Tides
Tidal Characteristics
single
waves - stretch across entire
ocean basins.
shallow-water waves
– wavelengths greatly exceed the depth of
the ocean.
complex
sun
interactions of moon and
Origin of the Tides
Unlike
wind-driven surface waves
and unlike tsunamis, tides are
caused by two principal factors:
–Gravitational attraction
–Centrifugal force
Gravitational Attraction
All
masses are drawn to each other.
The moon because of its closeness
to the Earth exerts a greater
gravitational effect on the Earth than
the Sun, despite the fact that the Sun
is much more massive than the
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
Earth
and Moon revolving around
one another.
unequal masses of Earth and Moon
– center of rotation lies beneath the
Earth’s surface.
second
tidal bulge
– ocean shifts away from center of
rotation
Centrifugal Forces: Center of Rotation
2 Bulges from Gravitational Attraction
& Centrifugal Force
Animation
Dynamic Model of the Tides
True Tidal Waves
(not tsunamis!)
The
motion of water around the basin
is a rotary wave.
– The crest is at one side of the basin.
– The trough is at the other side of the
basin.
Tides
have characteristics of both
progressive and standing waves.
Standing Waves
Sea,
swell, and surf are progressive
waves.
Standing waves (seiches)…
–Oscillate back and forth about a node,
i.e., a fixed point.
Node = Fixed Point
Antinode = maximum displacement
Dynamic Model of theTides
Dynamic Model of theTides
Earth rotates from
W to E
Tidal bulge to W
Pressure Gradient
Coriolis
cut-away
cut-away
Pressure Gradient
Coriolis
Pressure Gradient
Coriolis
Dynamic Model
 equilibrium
model of the tides is good,
BUT...
– not totally detailed and accurate
– oceans are separated by continents
– 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
High
tide : Wave crest
Low tide
: Wave trough
Tidal range : Wave height
Tidal periods depending on location:
– 12 hours, 25 minutes
– 24 hours, 30 minutes
Wave Parameters
Tidal Periods
 why
not exactly 24 hours or 12 hours?
Moon
moves forward in it’s orbit each
day.
– Takes 50 additional minutes for a spot on
the Earth’s surface to regain it’s position
relative to the Moon.
Classification by Daily Record
Diurnal:
have one high tide and one
low tide daily (high lat).
Semidiurnal: have two high tides
and two low tides daily (low lat).
Mixed: there are two high tides and
two low tides daily, but of unequal
shape (mid lat).
Classification by Monthly Record
Spring
tide: phase when tidal range
is maximal.
Neap tide: phase when tidal range is
minimal.
There are 2 spring and 2 neap tides
each month
Spring & Neap Tides
 Spring
tides occur when the Earth, Sun,
and Moon are aligned.
– New Moon and Full Moon phases
– Constructive interference
 Neap
tides occur when the Sun and
Moon are aligned at right angles to one
another.
– Quarter Moon phases
– Destructive interference
Sun/Moon/Earth - Spring & Neap
Can you read a tide table?
Marine Biorhythms
Cyclical
behaviors associated with
tidal rhythms.
– The diurnal activity of fiddler crabs.
– The spawning behavior of the grunion
fish.
Diurnal Activity of Fiddler Crabs
Natural
Searching for
food
in burrows
Movie Trivia
Jeopardy, 1953, Barbara Stanwyck