Chapter 11 Tides

1. Oceanography An Invitation to Marine Science, 7th Tom Garrison Chapter 11 Tides

2. Chapter 11 Study Plan • Tides Are the Longest of All Ocean Waves • Tides Are Forced Waves Formed by Gravity and Inertia • The Dynamic Theory of Tides Adds Fluid Motion Dynamics to the Equilibrium Theory • Most Tides Can Be Accurately Predicted • Tidal Patterns Can Affect Marine Organisms • Power Can Be Extracted from Tidal Motion

3. Chapter 11 Main Concepts • Tides are periodic short-term changes in ocean surface height. Tides are forced waves formed by gravity and inertia. • The equilibrium theory of tides explains tides by examining the balance of and effects of forces that allow our planet to stay in orbit around the sun, or the moon to orbit Earth. Because of its nearness to Earth, our moon has a greater influence on tides than the sun. • The dynamic theory of tides takes into account seabed contour, water’s viscosity, and tide wave inertia. • Together, the equilibrium and dynamic theories allow tides to be predicted years in advance. • Power can be extracted from tidal flow.

4. Tides Are the Longest of All Ocean Waves • What are the characteristics and causes of tides? • Tides are caused by the gravitational force of the moon and sun and the motion of earth. • The wavelength of tides can be half the circumference of earth and are the longest of all waves. • Tides are forced waves because they are never free of the forces that cause them.

5. The Movement of the Moon Generates Strong Tractive Forces A planet orbits the sun in balance between gravity and inertia. (a) If the planet is not moving, gravity will pull it into the sun. (b) If the planet is moving, the inertia of the planet will keep it moving in a straight line. (c) In a stable orbit, gravity and inertia together cause the planet to travel in a fixed path around the sun.

6. The Movement of the Moon Generates Strong Tractive Forces The moon does not rotate around the center of Earth. Earth and moon together – the Earth-moon system – rotate around a common center of mass about 1,650 kilometers (1,023 miles) beneath Earth’s surface.

7. The Movement of the Moon Generates Strong Tractive Forces The moon’s gravity attracts the ocean toward it. The motion of Earth around the center of mass of the Earth-moon system throws up a bulge on the side of Earth opposite the moon. The combination of the two effects creates two tidal bulges.

8. The Movement of the Moon Generates Strong Tractive Forces The action of gravity and inertia on particles at five different locations on Earth. At points (1) and (2), the gravitational attraction of the moon slightly exceeds the outward-moving tendency of inertia; the imbalance of forces causes water to move along Earth’s surface, converging at a point toward the moon. At points (3) and (4), inertia exceeds gravitational force, so water moves along Earth’s surface to converge at a point opposite the moon. Forces are balanced only at the center of Earth (point CE).

9. The Movement of the Moon Generates Strong Tractive Forces The formation of tidal bulges at points toward and away from the moon.

10. The Movement of the Moon Generates Strong Tractive Forces • How Earth’s rotation beneath the tidal bulges produces high and low tides. Notice that the tidal cycle is 24 hrs 50 minutes long because the moon rises 50 minutes later each day. • A graph of the tides at the island in (a).

11. The Movement of the Moon Generates Strong Tractive Forces A lunar day is longer than a solar day. A lunar day is the time that elapses between the time the moon is highest in the sky and the next time it is highest in the sky. In a 24-hour solar day, the moon moves eastward about 12.2°. Earth must rotate another 12.2° - 50 minutes – to again place the moon at the highest position overhead. A lunar day is therefore 24 hours 50 minutes long. Because Earth must turn an additional 50 minutes for the same tidal alignment, lunar tides usually arrive 50 minutes later each day.

12. Moon Earth North x Pole North x Pole North x Pole North x Pole North x Pole Rotation Tidal bulges Noon 8:00 P.M. 4:00 A.M. Noon 12:50 P.M. on Day 2 50 min 8 hours 8 hours 8 hours Start 1 Solar day 1 Lunar day The moon moves this much in 8 hours . . . . . . and this much in 24 hours Stepped Art Fig. 11-8, p. 302

13. The Movement of the Moon Generates Strong Tractive Forces Tidal bulges follow the moon. When the moon’s position is north of the equator, the gravitational bulge toward the moon is also located north of the equator and the opposite inertia bulge is below the equator.

14. The Movement of the Moon Generates Strong Tractive Forces How the changing position of the moon relative to Earth’s equator produces higher and lower high tides. Sometimes the moon is below the equator, and sometimes it is above.

15. Sun and Moon Influence Tides Together Relative positions of the sun, moon, and Earth during spring and neap tides. (a) At the new and full moons, the solar and lunar tides reinforce each other, making spring tides, the highest high and lowest low tides. (b) At the first-and third-quarter moons, the sun, Earth, and moon form a right angle, creating neap tides, the lowest high and the highest low tides.

16. Sun and Moon Influence Tides Together Tidal records for a typical month at (a) New York and (b) Port Adelaide, Australia. Note the relationship of spring and neap tides to the phases of the moon.

17. The Dynamic Theory of Tides • What are some key ideas and terms describing tides? • The dynamic theory of tides explains the characteristics of ocean tides based on celestial mechanics (the gravity of the sun and moon acting on Earth) and the characteristics of fluid motion. • Semidiurnal tides occur twice in a lunar day • Diurnal tides occur once each lunar day • Mixed tides describe a tidal pattern of significantly different heights through the cycle • Amphidromic points are nodes at the center of ocean basins; these are no-tide points.

18. Tidal Patterns Center on Amphidromic Points Common tide types. • A mixed tide pattern at Los Angeles, California. • A diurnal tide pattern at Mobile, Alabama. • A semidiurnal tide pattern at Cape Cod, Massachusetts. • The worldwide geographical distribution of the three tidal patterns. Most of the world’s ocean coasts have semidiurnal tides.

19. Tidal Patterns Center on Amphidromic Points The development of amphidromic circulation (a) A tide wave crest enters an ocean basin in the Northern Hemisphere. The wave trends to the right because of the Coriolis effect (b), causing a high tide on the basin’s eastern shore. Unable to continue turning to the right because of the interference of the shore, the crest moves northward, following the shoreline (c) and causing a high tide on the basin’s northern shore. The wave continues its progress around the basin in a counterclockwise direction (d), forming a high tide on the western shore and completing the circuit. The point around which the crest moves is an amphidromic point (AP).

20. Tidal Patterns Vary with Ocean Basin Shape and Size How do tides behave in confined basins? The tidal range is determined by basin configuration. (a) An imaginary amphidromic system in a broad, shallow basin. The numbers indicate the hourly positions of tide crests as a cycle progresses. (b) The amphidromic system for the Gulf of St. Lawrence between New Brunswick and Newfoundland, southeastern Canada. Dashed lines show the tide heights when the tide crest is passing.

21. Tidal Patterns Vary with Ocean Basin Shape and Size Tides in a narrow basin. (a) True amphidromic systems do not develop in narrow basins because there is no space for rotation. (b) Tides in the Bay of Fundy, Nova Scotia, are extreme because water in the bay naturally resonates (seiche) at the same frequency as the lunar tide.

22. Chapter 11 in Perspective In this chapter you learned that tides have the longest wavelengths of the ocean’s waves. They are caused by a combination of the gravitational force of the moon and the sun, the motion of Earth, and the tendency of water in enclosed ocean basins to rock at a specific frequency. Unlike the other waves, these huge shallow-water waves are never free of the forces that cause them and so act in unusual but generally predictable ways. Basin resonances and other factors combine to cause different tidal patterns on different coasts. The rise and fall of the tides can be used to generate electrical power, and tides are important in many physical and biological coastal processes. In the next chapter you will learn how the interaction of wind, waves, and weather affects the edges of the land – the coasts. Coasts are complex, dynamic places where the only constant is change.