eleventh edition - miracosta college
TRANSCRIPT
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Tides
Chapter 1 Clickers
Essentials of Oceanography Eleventh Edition
Alan P. Trujillo Harold V. Thurman
Chapter 9 Lecture
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Chapter Overview
• Tides are the rhythmic rise and fall of sea level.
• Tides are very long and regular shallow-water waves.
• Tides are caused by gravitational attraction of the Sun, Moon, and Earth.
• Different tidal patterns exist.
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What Are Tides?
• Tides – periodic raising and lowering of ocean sea level
• Occur daily • Isaac Newton’s gravitational laws explain
relationship • Tides recognized even by early sailors
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Tide-Generating Forces
• Tides caused by combination of gravity and motion between Earth, Moon, and Sun
• Barycenter between Moon and Earth – Common center of
mass or balance point – Beneath Earth’s
surface because of Earth’s greater mass
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Gravitational Forces
• Gravitational force derived from Newton’s Law of Universal Gravitation – Every object that has
mass in the universe is attracted to every other object.
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Gravitational Forces
• Gravitational force proportional to product of masses – Increase mass,
increase force • Inversely proportional
to square of separation distance
Fg =
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Gravitational Forces
• Greatest force at zenith – closest to moon • Least force at nadir
– furthest from moon and opposite zenith
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Centripetal Force
• Center-seeking force • Keeps planets in orbit
via gravitational attraction
• Tethers Earth and Moon to each other
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Resultant Forces
• Mathematical difference between gravitational and centripetal forces
• Relatively small
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Tide-Generating Forces
• Resultant force has significant horizontal component
• Lunar bulges – Result when force
pushes water into two simultaneous bulges
• One toward Moon • One away from Moon
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Idealized Tidal Bulges
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Tidal Phenomena
• Tidal period – time between high tides • Lunar day
– Time between two successive overhead moons – 24 hours, 50 minutes
• Solar day – 24 hours • High tides are 12 hours and 25 minutes apart
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Lunar Day
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Tidal Bulges – Sun’s Effect
• Similar to lunar bulges but much smaller
• Moon closer to Earth, exerts greater gravitational force
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Earth’s Rotation and Tides
• Flood tide – water moves toward shore • Ebb tide – water moves away from shore • Tidal bulges are fixed relative to the Sun’s
and Moon’s positions – Earth’s rotation moves different geographic
locations into bulges
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Monthly Tidal Cycle
• Tidal range – difference between high and low tides
• Syzygy – Moon, Earth, and Sun aligned • Quadrature – Moon in first or third quarter
phase
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Monthly Tidal Cycle
• Spring tides – New or full moons – Tidal range greatest – Syzygy
• Neap tides – Quarter moons – Tidal range least – Quadrature
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Monthly Tidal Cycle
• New Moon – Moon between Earth and Sun, cannot be seen from Earth
• Full Moon – Moon and Sun opposite
• Quarter Moon – Moon appears half lit
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Monthly Tidal Cycle
• Other Moon phases • 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
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Complicating Factors
• Declination – Angular distance of the Moon or Sun above or below Earth’s equator
• Sun to Earth: 23.5 degrees north or south of equator
• Moon to Earth: 28.5 degrees north or south of equator
• Lunar and solar bulges shift from equator – Unequal tides
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Declination and Tidal Bulges
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Complicating Factors
• Elliptical orbits • Earth around Sun:
– Tidal range greatest at perihelion (January) – Tidal range least at aphelion (July)
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Complicating Factors
• Moon around Earth: – Tidal range greatest at perigee (Moon closest to
Earth) – Tidal range least at apogee (Moon furthest from
Earth) – Perigee–apogee cycle is 27.5 days
• Proxigean tides – spring tide + perigee – Exceptionally high tidal range – Every 1.5 years or so
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Effects of Elliptical Orbits
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Idealized Tide Prediction
• Two high tides/two low tides per lunar day • Six lunar hours between high and low tides
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Predicted Idealized Tides
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Real Tides
• Continents and friction with seafloor modify tidal bulges.
• Tides are shallow-water waves with speed determined by depth of water.
• Idealized tidal bulges cannot form. – Cannot keep up with Earth’s rotation
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Real Tides
• Crests and troughs of tides rotate around amphidromic point. – No tidal range at amphidromic points
• Cotidal lines – connect simultaneous high tide points
• Tide wave rotates once in 12 hours.
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Cotidal Map
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Tidal Patterns
• Diurnal – One high tide/one low tide per day
• Semidiurnal – Two high tides/two low tides per day – Tidal range about same
• Mixed – Two high tides/two low tides per day – Tidal range different – Most common
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Tidal Patterns
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Monthly Tidal Curves
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Tides in Coastal Waters
• Standing Waves – Tide waves reflected by coast – Amplification of tidal range
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Tides in Coastal Waters
• Bay of Fundy in Nova Scotia – World’s largest tidal range
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Tides in Coastal Waters
• Tidal Bores – Tide-generated wall of water – Moves up certain rivers
• 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
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Coastal Tidal Currents
• 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
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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
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Coastal Tidal Currents
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Coastal Tidal Currents
• Whirlpool – Rapidly spinning
seawater – Restricted channel
connecting two basins with different tidal cycles
– Maelstrom near Arctic Norway
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Tides and Marine Life
• Grunion spawning – Small silvery fish – Come out of water in
California to spawn – Spawn only after each
night’s higher high tide has peaked on the three or four nights following the night of the highest spring high tide
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Tide-Generated Power
• Tides can be trapped one of 2 ways – Tidal water trapped behind coastal barriers in
bays and estuaries turns electrical turbines – Tidal currents in narrow channels turn
underwater turbines
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Tide-Generated Power
• Does not produce power on demand • Possible harmful environmental effects • Renewable resource • First Asian power plant in 2006 • United Kingdom proposed building world’s
largest tidal power plant.
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Power Plant at La Rance, France
• Successfully producing tidal power since 1967 • Potential usable tidal energy increases with
increasing tidal range.
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End of CHAPTER 9 Tides