1. Chapter 11 Tides

2. Tides
Tides Are the Longest of All Ocean Waves, hence they are Long waves
Tides Are Forced Waves Formed by Gravity and Inertia
They are responsible for the rise and fall in sea level.

3. Tides occur as shallow water waves and depend heavily upon topography.
One low-tide/high-tide cycle takes about 12 hrs and 25 min.
Tidal range is the difference in ocean level between high-tide and low-tide

4. 2 high tides = flood tides 2 low tides = ebb tides each day
What is the Tidal Range?
HT = 30 ft, LT = 20 ft
HT = 20 ft, LT = 12 ft
HT = 50 ft, LT = 20 ft `
2 high tides = flood tides
2 low tides = ebb tides
each day

5. What Causes Tides
The gravitational attractions of the Moon and Sun acting on the rotating Earth.

6. Tides
Tides result from the gravitational pull of the moon and, to a lesser degree, the sun.

7. Gravity and inertia
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.

8. Tidal bulges
The moon’s gravity pulls the ocean toward it. The motion of Earth around the center of mass of the Earth-moon system causes a bulge on the side of Earth opposite the moon. The combination of the two effects creates two tidal bulges.

9. Gravitational Effect of the Moon
Two big bulges of water form on the Earth:
one directly under the moon
another on the exact opposite side
As the Earth spins, the bulges follow the moon.

10. Lunar tidal bulge
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.

11. 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. (b) At the first-and third-quarter moons, the sun, Earth, and moon form a right angle, creating neap tides.

12. Spring tides
When the sun and the moon are aligned, their gravitational effects combine to increase the planetary water bulge.

13. Gravitational Effect of the Sun and Moon
Spring Tides -every 2 weeks
Earth, Moon, and Sun are lined up
High Tides ~20% higher than normal
Low Tides are lower than normal

14. Neap tides
When the moon and sun are at right angles (quarter moon stages), tides are weaker = neap tides.

15. Gravitational Effect of the Sun and Moon
Neap Tides- every 2 weeks
Earth, Moon, and Sun form right angles
High Tides are ~20% lower than normal and Low Tides are higher than normal, opposite interval to Spring Tide

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 magnitude of tides varies
Spring tide - higher high and lower low tides (more extreme)
Neap tide - weaker tide
Timing of spring and neap tides are important for forecasting hurricane storm surge – why?

18. NOAA tide demonstration
Ignite learning media

19. The Dynamic Theory of 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.

20. Tidal patterns Diurnal One high and one low tide each lunar day
Semidiurnal
Two high and two low tides of about the same height daily

21. Tidal Patterns of world coastlines
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.
Tidal Patterns of world coastlines

22. There are 140 amphidromic points in the world’s oceans.
Tides in the Ocean
Co-tidal map shows tides rotate around amphidromic points. vertical movement)
There are 140 amphidromic points in the world’s oceans.
More down-to-earth pattern of tides in the ocean
Amphidromic Points

23. Tidal Ranges increase with increasing distance from Amphidromatic points. Coriolis drives the process counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.
Red = most extreme
Blue = least extreme
White lines indicate tide waves that circulate around these points due to Coriolis.

24. Tidal Currents Ebb currents - produced by outgoing tides
Tidal currents occur in some bays and rivers due to a change in tides
Ebb currents - produced by outgoing tides
Flood currents - produced by incoming tide
LSW means Lower Slack Water
HSW means Higher Slack Water