1. Celestial Rhythms: Tides
- Tide Generating Forces – Inertia and Gravity
- Tides and Biology
Reading:
4th Ed., Ch 11, Secs 11-17, 24
5th Ed., Ch 11, Secs 2-8, 15
Graphics: High and low tides in the Gulf of Alaska. Cmmndr. G.B. Mills,
photographer. Courtesy of NOAA.
2. Wave Energy Revisited
Graphic: Garrison, Fig. 10.5, 4th Ed., pg 241, 5th Ed., pg 232.
3. Gravity and Tides
Gravitational attraction between the Earth and other astronomical bodies causes
tides
Strength of gravitational attraction depends on:
- masses of the two bodies
- distance between two bodies
The strongest tides are forced by:
- the Earth and moon
- the Earth and sun
Graphic: Montage of the Earth and moon. Courtesy of NASA/JPL and Northwestern
Univ.
4. The Moon’s Orbit
Gravity pulls the moon toward Earth
Inertia will keep the moon traveling in a straight line
The balance of gravity and inertia forms a circular orbit
Garrison, 5th Ed., Fig. 11.2, pg 259.
5. Rotation of the Earth-Moon System
The Earth and moon rotate about their common center of mass
Graphics: (top) see Garrison 4th Ed., Fig. 11.13, pg 271, 5th Ed., Fig. 11.3, pg
259, (bottom) see Garrison, 4th Ed., Fig. 11.15, 11.16, pg 271, 272, 5th Ed.,
Fig. 11.4 pg 260.
6. Tide Generating Forces - Gravity
Gravity pulls each point on Earth toward the center of the moon
Gravitational force (pink arrows):
- varies in strength (the arrows are different lengths)
- is not oriented the same direction everywhere
Graphic: Garrison, 4th Ed., Fig. 11.16, pg 272, see 5th Ed., Fig. 11.5, pg 260.
7. Tide Generating Forces - Inertia
Inertia exerts an outward pull away from the center of mass of the Earth and
moon
Inertial force (blue arrows):
- is the same strength everywhere
- is oriented in the same direction everywhere
Garrison, 4th Ed., Fig. 11.15, pg 271, see 5th Ed., Fig. 11.5, pg 260.
8. Combined Result of Inertia and Gravity
The ocean is pulled:
- toward the moon on the side of the Earth closest to the moon
- away from the moon on the side of the Earth opposite the moon
Garrison, 4th Ed., Fig. 11.17, pg 272, 5th Ed., Fig. 11.5, pg 260.
9. High and Low Tides
High and low tides occur as the Earth spins under tidal bulges
Garrison, 4th Ed., Fig. 11.19, pg 273, 5th Ed., Fig. 11.7, pg 261.
10. Solar and Lunar Tides
Lunar tides (Earth/moon)
Solar tides (Earth/sun)
Spring tide - solar and lunar tidal buldges add
Neap tide - solar and lunar tidal bulges cancel
Graphic: Garrison, 4th Ed., Fig. 11.23, pg 275, 5th Ed., Fig. 11.11, pg 263.
(animation)
11. Spring and Neap Tides
Spring tide:
- large difference between high and low tide
- Earth, moon and sun are lined up (new and full moon)
Neap tide:
- small difference between high and low tide
- Earth, moon and sun at right angles (1st and 3rd quarter moon)
Garrison, 4th Ed., Fig. 11.24, pg 276, 5th Ed., Fig. 11.12, pg 264.
12. Dynamic Tides
The previous explanation of tides (equilibrium theory):
- neglects blocking of tidal progression by continents
- assumes that tides can adjust infinitely fast
In reality, tides
- move within ocean basins
- behave like shallow water waves
- the speed of tidal motion is slowed by friction against the bottom of the
ocean
The number of high and low tides per day can vary from place to place due to
these “dynamic“ effects
13. Tides and the Coriolis Effect
Because of Coriolis, tide crests rotate around ocean basins
No tidal change occurs at centers of rotation (amphidromic points)
Garrison, 4th Ed., Fig. 11.27, pg 278, 5th Ed., Fig. 11.15, pg 266.
14. Amphidromic Points in the World Ocean
Lines show the location of the tide crests at different times
Colors show where tidal range is highest (red) and lowest (blue)
Garrison, 4th Ed., seeFig., 11.28, pg 279, 5th Ed., Fig. 11.16, pg 266.
(animation)
15. Semidiurnal, Diurnal and Mixed Tides
Because of dynamic effects, more complicated tide patterns occur
- Semidiurnal
- Diurnal
- Mixed
Graphics: (top) Garrison, 4th Ed., Fig. 11.25, pg 277, 5th Ed., Fig. 11.13, pg
265, (bottom) Garrison, 4th Ed., Fig. 11.26, pg 278, 5th Ed., Fig. 11.14, pg
265.
16. Tides and Biology
Many organisms time their reproductive cycles with the tides
Grunion “run” up on beaches to lay their eggs during the highest spring tides
Graphic: Garrison, 4th Ed. Fig. 11.33, pg 284, 5th Ed., Fig. 11.20, pg 271.
17. Intertidal Zones
Many organisms inhabit the zone between high and low tide
Communities of organisms usually form horizontal bands in this zone, according
to their tolerance for exposure to air
Graphic: Garrison, Fig. 16.6, 4th Ed., pg 420, 5th Ed., pg 396.
18. Preview of Next Lecture
Where Sea Meets Land: Coastlines and Beaches
Reading:
4th Ed., Ch 12, Secs 1-13, 15, 17, 19-21, 31-35, Ch 6 Sec 17
5th Ed., Ch 12, Secs 2-9, 11, 13-14, 16-19, 30-34, Ch 6 Sec 20
Graphic: Beachfront property along North Carolina's outer banks following a
Noreaster. Photographer, R.B.Mieremet, NOAA Senior Advisor. Courtesy of NOAA.