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TIDES

TIDES. periodic, short term changes in the height of the ocean surface at a particular place . Fig. 10-CO, p. 195. 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. Fig. 10-CO, p. 195

  3. 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

  4. 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

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

  6. 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)

  7. 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 10-1, p.197

  8. 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.

  9. 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)!

  10. 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

  11. Tides in basins, confined areas etc. Fig 10-14, pg. 205

  12. Different tides in different locations! Fig 10-10, p.202

  13. 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”

  14. 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) Fig 10-2, p.197

  15. Moon’s gravity attracts the ocean toward it, tidal bulge on opposite side(s). Fig 10-3, p.198

  16. 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

  17. Bulges follow the moon Fig 10-6, p.199

  18. Fig 10-4, p.198

  19. Fig. 10-5, p. 199

  20. 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)

  21. Fnft

  22. 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.

  23. Tides can be even higher and lower – All based on moon’s position. Fig 10-7, p.199

  24. 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

  25. Fig 10-10, p.202

  26. Fig 10-10, pg. 202

  27. 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

  28. Fig 10-8, p.200

  29. fnft

  30. Highest “high” tides = spring tides Fig 10.9, pg. 201

  31. 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

  32. A = narrow basin B= Bay of Fundy Tides in basins, confined areas etc. Fig 10-14, p.205

  33. Fig. 10-15b, p. 206

  34. Fig. 10-15a, p. 206

  35. 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

  36. Fig 10-16, p.208

  37. 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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