TIDES

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TIDES

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

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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

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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
  tideswavelengths of half the Earths
  circumference (max.)
• High tide is the crest of the wave and low tide
  is the trough

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Studies

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

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Equilibrium Theory of Tides

• Isaac Newton, 1600s
• 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)

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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
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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.

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Whats 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)!

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Dynamic Theory of tides

• Laplace, 1775 (1 century later)
• A modification to Newtons 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

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Tides in basins, confined areas etc.
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Different tides in different locations!
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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

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

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Bulges follow the moon
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Fig 10-7, p.231
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Fig. 10-8, p. 232
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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)

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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.

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Tides can be even higher and lower All based on
moons position.
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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

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Influence of the Sun

• The effects of the sun and moon cause wave
  interference
• constructive interferencehigher high tides and
  lower low tides spring tides
• destructive interferencelower high tides and
  higher low tides neap tides

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Fig 10-11, p.233
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Highest high tides spring tides
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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

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A narrow basin
B Bay of Fundy
Tides in basins, confined areas etc.
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France, 46, Fig. 10-1a, p. 229
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Fig. 10-1b, p. 229
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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

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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