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• Tides are single waves that stretch across ocean basins.
• They are also shallow-water waves because their wavelengths greatly exceed the depth of the ocean.
• They occur due to complex interactions of the moon and sun.

• Unlike wind-driven surface waves, tides are cause by two principal factors:
• Gravitational attraction
• Centrifugal force

• All masses are drawn to each other.
• The moon because of its closeness to the Earth exerts a greater gravitational effect on the Earth than the Sun, despite the fact that the Sun is much more massive than the Moon.

Gravitational Effect of Moon
(Figure 8-5a)

(courtesy of NASA JPL, Topex-Poseidon

• arise as the Earth and Moon revolve around one another.
• unequal masses of the Earth and Moon the center of rotation lies beneath the Earth’s surface.
• The water of the ocean shifts away from the center of rotation creating a second tidal bulge.

Centrifugal Forces: Center of Rotation
(Figure 8-5b)

• The motion of water around the basin is a rotary wave.
• The crest is at one side of the basin.
• The trough is at the other side of the basin.
• Tides have characteristics of both progressive and standing waves.

Dynamic Model of theTides
(Figure 8-8a)

Dynamic Model of the Tides
(Figure 8-8b)

Tidal waves are rotary waves w/in an ocean basin...
(courtesy of NASA JPL, Topex-Poseidon

• High tide : Wave crest
• Low tide : Wave trough
• Tidal range : Wave height
• Tidal periods depending on location:
• 12 hours, 25 minutes
• 24 hours, 30 minutes

• Diurnal: have one high tide and one low tide daily (high lat).
• Semidiurnal: have two high tides and two low tides daily (low lat).
• Mixed: there are two high tides and two low tides daily, but of unequal shape (mid lat).

• Spring tide: phase when tidal range is maximal.
• Neap tide: phase when tidal range is minimal.
• There are 2 spring and 2 neap tides each month

• Spring tides occur when the Earth, Sun, and Moon are aligned.
• New Moon and Full Moon phases
• Constructive interference
• Neap tides occur when the Sun and Moon are aligned at right angles to one another.
• Quarter Moon phases
• Destructive interference

Sun/Moon/Earth - Spring & Neap
(Figure 8-7a)

• The reason why tidal periods are not exactly 24 hours or 12 hours in length is that the Moon moves forward in it’s orbit each day. In fact it takes 50 additional minutes for a spot on the Earth’s surface to regain it’s position relative to the Moon.
• The equilibrium model of the tides is good, but it does not give us a detailed and accurate description of the tides in the real world, where the oceans are separated by continents and not infinitely deep.

• Cyclical behaviors associated with tidal rhythms.
• The diurnal activity of fiddler crabs.
• The spawning behavior of the grunion fish.