GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 17: Tides
POWERPOINT SLIDE SHOW NOTES
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Topic 17. Tides
Tide. Periodic means they happen regularly.
Only the moon and the sun are close enough to Earth to have major impacts on tides
Types of tidal patterns
A graph of water level versus time is called a “tidal record”
Naming water levels, diurnal and semidiurnal tides
Naming water levels, semidiurnal mixed tides
More Tide Terminology I
More Tide Terminology II
Tidal currents
Two ways to study tides
Equilibrium Tidal Theory
Dynamic Tidal Analysis
Let’s investigate the equilibrium tidal theory in detail
First, the moon
Earth-Moon system
The moon tide
Gravitational tidal bulge
Moon and Centrifugal Force
Centrifugal and gravitational tidal bulges balance
Water on Earth moves toward the moon (by gravitational attraction) and away from the
moon (by centrifugal force).
View looking down on the North Pole. Tides are caused by Earth rotating underneath the
tidal bulges. Tides are simply very long wavelength waves, a wavelength of one-half the
circumference of Earth. The high point of the gravitational bulge is one wave crest; the
high point of the inertial bulge is another wave crest. So there is one complete
wavelength on one side of Earth, a second complete wavelength on the other side.
Lunar Complications to Equilibrium Tidal Theory
The Lunar Day complicates the idealized picture. This is why a high tide or low tide occurs
about an hour later each day. Check it out next time you are at the beach for a couple of
days.
Declination complicates the idealized picture as well. Declination is simply the angle
above or below a plane passing through the center and equator of Earth. Sometimes the
moon is above the plane, sometimes below it. Check out this animation on the NOAA
web page http://oceanservice.noaa.gov/education/kits/tides/media/supp_tide04.html
Tidal bulges follow the moon.
Declination changes can change location of higher high water and lower high water for a
given position
Moon’s elliptical orbit also complicates the idealized picture
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Scale drawing showing range of variations of distance of moon from Earth and declination
(here called inclination band—the range of maximum and minimum declination)
Now let’s add the sun to the simple Earth-Moon picture. There is a set of solar tidal
bulges along with the set of lunar tidal bulges.
Lunar phases are caused by the changing alignment of Earth-Moon-Sun
As lunar phases change, the lunar and solar tidal bulges add together at new and full
moon, and subtract from each other at 1st- and 3rd-quarter moon.
Spring tides
Neap tides
Tide records from different tidal settings
Elliptical astronomical orbits. When the moon is closest to Earth, it is said to be at
perigee; when it is farthest it is at apogee. When the Earth-Moon system is closest to the
sun, it is at perihelion; when it is farthest it is at aphelion. The closer the moon and/or
the sun to Earth, the higher (and lower) the tides.
Perigean spring tides can be dangerous if coincide with storms.
Proxigean spring tides are rare, but also can be dangerous. Proxigee is the moon’s closest
perigee.
Elliptical orbit relationships of Moon with Earth, and Earth-Moon system with the Sun.
Now the other way to look at tides: Dynamic Tidal Analysis. Equilibrium tide theory is
needed to understand all the astronomical relationships and complications, but Dynamic
tidal analysis is needed to explain the real tides on the real Earth.
Real factors controlling tides. Lakes are so small that the difference in gravitational
attraction is negligible.
The tidal wave. Refer back to slide 22. Why are tides shallow-water waves?
Predicting tides. See http://tidesonline.nos.noaa.gov/
There are many types of tide gauges. They actually measure water level of which tides is
only one contributor along with processes such as waves, storm surge, and wind. Most
tide gauges today are digital and relay data directly to data centers.
Unlike the idealized Earth of equilibrium tide theory, Earth’s oceans are separated by
continents. And the real formation of tides occurs within individual ocean basins and the
Coriolis Effect.
A wave called a rotary wave rotates around each basin. The centerpoint of each rotary
wave is called the amphidromic point. At the amphidromic point, there is no change in
water level. High tide is simply the crest of the tidal wave moving around the basin. Low
tide is the trough.
The amphidromic system. In the cross-sectional view, note how the water level does not
change at the amphidromic point (called a node, just like in standing waves). Water level
moves up and down at the antinodes as the rotary tidal wave sweeps around the basin.
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In the map view on the left, the blue circles are called corange lines which have equal tidal
amplitude around the basin. The straight black lines are called cotidal lines showing the
timing of high tide around the basin.
Amphidromic systems in the global ocean. Rotary waves move counterclockwise around
the amphidromic points in the northern hemisphere.
Tidal bore
Photo of a small tidal bore
High-tide areas of the world
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Some of the world’s highest tides are in the Bay of Fundy, New Brunswick, Canada,
because of the funneling effect of the embayment.
High and low tide in the Bay of Fundy.