Transcript No Slide Title

Introduction to
Oceanography
Dynamic Oceanography: Tides
Tides and the Forces That Generate Them
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. Tides
are the longest of all waves.
Tides are forced waves because they are never free of the forces
that cause them.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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Tidal Characteristics
Tides have a wave form, but differ from other waves because
they are caused by the interactions between the ocean, Sun
and Moon.
• Crest of the wave form is high tide and trough is low tide.
• The vertical difference between high tide and low tide is the
tidal range.
• Tidal period is the time between consecutive high or low tides
and varies between 12 hrs 25 min to 24 hrs 50 min.
• There are three basic types of daily tides defined by their period
and regularity: Diurnal tides , Semidiurnal tides, and Mixed
tides.
• Over a month the daily tidal ranges vary systematically with the
cycle of the Moon.
• Tidal range is also altered by the shape of a basin and sea floor
configuration.
Sun and Moon Together
top: The positions of the Sun, the moon and Earth during a spring tide.
bottom: The positions of the Sun, the moon and Earth during a neap tide.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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Origin of the Tides
Tides result from gravitational attraction and
centrifugal effect.
• Gravity varies directly with mass, but inversely with distance.
• Although much smaller, the Moon exerts twice the gravitational
attraction and tide-generating force as the Sun because the
Moon is closer.
• Gravitational attraction pulls the ocean towards the Moon and
Sun, creating two gravitational tidal bulges in the ocean (high
tides).
• Centrifugal effect is the push outward from the center of
rotation.
The Equilibrium Theory of Tides
The action of gravity and inertia on particles at five different locations on
Earth. Forces are balanced only at point CE, the center of Earth. Note the
bulges that are aligned with the moon as Earth spins on its axis. The key
to understanding the equilibrium theory of tides is to see Earth turning
beneath these bulges.
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Origin of the Tides: Equilibrium model
• Latitude of the tidal bulges is determined by the declination, the
angle between Earth’s axis and the lunar and solar orbital
plane.
• Spring tides occur when Earth, Moon and Sun are aligned in a
straight line and the tidal bulges display constructive
interference, producing very high, high tides and very low, low
tides.
• Spring tides coincide with the new and full moon.
• Neap tides occur when the Earth, Moon and Sun are aligned
forming a right angle and tidal bulges displaying destructive
interference, producing low high tides and high low tides.
• Neap tides coincide with the first and last quarter moon.
• Earth on its axis and the Moon in its orbit both revolve eastward
and this causes the tides to occur 50 minutes later each day.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
The Dynamic Theory of Tides
Tide curves for three common types of tides.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
The Dynamic Theory of Tides
The worldwide distribution of the three tidal patterns.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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Origin of the Tides: Dynamic model
Movement of tides across ocean basins is deflected by Coriolis, blocked by
continental landmasses and forms a rotary wave, which each day completes
two cycles around the basin if the tide is semidiurnal or one cycle if it is
diurnal.
• High tide at the ocean basin’s western edge creates a pressure
gradient sloping downward towards the east.
• As water flows down the gradient, Coriolis deflects water
towards the equator, where it accumulates and establishes a
pressure gradient sloping downward towards the pole.
• Water flowing down this gradient is deflected eastward, forming
a pressure gradient sloping downward to the west.
• Westward flow along this gradient is diverted poleward forming
a pressure gradient sloping downward toward the equator.
• Finally, the flow toward the equator is deflected westward,
completing the cycle.
Origin of the Tides
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A rotary wave is part of an amphidromic system (rotary standing wave) in
which the wave progresses about a node (no vertical displacement) with the
antinode (maximum vertical displacement) rotating about the basin’s edges.
• Cotidal lines connect points on the rotary wave that experience
high tide at the same time.
• Cotidal lines are not evenly spaced because tides are shallow water waves and
their celerity depends upon water depth.
• Corange circles are lines connecting points which experience the
same tidal range.
– The lines form irregular circles which are concentric about the node.
– Tidal range increases outward from the node.
• Amphidromic systems rotate clockwise in the southern
hemisphere and counterclockwise in the northern hemisphere
because of the difference in the direction of Coriolis deflection.
• Irregular coastlines distort the rotary motion.
• Actual tide expressed at any location is a composite of 65
different tidal components.
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Tides in Small and Elongated Basins
In long and narrow basins tides can not rotate.
• Currents in these basins simply reverse direction between high
and low tide, flowing in with the high tide and out with the low
tide.
• Cotidal and corange lines are nearly parallel to each other.
• Tidal ranges increase if a bay tapers landward because water is
funneled towards the basin’s narrow end.
• Tidal resonance occurs if the period of the basin is similar to the
tidal period.
• Resonance can greatly enhance the tidal range.
• A tidal bore is a wall of water that surges upriver with the
advancing high tide.
Seiches
A seiche in Lake Geneva. The blue line represents the hypothetical
whole wave of which the seiche is a part.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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Tidal Currents
The movement of water towards and away from land
with the high and low tides, respectively, generates
tidal currents.
• Flood current is the flow of water towards the land with the
approaching high tide.
• Ebb current is the flow of water away from the land with the
approaching low tide.
• Far off shore the tidal currents inscribe a circular path over a
complete tidal cycle.
• Near shore the tidal currents produce simple landward and then
seaward currents.