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TIDES

TIDES. TIDES. Tides - periodic rise and fall of sea surface Generated by the gravitational attraction of the Sun and Moon on the oceans moon closer to earth, so has more pull than sun therefore the sun's pull is 46% of the Moon's. TIDES cont. Measurement of tide

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TIDES

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  1. TIDES

  2. TIDES • Tides - periodic rise and fall of sea surface • Generated by the gravitational attraction of the Sun and Moon on the oceans • moon closer to earth, so has more pull than sun therefore the sun's pull is 46% of the Moon's

  3. TIDES cont. • Measurement of tide • measuring pole attached to a pier • mechanical tide gauge • tidal curve - records of changing sea level

  4. TIDES cont. • Very long waves • 1. generated by the gravitational attraction between ocean water and the sun and moon • 2. are shallow-water waves because of the shallowness of ocean basins relative to wavelength ( depth < L/20 )

  5. TIDES cont, • Tidal period - time between successive high (or low) tides • remember wave period p 234

  6. TIDES cont. • Tidal day - time between transits of moon overhead (24 hours & 50 minutes) it takes the moon 50 min. to catch up to a specific spot on Earth (say 0º L) • 1. there are two high and two low tides per day • 2. this corresponds to the time between successive passes of the Moon over any point on earth

  7. Go to youtube and look at tides • Gravity and centrifugal force Fig 9.1, 9.2, 9.3 and 9.4 p 26 2- 264 • According to Newton’s law of universal gravitation force, the attractive force (F) between two bodies is proportional to the product of their masses (m1 and m2), and inversely proportional to the square of the distance (r) • between them: • m = mass, 1 and 2 • R = distance between the centers of masses in centimeters quantity squared • F= (m1) (m2) r-1

  8. Three types of tides in the ocean Fig 9.16 p 273 and S&A 122 • Semidiurnal - two high and two low tides per day and are approximately equal (New York Harbor) • Diurnal - one high and one low tide per day (gulf of Mexico)

  9. Three types of tides in the ocean cont. • Mixed - two high and two low tides per day - highs and lows different in one day - more difficult to predict • 1. higher high water, lower high water • 2. lower low water, higher low water

  10. Tidal record • Graph of tide over time Sept. graph Fig 9.17 p 274 • 1. tidal range - difference between the highest and lowest tide levels • 2. daily inequalities - difference between the heights of successive high or low tides

  11. Tidal record cont • 3. spring (forth) tides - the times of greatest tidal range (occur during full and new Moons) Fig 9.10 & 9.11 p 267 & 268 • 4. neap tides - the tidal range is least during the first and third quarters

  12. Show Hawaii calendar for January

  13. Theoretical approaches • Equilibrium Tide Theory - simplest approximation • accounts for daily inequalities, spring and neap tides

  14. Equilibrium Tide Theory cont. • makes the following assumptions • 1. Earth has two equal tidal bulges, one toward the Moon and one away S&A 124 p 284 this only happens in the model • 2. the ocean covers the whole Earth at a uniform depth • 3. there is no friction between sea water and seafloor • 4. the continents have no influence

  15. Theoretical approaches cont. • Dynamic tide theory - more complicated approximation (think standing waves) • 1. includes response of waters in ocean basins (shape and depth) to tide-generation forces - (no bulge)

  16. Dynamic tide theory cont. • 2. tides include water movement and must include the Coriolis effect - tide waves (progressive tide) rotate around fifteen amphidromic points (seven in the Pacific Ocean and four each in the Atlantic and Indian oceans) Fig 9.15 p 272

  17. Dynamic tide theory cont. • co-tidal lines Fig 9.15 p 272 • rotate counter clock wise in northern hemisphere clock wise in south • numbers represent hour after 00 GMT for 12 hour period

  18. Co-tidal lines

  19. Dynamic tide theory cont. • 3. can predict more aspects of tidal phenomena • 4. high-water side to the low-water side of a standing tide wave produces a rotating tidal current

  20. Tides in Ocean Basins • Looking at the Atlantic Ocean • 1. tide moves from southern ocean northward • 3. even if the Atlantic were completely isolated it would still have a tide that would be like a standing wave with some characteristics of a progressive wave • 3 the wave from the Antarctica tide also interacts with the independent tide generated in the Atlantic - amphidromic point Fig 9.15 p 272

  21. Tidal currents • horizontal water movements associated with passage of tides Fig 9.19 p 278 read this section carefully • More difficult to predict than tides T 99

  22. Reversing tidal currentFig 9.19 p 278

  23. Tidal currents cont. • The crest of the wave is high tide, and the trough is low tide • 1. orbital motions of water caused by tide are ellipses with their long axes parallel to the ocean bottom - horizontal motions with little vertical motion draw the orbitals like those seen for waves (recall the orbitals like those seen in waves)

  24. recall the orbitals like those seen in waves

  25. Tidal currents cont. • 2. Ocean tidal currents are rotary currents (semidiurnal mixed tide) S&A 134, Xerox copy • mixed tidal cycle • two unequal tidal cycles are shown • numbers indicate the consecutive hours • coriolis effect deflects the horizontal component of the water particles

  26. Tidal currents cont. • Reversing currents – p 277-8 • - water periodically flows in one direction for a while and then reverses to flow in the opposite direction • 1. occur in harbors • 2. flood current / ebb current • 3. slack current - no current (tell the eel story)

  27. Tidal currents cont. • Tidal currents are variable in estuaries and bays • 1. determined by the size and shape of the bay entrance • 2. can be modified by river discharges and by winds

  28. Tidal currents cont. • 3. Chesapeake Bay - tide behaves like progressive wave (wave moves in a constant direction) • 4. Long Island Sound - tide is standing wave • 5. Bay of Fundy Fig9.18 p 277

  29. Because the period of free oscillation in the Bay of Fundy is nearly the same as the tidal period resonance that is produced. A very large tidal range occurs at the northern end of Minas Basin due to constructive interference. The bay also narrows and becomes progressively shallower toward its northern end, which amplifies tidal effects. In addition, the Bay of Fundy is a right-curving bay, so the Coriolis effect in the Northern Hemisphere adds to the tidal range.

  30. Low tide Bay of Fundyhttp://www.youtube.com/watch?v=YiHP8EG_Wq4This is a quick video, so fish around for more

  31. Energy from the sea • Energy can be extracted from tides in area of high tidal ranges as well as the waves • Read text pages 278 - 281 • Try to integrate these two sources of energy from the ocean

  32. THE TIDE STOPS HERE

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