Geography and Oceanography - Chapter 20 pptx

Tides
Tides
Tidal Characteristics
Tidal Characteristics
◆
single waves - stretch across entire
single waves - stretch across entire
ocean basins.
ocean basins.
◆
shallow-water
shallow-water
waves
waves
–
wavelengths greatly exceed the depth of
wavelengths greatly exceed the depth of
the ocean.
the ocean.
◆
complex interactions of
complex interactions of
moon and
moon and
sun
sun
Origin of the Tides
Origin of the Tides
◆
Unlike wind-driven surface waves
Unlike wind-driven surface waves

and unlike tsunamis, tides are
and unlike tsunamis, tides are
caused by two principal factors:
caused by two principal factors:
–
Gravitational attraction
Gravitational attraction
–
Centrifugal force
Centrifugal force
Gravitational Attraction
Gravitational Attraction
◆
All masses are drawn to each other.
All masses are drawn to each other.
◆
The
The
moon
moon
because of its closeness
because of its closeness
to the Earth
to the Earth
exerts a greater
exerts a greater
gravitational effect on the Earth than
gravitational effect on the Earth than
the Sun
the Sun

, despite the fact that the Sun
, despite the fact that the Sun
is much more massive than the
is much more massive than the
Moon.
Moon.
Gravitational Effect of Moon
Gravitational Effect of Moon
Animation courtesy of NASA Goddard Space Flight Center: The Moon's gravity
tugs at the Earth, causing ocean water to slosh back and forth in predictable waves
called tides. We can visibly observe some of that energy dissipate at the beach, with
waves rolling across coastal shallows and shoals. Most of the energy dissipates due
to friction between the water and the shallow floor beneath it
Centrifugal Force
Centrifugal Force
◆
Earth and Moon revolving around
Earth and Moon revolving around
one another.
one another.
◆
unequal masses
unequal masses
of Earth and Moon
of Earth and Moon
–
center of rotation lies beneath the
center of rotation lies beneath the
Earth’s surface.
Earth’s surface.

◆
second tidal bulge
second tidal bulge
–
ocean shifts
ocean shifts
away
away
from center of
from center of
rotation
rotation
Centrifugal Forces: Center of Rotation
Centrifugal Forces: Center of Rotation
2 Bulges from Gravitational Attraction
2 Bulges from Gravitational Attraction
& Centrifugal Force
& Centrifugal Force
Animation
Animation
Dynamic Model of the Tides
Dynamic Model of the Tides
True Tidal Waves
True Tidal Waves
(
(
not
not
tsunamis!)
tsunamis!)

◆
The motion of water around the basin
The motion of water around the basin
is a
is a
rotary wave
rotary wave
.
.
–
The crest is at one side of the basin.
The crest is at one side of the basin.
–
The trough is at the other side of the
The trough is at the other side of the
basin.
basin.
◆
Tides have characteristics of both
Tides have characteristics of both
progressive and standing waves
progressive and standing waves
.
.
Standing
Standing
Waves
Waves
◆
Sea, swell, and surf are progressive

Sea, swell, and surf are progressive
waves.
waves.
◆
Standing waves (seiches)…
Standing waves (seiches)…
–
Oscillate back and forth about a
Oscillate back and forth about a
node
node
,
,


i.e., a fixed point.
i.e., a fixed point.
Node = Fixed Point
Node = Fixed Point
Antinode = maximum displacement
Antinode = maximum displacement
Dynamic Model of theTides
Dynamic Model of theTides
Dynamic Model of theTides
Dynamic Model of theTides
Earth rotates from
W to E
Tidal bulge to W
cut-away
Pressure Gradient

Coriolis
Pressure Gradient
Coriolis
Pressure Gradient
Coriolis
cut-away
Dynamic Model
Dynamic Model
◆
equilibrium model of the tides is good,
equilibrium model of the tides is good,
BUT
BUT
–
not totally detailed and accurate
not totally detailed and accurate
–
oceans are separated by continents
oceans are separated by continents
–
oceans not infinitely deep
oceans not infinitely deep
Animation courtesy of NASA Goddard Space Flight Center: Using 6 years of
data from TOPEX/Poseidon satellite altimeter, they derived a 16-day set of
predictive data, showing a synthetic view of how the tides move around the world's
oceans.
Key Features of Tides
Key Features of Tides
◆
High tide : Wave crest

High tide : Wave crest
◆
Low tide : Wave trough
Low tide : Wave trough
◆
Tidal range : Wave height
Tidal range : Wave height
◆
Tidal periods depending on location:
Tidal periods depending on location:
–
12 hours, 25 minutes
12 hours, 25 minutes
–
24 hours, 30 minutes
24 hours, 30 minutes
Wave Parameters
Wave Parameters
Tidal Periods
Tidal Periods
◆
why not exactly 24 hours or 12 hours?
why not exactly 24 hours or 12 hours?


◆
Moon moves forward in it’s orbit each
Moon moves forward in it’s orbit each
day.
day.

–
Takes 50 additional minutes for a spot on
Takes 50 additional minutes for a spot on
the Earth’s surface to regain it’s position
the Earth’s surface to regain it’s position
relative to the Moon.
relative to the Moon.
Classification by
Classification by
Daily
Daily
Record
Record
◆
Diurnal
Diurnal
: have one high tide and one
: have one high tide and one
low tide daily
low tide daily
(high lat).
(high lat).
◆
Semidiurnal
Semidiurnal
: have two high tides
: have two high tides
and two low tides daily
and two low tides daily
(low lat).

(low lat).
◆
Mixed
Mixed
: there are two high tides and
: there are two high tides and
two low tides daily, but of
two low tides daily, but of
unequal
unequal
shape
shape


(mid lat).
(mid lat).
Classification by
Classification by
Monthly
Monthly
Record
Record
◆
Spring tide
Spring tide
: phase when tidal range
: phase when tidal range
is
is
maximal

maximal
.
.
◆
Neap tide
Neap tide
: phase when tidal range is
: phase when tidal range is
minimal
minimal
.
.
◆
There are 2 spring and 2 neap tides
There are 2 spring and 2 neap tides
each month
each month
Spring & Neap Tides
Spring & Neap Tides
◆
Spring
Spring
tides occur when the Earth, Sun,
tides occur when the Earth, Sun,
and Moon are aligned.
and Moon are aligned.
–
New Moon and Full Moon phases
New Moon and Full Moon phases
–

Constructive interference
Constructive interference
◆
Neap
Neap
tides occur when the Sun and Moon
tides occur when the Sun and Moon
are aligned at right angles to one another.
are aligned at right angles to one another.
–
Quarter Moon phases
Quarter Moon phases
–
Destructive interference
Destructive interference
Sun/Moon/Earth - Spring & Neap
Sun/Moon/Earth - Spring & Neap