1. GEOL 2503 Introduction to Oceanography
Topic 17 Tides
GEOL 2503
Introduction to Oceanography

2. Tides
Periodic waves caused by the gravitational pull of the moon and sun on the earth
Timing set by earth's rotation
Long wave lengths—up to 1/2 the circumference of the earth

3. Major tidal influences on Earth
The tide-producing force (F) is proportional to the product of the masses (m1 and m2) over the CUBE of the distance (r).
Sun
Moon
Mass
2.0 x 1033 g
7.3 x 1025 g
Distance
150,000,000 km
385,000 km
Tidal effect
0.46
1.00

4. Types of Tide Patterns Diurnal tide—one high and one low daily
Semidiurnal—two cycles daily, with the two highs about the same height and the two lows drop to about the same level
Semidiurnal mixed—two cycles daily, but the two high tides reach different heights and the two low tides drop to different levels

6. Tide Levels: Diurnal or Semidiurnal
High water—greatest height to which the water rises on any day
Low water—lowest point to which the tide falls on any day

7. Tide Levels: Semidiurnal Mixed
Higher high water—higher of the two highs
Lower high water—lower of the two highs
Higher low water—higher of the two lows
Lower low water—lower of the two lows

8. More Tide Terminology I
Average (or mean) tides—average of all water levels taken over many years
Mean high water—average high water level
Mean low water—average low water level
Tidal datum—reference depth for reporting water depth (for navigation safety, mean low water is usually the tidal datum)

9. More Tide Terminology II
Minus tide—when water level falls below the mean value
Flood tide—when water level is rising
Ebb tide—when water level is falling

10. Tidal Currents Currents associated with rising or falling of the tides
Important currents for nearshore navigation
Flood tide current—water level rising and current is toward the land
Ebb tide current—water level falling and current is toward the sea
Slack water—turning tide, between ebb/flood or flood/ebb

11. Two Ways to Study Tides Equilibrium Tidal Theory
Mathematical idealized principles
Dynamic Tidal Analysis
Measure the real world

12. Equilibrium Tidal Theory
Mathematically ideal wave
Assumes uniform layer of water covering Earth
Used to simplify Earth, Moon, Sun relationships
Good for illustration of principles, but not good for predicting actual tides

13. Dynamic Tidal Analysis
Study of tides as they occur naturally
Modified by landmasses, shape of ocean basins, and Earth’s rotation

14. Equilibrium Tidal Theory
Equilibrium tide theory explains:
effects of the Sun’s and Moon’s gravity
effects of rotation
Consider Earth and Moon as a single unit, the Earth-Moon system orbiting the Sun

15. Moon Orbits Earth Held by Earth’s gravitational force
Force acting to pull Moon away from Earth is centrifugal force
The two forces balance

16. Earth-Moon System Earth and Moon rotate about a common center of mass
Held in orbit about the Sun by the Sun’s gravitational attraction
Sun’s gravitational attraction balanced by a centrifugal force acting to pull Earth-Moon system away from the sun

17. The Moon Tide
Water on Earth’s surface closer to the moon (side of Earth facing Moon) acted on by excess gravitational force
Water moves along Earth surface toward a point directly under the Moon
Produces a bulge in the water covering the Earth
Called the gravitational bulge

18. G

19. Moon and Centrifugal Force
Centrifugal force is equal to the gravitational attraction of the Moon, but operates in the opposite direction
Causes an opposing bulge on Earth’s surface away from the Moon
Called the centrifugal bulge or inertial bulge

20. G C C = Centrifugal Force G = Gravitational Attraction
The opposing gravitational and centrifugal forces create two tidal bulges

22. Earth’s rotation under the tidal bulge gives the observer two high tides and two low tides each day

23. Lunar Complications to Equilibrium Tidal Theory
Lunar Day
Caused by timing of Moon’s orbit of Earth
Moon’s declination
Moon above or below celestial plain
Moon’s elliptical orbit
Distance from Earth varies

24. Lunar Day (or Tidal Day) = Time for completion of each day’s entire tidal cycle, diurnal or semidiurnal, from high tide to the next day’s high tide. See the observer on the diagram below.
Observer begins at high tide, aligned with moon.
Earth rotates once in 24 hours, and Moon orbits Earth in about 28 days
After 24 hours, Moon has moved forward 1/28th of the way around Earth, which is 1/28th of 24 hours, or about 50 minutes
Thus, Earth must rotate 50 minutes more so the observer on Earth is aligned with the moon and thus at the next high tide
Thus, Lunar Day = 24 hours 50 minutes

25. Moon’s Declination Changes

28. When the moon is closest to Earth (perigee), the tide-producing force is increased by 20%
When the moon is farthest from Earth (apogee), the tide-producing force is decreased by 20%

29. This drawing is to scale
This drawing is to scale. Note the variation in the distance of the moon from the earth and the distance of the moon above and below the orbital plane.

30. Complications of the Sun
Like the Moon, the Sun also produces two equilibrium tidal bulges
Even though the Sun is huge, it is so far away that it has only 46% of the tide-producing effect of the Moon
Tides vary with phase of the Moon as Earth-Moon system orbits the Sun

31. As the Moon revolves around the Earth, the Earth-Moon system is revolving around the Sun. In a lunar month, the moon passes through a series of “phases” as seen from Earth.

33. Spring Tides Occur at New Moon and Full Moon phases
Earth, Moon, and Sun aligned
Moon’s tidal bulge adds to Sun’s
Higher high tides
Lower low tides
Nothing to do with seasons. The word “Spring” is from Old English word “springere” meaning to rise or spring up

34. Neap Tides Occur at 1st quarter and 3rd quarter phases
Earth, Moon, and Sun at right angles
Moon tidal bulge subtracts from Sun’s
Lower high tides
Higher low tides
Neap—acronym for near even as possible

35. Which tidal records show diurnal tides? Semidiurnal? Semidiurnal mixed?
Note the timing of spring and neap tides and lunar phases.

36. C. perigee
D. aphelion
A. apogee
B. perihelion

37. Perigean Spring Tides A spring tide during moon’s perigee.
Higher highs and lower lows
Occurs a few times per year
facts/perigean-spring-tide.html

38. Proxigean Spring Tide A rare, unusually high tide
Occurs when the moon is both unusually close to the Earth (at its closest perigee, called the proxigee) and in the New Moon phase (when the Moon is between the Sun and the Earth)
25% increase in tides
The proxigean spring tide occurs at most once every 1.5 years

40. Dynamic Tidal Analysis
Needed to explain the real tides on Earth
There are many complications with Equilibrium Tidal Theory

41. Real Factors Controlling Tides
Size of basin (lakes have no tides)
Width of continental shelf (wider shelves = higher tides)
Shape of shoreline, concave embayments = higher tides
Water depth variations deep water nearshore = higher tides

42. The Tidal Wave Wavelength is one-half Earth’s circumference
Shallow-water wave
Courtesy of Dr. Tom Garrison

43. Predicting Tides Can’t use Equilibrium Tidal Theory alone
Need to combine years of local observations with astronomical data
Minimum 19-year record to allow for 18.6-year declinational period of the Moon
Result is Tide Tables with local predictions of water level and timing of tides

45. What Really Happens
Tides in individual basins are deflected by the Coriolis Effect
Also blocked by continents
Results in a rotary wave going around the basin
Two daily cycles around the basin = semidiurnal
One daily cycle around the basin = diurnal

46. Rotary Tide Cycle called Amphidromic System

47. Amphidromic System Amphidromic Point (node)—no change in water level
Cotidal lines—tide occurs at the same time along each line
Corange lines—tides of same amplitude along each circle

48. Cotidal Lines

49. Tidal Bore
A tidal bore is a wall of water that surges upriver with the advancing high tide.

50. Tidal Bore

51. Tides