Chapter 9



Chapter 9

TIDES

I. Background of Tides

A. tide-generating force is inversely proportional to the cube of the distance between earth's points and the moon or sun

B. Newton’s law of gravitation: sun and moon attract ocean water toward them; because moon is closer, it has the largest effect on tides

C. tidal bulges: water is forced toward nadir and zenith by moon’s forces and inertia, which results in two massive hills of water that rotate around Earth and are the essential cause of tides; the bulges follow the moon around the Earth in its monthly journey; acts like a shallow-water wave and is slowed by friction with sea floor, so it travels at 700 km/h;

1. the sun creates a solar bulge, which is 46% of the size of the lunar bulge because the moon is much closer than the sun

D. equilibrium tide: theoretical tide that would exist if there was an ocean of uniform depth, no friction, and no continents

E. dynamic tide: actual tide that exists in the presence of factors such as ocean depth, continents, and friction

F. lunar/tidal day (= 24 hr 50 min): the time it takes a point on the Earth to start under the moon, then rotate back into position under it; this effects tidal period

G. moon terms

1. phases:

a. new moon: moon is between sun and earth (in conjunction)

b. full moon: moon is behind earth with respect to the sun (in opposition)

c. quarter moons: moon forms a right angle with the earth and sun

2. perigee (close): distance between Earth and moon is 375,200 km; tides are larger

3. apogee (far): distance between Earth and moon is 405,800 km; tides are smaller

4. anomalistic month (27.5 days): time for moon to complete one orbit around Earth relative to a fixed point

5. zenith: point on Earth closest to moon; marks one crest of the lunar bulge

6. nadir: point on Earth farthest from moon; marks one crest of the lunar bulge

7. lunar month (29.5 days): the time that elapses between successive passages of the moon across a certain point on Earth; it is the length of time that is the period of a full tidal cycle from spring tide back to spring tide or neap tide back to neap tide

H. sun terms

1. perihelion (close): distance between Earth and sun is 148.5 million km

2. aphelion (far): distance between Earth and sun is 152.2 million km

I. sun’s phases

1. vernal equinox (March 21): sun is over equator and is moving from south to north

2. summer solstice (June 21): sun reaches farthest northern point in sky at tropic of Cancer (23.5°N)

3. autumnal equinox (September 23): sun is over equator and is moving from north to south

4. winter solstice (December 22): sun reaches farthest southern point in sky above the tropic of Capricorn (23.5°S)

J. harmonic analysis: factors almost 400 tide-generating variables into a mathematical model; tides cannot be modeled purely mathematically because some real-world data is still needed due to their complexity

K. other terms

1. splash line: line where the highest high tides have reached

2. declination: angle between earth’s equatorial plane and the orbital plane of itself another object (max 23.5° for sun and Earth; max 28.5° for moon and Earth)

3. ecliptic: the plane of Earth’s orbital motion

4. tidal range: vertical difference between height of water at high and low tides

5. syzygy: 3 celestial bodies are in a row; this is what happens during conjunction, opposition, or spring tide

6. datum: the reference point of a tide; zero is usually defined as the average height; necessary when publishing tidal data

II. Tide Classifications

A. spring tide: tidal range is maximized because the sun, moon, and Earth are in a line (in conjunction or opposition) and the forces are working together; happens every 2 weeks; lunar and solar bulges are constructively interfering

B. neap tide: tidal range is minimized because the sun, Earth, and moon are at a right angle, and the forces are not working together; happens every 2 weeks; lunar and solar bulges are destructively interfering

C. semidiurnal tide: two high and two low tides each lunar day; tidal period is 12 hr 25 min; common along Atlantic coast of USA; tidal range is fairly constant

D. diurnal tide: single high and low tide each lunar day; tidal period is 24 hr 50 min; found in Gulf of Mexico and Southeast Asia

1. diurnal inequality: variation in heights of successive tides due to declination of sun and moon; tidal range varies rapidly

E. mixed semidiurnal tide: characteristics mixed between diurnal and semidiurnal tides; tidal range differs more each lunar day than either of the other types (high diurnal inequality), but the tidal period is usually 12 hr 25 min, like a semidiurnal tide; most common tide in the world; exists on Pacific coast of USA

F. proxigean tide: tide occurring when spring tides coincide with perigee, resulting in high tidal ranges

1. the absolute maximum tidal range occurs every 1600 years but requires more than just proxigean conditions; it last occurred in 1700 and will happen next in 3300

III. Tides: the periodic rise and fall of the local sea level due to the gravitational pull of the sun and moon

A. narrow basin (ex: bay): a tide-generated standing wave [seiche wave] forms in a narrow basin; a nodal line, where there is no vertical motion, runs perpendicular to the length of the basin

B. tidal motion

1. rotary current: tide in an open basin; tide travels radially around the amphidromic point once in a tidal period

a. amphidromic point: point that a rotary current tide travels around

b. cotidal line: marks the crest of a tidal wave; connects points that have high tide simultaneously

c. corange line: connects points with equal tidal range; perpendicular to cotidal lines, so they run in loops around the amphidromic point

2. reversing [alternating] current: the apparent motion of a tide toward and away from shore; only seen near coasts, not in the open ocean basins

a. ebb current: receding movement of water that occurs between the transition from high to low tide

b. flood current: oncoming movement of water that occurs between the transition from low to high tide

c. slack water: period when the water is not moving at high or low tide because it is between an ebb and flood current; high slack water occurs after a flood current; low slack water occurs after an ebb current

d. whirlpool: rapidly spinning body of water in shallow passages near the coast; occur when a tidal current is deflected by an obstacle

i. maelstrom: Scandinavian name for a very powerful whirlpool; its name came from the Nordic word male = to grind

ii. Saltstraumen, Norway: has the strongest whirlpools and tidal currents in the world

iii. Old Sow: largest whirlpool in North America; found just north of Maine

C. Kelvin wave: non-dispersive wave balanced by Coriolis force

D. Bay of Fundy: a long bay opening into the Atlantic Ocean in Canada that experiences tidal ranges that exceed 17 m (at the narrow north end of Minas Basin), which is the largest range in the world; large tidal range is caused by:

1. narrowing of basin

2. resonance of tide with incoming waves

E. tidal bore [Brazil = pororocas]: a steep wave traveling up a low-lying river as the result of an oncoming high tide [flood current]

1. Chientang River, China: highest tidal bores, which can have heights of 8 m

2. Amazon River, Brazil: tidal bores experienced here can have heights of 5 m and travel at 22 km/h; these bores go the farthest up the river of all bores

3. Cook Inlet, Alaska: experiences the largest tidal bores in the USA

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