Lesson Objectives

• Define waves and explain their formation.
• Describe what causes tides.
• Describe how surface currents form and how they affect the world’s climate.
• Describe the causes of deep currents.
• Relate upwelling areas to the food chain.

Vocabulary

• Coriolis effect
• downwelling
• gyre
• high tide
• longshore current
• low tide
• neap tide
• rip current
• spring tide
• storm surge
• surface current
• thermohaline circulation
• tidal range
• tide
• upwelling
• wave

Introduction

Ocean water is constantly in motion: north-south, east-west, alongshore, and vertically. Seawater motions are the result of waves, tides, and currents (Figure below). Ocean movements are the consequence of many separate factors: wind, tides, Coriolis effect, water density differences, and the shape of the ocean basins. Water movements and their causes will be discussed in this lesson.

Waves

Waves have been discussed in previous chapters in several contexts: seismic waves traveling through the planet, sound waves traveling through seawater, and ocean waves eroding beaches. Waves transfer energy and the size of a wave and the distance it travels depends on the amount of energy that it carries.

Wind Waves

This lesson studies the most familiar waves, those on the ocean’s surface. Ocean waves originate from wind blowing – steady winds or high storm winds – over the water. Sometimes these winds are far from where the ocean waves are seen. What factors create the largest ocean waves?

The largest wind waves form when the wind

• is very strong
• blows steadily for a long time
• blows over a long distance

The wind could be strong, but if it gusts for just a short time, large waves won’t form.

Wind blowing across the water transfers energy to that water. The energy first creates tiny ripples that create an uneven surface for the wind to catch so that it may create larger waves. These waves travel across the ocean out of the area where the wind is blowing.

Remember that a wave is a transfer of energy. Do you think the same molecules of water that starts out in a wave in the middle of the ocean later arrive at the shore?

Water molecules in waves make circles or ellipses (Figure below). Energy transfers between molecules but the molecules themselves mostly bob up and down in place.

In this animation, a water bottle bobs in place like a water molecule: http://www.onr.navy.mil/focus/ocean/motion/waves1.htm

An animation of motion in wind waves from the Scripps Institution of Oceanography: http://earthguide.ucsd.edu/earthguide/diagrams/waves/swf/wave_wind.html

An animation of a deep water wave is seen here: http://en.wikipedia.org/wiki/File:Deep_water_wave.gif.

An animation of a shallow water wave is seen here: http://commons.wikimedia.org/wiki/File:Shallow_water_wave.gif.

When does a wave break? Do waves only break when they reach shore? Waves break when they become too tall to be supported by their base. This can happen at sea but happens predictably as a wave moves up a shore. The energy at the bottom of the wave is lost by friction with the ground so that the bottom of the wave slows down but the top of the wave continues at the same speed. The crest falls over and crashes down.

Some of the damage done by storms is from storm surge. Water piles up at a shoreline as storm winds push waves into the coast. Storm surge may raise sea level as much as 7.5 m (25 ft), which can be devastating in a shallow land area when winds, waves, and rain are intense.

A wild video of “Storm Surge” can be seen on National Geographic Videos, Environment Video, Natural Disasters, Landslides, and more: http://video.nationalgeographic.com/video/player/environment/

KQED: Science of Big Waves

Maverick waves are massive. Learning how they are generated can tell scientists a great deal about how the ocean creates waves and especially large waves. Learn more at: http://www.kqed.org/quest/television/science-of-big-waves.

Tsunami

Tsunami are described in the Earthquakes chapter as damaging waves that result from the sharp jolt to the water from an undersea earthquake. Landslides, meteorite impacts, or any other jolt to ocean water may form a tsunami (Figure below). Tsunami can travel at speeds of 800 kilometers per hour (500 miles per hour).

Tsunami have small wave heights and long wavelengths so they are usually unnoticed at sea. As the wave rides up the continental shelf the wave height increases.

A video explanation of tsunami is here: http://www.youtube.com/watch?v=StdqGoezNrY

The wave speed of a tsunami is also slowed by friction with the shallower ocean floor, which causes the wavelength to decrease, creating a much taller wave.

Many people caught in a tsunami have no warning of its approach. Since the wavelength is long, a long time can pass between crests or troughs onshore. In 1755 in Lisbon, an offshore earthquake caused a great deal of damage on land. People rushed to the open space of the shore and discovered that the water was flowing seaward fast. The trough of the tsunami wave reached shore first. People who went out onto the open beach were drowned when the crest of the wave reached shore.

KQED: Science on the SPOT: Watching the Tides

Large tsunami in the Indian Ocean and more recently Japan have killed hundreds of thousands of people in recent years. The west coast is vulnerable to tsunami since it sits on the Pacific Ring of Fire. Scientists are trying to learn everything they can about predicting tsunamis before a massive one strikes a little closer to home. Learn more at: http://science.kqed.org/quest/video/scary-tsunamis/.

Tides

Tides are the daily rise and fall of sea level at any given place. The pull of the Moon’s gravity on Earth is the primarily cause of tides and the pull of the Sun’s gravity on Earth is the secondary cause (Figure below). The Moon has a greater effect because, although it is much smaller than the Sun, it is much closer. The Moon’s pull is about twice that of the Sun’s.

Daily Tide Patterns

To understand the tides it is easiest to start with the effect of the Moon on Earth. As the Moon revolves around our planet, its gravity pulls Earth toward it. The lithosphere is unable to move much but the water is pulled by the gravity and a bulge is created. This bulge is the high tide beneath the Moon. The Moon’s gravity then pulls the Earth toward it, leaving the water on the opposite side of the planet behind. This creates a second high tide bulge on the opposite side of Earth from the Moon. These two water bulges on opposite sides of the Earth aligned with the Moon are the high tides.

Since so much water is pulled into the two high tides, low tides form between the two high tides (Figure below). As the Earth rotates beneath the Moon, a single spot will experience two high tides and two low tides every day.

A detailed animation of lunar tides is shown here: http://www.pbs.org/wgbh/nova/venice/tides.html.

The tidal range is the difference between the ocean level at high tide and the ocean at low tide (Figure below). The tidal range in a location depends on a number of factors, including the slope of the seafloor. Water appears to move a greater distance on a gentle slope than on a steep slope.

Monthly Tide Patterns

If you look at the diagram of high and low tides in the Figure above, you’ll see that tides are waves. So when the Sun and Moon are aligned, what do you expect the tides to look like?

Waves are additive so when the gravitational pull of both bodies is in the same direction the high tides add and the low tides add (Figure below). Highs are higher and lows are lower than at other times through the month. These more extreme tides, with a greater tidal range, are called spring tides. Spring tides don’t just occur in the spring; they occur whenever the Moon is in a new-moon or full-moon phase, about every 14 days.

Here is a link to see these tides in motion: http://oceanservice.noaa.gov/education/kits/tides/media/tide06a_450.gif.

Neap tides are tides that have the smallest tidal range, and they occur when the Earth, the Moon, and the Sun form a 90^o angle (Figure below). They occur exactly halfway between the spring tides, when the Moon is at first or last quarter. How do the tides add up to create neap tides? The Moon’s high tide occurs in the same place as the Sun’s low tide and the Moon’s low tide in the same place as the Sun’s high tide. At neap tides, the tidal range relatively small.

A simple animation of spring and neap tides is found here: http://oceanservice.noaa.gov/education/kits/tides/media/supp_tide06a.html.

High tides occur about twice a day, about every 12 hours and 25 minutes. The reason is that the Moon takes 24 hours and 50 minutes to rotate once around the Earth so the Moon is over the same location 24 hours and 50 minutes later. Since high tides occur twice a day, one arrives each 12 hours and 25 minutes. What is the time between a high tide and the next low tide?

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

Some coastal areas do not follow this pattern at all. These coastal areas may have one high and one low tide per day or a different amount of time between two high tides. These differences are often because of local conditions, such as the shape of the coastline that the tide is entering.

Surface Currents

Ocean water moves in predictable ways along the ocean surface. Surface currents can flow for thousands of kilometers and can reach depths of hundreds of meters. These surface currents do not depend on weather; they remain unchanged even in large storms because they depend on factors that do not change.

Surface currents are created by three things:

• global wind patterns
• the rotation of the Earth
• the shape of the ocean basins

Surface currents are extremely important because they distribute heat around the planet and are a major factor influencing climate around the globe.

Global Wind Patterns

Winds on Earth are either global or local. Global winds blow in the same directions all the time and are related to the unequal heating of Earth by the Sun — that is, more solar radiation strikes the equator than the polar regions –- and the rotation of the Earth — that is, the Coriolis effect. The causes of the global wind patterns will be described in detail in the Earth’s Atmosphere chapter.

Water in the surface currents is pushed in the direction of the major wind belts:

• trade winds: east to west between the equator and 30^oN and 30^oS
• westerlies: west to east in the middle latitudes
• polar easterlies: east to west between 50^o and 60^o north and south of the equator and the north and south pole

Earth’s Rotation

Wind is not the only factor that affects ocean currents. The Coriolis effect describes how Earth’s rotation steers winds and surface ocean currents (Figure below). Coriolis causes freely moving objects to appear to move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The objects themselves are actually moving straight, but the Earth is rotating beneath them, so they seem to bend or curve.

An example might make the Coriolis effect easier to visualize. If an airplane flies 500 miles due north, it will not arrive at the city that was due north of it when it began its journey. Over the time it takes for the airplane to fly 500 miles, that city moved, along with the Earth it sits on. The airplane will therefore arrive at a city to the west of the original city (in the Northern Hemisphere), unless the pilot has compensated for the change. So to reach his intended destination, the pilot must also veer right while flying north.

As wind or an ocean current moves, the Earth spins underneath it. As a result, an object moving north or south along the Earth will appear to move in a curve, instead of in a straight line. Wind or water that travels toward the poles from the equator is deflected to the east, while wind or water that travels toward the equator from the poles gets bent to the west. The Coriolis effect bends the direction of surface currents to the right in the Northern Hemisphere and left in the Southern Hemisphere.

Coriolis effect is demonstrated using a metal ball and a rotating plate in this video. The ball moves in a circular path just like a freely moving particle of gas or liquid moves on the rotating Earth (5b): http://www.youtube.com/watch?v=Wda7azMvabE (2:04).

Shape of the Ocean Basins

When a surface current collides with land, the current must change direction. In the Figure below, the Atlantic South Equatorial Current travels westward along the equator until it reaches South America. At Brazil, some of it goes north and some goes south. Because of Coriolis effect, the water goes right in the Northern Hemisphere and left in the Southern Hemisphere.

You can see on the map of the major surface ocean currents that the surface ocean currents create loops called gyres (Figure below). The Antarctic Circumpolar Current is unique because it travels uninhibited around the globe. Why is it the only current to go all the way around?

This video shows the surface ocean currents set by global wind belts (5a): http://www.youtube.com/watch?v=Hu_Ga0JYFNg (1:20).

Local Surface Currents

The surface currents described above are all large and unchanging. Local surface currents are also found along shorelines (Figure below). Two are longshore currents and rip currents.

Rip currents are potentially dangerous currents that carry large amounts of water offshore quickly. Look at the rip-current animation to determine what to do if you are caught in a rip current: http://www.onr.navy.mil/focus/ocean/motion/currents2.htm. Each summer in the United States at least a few people die when they are caught in rip currents.

This animation shows the surface currents in the Caribbean, the Gulf of Mexico, and the Atlantic Ocean off of the southeastern United States: http://polar.ncep.noaa.gov/ofs/viewer.shtml?-gulfmex-cur-0-large-rundate=latest.

Effect on Global Climate

Surface currents play an enormous role in Earth’s climate. Even though the equator and poles have very different climates, these regions would have more extremely different climates if ocean currents did not transfer heat from the equatorial regions to the higher latitudes.

The Gulf Stream is a river of warm water in the Atlantic Ocean, about 160 kilometers wide and about a kilometer deep. Water that enters the Gulf Stream is heated as it travels along the equator. The warm water then flows up the east coast of North America and across the Atlantic Ocean to Europe (Figure below). The energy the Gulf Stream transfers is enormous: more than 100 times the world’s energy demand.

The Gulf Stream’s warm waters raise temperatures in the North Sea, which raises the air temperatures over land between 3 to 6^oC (5 to 11^oF). London, U.K., for example, is at the same latitude as Quebec, Canada. However, London’s average January temperature is 3.8^oC (38^oF), while Quebec’s is only -12^oC (10^oF). Because air traveling over the warm water in the Gulf Stream picks up a lot of water, London gets a lot of rain. In contrast, Quebec is much drier and receives its precipitation as snow.

Deep Currents

Thermohaline circulation drives deep ocean circulation. Thermo means heat and haline refers to salinity. Differences in temperature and in salinity change the density of seawater. So thermohaline circulation is the result of density differences in water masses because of their different temperature and salinity.

What is the temperature and salinity of very dense water? Lower temperature and higher salinity yield the densest water. When a volume of water is cooled, the molecules move less vigorously so same number of molecules takes up less space and the water is denser. If salt is added to a volume of water, there are more molecules in the same volume so the water is denser.

Changes in temperature and salinity of seawater take place at the surface. Water becomes dense near the poles. Cold polar air cools the water and lowers its temperature, increasing its salinity. Fresh water freezes out of seawater to become sea ice, which also increases the salinity of the remaining water. This very cold, very saline water is very dense and sinks. This sinking is called downwelling.

This video lecture discusses the vertical distribution of life in the oceans. Seawater density creates currents, which provide different habitats for different creatures (5d): http://www.youtube.com/watch?v=LA1jxeXDsdA (6:12).

Two things then happen. The dense water pushes deeper water out of its way and that water moves along the bottom of the ocean. This deep water mixes with less dense water as it flows. Surface currents move water into the space vacated at the surface where the dense water sank (Figure below). Water also sinks into the deep ocean off of Antarctica.

Since unlimited amounts of water cannot sink to the bottom of the ocean, water must rise from the deep ocean to the surface somewhere. This process is called upwelling (Figure below).

Generally, upwelling occurs along the coast when wind blows water strongly away from the shore. This leaves a void that is filled by deep water that rises to the surface.

Upwelling is extremely important where it occurs. During its time on the bottom, the cold deep water has collected nutrients that have fallen down through the water column. Upwelling brings those nutrients to the surface. Those nutrient support the growth of plankton and form the base of a rich ecosystem. California, South America, South Africa, and the Arabian Sea all benefit from offshore upwelling.

An animation of upwelling is seen here: http://oceanservice.noaa.gov/education/kits/currents/03coastal4.html.

Upwelling also takes place along the equator between the North and South Equatorial Currents. Winds blow the surface water north and south of the equator so deep water undergoes upwelling. The nutrients rise to the surface and support a great deal of life in the equatorial oceans.

Lesson Summary

• Ocean waves are energy traveling through the water.
• Most ocean waves are generated by wind. Tsunami are exceptionally long wavelength waves usually caused by earthquakes.
• Tides are produced by the gravitational pull of the Moon and Sun.
• Spring tides have large tidal ranges and occur at full and new moons, when Earth, Moon, and Sun are all aligned.
• Neap tides have low tidal ranges and occur at first and last quarter moons, when the Moon is at right angles to the Sun.
• Ocean surface currents are produced by global winds, the Coriolis effect and the shape of each ocean basin.
• The Pacific and Atlantic Oceans have a circular pattern of surface currents called gyres that circle clockwise in the Northern Hemisphere and counterclockwise in the Southern. The Indian Ocean only has a counterclockwise gyre.
• Surface ocean circulation brings warm equatorial waters towards the poles and cooler polar water towards the equator.
• Thermohaline circulation drives deep ocean currents.
• Upwelling of cold, nutrient-rich waters creates biologically rich areas where surface waters are blown away from a shore, or where equatorial waters are blow outward.

Review Questions

1. What factors of wind determine the size of a wave?

2. Tsunami are sometimes incorrectly called “tidal waves.” Explain why this is not correct.

3. Describe what causes the tides.

4. What is a tidal range? In what types of tides is the tidal range greatest? Lowest?

5. Why do some places have a greater tidal range than other places?

6. What causes the patterns of surface currents in the ocean?

7. How do ocean surface currents affect climate?

8. What is the Coriolis effect?

9. What process can make deep, dense water rise to the surface?

10. Why are upwelling areas important to marine life?

Further Reading / Supplemental Links

• Learn About Ocean Currents, 5 min. Life Videopedia http://www.5min.com/Video/Learn-about-Ocean-Currents-117529352
• NOAA’s Ocean Explorer program http://oceanexplorer.noaa.gov/edu/welcome.html
• A tutorial for grades 6 to 12 on currents from NOAA: http://oceanservice.noaa.gov/education/tutorial_currents/welcome.html
• Cool jobs: Oceanographer http://news.discovery.com/videos/cool-jobs-cool-jobs-oceanographer.html

Points to Consider

• Some scientists have hypothesized that if enough ice in Greenland melts, the Gulf Stream might be shut down. Why might this happen?
• If the Gulf Stream shuts down, what would be the result on climate in Europe?
• How do the movements of ocean water contribute to the ocean’s life?

Going Further – Applying Math

Tide Generating Force

In this exercise you will use an equation to calculate the tide generating force. Like the force of gravity, the pull of the tide generating force is directly related to the masses of the astronomical objects involved and inversely related to the square of the distance between them. Tides are caused by both the gravitational pull of the Moon and the gravitational pull of the Sun on surface water. Unlike the gravitational force, the tide generating force varies with the distance between the Moon (or the Sun) and the Earth cubed.

The equation for the tide generating force is: T = G (m1 . m2 / d3 ) where T is the tide generating force, G is the universal gravitational constant, m1 and m2 are the mass of the Earth and the mass of the Moon (or the mass of the Earth and the mass of the Sun), and d is the distance between them.

If we plug in values for the gravitational constant, the mass of the Earth and the mass of the Moon, we can calculate the tide generating force when the Moon is at apogee (farthest from the Earth in its orbit). Use G = 6.673 x 10 -11 m 3 / kg . s 2; m 1 = 7.35 x 10 22 kg for the mass of the Moon, m 2 = 5.974 x 10 24 kg for the mass of the Earth; and d = 405,500 km for the distance from the Earth to the Moon at apogee.

You could use all the same values but substitute in d = 363,300 km for the distance from the Earth to the Moon when the Moon is at perigee (point when the Moon is closest to the Earth) and compare the tide generating force each distance.

Tsunami Tag

Could you outrun a tsunami? How fast would you have to run? You can calculate how fast a tsunami travels in the ocean using the equation for the speed of a shallow water wave, which is: V = the square root of g x d, where V = wave speed (velocity), g = the acceleration of gravity: 9.8 meters / s 2, and d = the depth of the water. If you use d = 3,940m (the average depth of the Pacific Ocean), how fast does a tsunami travel? Do you think you could outrun this wave?