the Humboldt Current. This sea was obviously caused by a current and not simply raised by the wind. The water was green and cold and everywhere about us; the jagged mountains of Peru had vanished into the dense cloud banks astern.6

Thor Heyerdahl

Thor Heyerdahl was interested in the Peru (also known as the Humboldt) Current because of its ability to help carry him across the Pacific. However, this current is much more than just a means of transport. It has a signifi-cant influence on the climate and the marine ecology of the coast of South America. In addition, periodic changes in its movements are associated with far-reaching climate affects. You might be surprised to know how much this current in the South Pacific can affect where you live. Youll explore the effects of the Peru Current and how it relates to you in the next activity.

readingThe Peru CurrentThe Peru Current, shown in Figure 3.13, is an eastern boundary current and is relatively wide and slow. It carries cold, low-salinity water from Antarctic waters toward the equator.

As the Peru Current flows along the coast of Chile, Peru, and southern Ecuador, it cools the air and thus the climate of these coastal areas. Cool air doesnt hold much water, so there is very little precipitation along the coast. Because of the cool air and the posi-tion of the Andes Mountains to the east, which also prevent moisture from reaching the coast, the strip of land along the coast between the ocean and the mountains is one of the driest deserts in the world. In fact, there is evidence that the Atacama Desert in northern Chile, shown in Figure3.14, had no signifi-cant rainfall from 1570 to 1971.


Peru Current

Peru

figure 3.13The Peru Current flows from south to north along the west coast of South America.



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However, an amazing contrast exists along this parched coast. This barren desert borders one of the most productive marine ecosystems in the world, supporting an incredible abun-dance of marine life. How is this possible?

As it turns out, under normal conditions, the prevailing winds along the coast of Peru push the water away from the land, causing a phenom-enon called upwelling (the upward movement of ocean water). As water is moved away from the coast, cold, nutrient-rich water rises from below to replace it. These nutrients, when brought to the surface by upwelling, act as plant fertilizers and promote the growth of photosynthesizing organisms such as phytoplankton (photosyn-thesizing microscopic organisms that float in sea water), which in turn provide a food source for millions of fish. The abundance of fish in the Peru Current supplied Thor Heyerdahl and his com-panions with a rich diet as they drifted on their raft. In fact, this current is so full of life that it feeds much of the world. Approximately 18%20% of the worlds fish catch comes from the Peru Current Large Marine Ecosystem (LME). As such, if youre a fish eater, theres a good chance this current has provided a meal for you.

Think About It The two images in Figure 3.15 show the sea-surface temperature and chlorophyll concentrations off the coast of Peru. When chlorophyll is detected in the seawater, it indicates that photosynthesizing plants, such as phytoplankton, are present. What evidence of upwelling do you see in these images?

However, the connection of the Peru Current to the rest of the world doesnt stop there. The

3861 EDPS Earth Science Student Book, Part 1Figure: 3861 EDPS EaSci SB03_15aCronos Pro Regular 8/9

5N

Eq

5S

10S

15S

20S

25S

87W 81W 75W 69W

18

19

20

21

22

23

24

25

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28

Sea

surf

ace

tem

pera

ture

(deg

rees

Cel

sius)

Columbia

BrazilPeru

Chile

Bolivia

Ecuador

3861 EDPS Earth Science Student Book, Part 1Figure: 3861 EDPS EaSci SB03_15bCronos Pro Regular 8/9

5 N

Eq

5 S

10 S

15 S

20 S

25 S

87 W 81 W 75 W 69 W

Chl

orop

hyll

conc

entr

atio

n (m

g/m

3 )

0.05

3.487

6.924

10.361

27.545

30.0

13.798

17.234

20.671

24.108

Columbia

BrazilPeru

Chile

Bolivia

Ecuador

figure 3.15Sea-surface temperatures (top) and chlorophyll concentrations (bottom) off the coast of Peru during December of 2004. (These measurements were taken by instruments on satellites. The white areas in the chlorophyll map are areas where no data could be collected because of clouds.) 7

figure 3.14The Atacama Desert along the coast of northern Chile receives virtually no rainfall.



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figure 3.16Under normal conditions, strong trade winds help to drive the flow of the Peru Current and coastal upwelling.

figure 3.17The patterns of wind and water circulation in the South Pacific change during an El Nio Southern Oscillation (ENSO) event.

surface currents of the South Pacific Gyre are set in motion in large part by the equatorial trade winds, which blow generally from east to west, as shown in Figure3.16.

Periodically, however, for reasons that scientists are still trying to fully understand, these trade winds weaken, and a strong counter current forms along the equator, as shown in Figure 3.17. This weakens the Peru Current and disrupts upwelling along the coast of South America. This periodic change in atmospheric and ocean circulation in the South Pacific is called the El Nio Southern Oscillation (ENSO).

The effects of El Nio are far reaching. The upwelling is blocked by warm water flowing in from the west. Without the continual supply of nutrients, fish starve and the fishing industry crashes. Marine birds and mammals that


strong trade winds

Perustrong trade winds

high pressure

polar jet

strong Peruvian current

cool water

North America

subtropical jet

Asia

Australia

equatorial currents (strong)

warm water

low pressure

3861 EDPS Earth Science Student Book, Part 1Figure: 3861 EDPS EaSci SB03_17 alternateCronos Pro Regular 8/9

weak trade winds

Peruweak trade winds

high pressure

polar jet

weak Peruvian current

warm water

North America

wetter than average winter

Asia

strong counter current

dryer than average

pressure increases

subtropical jet

Australia

warmer than average winter



75

depend on the fish for food die off in great numbers as well. However, the most significant global effects are related to changes in patterns of precipitation and temperature.

A zone of high rainfall that normally exists in the western Pacific shifts to the central Pacific during an ENSO event. This brings drought conditions to Indonesia and Australia. At the same time, precipita-tion typically increases in other areas of the globe. For example, normally dry areas of the United States experience wet conditions, and warmer-than-normal winters occur in the northern United States and Canada. During an El Nio event in 1991, Texas received more than twice its normal December precipitation, and winter temperatures in western Canada were well above normal. A 1993 El Nio event may have contributed to extensive flooding in the midwestern United States, costing lives and mil-lions of dollars.

About the ReadingWrite your responses to the following questions in your notebook. Be prepared to discuss your answers with the class.

1. Why would shifts in sea-surface temperature be connected to changes in rainfall?

2. Draw a diagram that shows how an El Nio event demonstrates the connections among the hydrosphere, atmosphere, and biosphere.

activity 4An Influential CurrentIn this activity, youll learn more about the Peru Current and then do research of your own to obtain up-to-date information. Investigate how the movements of this current may be affecting you now or perhaps in the near future.

Procedure1. Research how El Nio Southern Oscillation (ENSO) events have

affected the United States and your region in the past and potentially inthe near future.

a. When was the last ENSO event?b. How did this ENSO event affect the United States? Were there any

noticeable effects on your region?

materialsFor each Student or team oF StudentS

access to the Internet or copies of information from Web sources such as the national oceanic and Atmospheric Administra-tion (noaa), national Aeronautics and Space administration (nasa), and american meteoro-logical Association

Biogeology: Upwelling and Downwelling

upwelling and downwelling occur when water is pushed either away from shore (upwelling) or toward shore (downwelling) by prevailing winds. These prevailing winds actually blow parallel to the coast, but due to the combined forces on the water, particularly in areas where the ocean bottom drops off sharply, the water is pushed in a direction 90 degrees from this wind direction. upwelling commonly occurs in the eastern regions of the oceans. In the Southern Hemisphere, the winds must blow north for upwelling to occur. upwelling brings nutrients from the ocean floor to the surface, and these nutrients sup-port a rich diversity of marine life.

When water is pushed toward the shore, water piles up next to shore and is driven downward by gravity. Downwelling transports oxygen-rich surface water to depth, where it is needed by animals that live in the deepocean.



76

c. What are the signs that an ENSO event is happening? Are there any indications that another ENSO event is happening now or will in the coming year?

As you do your research, keep track of your sources of information. You will communicate your findings in a format determined by your teacher.

2. Write answers to the Analysis questions and be prepared to discuss them with the class.

Analysis1. Describe how an ENSO event might have affected Thor Heyerdahls

journey.

Digging Deeper

The concepts you have been studying in this chapter play out in many dif-ferent ways in the world. If youre interested in exploring more about these concepts, below are some interesting topics to investigate or research. Research some of the methods used by scientists to study ocean currents.

For example, diving robots monitor the physical properties of ocean water throughout the worlds oceans and relay data to scientists via satellites as part of the Argo program. How do these robots work? Where are they located? What type of data do they collect?

Periodically, cargo ships carrying floating objects such as rubber shoes or bathtub toys are accidentally spilled into the ocean. These objects flow with ocean currents, and some of them arrive on distant shores. Find informa-tion about one of these events, and research how scientists have used these accidental data to learn about the motions of ocean currents.

The life cycle of the eel is intimately connected to ocean currents and is a pretty amazing story. Research how this species relies on currents through different stages of its life.

review The water in Earths oceans is continually circulating in ocean currents,

which carry water from one area of the oceans to another.

The flow of ocean currents follows predictable patterns. For example, in each of the major ocean basins, currents flow in circular gyres. These gyres move in a clockwise direction in the Northern Hemisphere and a counter-clockwise direction in the Southern Hemisphere.

The uneven heating of Earth by the Sun provides the energy that drives the ocean currents. Movements of the air in the atmosphere and water in the oceans redistribute heat from the equator toward the poles in an attempt to reach equilibrium.


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Chapter 3 rivers of the sea: oCean Currents

Wind is the primary driving force of surface currents. Once set in motion, their direction is affected by Earths rotation (the Coriolis effect) and the position of continents.

Deep ocean currents are driven primarily by density differences. These den-sity differences are related to variations in temperature and salinity of the ocean water.

Currents have a significant effect on the climate of the continents they flow past, raising or lowering the average temperature of communities, or even changing the amount of precipitation.

Changes in the flow patterns of ocean currents can significantly alter regional and global climate and have far-reaching effects on Earths biosphere.

aSSeSSment1. Roughly what portion of Earth is covered by ocean?

a. 50%b. 33%c. 12% d. 70%

2. ______________ causes major ocean gyres in the Northern Hemisphere to flow in a clockwise direction and the gyres in the Southern Hemi-sphere to flow in a counterclockwise direction.a. Earths magnetic fieldb. the effect of Earths rotation (the Coriolis effect)c. the gravitational pull of the Moond. the effect of Earths axial tilt

3. Surface temperatures are warmer at Earths equator than at the poles because ofa. Earths rotationb. patterns of circulation in the atmospherec. Earths spherical shape, which affects the angle that the Suns energy

strikes the surfaced. Earths axial tilt

4. What is the primary driving force behind surface currents?a. prevailing windsb. uneven sloping of the oceanic crustc. the Coriolis effectd. density differences due to variations in temperature and salinity



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5. What is the primary driving force behind deep water currents?a. prevailing windsb. uneven sloping of the oceanic crustc. the Coriolis effectd. density differences due to variations in temperature and salinity

6. Which of the following factor(s) affect(s) the flow of ocean currents?a. the position of continentsb. gravityc. topographic features on the ocean floord. all of the above

7. Which of the following properties represent water with the highest density?a. cold, low salinityb. cold, high salinityc. warm, low salinityd. warm, high salinity

8. Especially dense water forms in deep water formation areas because ofa. atmospheric cooling of the waterb. the freezing of waterd. winds, which cause evaporationd. all of the above

9. Deep water formation areas drive the global conveyor belt, which (select best answer)a. circulates water primarily within the Atlantic Oceanb. flows beneath the continentsc. connects the water of all Earths oceansd. stops circulating during El Nio years

10. Under normal conditions, the prevailing winds along the coast of Peru push the water away from the land, causing

a. upwelling (the upward movement of ocean water).b. downwelling (the downward movement of ocean water).c. significant sea level rise along the coast.d. a significant drop in sea level along the coast.

11. Describe how the uneven heating of the surface by the Sun affects Earths oceans.

12. How might a change in the location of an ocean current affect the climate of a region? For example, what if the Gulf Stream did not reach as far north?


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unIT 2Atmosphere and Climate


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