CHAPTER 9 CHAPTER 9
Tides
Tides
Chapter 9: Tides - Contents Chapter 9: Tides - Contents
Tide-Generating ForcesTide-Generating Forces
- Gravitational- Gravitational
- Centripetal- Centripetal
- Lunar vs Solar- Lunar vs Solar
Monthly Tidal CycleMonthly Tidal Cycle
- Terms & Definitions- Terms & Definitions
- Phases of the Moon- Phases of the Moon
- Spring vs Neap Tides- Spring vs Neap Tides
Ideal vs Real TidesIdeal vs Real Tides
- Complicating Factors- Complicating Factors
o Moon’s OrbitMoon’s Orbit
o DeclinationDeclination
o Elliptical OrbitsElliptical Orbits
- Tidal Cells & Patterns- Tidal Cells & Patterns
Tides in Coastal WatersTides in Coastal Waters
- Tidal Bores - Tidal Bores
- Whirlpools - Whirlpools
Overview of Tides Overview of Tides
Rhythmic daily rise and fall of sea level Rhythmic daily rise and fall of sea level
Classified as regular shallow-water waves Classified as regular shallow-water waves
Very long wavelengths (thousands of kms) Very long wavelengths (thousands of kms)
Very long wave periods (12 Very long wave periods (12 24 hrs) 24 hrs)
Wave heights may surpass 50 feet Wave heights may surpass 50 feet
Caused by the gravitational attraction of the Sun, Caused by the gravitational attraction of the Sun, Moon, and Earth
Moon, and Earth
F = F = G m G m
11m m
2 (Newton’s Law of Universal Gravitation)2 (Newton’s Law of Universal Gravitation)r r
22 Sun is bigger, but moon is much closer Sun is bigger, but moon is much closer Sun’s Sun’s tide-generating force on the Earth is only 46% of tide-generating force on the Earth is only 46% of
that of the Moon’s tide-generating force
that of the Moon’s tide-generating force
Gravitational Forces (gravity) Gravitational Forces (gravity)
Centripetal Forces (motion) Centripetal Forces (motion)
Combination Combination of the 2 produce a resultant force of the 2 produce a resultant force
Tide-Generating Forces Tide-Generating Forces
•
Barycenter between Moon and Earth
–
Common center of mass or balance point
–
Beneath Earth’s surface because of Earth’s
greater mass
• Gravitational force
derived from Newton’s Law of Universal
Gravitation
– Every object that has mass in the universe is attracted to every other object.
Gravitational Forces
• Gravitational force proportional to product of masses
– Increase mass, increase force
• Inversely proportional to square of separation
distance
Gravitational Force (Earth & Moon) Gravitational Force (Earth & Moon)
Every particle attracts every other particle Every particle attracts every other particle
The closer to the Moon, the more of a force The closer to the Moon, the more of a force
Zenith (Z) Zenith (Z) point closest to Moon greatest point closest to Moon greatest gravitational attraction
gravitational attraction
Nadir (N) Nadir (N) point farthest point farthest least attraction least attraction
Different points on Earth have different
gravitational forces acting on them in
different
Centripetal Force (Earth & Moon) Centripetal Force (Earth & Moon)
Center-seeking force Center-seeking force
Analogy Analogy Swing a ball-on-a-string over your head Swing a ball-on-a-string over your head string pulls ball string pulls ball toward your hand
toward your hand string provides a centripetal force on ball forcing ball string provides a centripetal force on ball forcing ball to seek the center of its orbit
to seek the center of its orbit
Gravity (string) tethers Earth and Moon to each other Gravity (string) tethers Earth and Moon to each other
All points
experiencing the same
centripetal
force oriented in the same direction
(towards Moon)
Tide-Generating Forces Tide-Generating Forces
Resultant force Resultant force (Net Force) = difference between (Net Force) = difference between centripetal and gravitational force
centripetal and gravitational force
line from tip of red arrow to tip of black arrow (starting line from tip of red arrow to tip of black arrow (starting from the origin of both arrows)
from the origin of both arrows)
• Resultant force has significant horizontal component
• Lunar bulges
– Result when force pushes water into two simultaneous bulges
• One toward Moon
• One away from Moon
Tide-Generating Forces
• Solar bulges
• Similar to lunar bulges but less than half the size
• Moon closer to Earth, exerts greater gravitational force
Tidal Bulges (Solar)
• Tidal period – time between high tides
• Flood tide (High Tide) – water moves toward shore
• Ebb tide (Low Tide) – water moves away from shore
• Tidal range – difference in distance between high & low tides
• Tidal bulges are fixed relative to the Sun’s and Moon’s positions
– Earth’s rotation moves different geographic locations into bulges
Earth’s Rotation and Tides
Monthly Tidal Cycle
• New (or No) Moon – Moon
between Earth and Sun, cannot be seen from Earth (bright
around the fringe)
• Full Moon – Moon and Sun opposite
• Quarter Moon – Moon appears half lit
• Waxing crescent – Moon
moving from new to first quarter
• Waxing gibbous – Moon
moving from first quarter to full
• Waning gibbous – Moon
moving from full to last quarter
• Waning crescent – Moon
moving from last quarter to new moon
Monthly Tidal Cycle
• Spring tides
– New or full moons
– Tidal range greatest
– Syzygy
• Neap tides
– Quarter moons
– Tidal range least
– Quadrature
Monthly Tidal Cycle
Idealized Tidal Bulges
Ideal Earth Ideal Earth covered only by ocean covered only by ocean
Two tidal bulges Two tidal bulges
Two high tides, 12 hrs apart Two high tides, 12 hrs apart
Two low tides, 12 hrs apart Two low tides, 12 hrs apart
Complicating Factors: Moon’s Orbit Complicating Factors: Moon’s Orbit
Successive high tides are actually not exactly 12 hours apart Successive high tides are actually not exactly 12 hours apart 12 hours and 25 minutes
12 hours and 25 minutes WHY? WHY?
Moon orbits Earth Moon orbits Earth it takes 24 hours 50 minutes for observer to it takes 24 hours 50 minutes for observer to see subsequent Moons directly overhead (lunar day)
see subsequent Moons directly overhead (lunar day)
Solar day Solar day 24 hours 24 hours
Why the difference? Why the difference?
Because during the Earth’s 24 hr rotating period, the moon has Because during the Earth’s 24 hr rotating period, the moon has moved an extra 12.2 degrees in its revolution around the Earth moved an extra 12.2 degrees in its revolution around the Earth
Earth has to catch up (needs extra 50 min.) Earth has to catch up (needs extra 50 min.)
This is why high tides and low tides don’t occur at the same time This is why high tides and low tides don’t occur at the same time every day.
every day.
Another Complicating Factor:
Another Complicating Factor: Declination Declination
Moon & Sun Moon & Sun not always directly over equator not always directly over equator
Usually at an angle (tilt) Usually at an angle (tilt) Declination Declination
Angular distances vary Angular distances vary
Sun to Earth: 23.5 Sun to Earth: 23.5
ooN or S of equator N or S of equator
Moon to Earth: 28.5 Moon to Earth: 28.5
ooN or S of equator (Moon’s N or S of equator (Moon’s orbit is tilted 5 degrees more wrt the ecliptic)
orbit is tilted 5 degrees more wrt the ecliptic)
Shifts lunar and solar bulges from equator Shifts lunar and solar bulges from equator
Unequal tides Unequal tides
Since the Moon affects tides more, the center of
Declination Declination
Unequal tides Unequal tides Unequal tidal ranges Unequal tidal ranges
More Complicating Factors: Elliptical Orbits More Complicating Factors: Elliptical Orbits
Earth & Moon not always the same distance apart Earth & Moon not always the same distance apart
Earth & Sun not always the same distance apart Earth & Sun not always the same distance apart
Tidal range greatest at perihelion Tidal range greatest at perihelion (January 3) & perigee (January 3) & perigee
Tidal range least at aphelion Tidal range least at aphelion (July 4) & apogee (July 4) & apogee
Perigee–Apogee
cycle is ~ 27.5 days
Size comparison of Moon at Apogee and
Perigee
Real Tides Real Tides
Earth not covered completely by ocean Earth not covered completely by ocean
Continents and friction with seafloor modify tidal Continents and friction with seafloor modify tidal bulges (turbulence & mixing)
bulges (turbulence & mixing)
Tidal bulges move as “forced” waves Tidal bulges move as “forced” waves
Tides are shallow water waves with speed Tides are shallow water waves with speed determined by depth of water
determined by depth of water
Tidal bulges cannot form (cannot keep up with Tidal bulges cannot form (cannot keep up with Earth’s rotation)
Earth’s rotation)
Instead they break up into distinct units
Instead they break up into distinct units cells cells
Tidal Cells in World Oceans Tidal Cells in World Oceans
Amphidromic point Amphidromic point center of each cell center of each cell
No tidal range at this point, but radiating No tidal range at this point, but radiating out from the center are Cotidal Lines
out from the center are Cotidal Lines
Cotidal lines Cotidal lines
lines which connect points where high tide lines which connect points where high tide occurs all at the same time
occurs all at the same time
Tide wave rotates once in 12 hours Tide wave rotates once in 12 hours
Counterclockwise in NH Counterclockwise in NH Clockwise in SH
Clockwise in SH
Tidal Patterns Tidal Patterns
May be different for different places due to the various May be different for different places due to the various depths, sizes, and shapes of ocean basins
depths, sizes, and shapes of ocean basins
Diurnal Diurnal
One high tide/one low tide per day One high tide/one low tide per day
Semidiurnal Semidiurnal
Two high tides/two low tides per day Two high tides/two low tides per day
Tidal range about same Tidal range about same
Mixed Mixed
Two high tides/two low tides per day Two high tides/two low tides per day
Tidal range different Tidal range different
Most common
Tidal Patterns
• Bay of Fundy in Nova Scotia
– World’s largest tidal range
Tides in Coastal Waters
Tidal Bores
– Tide-generated wall of water
– Moves up certain rivers (low gradient)
•Conditions needed for tidal bores
– Large spring tidal range of at least 6 m (20 ft) – Abrupt flood tide and short ebb tide phases – Low-lying river with seaward current
– Shallowing of landward sea floor – Narrowing of basin in upper reaches
Tides in Coastal Waters
Tidal Bore in Nova Scotia
Tidal Bore in Nova Scotia
• Rotary Current
– Current that accompanies the slowly turning tide crest in a Northern Hemisphere basin
– Rotates counterclockwise
• Reversing current
– Alternating current
– Moves in and out of narrow coastal passages
Coastal Tidal Currents
• Flood current
– Water rushes up a bay or river with incoming tide
• Ebb current
– Water drains from bay or river as tide goes out
• High slack water
– Peak of each high tide with no current motion
• Low slack water
– Peak of each low tide with no current motion
Coastal Tidal Currents
• Whirlpool
– Rapidly spinning seawater – Restricted
channel connecting two basins with different tidal cycles – Maelstrom near Arctic
Coastal Tidal Currents
Grunion Spawning Grunion Spawning ( ( March to September March to September ) )
So.Cal. & Baja Beaches
mixed tides
During spawning a female grunion wiggles vertically into wet sand to deposit eggs
while males curl around her body, releasing milt
(sperm-containing semen of a male fish) fertilizing the eggs.
Tidal Power Tidal Power
Tidal water trapped in bays Tidal water trapped in bays released to turn released to turn turbines (like hydroelectric)
turbines (like hydroelectric)
Renewable resource Renewable resource
