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Chapter 8 Tides. Tidal Characteristics. Tides are single waves that stretch across entire 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 . Origin of the Tides.

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Chapter 8 Tides

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tidal characteristics
Tidal Characteristics
  • Tides are single waves that stretch across entire 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.
origin of the tides
Origin of the Tides
  • Unlike wind-driven surface waves, unlike tsunamis, tides are caused by two principal factors:
    • Gravitational attraction
    • Centrifugal force
gravitational attraction
Gravitational Attraction
  • 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.

Animation courtesy of NASA Goddard Space Flight Center: The Moon's gravity tugs at the Earth, causing ocean water to slosh back and forth in predictable waves called tides. We can visibly observe some of that energy dissipate at the beach, with waves rolling across coastal shallows and shoals. Most of the energy dissipates due to friction between the water and the shallow floor beneath it

centrifugal forces
Centrifugal Forces
  • 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.
true tidal waves not tsunamis
True Tidal Waves (not tsunamis!)
  • 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.
standing waves
Standing Waves
  • Seas, swell, and surf are progressive waves.
  • Standing waves (seiches)…
    • Oscillate back and forth about a nodei.e., a fixed point.
dynamic model of thetides figure 8 8b
Dynamic Model of theTides(Figure 8-8b)

Earth rotates from

W to E

Tidal bulge to W

Pressure Gradient



Pressure Gradient


Pressure Gradient



dynamic model
Dynamic Model
  • equilibrium model of the tides is good, BUT...
    • not totally detailed and accurate
    • oceans are separated by continents
    • oceans not infinitely deep

Animation courtesy of NASA Goddard Space Flight Center: Using 6 years of data from TOPEX/Poseidon satellite altimeter, they derived a 16-day set of predictive data, showing a synthetic view of how the tides move around the world's oceans.

key features of tides
Key Features of Tides
  • 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
tidal periods
Tidal Periods
  • why not exactly 24 hours or 12 hours?
  • Moon moves forward in it’s orbit each day.
    • Takes 50 additional minutes for a spot on the Earth’s surface to regain it’s position relative to the Moon.
classification by daily record
Classification by Daily Record
  • 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).
classificaton by monthly record
Classificaton by Monthly Record
  • 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 neap tides
Spring & Neap Tides
  • 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
marine biorhythms
Marine Biorhythms
  • Cyclical behaviors associated with tidal rhythms.
    • The diurnal activity of fiddler crabs.
    • The spawning behavior of the grunion fish.
diurnal activity of fiddler crabs figure b8 4
Diurnal Activity of Fiddler Crabs(Figure B8-4)


Searching for food

in burrows