CHAPTER 9 Tides

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

Barycenter between Moon and Earth Barycenter between Moon and Earth Mutual orbit due to gravity and motion Mutual orbit due to gravity and motion Fig. 9.1 Tide-generating forces

Gravitational forces Every particle attracts every other particle Every particle attracts every other particle Gravitational force proportional to product of masses Gravitational force proportional to product of masses Inversely proportional to square of separation distance Inversely proportional to square of separation distance Fig. 9.2

Centripetal force Center-seeking force Center-seeking force Tethers Earth and Moon to each other Tethers Earth and Moon to each other Fig. 9.3

Tide-producing forces Resultant forces = differences between centripetal and gravitational forces Resultant forces = differences between centripetal and gravitational forces Tide-generating forces are horizontal components Tide-generating forces are horizontal components Fig. 9.4

Tidal bulges (lunar) Fig. 9.6 Small horizontal forces push seawater into two bulges Small horizontal forces push seawater into two bulges Opposite sides of Earth Opposite sides of Earth One bulge faces Moon One bulge faces Moon Other bulge opposite side Earth Other bulge opposite side Earth

Tidal bulges (lunar) Moon closer to Earth so lunar tide- producing force greater than that of Sun Moon closer to Earth so lunar tide- producing force greater than that of Sun Ideal Earth covered by ocean Ideal Earth covered by ocean Two tidal bulges Two tidal bulges Two high tides, 12 hours apart Two high tides, 12 hours apart High tide, flood tide, seawater moves on shore High tide, flood tide, seawater moves on shore Low tide, ebb tide, seawater moves offshore Low tide, ebb tide, seawater moves offshore

Lunar Day Moon orbits Earth Moon orbits Earth 24 hours 50 minutes for observer to see subsequent Moons directly overhead 24 hours 50 minutes for observer to see subsequent Moons directly overhead High tides are 12 hours and 25 minutes apart High tides are 12 hours and 25 minutes apart Fig. 9.7

Tidal bulges (solar) Similar to lunar bulges but much smaller Similar to lunar bulges but much smaller Moon closer to Earth Moon closer to Earth New/full moon – tidal range greatest – spring tide New/full moon – tidal range greatest – spring tide Quarter moons – tidal range least – neap tide Quarter moons – tidal range least – neap tide Time between spring tides about two weeks Time between spring tides about two weeks

Earth-Moon-Sun positions and spring and neap tides Fig. 9.9

Other complicating factors: declination Angular distance Moon or Sun above or below Earth’s equator Angular distance Moon or Sun above or below Earth’s equator Sun to Earth: 23.5 o N or S of equator Sun to Earth: 23.5 o N or S of equator Moon to Earth: 28.5 o N or S of equator Moon to Earth: 28.5 o N or S of equator Shifts lunar and Shifts lunar and solar bulges from equator solar bulges from equator Unequal tides Unequal tides Fig. 9.11

Declination and tides Unequal tides (unequal tidal ranges) Unequal tides (unequal tidal ranges) Fig. 9.13

Other complicating factors: elliptical orbits Fig Tidal range greatest at perihelion (January) and perigee Tidal range greatest at perihelion (January) and perigee Tidal range least at aphelion (July) and apogee Tidal range least at aphelion (July) and apogee Perigee and apogee cycle 27.5 days Perigee and apogee cycle 27.5 days

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

Real tides Earth not covered completely by ocean Earth not covered completely by ocean Continents and friction with seafloor modify tidal bulges Continents and friction with seafloor modify tidal bulges Tides are shallow water waves with speed determined by depth of water Tides are shallow water waves with speed determined by depth of water Tidal bulges cannot form (too slow) Tidal bulges cannot form (too slow) Tidal cells rotate around amphidromic point Tidal cells rotate around amphidromic point

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

Fig. 9.14

Tidal patterns 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 Most common

Tides in coastal waters Standing waves Standing waves Tide waves reflected by coast Tide waves reflected by coast Amplification of tidal range Amplification of tidal range Example, Bay of Fundy maximum tidal range 17 m (56 ft) Example, Bay of Fundy maximum tidal range 17 m (56 ft)

Tides in coastal waters Tidal bore in low- gradient rivers Tidal bore in low- gradient rivers Fig. 9A

Coastal tidal currents Reversing current Reversing current Flood current Flood current Ebb current Ebb current High velocity flow in restricted channels High velocity flow in restricted channels Fig. 9.18

Coastal tidal currents Whirlpool Whirlpool Rapidly spinning seawater Rapidly spinning seawater Restricted channel connecting two basins with different tidal cycles Restricted channel connecting two basins with different tidal cycles Fig. 9.19

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

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

End of CHAPTER 9 Tides Fig. 9.21