Title: Garrison Oceanography 7e Chapter 11
1 Fig. 11-1a, p. 299
2 Fig. 11-1b, p. 299
3Chapter 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
4Chapter 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, waters 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.
5Tides 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.
6 Fig. 11-2a, p. 300
7 Fig. 11-2b, p. 300
8 Motion due to inertia
Combined effect
Motion due to gravity
c
Fig. 11-2c, p. 300
9The 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.
10The 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
Earths surface.
11The Movement of the Moon Generates Strong
Tractive Forces
- The moons 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.
12The 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 Earths surface, converging at a point
toward the moon. At points (3) and (4), inertia
exceeds gravitational force, so water moves along
Earths surface to converge at a point opposite
the moon. Forces are balanced only at the center
of Earth (point CE).
13The Movement of the Moon Generates Strong
Tractive Forces
- The formation of tidal bulges at points toward
and away from the moon.
14The Movement of the Moon Generates Strong
Tractive Forces
- How Earths 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).
15The 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.
16The Movement of the Moon Generates Strong
Tractive Forces
- Tidal bulges follow the moon. When the moons
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.
17The Movement of the Moon Generates Strong
Tractive Forces
- How the changing position of the moon relative to
Earths equator produces higher and lower high
tides. Sometimes the moon is below the equator,
and sometimes it is above.
18Sun 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.
19Sun 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.
20The 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.
21Tidal 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 worlds ocean
coasts have semidiurnal tides.
22Tidal 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 basins 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 basins 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).
23Fig. 11.15, p. 307
24Tidal 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.
25Tidal 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.
26Fig. 11.18, p. 309
27Fig. 11.18, p. 309
28Fig. 11.19, p. 309
29Fig. 11.20, p. 311
30Fig. 11.21, p. 311
31Fig. 11.22, p. 312
32Fig. 11.22, p. 312
33Chapter 11 in Perspective
- In this chapter you learned that tides have the
longest wavelengths of the oceans 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.