Tides Overview n n n Rhythmic rise and

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Tides

Tides

Overview n n n Rhythmic rise and fall of sea level Very long and

Overview n n n Rhythmic rise and fall of sea level Very long and regular shallow-water waves Caused by gravitational attraction of Sun, Moon, and Earth

Tide-generating forces n n Barycenter gravity

Tide-generating forces n n Barycenter gravity

Barycenter n n n center of mass point at which system's mass behaves as

Barycenter n n n center of mass point at which system's mass behaves as if it were concentrated function only of the positions and masses of the particles that comprise the system

Barycenter between Moon and Earth n Mutual orbit due to gravity and motion n

Barycenter between Moon and Earth n Mutual orbit due to gravity and motion n Fig. 9. 1

Newton’s law of Gravity

Newton’s law of Gravity

Gravitational forces n n n Every particle attracts every other particle Gravitational force proportional

Gravitational forces n n n Every particle attracts every other particle Gravitational force proportional to product of masses Inversely proportional to square of separation distance Fig. 9. 2

Centripetal force n n n external force required to make a body follow a

Centripetal force n n n external force required to make a body follow a circular path at constant speed directed inward, toward the center of the circle it is a force requirement, not a particular kind of force.

Centripetal force n n Center-seeking force Tethers Earth and Moon to each other Fig.

Centripetal force n n Center-seeking force Tethers Earth and Moon to each other Fig. 9. 3

Tide-producing forces n n Resultant forces = differences between centripetal and gravitational forces Tide-generating

Tide-producing forces n n Resultant forces = differences between centripetal and gravitational forces Tide-generating forces are horizontal components Fig. 9. 4

Tidal bulges (lunar) n n Small horizontal forces push seawater into two bulges Opposite

Tidal bulges (lunar) n n Small horizontal forces push seawater into two bulges Opposite sides of Earth Fig. 9. 6

Tidal bulges (lunar) n Moon closer to Earth so lunar tideproducing force greater than

Tidal bulges (lunar) n Moon closer to Earth so lunar tideproducing force greater than that of Sun

Ideal Earth n n n covered by ocean Two tidal bulges Two high tides,

Ideal Earth n n n covered by ocean Two tidal bulges Two high tides, 12 hours apart

High tide, flood tide n seawater moves on shore

High tide, flood tide n seawater moves on shore

Low tide, ebb tide n seawater moves offshore

Low tide, ebb tide n seawater moves offshore

Lunar Day n n n Moon orbits Earth 24 hours 50 minutes for observer

Lunar Day n n n Moon orbits Earth 24 hours 50 minutes for observer to see subsequent Moons directly overhead High tides are 12 hours and 25 minutes apart Fig. 9. 7

Tidal bulges (solar) n n Similar to lunar bulges but much smaller Moon closer

Tidal bulges (solar) n n Similar to lunar bulges but much smaller Moon closer to Earth

spring tide n New/full moon – tidal range greatest Time between spring tides about

spring tide n New/full moon – tidal range greatest Time between spring tides about two weeks

neap tide n Quarter moons – tidal range least

neap tide n Quarter moons – tidal range least

Earth-Moon-Sun positions and spring and neap tides Fig. 9. 9

Earth-Moon-Sun positions and spring and neap tides Fig. 9. 9

Other complicating factors: declination n n Angular distance Moon or Sun above or below

Other complicating factors: declination n n Angular distance Moon or Sun above or below Earth’s equator Sun to Earth: 23. 5 o N or S of equator Fig. 9. 11

Moon to Earth: 28. 5 o N or S of equator

Moon to Earth: 28. 5 o N or S of equator

declination n Shifts lunar and solar bulges from equator n Unequal tides

declination n Shifts lunar and solar bulges from equator n Unequal tides

Declination and tides n Unequal tides (unequal tidal ranges) Fig. 9. 13

Declination and tides n Unequal tides (unequal tidal ranges) Fig. 9. 13

Other complicating factors: elliptical orbits n n n Tidal range greatest at perihelion (January)

Other complicating factors: elliptical orbits n n n Tidal range greatest at perihelion (January) and perigee Tidal range least at aphelion (July) and apogee Perigee and apogee cycle 27. 5 days Fig. 9. 12

Lunar day n n n 24 hours and 50 minutes moon revolves around the

Lunar day n n n 24 hours and 50 minutes moon revolves around the Earth in the same direction that the Earth is rotating on its axis it takes the Earth an extra 50 minutes to メcatch up to the moon

Lunar day n n Earth rotates through two tidal bulges every lunar day we

Lunar day n n Earth rotates through two tidal bulges every lunar day we experience two high and two low tides every 24 hours and 50 minutes

Idealized tide prediction n n Two high tides/two low tides per lunar day Six

Idealized tide prediction n n Two high tides/two low tides per lunar day Six lunar hours between high and low tides

Real tides n n n Earth not covered completely by ocean Continents and friction

Real tides n n n Earth not covered completely by ocean Continents and friction with seafloor modify tidal bulges Tides are shallow water waves with speed determined by depth of water

amphidromic point n n Tidal cells rotate around them occur because of the coriolis

amphidromic point n n Tidal cells rotate around them occur because of the coriolis effect and interference with ocean basins

At the amphidromic point, there is almost no vertical movement.

At the amphidromic point, there is almost no vertical movement.

Tidal cells in world ocean n n Cotidal lines Tide wave rotates once in

Tidal cells in world ocean n n Cotidal lines Tide wave rotates once in 12 hours n Counterclockwise in Northern Hemisphere

Fig. 9. 14

Fig. 9. 14

Tidal patterns n n n Diurnal n One high tide/one low tide per day

Tidal patterns n n n Diurnal n One high tide/one low tide per day Semidiurnal n Two high tides/two low tides per day n Tidal range about same Mixed n Two high tides/two low tides per day n Tidal range different n Most common

Tides in coastal waters n n n Standing waves remains in a constant position

Tides in coastal waters n n n Standing waves remains in a constant position result of interference between two waves travelling in opposite directions.

Standing wave n Tide waves reflected by coast n Amplification of tidal range n

Standing wave n Tide waves reflected by coast n Amplification of tidal range n Example, Bay of Fundy maximum tidal range 17 m (56 ft)

Tides in coastal waters n Tidal bore in lowgradient rivers Fig. 9 A

Tides in coastal waters n Tidal bore in lowgradient rivers Fig. 9 A

Coastal tidal currents n Reversing current n Flood current n Ebb current n High

Coastal tidal currents n Reversing current n Flood current n Ebb current n High velocity flow in restricted channels Fig. 9. 18

Coastal tidal currents n Whirlpool n Rapidly spinning seawater n Restricted channel connecting two

Coastal tidal currents n Whirlpool n Rapidly spinning seawater n Restricted channel connecting two basins with different tidal cycles Fig. 9. 19

Tides and marine life n n Fig. 9 C Tide pools and life Grunion

Tides and marine life n n Fig. 9 C Tide pools and life Grunion spawning

Tide-generated power n n n Renewable resource Does not produce power on demand Possible

Tide-generated power n n n Renewable resource Does not produce power on demand Possible harmful environmental effects