14.2 Ocean Movements

Lesson Objectives

• Describe how waves move through water.
• Explain what causes tides.
• Give an overview of surface currents.
• Identify the cause of deep currents.
• Describe upwelling.

Vocabulary

• convection current
• Coriolis effect
• deep current
• density
• neap tide
• spring tide
• surface current
• tide
• upwelling
• wave

Introduction

If you’ve ever visited an ocean shore, then you know that ocean water is always moving. Waves ripple through the water, as shown in Figure below. The water slowly rises and falls because of tides. You may see signs warning of currents that flow close to shore. What causes all these ocean motions? Different types of motions have different causes.

Waves cause the rippled surface of the ocean.

Waves

Most ocean waves are caused by winds. A wave is the transfer of energy through matter. A wave that travels across miles of ocean is traveling energy, not water. Ocean waves transfer energy from wind through water. The energy of a wave may travel for thousands of miles. The water itself moves very little. Figure below shows how water molecules move when a wave goes by.

A wave travels through the water. How would you describe the movement of water molecules as a wave passes through?

The Size of Waves

Figure above also shows how the size of waves is measured. The highest point of a wave is the crest. The lowest point is the trough. The vertical distance between a crest and a trough is the height of the wave. Wave height is also called amplitude. The horizontal distance between two crests is the wavelength. Both amplitude and wavelength are measures of wave size.

The size of an ocean wave depends on how fast, over how great a distance, and how long the wind blows. The greater each of these factors is, the bigger a wave will be. Some of the biggest waves occur with hurricanes. A hurricane is a storm that forms over the ocean. Its winds may blow more than 150 miles per hour! The winds also travel over long distances and may last for many days.

Breaking Waves

Figure below shows what happens to waves near shore. As waves move into shallow water, they start to touch the bottom. The base of the waves drag and slow. Soon the waves slow down and pile up. They get steeper and unstable as the top moves faster than the base. When they reach the shore, the waves topple over and break.

Waves break when they reach the shore.

Tsunamis

Not all waves are caused by winds. A shock to the ocean can also send waves through water. A tsunami is a wave or set of waves that is usually caused by an earthquake. As we have seen in recent years, the waves can be enormous and extremely destructive. Usually tsunami waves travel through the ocean unnoticed. But when they reach the shore they become enormous. Tsunami waves can flood entire regions. They destroy property and cause many deaths. Figure below shows the damage caused by a tsunami in the Indian Ocean in 2004.

A 2004 tsunami caused damage like this all along the coast of the Indian Ocean. Many lives were lost.

Tides

Tides are daily changes in the level of ocean water. They occur all around the globe. High tides occur when the water reaches its highest level in a day. Low tides occur when the water reaches its lowest level in a day. Tides keep cycling from high to low and back again. In most places the water level rises and falls twice a day. So there are two high tides and two low tides approximately every 24 hours.

In Figure below, you can see the difference between high and low tides. This is called the tidal range.

Where is the intertidal zone in this picture?

Why Tides Occur

Figure below shows why tides occur. The main cause of tides is the pull of the Moon’s gravity on Earth. The pull is greatest on whatever is closest to the Moon. Although the gravity pulls the land, only the water can move. As a result:

• Water on the side of Earth facing the Moon is pulled hardest by the Moon’s gravity. This causes a bulge of water on that side of Earth. That bulge is a high tide.
• Earth itself is pulled harder by the Moon’s gravity than is the ocean on the side of Earth opposite the Moon. As a result, there is bulge of water on the opposite side of Earth. This creates another high tide.
• With water bulging on two sides of Earth, there’s less water left in between. This creates low tides on the other two sides of the planet.

High and low tides are due mainly to the pull of the Moon’s gravity.

Spring Tides and Neap Tides

The Sun’s gravity also pulls on Earth and its oceans. Even though the Sun is much larger than the Moon, the pull of the Sun’s gravity is much less because the Sun is much farther away. The Sun's gravity strengthens or weakens the Moon’s influence on tides.

Figure below shows the position of the Moon relative to the Sun at different times during the month. The positions of the Moon and Sun relative to each other determines how the Sun affects tides. This creates spring tides or neap tides.

• Spring tides occur during the new moon and full moon. The Sun and Moon are in a straight line either on the same side of Earth or on opposite sides. Their gravitational pull combines to cause very high and very low tides. Spring tides have the greatest tidal range.
• Neap tides occur during the first and third quarters of the Moon. The Moon and Sun are at right angles to each other. Their gravity pulls on the oceans in different directions so the highs and lows are not as great. Neap tides have the smallest tidal range.

The Sun and Moon both affect Earth’s tides.

This animation shows the effect of the Moon and Sun on the tides: http://www.onr.navy.mil/focus/ocean/motion/tides1.htm.

Surface Currents

Another way ocean water moves is in currents. A current is a stream of moving water that flows through the ocean. Surface currents are caused mainly by winds, but not the winds that blow and change each day. Surface currents are caused by the major wind belts that blow in the same direction all the time.

The major surface currents are shown in Figure below. They flow in a clockwise direction in the Northern Hemisphere. In the Southern Hemisphere, they flow in the opposite direction.

Earth’s surface currents flow in the patterns shown here.

Coriolis Effect

Winds and surface currents tend to move from the hot equator north or south toward the much cooler poles. That’s because of differences in the temperature of air masses over Earth’s surface. But Earth is spinning on its axis underneath the wind and water as they move. The Earth rotates from west to east. As a result, winds and currents actually end up moving toward the northeast or southeast. This effect of Earth’s rotation on the direction of winds and currents is called the Coriolis effect.

Surface Currents and Climate

Large ocean currents can have a big impact on the climate of nearby coasts. The Gulf Stream, for example, carries warm water from near the equator up the eastern coast of North America. Look at the map in Figure below. It shows how the Gulf Stream warms both the water and land along the coast.

In this satellite photo, different colors indicate the temperatures of water and land. The warm Gulf Stream can be seen snaking up eastern North America.

Deep Currents

Currents also flow deep below the surface of the ocean. Deep currents are caused by differences in density at the top and bottom. Density is defined as the amount of mass per unit of volume. More dense water takes up less space than less dense water. It has the same mass but less volume. Water that is more dense sinks. Less dense water rises. What can make water more dense?

Water becomes more dense when it is colder and when it has more salt. In the North Atlantic Ocean, cold winds chill the water at the surface. Sea ice grows in this cold water, but ice is created from fresh water. The salt is left behind in the seawater. This cold, salty water is very dense, so it sinks to the bottom of the North Atlantic. Downwelling can take place in other places where surface water becomes very dense (see Figure below).

Deep currents flow because of differences in density of ocean water.

When water sinks it pushes deep water along at the bottom of the ocean. This water circulates through all of the ocean basins in deep currents.

Upwelling

Sometimes deep ocean water rises to the surface. This is called upwelling. Figure below shows why it happens. Strong winds blow surface water away from shore. This allows deeper water to flow to the surface and take its place.

An upwelling occurs when deep ocean water rises to the surface.

When water comes up from the deep, it brings a lot of nutrients with it. Why is deep water so full of nutrients? Over time, dead organisms and other organic matter settle to the bottom water and collect. The nutrient-rich water that comes to the surface by upwelling supports many living things.

Lesson Summary

• Most ocean waves are caused by winds. The size of a wave depends on how fast, how far, and how long the wind blows. Tsunamis are waves caused by earthquakes.
• Tides are daily changes in the level of ocean water. They are caused mainly by the pull of the Moon’s gravity on Earth and its oceans. The Sun’s gravity also influences tides.
• Surface currents are like streams flowing through the surface of the ocean. They are caused mainly by winds. Earth’s rotation influences their direction. This is called the Coriolis effect. Surface currents may affect the climate of nearby coasts.
• Deep currents are convection currents that occur far below the surface. They are caused by differences in the density of ocean water.
• Upwelling occurs when deep ocean water rises to the surface. The water brings nutrients with it. These nutrients support many organisms.

Lesson Review Questions

Recall

1. Identify two causes of ocean waves.

2. What factors determine how big a wave is?

3. What is the Coriolis effect?

4. Define density. How is the density of water related to its temperature?

5. Describe upwelling. State why it occurs.

Apply Concepts

6. The crest of an ocean wave is 3 meters above the still water level. The trough is 3 meters below the still water level. The horizontal distance between the crest and trough is 8 meters. Draw a diagram of this wave. Label the crest, trough, and distances. Then calculate the wave’s amplitude and wavelength.

7. Assume that a spring tide occurs on September 1. Predict when the next neap tide will occur. When will the next spring tide occur? Explain your answers.

Think Critically

8. Explain why waves break on the shore.

9. If the tidal cycle was actually 12 hours then high tides would occur at the same time every day. In reality, high tides occur about every 12 hours and 25 minutes. Can you think of why this would be the case?

10. Are deep currents the same as surface currents except that they are near the ocean bottom?

Points to Consider

Upwelling brings nutrients to the surface from the ocean floor. Nutrients are important resources for ocean life. However, they aren’t the only resources on the ocean floor.

• What other resources do you think might be found on the ocean floor?
• It’s hard to get resources from the ocean floor. Can you explain why?