CH 3 Waves and Tides

1. CH 3 Waves and Tides

2. WavesThe wind not only drives surface currents, it causes waves

3. Fundamental Features of Water Waves Wave Crest: is the highest portion of the wave. Wave Trough: is the lowest portion of the wave. Wavelength: is the linear distance separating wave crests (or separating wave troughs). Wave Amplitude: is the displacement of a crest or trough about the mean position or water level. Wave Height: is the total vertical distance from crest to trough (equal to twice the amplitude). Wave Period: is the time required for successive wave crests (or troughs) to pass a fixed point. from An Introduction to the World's Oceans, 8th edition by K.A. Sverdrup, A.C. Duxbury, and A.B. Duxbury (2004)

4. Fig. 3.27

5. When water molecules under the wave • Crest move up and forward • Under the trough moves down and back • The water molecules don’t move anywhere they just move in circles transferring energy. • They don’t actually transport water. • Waves form as soon as wind blows

6. Fig. 3.26

7. The faster and longer the wind blows, the larger the waves get. • The size of the wave also depends on fetch, how much open water wind affects

8. Wind blows and pushes the wave crests up into sharp peaks and troughs are spread out (seas) • Waves move away from where they are generated, slightly faster than the wind that generated them. Once they have moved away from the source they begin to spread out (swells), which are smooth waves

9. Once waves begin move into the shallows, they begin to “feel” the land below them. • The bottom forces the water molecules to drag along elongating the movement of the molecules slowing the wave. • As the waves behind the front wave catch up the waves get closer together decreasing the wavelength • They begin to pile up getting taller and steeper until they fall forward and break creating a surf.

10. Fig. 3.29

11. Surface at any location can be affected by wind and therefore cause waves. • Waves coming from different directions, strengths, and distances can result in these waves interacting and wave cancellation. • Wave cancellation occurs when the crest of one wave meets the trough of another and they combine resulting in an intermediate wave. • If the crest of the two waves collides they can result in a larger wave, which is called wave reinforcement. These rogue waves can be huge sometimes consuming large ships.

12. Waves spread or disperse upon passing through the gaps in barriers. This process is called "wave diffraction“ Barriers with many gaps scatter wave energy, diminish wave height and power

13. Tsunamis are a form of rogue waves, also known as tidal or harbor waves, have nothing to do with tides. • Produced by earthquakes, landslides, volcanoes, and other seismic disturbances of the sea floor • Tsunamis are much longer and faster than ordinary wind driven waves

14. Tsunamis have wavelengths ranging from 100 – 200 km! • Tsunamis are, therefore, shallow-water waves (even in the open ocean)! • Though tsunamis have very long wavelengths, their amplitude in the open ocean is often relatively small - commonly only a meter or two - and this amplitude is distributed over the very long wavelength so that tsunamis are quite imperceptible on the surface of the ocean. • In the deep ocean, tsunamis travel at speeds up to 750 km per hour (465 mi/hr)!

15. Tsunamis are waves generated by displacement of the ocean by impulsive events • Events known to generate tsunamis: • Submarine earthquakes • Explosive volcanic eruptions • Submarine landslides • Terrestrial landslides that enter water bodies • Impacts of large extraterrestrial objects (e.g. asteroids or comets) in the ocean

16. BR: CE Describe how the Coriolis Effect impacts currents.

17. Tide • The sea surface has been rising and falling in the rhythmic patterns known as tides • Tides alternately expose and submerge organisms, drive the circulation of bays • Tides are caused by the gravitational pull of the moon and sun and by the rotations of the earth, moon, and sun.

18. The moon’s gravity is the strongest on the side of the earth closest to the moon. • The moon’s gravity pulls the water in the ocean toward the moon • On the opposite side of the earth, further away from the moon, the pull is weaker so the water does not move toward the moon, it actually bulges away from the moon because of centrifugal force

19. Fig. 3.31

20. Centrifugal force, the moon does not revolve around the earth, instead the earth and moon both rotate • There is a slight wobble, creating a centrifugal force that pushes water away from the moon • Pull towards the moon and pull toward the other side (bulging), deep water under the bulge and shallow away from the bulge

21. Fig. 3.31

22. As the earth rotates the planet’s surface will alternately lie under the bulge and then away from it • High tide occurs with the point is under the bulge. Earth takes 24 hours to complete a rotation, the pt will have two high tides and two low tides every day • The moon advances a little during the earth’s rotation, so it takes earth an additional 50 minutes to come directly in line with the moon again.

23. Fig. 3.32

24. A full tidal cycle takes 24 hours and 50 minutes • Sun produces tidal bulges, the effect of the sun is only half as strong as the moon • When the sun and moon are in line with each other (full and new moon), full tidal range called spring tides. (Surge like a spring) • When the sun and moon are at right angles their efforts partially can each other, neap tides. Neap tides are small. The moon is at the first and third quarter.

25. Fig. 3.33

26. Tides in the Real World • Most places do have semidiurnal, two high tides and two low tides a day • Diurnal tides, only one high tide and one low tide every day. Diurnal tides are pretty uncommon, occurring in Antarctica, Gulf of Mexico, Caribbean, and Pacific • Tides vary from place to place • To predict the tides at a given place equations hare use to produce tide tables that predict the time and height of high and low tides