Transcript Tides, lecture 9

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
• 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
Studies
• 2 scientists (combined) explain the effects
and daily occurrence of tides
• Newton
• Laplace
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)
Newton: Equilibrium theory
Planets orbit the sun in balance between gravity and inertia:
(a) 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
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.
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)!
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
Tides in basins, confined areas etc.
Different tides in different locations!
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”
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)
Moon’s gravity attracts the ocean toward it, tidal bulge
on opposite side(s).
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
Bulges follow
the moon
Fig 10-7, p.231
Fig. 10-8, p. 232
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)
Fnft
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.
Tides can be
even higher
and lower –
All based on
moon’s position.
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
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
Fig 10-11, p.233
Highest “high” tides = spring tides
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
A = narrow basin
B= Bay of Fundy
Tides in basins, confined areas etc.
France, 46’, Fig. 10-1a, p. 229
Fig. 10-1b, p. 229
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
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