http://www.bbso.njit.edu/espr/sci_images/composite_earth1_red.gif

Chapter 9Tides

Overview

Rhythmic rise and fall of sea level Very long and regular shallow-water

waves Caused by gravitational attraction of

Sun, Moon, and Earth

BarycenterBarycenter between Moon and Earth – center of gravity of “Earth-Moon System”

Mutual orbit due to gravity and motion

Fig. 9.1

Tide-generating Tide-generating forcesforces

Gravitational forces Every particle attracts every other particle Gravitational force proportional to product of

masses, toward center of gravity of Moon Inversely proportional to square of separation

distance – decrease the farther away from Moon (look at red circles)

Fig. 9.2

Centripetal force Center-seeking force – force equal at all

points Tethers Earth and Moon to each other

Fig. 9.3

Tide-producing forces Resultant forcesResultant forces =

differences between centripetal and gravitational forces (blue arrows)○ Near moon, gravitational forces are greater○ Away from moon, centripetal forces are greater

Tide-generating forcesTide-generating forces are horizontal components resulting in “bulges”

Fig. 9.4

Tidal bulges (lunar) Small horizontal forces push seawater into

two bulges on opposite sides of EarthOne bulge faces MoonOther bulge opposite side Earth

Tidal bulges (lunar)

Moon closer to Earth so lunar tide-producing force greater than that of Sun

Ideal Earth covered by ocean○ If the entire earth was covered by ocean and

the ocean depth and bottom was uniform○ This is not the case (continents, etc)

Two tidal bulgesTwo high tides, 12 hours apartHigh tide, flood tideHigh tide, flood tide, seawater moves on

shoreLow tide, ebb tideLow tide, ebb tide, seawater moves offshore

Lunar Day

Moon orbits Earth 24 hours 50 minutes for observer to see

subsequent Moons directly overhead High tides are 12 hours and 25 minutes apart

○ Tides are based on lunar cycle

Tidal bulges (solar)

Sun effects tides, also Similar to lunar bulges but

much smaller because sun is farther from earth

Understand this figure

Other complicating factors: declination

• Angular distance Moon or Sun above or below Earth’s equator• Sun to Earth: 23.5o N

or S of equator• Moon to Earth: 28.5o

N or S of equator

• Shifts lunar and solar bulges from equator unequal tides at given latitude

Fig. 9.11

http://www.nos.noaa.gov/education/kits/tides/media/

Declination and tides Unequal tidesUnequal tides (unequal tidal ranges at given

latitude)

Fig. 9.13

Other complicating factors: elliptical orbits

Fig. 9.12

Tidal range greatest at perihelion (January) and perigee Perihelion – Earth closest to sun Perigee – Earth is closest to moon

Tidal range least at aphelion (July) and apogee Aphelion – Earth farthest from sun Apogee – Earth farthest from moon

Moon’s perigee and apogee cycle 27.5 days

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

http://www.aztecsailing.co.uk/theory/Ch2-Fig-2-tidal-cycle.gif

Real tides affected by many factors Tides extreme example of shallow-water

waves 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

Tidal bulges cannot form (too slow) Ocean is not deep enough Speed of tides (which are shallow water waves)

cannot keep up with rotation of Earth Therefore:

Tidal cells “slosh” around amphidromic point in ocean basinsamphidromic point in ocean basins○ Crests and troughs of tides rotate around this pointCrests and troughs of tides rotate around this point○ Tidal range is zero at this pointTidal range is zero at this point

Tidal cells in world ocean Cotidal linesCotidal lines

Connects all points experiencing the same phase of the tide

Radiates from amphidromic point

Tidal patterns SemidiurnalSemidiurnal

Two high tides/two low tides per day

Tidal range about same

DiurnalDiurnalOne high tide/one low

tide per dayRarest

MixedMixed Two high tides/two

low tides per dayTidal range differentMost common

http://www.fhwa.dot.gov/engineering/hydraulics/images/h25_b6.jpg

Global distribution of tides

www.nos.noaa.gov/.../ tides/media/

Tidal Pattern for Fort Pierce

Tidal Phenomena Standing wavesStanding waves

Tide waves reflected by coastAmplification of tidal rangeExample, Bay of Fundy maximum tidal

range 17 m (56 ft)

http://academics.sru.edu/GGE/pictures/Newfoundland/Images

Tides in coastal waters

Tidal boreTidal bore in low-gradient rivers

○ Come in to shallow rivers

○ Example is Amazon River Can rise 5ft

Fig. 9A

Tides and coastal areas like the Indian River Lagoon

Reversing currentReversing currentFlood current Flood current oror tidetide – water rushes into harbor

with incoming tide

Ebb current Ebb current oror tidetide – water drains out of harbor or bay with out-going tide

Slack tide Slack tide – current stops at peak of high and low tides

High velocity flow in restricted channels

Fig. 9.18

Tidal range is not a good predictor of tidal currents; altered by:○ Shape and volume of basin○ Restriction of flow at basin mouth○ Winds○ Currents stronger at channel center than at

edges

Coastal tidal currents WhirlpoolWhirlpool

Rapidly spinning seawater

Restricted channel connecting two basins with different tidal cycles

Not as deadly as legend suggests

Fig. 9.19

Tides and marine life Tide pools and life Organisms living in

intertidal have to be adapted to spending long periods underwater and long periods exposed to sun and drying out

Fig. 9Chttp://www.stripersonline.com/surftalk/attachment.php?

attachmentid=30481&stc=1&d=1169261518

Tide-generated power Renewable resource – mostly

hypothetical at this point Does not produce power on

demand Possible harmful

environmental effects – alternating water flow

http://www.bluenergy.com/technology.html

Misconceptions

Distance from the sun causes the seasons.

All tides happen in bodies of water that are very large.

Outer space doesn’t really affect Earth.

The sun revolves around the Earth.

Ocean Literacy Principles 1.c - Throughout the ocean there is one interconnected circulation system

powered by wind, tides, the force of the Earth’s rotation (Coriolis effect), the Sun, and water density differences. The shape of ocean basins and adjacent land masses influence the path of circulation.

1.d - Sea level is the average height of the ocean relative to the land, taking into account the differences caused by tides. Sea level changes as plate tectonics cause the volume of ocean basins and the height of the land to change. It changes as ice caps on land melt or grow. It also changes as sea water expands and contracts when ocean water warms and cools.

5.h - Tides, waves and predation cause vertical zonation patterns along the shore, influencing the distribution and diversity of organisms.

Florida Sunshine State Standards SC.6.P.13.2 - Explore the Law of Gravity by recognizing that every object exerts

gravitational force on every other object and that the force depends on how much mass the objects have and how far apart they are.

SC.8.E.5.9 - Explain the impact of objects in space on each other including: 1. the Sun on the Earth including seasons and gravitational attraction 2. the Moon on the Earth, including phases, tides, and eclipses, and the relative position of each body.

SC.912.L.17.3 - Discuss how various oceanic and freshwater processes, such as currents, tides, and waves, affect the abundance of aquatic organisms.