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EAS 212: The Oceans > 13. Tides > Flashcards

13. Tides Flashcards

1
Q

Tides=

A

periodic raising and lowering of sea level that occurs daily

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2
Q

Explain the length and depth of waves associated with tides?

A

very long (wavelength and period)
shallow-water waves

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3
Q

high tides are associated with the wave ____, while low tides are associated with the wave _____

A

crest

trough

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4
Q

Vertical motion associated with tides can be as much as __-__m, and can happen twice/ day in some places

A

10-15

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5
Q

t/f
currents are associated with the tidal processes

A

true
- flood & ebb currents

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6
Q

Flood currents=

A

incoming currents associated with riding tides

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7
Q

ebb currents=

A

outgoing currents associated with decreasing tides

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8
Q

Give some examples of the importance of ebb and flood currents

A
  • re-suspension of bottom sediment
  • internal waves generated over topography/ mixing
  • energy!
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9
Q

It’s been known for a long time that tides are related to the movements of the sun and moon. Who fully explained this relationship?

A

Newton
- Universal Law of Gravitation

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10
Q

Describe the relationship between the gravitational attraction of a body and:
- its mass
- the distance between 2 bodies

A
  • as a body’s mass increases, so does its gravitational force
  • gravitational force between 2 bodies greatly decreases with increasing distance
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11
Q

Give the equation for Newton’s universal law of gravitation

A

Fg= G(m1)(m2)/ r^2

  • G is the gravitational constant (doesn’t change)
  • m is mass (body 1 and body 2)
  • r is distance
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12
Q

“every object that has mass in the universe is attracted to every other object”

How does this relate to tides?

A

This means the earth, sun , and moon are all attracted to each other, causing tides

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13
Q

“the moon orbits the earth”

Explain how this sentence could be fixed to make it more correct

A

It’s not that simple; it’s a MUTUAL orbit due to gravity and motion
- 2 bodies rotate around the braycenter

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14
Q

Braycenter=

Where is the braycenter in the earth-moon system?

A

= the common center of mass in a system; the balance point between the bodies

1600km beneath the earth’s surface

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15
Q

Why is the braycenter not halfway between the earth and the moon?

A

Because the earth it much heavier than the moon

braycenter is 1600km below earth’s sfc

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16
Q

How long does one rotation of the earth-moon system take?

A

approx 1 month: 27.3 days

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17
Q

____ keeps the earth and moon in orbit in relation to each other

A

gravity

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18
Q

Does the average distance between the earth and moon remain constant?

A

yes
b/c of a balance of forces
gravity would move them closer without forces counterbalancing

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19
Q

Why don’t bodies collide?

A

because the body is in motion: orbital velocity
AND they’re spinning around something= centripetal force

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20
Q

The balance between which 2 forces keeps orbits constant?

A

centripetal force
orbital velocity

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21
Q

zenith=
nadir=

Which has the greatest vs least gravitational attraction to the moon?

A

zenith= the point on earth closest to the moon
- greatest grav. attraction

nadir= the point on earth furthest from the moon
- smallest grav. attraction

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22
Q

The earth has orbital velocity, so it needs an equal ____ force over its entire body to keep the earth-moon orbit constant

A

centripetal

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23
Q

The centripetal force in the earth-moon system is balanced by what?

A

the gravitational attraction of the moon

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24
Q

Use concepts of GAM (gravitational attraction of the moon) and centripetal force to describe how tides are generated

A
  • GAM and centripetal force balance each other out

GAM is different from the required centripetal force because GAM varies with distance

  • the difference b/w GAM and the centripetal force lead to the creation of a resultant force, which ultimately generates tides
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25
Q

resultant force minus the gravitational attraction of the earth gives:

A

TGF (tide generating force)

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26
Q

TGF are based on which force?

A

gravitational attractive force

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27
Q

TGF formula??? revisit (slide 15)

A

proportional

inc w mass and dec with distance?

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28
Q

The sun is __x further away from earth than the moon

The sun is __ ____x more massive than the moon

A

390

27 million

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29
Q

Which exerts more gravitational pull on earth, the sun or the moon? Explain why

A

The moon exerts over 2x the grav pull of the sun on tides!!

Moon has more control of the tides b/c it’s much closer to earth (distance matters more than mass!)

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30
Q

For objects on earth, does the gravitational force of earth or larger bodies like the sun impact them more?

A

earth’s

b/c the sun is so far away, gravitational force is less

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31
Q

resultant forces are _____ of earth’s g

A

one millionth!

32
Q

Where is TGF zero? Where is TGF maximum?

A

zero where resultant is purely vertical (b/c it competes with g)
- at nadir and zenith, & along a line midway b/w them

maximum where resultant has a larger component tangential to earth’s surface
- ie resultant forces have a significant horizontal component

*know where these are on a globe

33
Q

Give 4 assumptions about the earth-moon system

A
  • earth is smooth & covered by a single ocean of uniform depth
  • moon is directly above earth’s equator
  • the moon is always above the same point on the earth’s sfc
  • system balances on barycenter (1700m below earth’s sfc)
34
Q

Explain tidal bulges
- where do they occur?

A

= water gets squeezed toward the equator by the TGF

  • 2 bulges occur: under and opposite to the moon
    –> these bulges stay aligned with the moon as it rotates
  • bulge closest to moon bulges towards the moon, vise versa
35
Q

T/F

On earth, gravitational forces are in excess further from the moon

A

false
On earth, gravitational forces are in excess closer to the moon

grav forces are weaker on the far side

36
Q

“an apparent force that is felt by an object moving in a curved path that acts outwardly away from the center of rotation”

What does this define?

A

Centrifugal force

37
Q

Gravitational forces on earth are weaker on the side facing away from the moon; thus there is an excess for the ____ force

A

centrifugal

38
Q

Grav attraction is strongest on the side of earth facing the moon. What does this cause the water to do?

A

Tidal bulge!

water on the side of the earth facing the moon is pulled towards the moon
- inertial forces try to keep the water in place, but grav force is larger
= bulge appears facing the moon

39
Q

On the side of the earth that’s not facing the moon, the grav force is weaker. What does this cause the water there to do?

A

grav attraction weaker= inertial (centrifugal) forces exceed grav force

  • so, water moves away from the moon, forming another bulge
    TIDAL BLUGE!
40
Q

Solar day=

A

time for a specific point on the earth to rotate from an exact point under the sun back to that same point (24hr)

41
Q

lunar day=

A

time it takes for a specific site on earth to rotate from an exact point under the moon to the same point under the moon (24h 50min)

42
Q

Why is the lunar day longer than the solar day?

A

b/c the moon revolves around earth in the same direction as earth rotates around its axis; needs an extra 50min to ‘catch up’

43
Q

Earth rotates through two bulges per lunar day. How many high tides occur?

A

2 high tides each lunar day

44
Q

If you moved N or S of the equator, what tidal period would you experience? Would the tides be higher or lower than at the equator?

A

Same tidal period everywhere: 12hr 25min

Tides less high as you move away from the equator

45
Q

High tide occurs __min later each day. Why?

A

50min
b/c the moon rises 50min later in the sky every day

46
Q

Although smaller than the moon, the sun does exert a significant tidal force. What’s it usually expressed as?

A

a variation of the lunar pattern

47
Q

Explain what spring tides are and how they occur

A

when the sun and moon are aligned, the solar bulge has an additive effect on the lunar bulge, creating extra high and very low tides = spring tides

vertical difference (tidal range) b/w high and low tides is large

48
Q

Explain what neap tides are and how they occur

A

when the sun & moon are at right angles to each other, their bulges partially cancel out, producing more moderate tides
= neap tides

  • tidal range is lower (lower high tides and higher low tides)
49
Q

The monthly tidal cycle is ___ days. There are __ spring tides and __ neap tides per cycle

A

29.5
2
2

50
Q

declination=

A

the angular distance of the sun or moon above or below earth’s equatorial plane

51
Q

T/F

the moon and sun are usually directly overhead at the equator

A

false
most of the year they’re either N or S of the equator
The distance of the sun or the moon from equator= declination

52
Q

What’s the maximum declination of the sun relative to earth’s equator? What about the moon?

A

23.5 degrees
moon: 28.5 degrees

53
Q

Due to the moon’s declination, tidal bulges move from ___ degrees N to ___ degrees S during multiple lunar cycles within one year

A

28.5
28.5

54
Q

What impact do elliptical orbits have on the spring tides?

A

spring tides have greater ranges during the NH winter and when they coincide with perigee (when moon is closest to earth in its orbit)

55
Q

The earth has an ellptical orbit around the sun; the distance varies __% over the year
The moon’s orbit is also elliptical, with __% variance in distance

A

sun= 2.5% variance
moon= 8% variance

56
Q

Any location will have 2 high tides and 2 low tides per lunar day. Which locations are the exception?

A

the poles

57
Q

Neither the two high tides nor the two low tides each day will be the same height. Why?

What is the exception?

A

because of declination

exception: rare occasions when the moon and sun are simultaneously over the equator

58
Q

T/F

tidal range varies with earth’s distance to the moon and the sun

A

true

59
Q

Tides are shallow water waves, which means their speed is restricted by the ocean’s depth. Can tidal bulges exist if they can’t keep up with earth’s rotation?

A

No!
Instead, ocean tides break up into distinct large circulation units (cells)

60
Q

Give up to 4 factors that deviate real tides from the model

A
  1. the presence of landmasses prevents the tide wave from travel around the world
  2. the latitudinal variation of earth’s linear velocity; although at low lats the tide wave is slower than the earth’s linear velocity, at higher lats the tide wave is faster than the earth’s
  3. being a shallow water wave, tides are also subject to refraction and reflection, creating complicated patterns at some regions
  4. coastline shape and offshore depth also influence tides –> can amplify ranges
61
Q

water moving within a tide is subject to the Coriolis effect. What effect does this have?

A

high tide (crests) and low tide (trough) points move around the basin in a rotary path
- counterclockwise in NH and clockwise in SH

62
Q

Amphidromic point=

A

no tide point in the ocean around which the crest of the tide waves rotates during one tidal period
- in the open ocean
- crests and troughs of the tide wave rotate around this point (at the center of each cell)

63
Q

T/F
there is no tidal range at the amphidromic point

A

true

64
Q

What limits the size of tidal cells?

A

the wave must complete one rotation during the tidal period (12h25min), which limits the size

65
Q

If high tide is occurring along line 10, which line is low tide occurring on?
*i don’t really get this

A

4

66
Q

cotidal lines=

A

times of the main lunar daily high tide in lunar hours after the moon has crossed the Greenwich meridian (0 deg longitude)

67
Q

Where do cotidal lines terminate? Maximum tidal range will be near which point on the line?

A

amphidromic points (both ends terminate at an AP)
- midpoints of the lines

68
Q

In theory, most of earth should experience 2 low tides and 2 high tides during a lunar day.
In practice though, there are 3 different patterns seen throughout the world:

  • what causes these instead of the expected 2HT and 2LT per day?
A
  • diurnal
  • semidiurnal
  • mixed

different depths, sizes, and shapes of ocean basins modify tides

69
Q

How many tides do diurnal tides have per lunar day?
What is the tidal period for diurnal tides?

A

one high tide and one low tide per lunar day

tidal period= 24hr50min

70
Q

How many tides do semidiurnal tides have per lunar day?
What is the tidal period for semidiurnal tides?

A

two high tides and two low tides each lunar day

  • the heights of successive high and low tides are about the same
  • tidal period= 12hr25min
71
Q

Explain the characteristics of mixed tides

A
  • has characteristics of both diurnal and semidiurnal tides
  • successive high and/or low tides will have significantly different heights
  • tidal period= 12hr25min but may also exhibit diurnal periods
72
Q

Give the tidal pattern for each of these locations:
- west coast of north america
- east coast of north america
- east coast of europe
- gulf of mexico

A
  • west coast of north america: mixed
  • east coast of north america: semidiurnal
  • east coast of europe: semidiurnal
  • gulf of mexico: diurnal
73
Q

T/F

Even though a tide at any location follows a single tidal pattern, it still may pass through stages of the other tidal patterns

A

true

(but typically, tidal pattern for a location stays the same throughout the year)

74
Q

T/F
tidal patterns show clear switches b/w spring and neap tide cycles

A

true

75
Q

The shape of the shoreline can strongly affect tides. Explain how the following affect tides:
- wide continental margins
- mid-oceanic islands
- funnel-shaped bays
- local wind/ weather

A
  • wide continental margins: increase tide amplitudes
  • mid-oceanic islands: generally experience small tides
  • funnel-shaped bays: can dramatically alter tide magnitude
  • local wind/ weather can affect tides in many ways
76
Q

The Bay of Fundy in Nova Scotia experiences very extreme tides. Give 3 reasons why this occurs.

A
  1. resonance: period of free oscillation of water in the bay is almost = to the tidal period. Results in constructive interference, amplifying tidal energy
  2. bay shape & bottom topography: bay becomes narrower and shallower toward the upper bay, forcing the water higher up on the shores
  3. bay orientation: bay curves to the right so the Coriolis effect in the NH adds to the extreme tidal range
77
Q
A

EAS 212: The Oceans (18 decks)

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