18 ocean motion 3 - clinton middle...

Ask Authentic QuestionsBefore you read, write downquestions you may have aboutcurrents in the ocean. Then, tryto answer them from thematerial in this section.

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Ocean Motion

section ● Ocean Currents2

What You’ll Learn■ the Coriolis effect■ what influences surface

currents■ the temperature of

coastal waters■ about density currents

Read to Learn

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Before You ReadImagine that you are stirring chocolate into a glass of milkwith a spoon. How does the milk move? What happenswhen you stir faster?

Surface CurrentsWhen you stir chocolate into a glass of milk, the milk

swirls around the glass in a circle. This is similar to the wayan ocean current moves. Ocean currents are a massmovement, or flow, of ocean water. Think of an oceancurrent as a river moving within the ocean.

A surface current is a current that moves waterhorizontally, or parallel to Earth’s surface. Surface currentsare powered by wind blowing over the water. The windforces the water in the ocean to move in huge, circularpatterns. In fact, the currents on the ocean’s surface arerelated to the circulation of the winds on Earth. However,these currents don’t affect the deep sections of the ocean.They move only the upper few hundred meters of seawater.

Some seeds and plants are carried between continents bysurface currents. Sailors have relied on surface currents andwinds to make sailing easier. You can see some surfacecurrents in the figure on the next page. The arrows showthe circular direction that the currents follow. Some of thecurrents are caused by warm winds and some are caused bycool winds.

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280 Ocean Motion

N

Equator

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Clockwise currents

Counter- clockwise currents

●B Organize InformationMake the following Foldablefrom a half sheet of notebookpaper to summarize informationabout currents in the northernand southern hemispheres.

517_p01-16_MSS05_RE 8/17/04 4:10 PM Page 6 impos05 301:goscanc:Reading Essentials - ES - 1:layouts:

2. Summarize How cansurface currents be helpfulto ships?

How do surface currents form?Surface ocean currents and surface winds are affected by

the Coriolis (kor ee OH lus) effect. The Coriolis effect isthe shifting of winds and surface currents from theirexpected paths because of Earth’s rotation.

Earth rotates toward the east. Because of this, winds inthe northern hemisphere turn to their right and winds inthe southern hemisphere turn to their left. These surfacewinds can cause water to pile up in certain parts of theocean. When gravity pulls water off the pile, the Corioliseffect turns the water. This causes surface water in oceans tospiral, or circle, around the piles of water.

Look again at the map of major surface currents. Thecircular patterns that you see are caused by the Corioliseffect. The currents north of the equator circle to theirright. Currents south of the equator circle to their left.

What is the Gulf Stream?Much of what is known about surface currents comes

from records that were kept by sailors in the nineteenthcentury. Sailors always have used surface currents to maketraveling easier. Sailors heading west use surface currentsthat flow west. Sailors heading east use currents such as theGulf Stream. The Gulf Stream is a 100-km-wide surfacecurrent in the Atlantic Ocean. When America was still acolony of England, sailors noticed that trips to England werefaster than trips from England. Going eastward with theGulf Stream made the journey quicker.

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Reading Essentials 281

Labrador

West Wind Drift West Wind Drift Falk

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GulfS tream

Equatorial

Peru

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Atlantic

Drift

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Caribbean

Equatorial

California

North Pacific

Equatorial Counter

North Equatorial

Can

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North Equatorial

North Equatorial Counter

Equatorial Counter

Major Surface Currents of Earth’s Oceans

Picture This1. Identify Name one

current that affects theoceans around NorthAmerica’s coasts.


3. Infer What couldscientists learn aboutcurrents from a driftbottle’s trip?

How are surface currents tracked?Items that wash up on beaches, such as bottles, can provide

information about ocean currents. One method used to tracksurface currents is to release drift bottles into the ocean. Driftbottles are released from a variety of coastal locations.

Inside each bottle, a message and a numbered card statewhere and when the bottle was released. When the bottlewashes ashore, the person who finds it may notice the cardinside. The person will fill out the card with the informationabout when and where it washed ashore. The card is returnedto the research team and provides valuable informationabout the surface currents that carried the bottle.

How do warm and cold surface currentsaffect the climate?

Look at the map of surface currents again. Notice that somecurrents start near the north and south poles, and othercurrents start near the equator. Currents on the west coasts ofcontinents begin near the poles where the water is colder.The California Current is an example of such a current. Itstarts near the north pole and is a cold surface current.

Currents on the east coast of continents start near theequator where the water is warmer. The Gulf Stream startsin waters near the equator and is a warm surface current.

As a warm surface current flows away from the equator,heat is released to the atmosphere. The atmosphere iswarmed. The transfer of heat helps determine climate.

UpwellingRecall that surface currents carry water horizontally—

parallel to Earth’s surface. Water also travels vertically, fromthe bottom to the top of the ocean. Upwelling is a verticalcirculation in the ocean that brings deep, cold water to theocean surface.

Along some coasts of continents, wind blowing parallel tothe coast carries water away from the land because of theCoriolis effect. Cold water from deep in the ocean rises upto replace it. The cold water is full of nutrients fromorganisms that died, sank to the bottom, and decayed. Fishare attracted to these nutrient-rich areas. Areas of upwellingare important fishing grounds. The figure on the next pageillustrates upwelling off the coast of Peru.

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282 Ocean Motion

4. Explain What happensduring upwelling?


6. Sequence of EventsNumber the events to showthe order in which a densitycurrent forms in Antarctica.

_____ seawater freezes

_____ unfrozen seawatersinks

_____ dense seawaterspreads alongocean floor

Density CurrentsDeep in the ocean, there is no wind to move the water.

Instead, differences in density cause water to circulate ormove. Cold water is more dense than warm water. Saltywater is more dense than less salty water.

A density current forms when a mass of seawaterbecomes more dense than the surrounding water. Gravitycauses this dense water to sink beneath less dense seawater.The deep, dense water spreads to the rest of the ocean.Changes in temperature and salinity work together tocreate density currents. A density current moves watervery slowly.

Where are density currents found?One important density current begins in Antarctica. In

winter, the seawater there is more dense than at any othertime. When seawater freezes, the salt is left behind in theunfrozen water. This extra salt increases the seawater’sdensity and causes it to sink. Slowly, the water begins tospread along the ocean bottom toward the equator forminga density current. In the Pacific Ocean, it could take up to1,000 years for the water in this density current to reachthe equator.

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Southerly wind

Water movement

Upwelling

Upwelling

Picture This5. Infer Why does upwelling

around Peru make Peru arich fishing ground?


North Atlantic Deep Water Another density current startsin the North Atlantic Ocean. Around Norway, Greenland,and Labrador, cold, dense waters form and sink. They formwhat is known as North Atlantic Deep Water. This watercovers the floor of the northern one-third to one-half of theAtlantic Ocean. In the southern part of the Atlantic Ocean,this current meets the density current from Antarctica. TheAntarctic density current is colder and denser. The NorthAtlantic Deep Water floats just above it. Density currentscirculate more quickly in the Atlantic Ocean than in thePacific Ocean. In the Atlantic, a density current couldcirculate in 275 years.

Do density currents affect other waters?Density currents also occur in the Mediterranean Sea. The

sea connects to the Atlantic Ocean by a narrow passagecalled the Strait of Gibraltar. Warm temperatures and dryair in the Mediterranean region cause the seawater toevaporate. The salts remain behind. This increases thesalinity and density of the sea. The dense, salty water travelsthrough the Straits of Gibraltar into the Atlantic Ocean.Because it is much denser than water at the surface of theocean, it sinks. However, it is not as dense as the very cold,salty water of the North Atlantic Deep Water. So, the waterfrom the Mediterranean floats above it. It forms a middlelayer known as the Mediterranean Intermediate Water. Youcan see the different water layers in the figure below.

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284 Ocean Motion

7. Think Critically Whichis more dense, the Antarcticcurrent or the North AtlanticDeep Water?

Picture This8. Interpret Scientific

Illustrations Whichlayer of water shown in thefigure is most dense?

Surface water

MediterraneanIntermediateWater

North AtlanticDeep Water

EvaporationMediterranean Sea


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Mini Glossary

End ofSection

Visit earth.msscience.com to access your textbook, interactivegames, and projects to help you learn more about ocean currents.


1. Review the terms and their definitions in the Mini Glossary. Write a sentence thatexplains where density currents and surface currents are found.

2. Complete the spider map about the Coriolis effect. List some of the results on oceancurrents of the Coriolis effect.

3. Before you read this section, you wrote down questions you had about ocean currents.Were you able to answer any of those questions? What information would you still liketo learn about ocean currents?

Coriolis effect: the shifting of winds and surface currents fromtheir expected paths that is caused by Earth’s rotation

density current: a current that forms in the ocean becausea mass of seawater becomes more dense than thesurrounding water and sinks

surface current: a current in the ocean that moves waterhorizontally, or parallel to Earth’s surface

upwelling: a vertical circulation in the ocean that bringsdeep, cold water to the ocean surface

Coriolis effect causes

currents to circle to their __________________________in the northern hemisphere

currents to circle to their left in the __________________hemisphere

surface water to _________________________________or circle


18 ocean motion 3 - clinton middle...

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