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TIDES

TIDES. periodic, short term changes in the height of the ocean surface at a particular place . What causes tides?. a combination of the gravitational force of the moon and sun and the rotation of the earth moon exerts greater effect than the sun. Tides are waves.

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TIDES

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  1. TIDES • periodic, short term changes in the height of the ocean surface at a particular place

  2. What causes tides? • a combination of the gravitational force of the moon and sun and the rotation of the earth • moon exerts greater effect than the sun

  3. Tides are waves • Tides are simply specific types of “waves,” we now know that waves can be either deep or shallow and long or short… • Thus, tides are the longest of all (shallow water) wave examples • longest and largest of all waves in the ocean = tides…wavelengths of half the Earth’s circumference (max.) • High tide is the crest of the wave and low tide is the trough

  4. Studies • 2 scientists (combined) explain the effects and daily occurrence of tides • Newton • Laplace

  5. Equilibrium Theory of Tides • Isaac Newton, 1600’s • Would accurately describe tides on a planet uniformly covered with water through a gravitational model • Does not consider the effects of continents, ocean basin shape and ocean depth • Explains the effects of the gravitational attractions of Earth, moon and sun (with the influence of the moon being 50% greater than that of the sun)

  6. Newton: Equilibrium theory • Planets orbit the sun in balance between gravity and inertia: • Not moving, gravity pulls planet to sun; (b) if planet • is moving inertia will maintain straight line; (c) together • you get a fixed path for planet orbit. Fig 10-2, p.229

  7. Newton/Equilibrium theory • Assumes the ocean conforms instantly to the forces that affect the position of its surface – i.e. the oceans surface is always in equilibrium (balance) with the forces acting on it.

  8. What’s missing? • There are continents in the way of the water moving! • The ocean basin is not flat and/or uniform! • Average tidal range = 7’ for world (thus NOT at equilibrium)!

  9. Dynamic Theory of tides • Laplace, 1775 (1 century later) • A modification to Newton’s gravitational model = the Dynamic Theory • Now considers ocean depth, continents, the shape of the basin AND what that means to long-wavelength tides/waves in shallow waters

  10. Tides in basins, confined areas etc.

  11. Different tides in different locations!

  12. Put both theories together… • Newton explains HOW the tides occur • Laplace explains how they INTERACT with the surrounding environment (continental margins and shores and even ocean basin depth) • Result: What we now know as “daily tides”

  13. Moon doesn’t rotate around the center of earth but instead around its center of mass (illustrated); they, in turn, together rotate around this center point. (complete cycle every 27.3 days)

  14. Moon’s gravity attracts the ocean toward it, tidal bulge on opposite side(s).

  15. How tides occur... • The bulges stayes aligned with the moon as the earth rotates on its axis, • as points on the earth pass through the bulges, they experience high tide

  16. Bulges follow the moon

  17. Fig 10-7, p.231

  18. Fig. 10-8, p. 232

  19. Other factors affecting tides... • #1 - the moon rises 50 min. later each day • Thus high tides occur about 50 min later each day. Remember (in the NE) there are 2 high and 2 low tides per tidal cycle (roughly 6 hrs. apart)

  20. Fnft

  21. Other factors affecting tides... • #2 - the moon moves each month from a position 28.5o above to 28.5o below the equator • leads to unequal tides – a high high and a lower high that same day.

  22. Tides can be even higher and lower – All based on moon’s position.

  23. Tidal regimes • Semidiurnal tides = two high tides and two low tides of nearly equal level each lunar day • Diurnal tides = one high and one low tide each lunar day • Mixed tides = when successive high or low tides are significantly different in height

  24. Influence of the Sun • The effects of the sun and moon cause wave interference • constructive interference…higher high tides and lower low tides = spring tides • destructive interference…lower high tides and higher low tides = neap tides

  25. Fig 10-11, p.233

  26. fnft

  27. Highest “high” tides = spring tides

  28. Interesting “additions” to tides • Tidal range is different/unique in each location depending upon the topograpgy of the ocean basin underneath the water AND the continental margin(s) shape. • Tidal range = high-water to low-water height differences in an area

  29. A = narrow basin B= Bay of Fundy Tides in basins, confined areas etc.

  30. France, 46’, Fig. 10-1a, p. 229

  31. Fig. 10-1b, p. 229

  32. Effects? …Tidal currents • Rise and fall in sea-level as the tidal crest passes will cause currents • Water rushing toward shore due to rising sea-level is called a flood current • Water rushing away from shore is called an ebb current • Slack water – a time of no current occurs at high or low tide

  33. And finally – tides effect marine life! • Tides produce severe zonation • Currents move fish/planktonic organisms to extreme locations (even more severe during storms) • Seasons effect temperature and that effects everything! • Ex. Grunion: swim up on shore to deposit eggs during an extreme spring tide each year

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