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Introduction Introduction From the observations of Lt. Pelham aboard the Each of the half-hour video episodes is accompa- H.M.S. Challenger to the mapping of oceans by satel- nied by a complementary lesson containing the fol- lites and undersea vehicles, the study of the ocean is lowing features to help you master your study of as rich and dynamic as the ocean itself. The Endless oceanography: Voyage will bring you face to face with exotic deni- — An Overview summarizes each lesson’s main zens of the deep and the peculiarities of their watery topics. world. It will take you on virtual field trips to lead- ing research institutions fromWoods Hole, Massa- — Learning Objectives identify the major concepts, chusetts to La Jolla, California. It will introduce you ideas, and factual data that you should recall and to scientists and academicians involved in the sci- understand after viewing the video and reading ence and study of oceanography in a wide range of the required selections from the text. Many test disciplines who share their ideas and insights with questions are derived from these objectives. you. — Assignments that link the video lesson with This guide is one part of the total package avail- related sections of the text. able to you when you embark upon the 26 half-hour videos that comprise The Endless Voyage. Each video —Video Focus Points help you follow and analyze episode will introduce you to academicians, information in the video and integrate the infor- researchers, physical oceanographers, chemists, mation with your readings. marine biologists, tsunami specialists, geologists, — Text Focus Points are intended to guide your and a host of others—people whose contributions reading of the selections for each assignment. are helping to interpret current understanding of the Earth’s ocean and shape the our environmental — Key Terms and Concepts help you to focus on the future. words and ideas important to understanding the These video episodes, in conjunction with the language of oceanography as you work through guidance of your instructor, are closely integrated each lesson. with Tom Garrison’s text, Oceanography: An Invita- — A Self Test enables you to check your under- tion to Marine Science. The fifth edition of this popu- standing of the material in the video and text lar text provides a richly illustrated foundation of assignments. information upon which to base your study. An interactive web component enhances independent — Supplemental Activities provide opportunities learning. for further examination of the issues raised by the video and readings in the text. v Lesson 13 Surf’s Up Overview Do waves conjure an image of lazy sunsets and clear mental image of this orbital activity to set the the gentle slap of water on the sand? Or maybe stage for understanding the forms and behaviors the roar of water littered with surfers? Waves can of the various types of wind waves. The diagrams range from soothing to destructive. In its broadest found on pages 230 and 231 of your text will help. definition, the term wave describes a disturbance Ocean waves are classified and named accord- caused by the movement of energy from a source, ing to the disturbing forces that cause them, and through a medium—solid, liquid, or gas. As that the restoring forces, or surface tension and grav- energy, or disturbing force, travels, it affects the ity, that attempt to restore the surface to smooth- medium in specific ways. The focus of this lesson ness. Wind is a major disturbing force, but is how ocean water is affected by a particular type earthquakes, landslides, erupting volcanoes, and of energy; namely, the wind. tides can also cause waves. Waves can occur in Ocean waves seem like a moving ridge of any ocean, but are most common around the equa- water that can be ridden to shore. By definition, tor and in the Southern Hemisphere, where most however, a wave is a moving ridge of energy, not storm energy originates. Large waves originating water. Look at Figure 10.1 on page 230 in your in these areas tend to move northward, affecting text, in which a seagull sitting on the water’s sur- the ocean and shorelines in the Northern Hemi- face stays in essentially the same lateral position sphere. but is moved vertically as the forward-moving In order to discuss and understand wind wave energy changes the wave form, or shape, from formation and behavior, there are a number of crests to troughs to crests. When energy contacts descriptive terms to be learned. Wave shape con- the sea surface as wind, some of it is transferred to sists of the crest, or high point, and trough, or low the water beneath the surface, causing the water point. The period of a wave is its travel time molecules comprising the wave form to move. expressed in seconds and measured based on the These tend to form a circular, or orbital, pattern as time it takes successive wave crests to pass a fixed the energy passes, but the water itself does not point. Wave size refers to its height and length. progress forward. This orbital wave concept Wavelength is measured by the distance applies to most of the more common types of between adjacent crests or adjacent troughs. The waves. speed of a wave is based on the ratio of the length The orbital motion of the water continues of the wave to its period. It is ultimately controlled beneath the surface, but the diameter of the orbit by gravity, wavelength, and water depth. As a gen- is controlled by the depth of the water, rapidly eral rule, the longer the wavelength is, the faster diminishing when the depth of the water is less the wave energy will move through the water. than half of the wavelength. Most ocean waves Height is an important characteristic of wind have moderate wavelengths, so the circular distur- waves, and is measured from the trough to a line bance that propagates the wave affects only the connecting the two adjacent crests. Moderately uppermost water mass. It is important to have a sized waves usually have about a 1:7 ratio of wave 93 94 The Endless Voyage height to length. Waves with a ratio higher than Not all waves are surface waves; subsurface this will often break at sea, dissipating their internal waves may form where water of different energy as whitecaps or combers. The highest wave density masses meet. You may recall that most accurately measured and recorded to date was 34 surface winds in the tropical Pacific, such as trade meters (112 feet). winds, move from east to west, but that at irregu- Wind waves begin as small capillary waves, lar intervals they can reverse direction and move when some wind energy is transferred from the from west to east. This is called the Southern atmosphere to the sea surface. If the wind contin- Oscillation, and it causes Ekman transport in the ues and grows stronger, the orbital motion gains area to change direction, too. Instead of flowing up momentum, while the surface tension and grav- and away from the equator it moves down and ity—the restoring forces—try to flatten them out, toward it. The warm water of the upper layer of creating a larger wave called a gravity wave. the Pacific downwells into the lower layer, result- Deep-water waves are those that move ing in an oscillation where those two density- through water that is deeper than one-half their different layers meet. This oscillation forms an wavelength; shallow-water waves are found internal wave at the base of the pycnocline, espe- where the water is less than one-twentieth of the cially at the bottom of a steep thermocline. wavelength. Deep-water waves can become larger Internal waves are also important in trans- as the wind continues to blow, forming into a cha- porting and mixing nutrients with surface waters, otic mix of waves of different lengths, called a sea. affecting both midwater and coastal planktonic These storm waves tend to sort themselves out organisms. Although deep-ocean internal waves into groups with similar lengths and speeds, set- are less well known than those along the coasts, tling into a smooth surface undulation called space shuttle and TOPEX/Poseidon satellite data swell, which may develop into progressive groups have recently documented their existence and called wave trains. Wave trains coming from dif- basic behaviors. ferent directions sometimes meet in an interaction Since they are such a fundamental component known as interference. It is called destructive of the oceanic system, waves have been observed interference if they counteract each other and con- and studied since the early voyagers. Wave study structive interference if they reinforce each other. became even more important in the mid-1940s If high crests from different wave trains meet, a when amphibious landings on the coasts of rogue wave—one that is higher than is theoreti- Europe were being planned during World War II. cally possible—may form. Rogue waves can also Since then significant progress has been made in form when a wind wave meets a swift current. the effort to describe, monitor, and predict wave As a deep-water wave approaches a shoreline formation and propagation using a number of it will eventually “feel” the bottom, which initiates highly innovative techniques and devices. One a series of events that change the form and speed such device, called a pressure sensor, is mounted of the wave, eventually forming surf as the wave on the sea floor and actually records wave height dissipates its water and energy on the beach.
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