Chapter 10: Waves Insert: Textbook cover photo

Chapter 10 Five Main Concepts Waves transmit energy, not water mass, across the ocean’s surface. Ocean waves are classified by the disturbing force that creates them. The speed (celerity) of an ocean wave is proportional to its wavelength. The orbits of water molecules in waves moving through water deeper than half the wavelength are unaffected by the bottom. Water displacement causes tsunami and seismic sea waves.

Ocean Waves Move Energy across the Sea Surface Ocean waves are visual proof of the transmission of energy across the surface of the ocean. A floating 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 circular pattern.

Ocean Waves Move Energy across the Sea Surface The anatomy and parts of a progressive wave. The orbital motion of water particles in a wave, which extends to a depth of about half of the wavelength. 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.

Waves Are Classified by Their Physical Characteristics Ocean waves are classified by: the disturbing force that creates them the restoring force that tries to flatten them their wavelength Wave energy in the ocean as a function of the wave period.

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.

Wave Behavior Is Influenced by Water Depth 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.

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. Capillary waves become gravity waves as their wavelength exceeds 1.74 centimeters. These wind-induced gravity waves (wind waves) continue to grow as long as the wind above them exceeds their speed.

Larger Swell Move Faster Wave separation, or dispersion, is a function of wavelength. Waves with the longest wavelength move the fastest. The smooth undulation of ocean water caused by wave dispersion is called swell. (LEFT) Waves travel in groups called wave trains. The leading wave in the wave train continuously disappears, while a new wave is continuously formed at the back of the train. The wave train travels at half the speed of any individual wave.

5 4 3 2 1 6 5 4 3 2 1 6 5 4 3 2 7 6 5 4 3 7 6 5 4 3 7 6 5 4 Figure 10.9 Growth and progress of wind waves. 8 7 6 5 8 7 6 5 Stepped Art Figure 10-9d p305

Many Factors Influence Wind Wave Development Three factors affect the growth of wind waves 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

Many Factors Influence Wind Wave Development 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.

Interference Produces Irregular Wave Motions 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

Interference Produces Irregular Wave Motions Constructive and destructive interference: Two overlapping waves of different wavelength are shown, one in blue and one in red. Note that the wave show in blue has a slightly longer wavelength. If both are present in the ocean at the same time, they will interfere with each other to form a composite wave (green).

Deep-Water Waves Change to Shallow-Water Waves As They Approach Shore How a wave train breaks against the shore.

Deep-Water Waves Change to Shallow-Water Waves As They Approach Shore Waves break against the shore in different ways, depending, in part, on the slope of the bottom. 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.

Deep-Water Waves Change to Shallow-Water Waves As They Approach Shore Wave direction can change when it comes in contact with the seafloor or an obstacle. 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.

Waves Refract When They Approach a Shore at an Angle (LEFT) Wave refraction. Diagram showing the elements that produce refraction. (RIGHT) Wave refraction around Maili Point, O’ahu, Hawai’i. Note how the wave crests bend almost 90｡ as they move around the point.

Waves Can Diffract When Wave Trains Are Interrupted (LEFT) Diffraction of waves at a breakwater gap at Morro Bay, California. (RIGHT) Wave diffraction past an island chain. Polynesian navigators used diffraction patterns to sense the presence of islands out of sign over the horizon.

Internal Waves Can Form between Ocean Layers of Differing Densities Subsurface waves that form at the boundary between water layers of different densities are called internal waves. Internal waves occur in the ocean at the base of the pycnocline, especially at the bottom edge of a steep thermocline. Internal waves are generated by wind energy, tidal energy, and ocean currents. They may mix nutrients into surface water and trigger plankton blooms.

Storm Surges Form beneath Strong Cyclonic Storms 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.

Storm Surges Form beneath Strong Cyclonic Storms A storm surge: The low pressure and high winds generated within a hurricane can produce a storm surge up to 9 meters (30 feet) high.

Water Can Rock in a Confined Basin 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. Seiches rock back and forth at a specific resonant frequency in a confined area. 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.

Water Displacement Causes Tsunami and Seismic Sea Waves Tsunami are long-wavelength, progressive waves caused by the rapid displacement of ocean water. Tsunami are always shallow-water waves. Tsunami move at a very high rate of speed. What else can generate tsunami? Landslides Icebergs falling from glaciers Volcanic eruptions Asteroid impacts Other direct displacements of the water surface

Water Displacement Causes Tsunami and Seismic Sea Waves Seismic waves can reach tremendous size, causing destruction and loss of life. 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 Have a Long and Destructive History Twelve destructive tsunami have claimed more than 180,000 lives since 1990.