Tides
Bay of Fundy (Canada)
Tides
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Measuring tides
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General description
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Physical mechanism
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Characteristics in different locations
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Tidal resonance and tidal bore
Measuring tides
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Records of the position of sea surface with
respect to a fixed level (a datum)
tidal pole
float tidal gauge
Measuring tides
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Time series of sea surface height
Hilo tide observations and predictions (PacIOOS)
Measuring tides
Examples of tidal time series
General description
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Up and down motion of sea level
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Back and forth currents
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Periodic (~12 hours and 24 hours)
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Amplitude and timing vary from
one location to another
Propagating shallow-water wave
of very long wavelength
(~10,000km)
Physical mechanism
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due to the gravitational attraction of Moon and
Sun on Earth and its ocean
and the rotation of Moon around Earth and the
Earth-Moon system around the Sun
Sun
Moon
Earth
Physical mechanism
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due to the gravitational attraction of Moon and
Sun on Earth and its ocean
and the rotation of Moon around Earth and the
Earth-Moon system around the Sun
Sun
Moon
“It's complicated!”
Earth
Theory of equilibrium tides
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Simplified problem
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Earth-Moon system only
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No rotation of Earth around its own axis
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Earth covered with an ocean (no continents)
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No friction
Theory of equilibrium tides
Centrifugal Force
(varies withr)
r
d
Gravitational Force
varies with 1/d2)
Theory of equilibrium tides
Theory of equilibrium tides
Theory of equilibrium tides
Theory of equilibrium tides
Theory of equilibrium tides
Tidal Force
Theory of equilibrium tides
Tidal Force
Theory of equilibrium tides
Tidal Force
Two Lunar Bulges
With Earth's rotation
Semi-diurnal Tide
B
A
C
~ 1 day
D
Two Lunar Bulges
A
B
C
D
A
Lunar day
Lunar day is about 24 hours and 50 min
Semi-diurnal tide is 12 hours and 25 min
With Earth's rotation
Semi-diurnal Tide
B
A
C
1 lunar day
D
Two Lunar Bulges
A
B
C
D
A
With Earth's rotation
Semi-diurnal Tide
D
C
A
equator
1 lunar day
Two Lunar Bulges
A
B
C
D
A
With Earth's declination
Diurnal Tide
C
A
B
or
t
a
equ
1 lunar day
Two Lunar Bulges
A
B
C
D
A
With Earth's declination
Mixed Tide
B
A
C
or
t
a
equ
1 lunar day
Two Lunar Bulges
A
B
C
D
A
With Earth's declination
Semi-diurnal Tide
C
or
t
a
equ
B
1 lunar day
A
Two Lunar Bulges
A
B
C
D
A
Three types of tides
Diurnal Tide
Semi-diurnal Tide
Mixed Tide
Spring and neap tides
Spring Tide
Neap Tide
Two Spring/Neap Tides every 29 ½ days
Spring and neap tides
Additional variations
Greatest tidal range
Lowest tidal
range
Additional variations
Spring Tide +
Perigee =
Proxigean Tides
Greatest tidal range
Lowest tidal
range
Real tides
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Tides are shallow-water waves
(period 12-24 hours, size of an ocean basin)
Propagates counterclockwise in N.H. and
clockwise in S.H.
Amphidromic points (weak tides at these locations)
Real tides
Bay of Fundy: Case of
tidal resonance
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Off-shore tides excite a natural mode of variability
of the continental shelf or the bay
Tacoma Narrows Bridge Collapse
Tidal bore
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Incoming tide forms a wave that travels up a river
against the river current
Watch the "Bono" in Indonesia: A case of river
surfing (Kampar River)