Unit 8 - Tides

What are Tides?• Tides are the periodic raising and lowering of

ocean sea level• Tides are very long shallow water waves• Caused by combination of gravity and motion

between Earth, Moon, and Sun• Isaac Newton’s gravitational laws explain the

tides

Gravitational Forces• Gravitational force is greatest at zenith – closest to moon• Gravitational force is lowest at nadir – furthest from

moon and opposite zenith

Centripetal Forces• Centripetal force tethers the Earth and Moon to each

other (keeps them in orbit)• Centripetal force is equal on all parts of the earth

Resultant Forces produce tides• The resultant force is the difference between

gravitational and centripetal forces• It is the relatively small force that produces the

tides

Resultant Forces• The resultant force pushes water into two

simultaneous bulges– One toward Moon– One away from Moon

Tidal Bulges – Moon’s Effect

Tidal Periods• Tidal period – time between high tides• Lunar day - Time between two successive overhead moons

equal to 24 hours, 50 minutes• High tides are 12 hours and 25 minutes apart (2 per lunar day)

Tidal Bulges – Sun’s Effect• Similar to lunar bulges but much smaller (only 46% of the lunar

bulge)• The moons effect on tides is greater because it is so much closer

than the sun ( Video )

Monthly Tidal Cycle• Spring tides

– New or full moons when sun and moon are in alignment– Tidal range is the greatest– Syzygy – when celestial bodies are in alignment

• Neap tides– Quarter moons when sun and moon are at right angles

relative to the Earth.– Tidal range is the lowest– Quadrature – when celestial bodies are at right angles

Spring and Neap Tides

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• This produces unequal (mixed) tides at

different spots around the world

Declination and Tidal Bulges

Effects of declination on tides

Elliptical Orbits add Complication• Earth around Sun:

– Tidal range greatest at perihelion (January)– Tidal range least at aphelion (July)

• 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

Relative sizes of the moon

Effects of Elliptical Orbits

Actual tides are more complex• 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 because they

cannot keep up with Earth’s rotation

Where do the tides start?• Tides start in the centers of the world’s oceans.• Tides act like water swirling in a bowl. • There will be a node in the center where the water

level never rises or falls.

Amphidromic Points• Cotidal map shows tides rotate around amphidromic points.• There are 140 amphidromic points in the world’s oceans.

Figure 9-14

Amphidromic Points

Cotidal Lines• Cotidal lines show where high tides occur at

the same time

“if you don’t understand amphidromic points I’m going to bite you!”

Spidey Says…

Tidal circulation

• Tides progress around basins, clockwise in S hemisphere and counterclockwise in N hemisphere

Animation of Tidal Elevations in the Pacific

Tidal Patterns

• Diurnal One high tide and one low tide per day

• Semidiurnal Two equal high and low tides per day

• Mixed Two unequal high and low tides per day (most common type)

Tidal Patterns in the U.S.

Tidal Graphs

• Tidal graphs show all the tidal patterns for a location over time.

• Spring and neap tides• Diuranal, mixed, or

semidiuranal tides.

Flood and Ebb Currents• Flood current

is when the tide is coming in.

• Ebb current is when the tide is going out

Tides in Coastal Waters• Tide waves are reflected by coastlines

producing complicated effects• This can amplify the tidal range• Cook Inlet Alaska and the Bay of Fundy in

Nova Scotia are examples of extreme tides caused by coastal effects

Cook Inlet Tides

The Bay of Fundy• Tidal Range is the difference between high tide and low

tide. The Bay of Fundy in Canada has the greatest tidal range on the planet -56 ft.

• Nova Scotia bends when the tide comes in! • As 14 billion tons of sea water flow into Minas Basin

twice daily, the Nova Scotia countryside actually tilts lightly under the immense load ! Video

Bay of Fundytidal bore

Bay of Fundy – World’s Largest Tidal Range

• The wave on the incoming (flood) tide in certain rivers is known as a Tidal Bore.

• Tidal bores occur in about 100 rivers throughout the world. • The Qiantang Bore in China reaches heights of 15 ft. and travels 15 mph.

Video • The Pororoca, in the Amazon River, forms waves 12 ft. high and can reach

speeds of 20 mph. • The Severn Bore in England is a popular one to surf as shown below.

Qiantang Bore Surfing the Severn Bore

Tidal Bores

Whirlpools!• Rapidly spinning seawater• Occur in restricted

channels connecting two basins with different tidal cycles

• The Maelstrom in Norway is the world’s largest whirlpool Video

Tide-Generated Power• Renewable energy source but not yet widely used.• Possible harmful environmental effects from blocking tidal currents• Oldest was built in France in the 1960s• Largest was finished in South Korea this year (2011)

Power Plant at La Rance, France