1. Chapter 8 Tides

2. Tidal Characteristics
single waves - stretch across entire ocean basins.
shallow-water waves
wavelengths greatly exceed the depth of the ocean.
complex interactions of moon and sun

3. Origin of the Tides
Unlike wind-driven surface waves and unlike tsunamis, tides are caused by two principal factors:
Gravitational attraction
Centrifugal force

4. 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.

5. Gravitational Effect of Moon (Figure 8-5a)

6. 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

7. Centrifugal Force Earth and Moon revolving around one another.
unequal masses of Earth and Moon
center of rotation lies beneath the Earth’s surface.
second tidal bulge
ocean shifts away from center of rotation
AWAY from center
can be observed in action on a passenger riding in a car. If the car swerves around a corner, the passenger's body pushes against the outer edge of the car

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

9. 2 Bulges from Gravitational Attraction & Centrifugal Force (Figure 8-5c)

10. Animation

11. Dynamic Model of the Tides (Figure 8-8a)

12. 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.

13. Standing Waves Sea, swell, and surf are progressive waves.
Standing waves (seiches)…
Oscillate back and forth about a node, i.e., a fixed point.

14. Node = Fixed Point Antinode = maximum displacement (Figure 7-11)
Form when winds blow in one direction which causes water to pile up at one end of a basin.

15. Dynamic Model of theTides (Figure 8-8a)

16. Dynamic Model of theTides (Figure 8-8b)
Earth rotates from
W to E
Tidal bulge to W
Pressure Gradient
Coriolis
cut-away
Pressure Gradient
Coriolis
Pressure Gradient
Coriolis
cut-away

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

18. 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.

19. 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

20. Wave Parameters (Figure 7-1a)

21. Tidal Periods Moon moves forward in it’s orbit each day.
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.

22. 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).

23. 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

24. 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

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

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

27. Diurnal Activity of Fiddler Crabs (Figure B8-4)
Natural
Searching for food
in burrows