Garrison Oceanography 7e Chapter 11

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Fig. 11-1a, p. 299
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Fig. 11-1b, p. 299
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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

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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, 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.

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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.

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Fig. 11-2a, p. 300
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Fig. 11-2b, p. 300
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Motion due to inertia
Combined effect
Motion due to gravity
c
Fig. 11-2c, p. 300
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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.

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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
  Earths surface.

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The 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.

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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 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).

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The Movement of the Moon Generates Strong
Tractive Forces

• The formation of tidal bulges at points toward
  and away from the moon.

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The Movement of the Moon Generates Strong
Tractive Forces

1. 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.
2. A graph of the tides at the island in (a).

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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.

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The 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.

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The 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.

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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.

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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.

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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.

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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 worlds ocean
  coasts have semidiurnal tides.

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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 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).

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Fig. 11.15, p. 307
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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.

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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.

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Fig. 11.18, p. 309
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Fig. 11.18, p. 309
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Fig. 11.19, p. 309
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Fig. 11.20, p. 311
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Fig. 11.21, p. 311
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Fig. 11.22, p. 312
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Fig. 11.22, p. 312
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Chapter 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.