Chapter 10Waves

OceanographyOceanographyAn Invitation to Marine Science, 7thAn Invitation to Marine Science, 7th

Tom GarrisonTom Garrison

Chapter 10 Study Plan

• Ocean Waves Move Energy across the Sea Surface• Waves Are Classified by Their Physical Characteristics• The Behavior of Waves Is Influenced by the Depth of Water

through Which They Are Moving• Wind Blowing over the Ocean Generates Waves• Interference Produces Irregular Wave Motions• Deep-Water Waves Change to Shallow-Water Waves As

They Approach Shore• Internal Waves Can Form between Ocean Layers of

Differing Densities• “Tidal Waves” Are Probably Not What You Think• Storm Surges Form beneath Strong Cyclonic Storms• Water Can Rock in a Confined Basin• Water Displacement Causes Tsunami and Seismic Sea

Waves

Chapter 10 Main Concepts

• Waves transmit energy, not water mass, across the ocean’s surface. Ocean waves are orbital waves in which water molecules move in closed circles (orbits) as the wave passes.

• Ocean waves are classified by the disturbing force that creates them, the extent to which the disturbing force continues to influence them once they are formed, and by their wavelength.

• The speed (celerity) of an ocean wave is proportional to its wavelength. Most characteristics of ocean waves depend on the relationship between their wavelength and water depth.

• The orbits of water molecules in waves moving through water deeper than half the wavelength are unaffected by the bottom. The wavelength of tsunami and tides are so great that they are always in shallow water.

• Water displacement causes tsunami and seismic sea waves. Unnoticeable in the open sea, tsunami rush ashore like a sudden and very high onrushing tide. These huge shallow-water waves are among the most lethal of our planet’s natural phenomena.

Ocean Waves Move Energy across the Sea Surface

Ocean waves are visual proof of the transmission of energy across the surface of the ocean.

(right) A floating sea gull demonstrates that wave forms travel but the water itself does not.

In this sequence, a wave moves from left to right as the gull (and the water in which it is resting) revolves in a circle, moving slightly to the left up the front of an approaching wave, then to the crest, then sliding to the right down the back of the wave.

Fig. 10-1, p. 286

Wave directionWave

Stepped Art

(right) Progressive waves are waves of moving energy in which the wave form moves in one direction along the surface (or junction) of the transmission medium. Orbital waves are a type of progressive wave because the waveform moves forward.

(left) Orbital waves are waves in which the particles of water move in closed circles as the wave passes.

Note that the water molecules in the crest of the wave move in the same direction as the wave, but molecules in the trough move in the opposite direction.

Ocean Waves Move Energy across the Sea Surface

Wavelength Is the Most Useful Measure of Wave Size

Waves transmit energy across the ocean’s surface. Wave energy in the ocean as a function of the wave period. As the graph shows, most wave energy is typically concentrated in wind waves. However, large tsunami, rare events in the ocean, can transmit more energy than all wind waves for a brief time. Ides are waves – their energy is concentrated at periods of 12 and 24 hours.

Wave Behavior Is Influenced by Water Depth

Progressive waves. Classification depends on their wavelength relative to the depth of water through which they are passing. Note the importance of the relationship between wavelength and depth in determining wave type.

The theoretical relationship among speed, wavelength, and period in deep-water waves.

Speed is equal to wavelength divided by period. If one characteristic of a wave can be measured, the other two can be calculated.

The easiest to measure exactly is period in the example shown in red, the speed of a wave with a wavelength of 233 meters and a period of 12 seconds is 19.4 meters per second.

Wave Behavior Is Influenced by Water Depth

Wind Blowing over the Ocean Generates Waves

Wind waves are gravity waves formed by the transfer of wind energy into water. Wind forces convert capillary waves to wind waves.

A capillary wave interrupts the smooth sea surface, deflecting surface wind upward, slowing it, and causing some of the wind’s energy to be transferred into the water to drive the capillary wave crest forward (point a). The wind may eddy briefly downwind of the tiny crest, creating a slight partial vacuum (-). Atmospheric pressure (+) pushes the trailing crest forward (downwind) toward the trough (point b), adding still more energy to the water surface.

Larger Swell Move Faster

Wave separation, or dispersion, is a function of wavelength. Waves with the longest wavelength move the fastest and leave the area of wave formation sooner. The smooth undulation of ocean water caused by wave dispersion is called swell.

(left) Waves travel in groups called wave trains. As the leading wave of the group travels forward, it transfers half of its energy forward to initiate motion in the undisturbed surface ahead. The other half is transferred to the wave behind to maintain wave motion. The leading wave in the wave train continuously disappears, while a new wave is continuously formed at the back of the train. Follow wave number 5 in this diagram. The wave train travels at half the speed of any individual wave, a speed known as group velocity (v)

Fig. 10-9, p. 273

Stepped Art

7 6 5 4 3

6 5 4 3 2

8 7 6 5

8 7 6 5

7 6 5 4 3

5 4 3 2 1

7 6 5 4

6 5 4 3 2 1

Many Factors Influence Wind Wave Development

• What factors affect wind wave development?

• Wind strength - wind must be moving faster than the wave crests for energy transfer to continue

• Wind duration - winds that blow for a short time will not generate large waves

• Fetch - the uninterrupted distance over which the wind blows without changing direction

The fetch is the uninterrupted distance over which the wind blows without significant change in direction. Wave size increases with increased wind speed, duration, and fetch. A strong wind must blow continuously in one direction for nearly three days for the largest waves to develop fully.

Many Factors Influence Wind Wave Development

Global wave height acquired by a radar altimeter aboard the TOPEX/Poseidon satellite in October 1992. In this image, the highest waves occur in the southern ocean, where waves were more than 6 meters (19.8 feet) high (represented in white). The lowest waves (indicated by dark blue) are found in the tropical and subtropical ocean, where wind speed is lowest.

Many Factors Influence Wind Wave Development

Interference Produces Irregular Wave Motions

• What happens when waves from different storm systems exist simultaneously?

• When waves meet, they interfere with one another.• Wave interference can be:

– Destructive interference – two waves that cancel each other out, resulting in reduced or no wave

– Constructive interference – additive interference that results in waves larger than the original waves

• Rogue waves - these freak waves occur due to interference and result in a wave crest higher than the theoretical maximum

(above) Constructive and destructive interference.

(a) Two overlapping waves of different wavelength are shown, one in blue and one in green. Note that the wave show in blue has a slightly longer wavelength. (b) If both are present in the ocean at the same time, they will interfere with each other to form a composite wave. At the position of line 1, the two waves in (a) will constructively interfere to form very large crests and troughs, as shown in (b). At the position of line 2, the two waves will destructively interfere, and the crests and troughs will be very small (again shown in b).

Interference Produces Irregular Wave Motions

Deep-Water Waves Change to Shallow-Water Waves As They Approach Shore

What happens when a wave train breaks against the shore?

(1) The swell “feels” bottom when the water is shallower than half the wavelength. (2) The wave crests become peaked because the wave’s energy is packed into less water depth. (3) Constraint of circular wave motion by interaction with the ocean floor slows the wave, while waves behind it maintain their original rate. (4) The wave approaches the critical 1:7 ratio of a wave height to wavelength. (5) The wave breaks when the ratio of wave height to water depth is about 3:4. The movement of water particles is shown in red. Note the transition from a deep-water wave to a shallow-water wave.

Deep-Water Waves Change to Shallow-Water Waves As They Approach Shore

• What different ways can waves break against the shore?

• Plunging waves break violently against the shore, leaving an air-filled tube, or channel, between the crest and foot of the wave. Plunging waves are formed when waves approach a shore over a steeply sloped bottom.

• Spilling waves occur on gradually sloping ocean bottoms. The crest of a spilling wave slides down the face of the wave as it breaks on shore.

Waves Refract When They Approach a Shore at an Angle

• What can affect the way that waves travel?

• Wave refraction - the slowing and bending of waves in shallow water.

• Wave diffraction - propagation of a wave around an obstacle

• Wave reflection - occurs when waves “bounce back” from an obstacle they encounter. Reflected waves can cause interference with oncoming waves, creating standing waves.

Wave refraction. Diagram showing the elements that produce refraction.

Waves Refract When They Approach a Shore at an Angle

(below) Wave refraction. Diagram showing the elements that produce refraction.

(above) Wave diffraction past an island chain. Polynesian navigators used diffraction patterns to sense the presence of islands out of sign over the horizon.

Waves Refract When They Approach a Shore at an Angle

Storm Surges Form beneath Strong Cyclonic Storms

• What are the characteristics of a storm surge?

• A storm surge is an abrupt bulge of water driven on shore by a tropical cyclone or a frontal storm.

• Storm surges are short-lived.

• Storm surges consist of only a crest, so they cannot be assigned a period or wavelength, and cannot be called a wave.

• Storm surges are sometimes called storm tides.

A storm surge.

(a) The low pressure and high winds generated within a hurricane can produce a storm surge up to 9 meters (30 feet) high.

Storm Surges Form beneath Strong Cyclonic Storms

Dome of water

Low atmospheric pressure

CCW spin

Direction of storm and storm surge

Stepped Art

Fig. 10-25a, p. 284

Storm Surges Form beneath Strong Cyclonic Storms

• What are the characteristics of a seiche?

• Water rocking back and forth at a specific resonant frequency in a confined area is a seiche.

• A seiche is also called a standing wave.

• The node is the position in a standing wave where water moves sideways, but does not rise or fall.

(a) A seiche is a long wave in a lake or ocean basin that sloshes back and forth from one end of the basin to another. The rocking frequency depends on the length of the basin. At the node, water moves sideways and does not rise or fall.

(b) A graph of a seiche in Lake Erie. Strong westerly winds in November 2003 caused a seiche with more than 4 meters (13 feet) of difference in water level from Toledo on the western end of the lake to Buffalo on the eastern end.

Storm Surges Form beneath Strong Cyclonic Storms

Tsunami and Seismic Sea Waves Are Caused by Water Displacement• Tsunami are long-wavelength, shallow-water,

progressive waves caused by the rapid displacement of ocean water. Tsunami generated by the vertical movement of earth along faults are seismic waves.

• What else can generate tsunami?– Landslides– Icebergs falling from glaciers– Volcanic eruptions– Asteroid impacts– Other direct displacements of the water surface

Tsunami Are Always Shallow-Water Waves

Seismic waves can reach tremendous size, causing destruction and loss of life.

(right) The great Indian Ocean tsunami of 26 December 2004 began when a rupture along a plate junction lifted the sea surface above. The wave moved outward at speeds of 212 meters per second (472 miles per hour).

At this speed, it took only about 15 minutes to reach the nearest Sumatran coast and 28 minutes to travel to the city of Banda Ache.

Tsunami Move at High Speed

• How can the speed of a tsunami be calculated?• Because tsunami have extremely long wavelengths,

they always behave as shallow water waves.• The speed of a tsunami can be calculated using the

same formula used for other shallow-water waves:• C = √(gd)• g = 9.8 meters per second (acceleration due to

gravity)• d = depth (a typical Pacific abyssal depth is 4,600

meters)

Tsunami Have a Long and Destructive History

Eleven destructive tsunami have claimed more than 180,000 lives since 1990.

Chapter 10 in Perspective

In this chapter you learned that waves transmit energy, not water mass, across the ocean’s surface. The speed of ocean waves usually depends on their wavelength, with long waves moving fastest. Arranged from short to long wavelengths (and therefore from slowest to fastest), ocean waves are generated by very small disturbances (capillary waves), wind (wind waves), rocking of water in enclosed spaces (seiche), seismic and volcanic activity or other sudden displacements (tsunami), and gravitational attraction (tides). The behavior of waves depends largely on the relation between a wave’s size and the depth of water through which it is moving. Waves can refract and reflect, break, and interfere with one another.

Wind waves can be deep-water waves if the water is more than half their wavelength deep. The waves of very long wavelengths are always in “shallow water” (water less than half their wavelength deep). These long waves travel at high speeds, and some may have great destructive power.

In the next chapter you will learn that even greater waves exist: the tides. You may be interested to know that two “tidal waves” move along most coasts each day. You don’t read of daily mayhem and destruction from these passages because the crests are the day’s high tides; and the troughs, the day’s low tides. Tides are shallow-water waves no matter how deep the ocean they’re moving through. Tides can be destructive, but among all waves their ability to cause damage is fortunately not proportional to their wavelengths.