Chapter 12 Coasts Oceanography An Invitation to Marine Science, 7th Tom Garrison.
An Invitation to Marine Science, 7th
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Transcript of An Invitation to Marine Science, 7th
An Invitation to Marine Science, 7th
Oceanography An Invitation to Marine Science, 7th Tom Garrison
Chapter 11 Tides 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
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.
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. 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. 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.
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. 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). The
Movement of the Moon Generates Strong Tractive Forces
The formation of tidal bulges at points toward and away from the
moon. The 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). 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. The moon moves this
much in 8 hours . . .
. . . and this much in 24 hours Start North x Pole Moon Earth Tidal
bulges Noon Rotation North x Pole 8:00 P.M. 8 hours North x Pole
4:00 A.M. 8 hours North x Pole Noon 1 Solar day 8 hours North x
Pole 12:50 P.M. on Day 2 50 min 1 Lunar day Figure 11.8: 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 12.250
minutesto 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. Stepped Art Fig.
11-8, p. 302 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. 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. 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. 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. 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. 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. 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). 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. 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. 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.