chapter 18 3 Ocean Motion section 2 Ocean Currents What You ll Learn the Coriolis effect what influences surface currents the temperature of coastal waters about density currents Before You Read Imagine that you are stirring chocolate into a glass of milk with a spoon. How does the milk move? What happens when you stir faster? Study Coach Ask Authentic Questions Before you read, write down questions you may have about currents in the ocean. Then, try to answer them from the material in this section. B Organize Information Make the following Foldable from a half sheet of notebook paper to summarize information about currents in the northern and southern hemispheres. N Clockwise currents Equator Counterclockwise currents S Read to Learn Surface Currents When you stir chocolate into a glass of milk, the milk swirls around the glass in a circle. This is similar to the way an ocean current moves. Ocean currents are a mass movement, or flow, of ocean water. Think of an ocean current as a river moving within the ocean. A surface current is a current that moves water horizontally, or parallel to Earth s surface. Surface currents are powered by wind blowing over the water. The wind forces the water in the ocean to move in huge, circular patterns. In fact, the currents on the ocean s surface are related 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 by surface currents. Sailors have relied on surface currents and winds to make sailing easier. You can see some surface currents in the figure on the next page. The arrows show the circular direction that the currents follow. Some of the currents are caused by warm winds and some are caused by cool winds. 280 Ocean Motion
517_p01-16_MSS05_RE 8/17/04 4:10 PM Page 7 impos05 301:goscanc:Reading Essentials - ES - 1:layouts: Major Surface Currents of Earth s Oceans Per West Wind Drift u West Wind Drift How do surface currents form? Surface ocean currents and surface winds are affected by the Coriolis (kor ee OH lus) effect. The Coriolis effect is the shifting of winds and surface currents from their expected paths because of Earth s rotation. Earth rotates toward the east. Because of this, winds in the northern hemisphere turn to their right and winds in the southern hemisphere turn to their left. These surface winds can cause water to pile up in certain parts of the ocean. When gravity pulls water off the pile, the Coriolis effect turns the water. This causes surface water in oceans to spiral, or circle, around the piles of water. Look again at the map of major surface currents. The circular patterns that you see are caused by the Coriolis effect. The currents north of the equator circle to their right. Currents south of the equator circle to their left. u ela ng Br az il Equatori al or ea th Equatorial m an Equatorial Counter Equato h rial ut So Falkland Agul has ib North Equatorial Counter South C a n a ry G Car ni a North Equator ial r Equatorial Cou nte q E u h atorial Sout be l oria u at North E q N ul f S tr Ala sk a No rth r ado L ab r r ifo C al North Pacific rift ic D t lan At Be Picture This 1. Identify Name one current that affects the oceans around North America s coasts. 2. Summarize How can surface currents be helpful to ships? What is the Gulf Stream? Much of what is known about surface currents comes from records that were kept by sailors in the nineteenth century. Sailors always have used surface currents to make traveling easier. Sailors heading west use surface currents that flow west. Sailors heading east use currents such as the Gulf Stream. The Gulf Stream is a 100-km-wide surface current in the Atlantic Ocean. When America was still a colony of England, sailors noticed that trips to England were faster than trips from England. Going eastward with the Gulf Stream made the journey quicker. Reading Essentials 281
3. Infer What could scientists learn about currents from a drift bottle s trip? 4. Explain What happens during upwelling? How are surface currents tracked? Items that wash up on beaches, such as bottles, can provide information about ocean currents. One method used to track surface currents is to release drift bottles into the ocean. Drift bottles are released from a variety of coastal locations. Inside each bottle, a message and a numbered card state where and when the bottle was released. When the bottle washes ashore, the person who finds it may notice the card inside. The person will fill out the card with the information about when and where it washed ashore. The card is returned to the research team and provides valuable information about the surface currents that carried the bottle. How do warm and cold surface currents affect the climate? Look at the map of surface currents again. Notice that some currents start near the north and south poles, and other currents start near the equator. Currents on the west coasts of continents begin near the poles where the water is colder. The California Current is an example of such a current. It starts near the north pole and is a cold surface current. Currents on the east coast of continents start near the equator where the water is warmer. The Gulf Stream starts in 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 is warmed. The transfer of heat helps determine climate. Upwelling Recall that surface currents carry water horizontally parallel to Earth s surface. Water also travels vertically, from the bottom to the top of the ocean. Upwelling is a vertical circulation in the ocean that brings deep, cold water to the ocean surface. Along some coasts of continents, wind blowing parallel to the coast carries water away from the land because of the Coriolis effect. Cold water from deep in the ocean rises up to replace it. The cold water is full of nutrients from organisms that died, sank to the bottom, and decayed. Fish are attracted to these nutrient-rich areas. Areas of upwelling are important fishing grounds. The figure on the next page illustrates upwelling off the coast of Peru. 282 Ocean Motion
Upwelling Picture This 5. Infer Why does upwelling around Peru make Peru a rich fishing ground? Water movement Southerly wind Upwelling Density Currents Deep in the ocean, there is no wind to move the water. Instead, differences in density cause water to circulate or move. Cold water is more dense than warm water. Salty water is more dense than less salty water. A density current forms when a mass of seawater becomes more dense than the surrounding water. Gravity causes 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 to create density currents. A density current moves water very slowly. Where are density currents found? One important density current begins in Antarctica. In winter, the seawater there is more dense than at any other time. When seawater freezes, the salt is left behind in the unfrozen water. This extra salt increases the seawater s density and causes it to sink. Slowly, the water begins to spread along the ocean bottom toward the equator forming a density current. In the Pacific Ocean, it could take up to 1,000 years for the water in this density current to reach the equator. 6. Sequence of Events Number the events to show the order in which a density current forms in Antarctica. seawater freezes unfrozen seawater sinks dense seawater spreads along ocean floor Reading Essentials 283
7. Think Critically Which is more dense, the Antarctic current or the North Atlantic Deep Water? Picture This 8. Interpret Scientific Illustrations Which layer of water shown in the figure is most dense? North Atlantic Deep Water Another density current starts in the North Atlantic Ocean. Around Norway, Greenland, and Labrador, cold, dense waters form and sink. They form what is known as North Atlantic Deep Water. This water covers the floor of the northern one-third to one-half of the Atlantic Ocean. In the southern part of the Atlantic Ocean, this current meets the density current from Antarctica. The Antarctic density current is colder and denser. The North Atlantic Deep Water floats just above it. Density currents circulate more quickly in the Atlantic Ocean than in the Pacific Ocean. In the Atlantic, a density current could circulate 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 passage called the Strait of Gibraltar. Warm temperatures and dry air in the Mediterranean region cause the seawater to evaporate. The salts remain behind. This increases the salinity and density of the sea. The dense, salty water travels through the Straits of Gibraltar into the Atlantic Ocean. Because it is much denser than water at the surface of the ocean, it sinks. However, it is not as dense as the very cold, salty water of the North Atlantic Deep Water. So, the water from the Mediterranean floats above it. It forms a middle layer known as the Mediterranean Intermediate Water. You can see the different water layers in the figure below. Mediterranean Sea Surface water Mediterranean Intermediate Water North Atlantic Deep Water Evaporation 284 Ocean Motion
After You Read Mini Glossary Coriolis effect: the shifting of winds and surface currents from their expected paths that is caused by Earth s rotation density current: a current that forms in the ocean because a mass of seawater becomes more dense than the surrounding water and sinks surface current: a current in the ocean that moves water horizontally, or parallel to Earth s surface upwelling: a vertical circulation in the ocean that brings deep, cold water to the ocean surface 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains where density currents and surface currents are found. 2. Complete the spider map about the Coriolis effect. List some of the results on ocean currents of the Coriolis effect. currents to circle to their in the northern hemisphere surface water to or circle Coriolis effect causes currents to circle to their left in the hemisphere 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 like to learn about ocean currents? Visit earth.msscience.com to access your textbook, interactive games, and projects to help you learn more about ocean currents. End of Section Reading Essentials 285