13. Tides Flashcards
Tides=
periodic raising and lowering of sea level that occurs daily
Explain the length and depth of waves associated with tides?
very long (wavelength and period)
shallow-water waves
high tides are associated with the wave ____, while low tides are associated with the wave _____
crest
trough
Vertical motion associated with tides can be as much as __-__m, and can happen twice/ day in some places
10-15
t/f
currents are associated with the tidal processes
true
- flood & ebb currents
Flood currents=
incoming currents associated with riding tides
ebb currents=
outgoing currents associated with decreasing tides
Give some examples of the importance of ebb and flood currents
- re-suspension of bottom sediment
- internal waves generated over topography/ mixing
- energy!
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?
Newton
- Universal Law of Gravitation
Describe the relationship between the gravitational attraction of a body and:
- its mass
- the distance between 2 bodies
- as a body’s mass increases, so does its gravitational force
- gravitational force between 2 bodies greatly decreases with increasing distance
Give the equation for Newton’s universal law of gravitation
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
“every object that has mass in the universe is attracted to every other object”
How does this relate to tides?
This means the earth, sun , and moon are all attracted to each other, causing tides
“the moon orbits the earth”
Explain how this sentence could be fixed to make it more correct
It’s not that simple; it’s a MUTUAL orbit due to gravity and motion
- 2 bodies rotate around the braycenter
Braycenter=
Where is the braycenter in the earth-moon system?
= the common center of mass in a system; the balance point between the bodies
1600km beneath the earth’s surface
Why is the braycenter not halfway between the earth and the moon?
Because the earth it much heavier than the moon
braycenter is 1600km below earth’s sfc
How long does one rotation of the earth-moon system take?
approx 1 month: 27.3 days
____ keeps the earth and moon in orbit in relation to each other
gravity
Does the average distance between the earth and moon remain constant?
yes
b/c of a balance of forces
gravity would move them closer without forces counterbalancing
Why don’t bodies collide?
because the body is in motion: orbital velocity
AND they’re spinning around something= centripetal force
The balance between which 2 forces keeps orbits constant?
centripetal force
orbital velocity
zenith=
nadir=
Which has the greatest vs least gravitational attraction to the moon?
zenith= the point on earth closest to the moon
- greatest grav. attraction
nadir= the point on earth furthest from the moon
- smallest grav. attraction
The earth has orbital velocity, so it needs an equal ____ force over its entire body to keep the earth-moon orbit constant
centripetal
The centripetal force in the earth-moon system is balanced by what?
the gravitational attraction of the moon
Use concepts of GAM (gravitational attraction of the moon) and centripetal force to describe how tides are generated
- 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
resultant force minus the gravitational attraction of the earth gives:
TGF (tide generating force)
TGF are based on which force?
gravitational attractive force
TGF formula??? revisit (slide 15)
proportional
inc w mass and dec with distance?
The sun is __x further away from earth than the moon
The sun is __ ____x more massive than the moon
390
27 million
Which exerts more gravitational pull on earth, the sun or the moon? Explain why
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!)
For objects on earth, does the gravitational force of earth or larger bodies like the sun impact them more?
earth’s
b/c the sun is so far away, gravitational force is less
resultant forces are _____ of earth’s g
one millionth!
Where is TGF zero? Where is TGF maximum?
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
Give 4 assumptions about the earth-moon system
- 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)
Explain tidal bulges
- where do they occur?
= 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
T/F
On earth, gravitational forces are in excess further from the moon
false
On earth, gravitational forces are in excess closer to the moon
grav forces are weaker on the far side
“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?
Centrifugal force
Gravitational forces on earth are weaker on the side facing away from the moon; thus there is an excess for the ____ force
centrifugal
Grav attraction is strongest on the side of earth facing the moon. What does this cause the water to do?
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
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?
grav attraction weaker= inertial (centrifugal) forces exceed grav force
- so, water moves away from the moon, forming another bulge
TIDAL BLUGE!
Solar day=
time for a specific point on the earth to rotate from an exact point under the sun back to that same point (24hr)
lunar day=
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)
Why is the lunar day longer than the solar day?
b/c the moon revolves around earth in the same direction as earth rotates around its axis; needs an extra 50min to ‘catch up’
Earth rotates through two bulges per lunar day. How many high tides occur?
2 high tides each lunar day
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?
Same tidal period everywhere: 12hr 25min
Tides less high as you move away from the equator
High tide occurs __min later each day. Why?
50min
b/c the moon rises 50min later in the sky every day
Although smaller than the moon, the sun does exert a significant tidal force. What’s it usually expressed as?
a variation of the lunar pattern
Explain what spring tides are and how they occur
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
Explain what neap tides are and how they occur
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)
The monthly tidal cycle is ___ days. There are __ spring tides and __ neap tides per cycle
29.5
2
2
declination=
the angular distance of the sun or moon above or below earth’s equatorial plane
T/F
the moon and sun are usually directly overhead at the equator
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
What’s the maximum declination of the sun relative to earth’s equator? What about the moon?
23.5 degrees
moon: 28.5 degrees
Due to the moon’s declination, tidal bulges move from ___ degrees N to ___ degrees S during multiple lunar cycles within one year
28.5
28.5
What impact do elliptical orbits have on the spring tides?
spring tides have greater ranges during the NH winter and when they coincide with perigee (when moon is closest to earth in its orbit)
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
sun= 2.5% variance
moon= 8% variance
Any location will have 2 high tides and 2 low tides per lunar day. Which locations are the exception?
the poles
Neither the two high tides nor the two low tides each day will be the same height. Why?
What is the exception?
because of declination
exception: rare occasions when the moon and sun are simultaneously over the equator
T/F
tidal range varies with earth’s distance to the moon and the sun
true
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?
No!
Instead, ocean tides break up into distinct large circulation units (cells)
Give up to 4 factors that deviate real tides from the model
- the presence of landmasses prevents the tide wave from travel around the world
- 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
- being a shallow water wave, tides are also subject to refraction and reflection, creating complicated patterns at some regions
- coastline shape and offshore depth also influence tides –> can amplify ranges
water moving within a tide is subject to the Coriolis effect. What effect does this have?
high tide (crests) and low tide (trough) points move around the basin in a rotary path
- counterclockwise in NH and clockwise in SH
Amphidromic point=
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)
T/F
there is no tidal range at the amphidromic point
true
What limits the size of tidal cells?
the wave must complete one rotation during the tidal period (12h25min), which limits the size
If high tide is occurring along line 10, which line is low tide occurring on?
*i don’t really get this
4
cotidal lines=
times of the main lunar daily high tide in lunar hours after the moon has crossed the Greenwich meridian (0 deg longitude)
Where do cotidal lines terminate? Maximum tidal range will be near which point on the line?
amphidromic points (both ends terminate at an AP)
- midpoints of the lines
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?
- diurnal
- semidiurnal
- mixed
different depths, sizes, and shapes of ocean basins modify tides
How many tides do diurnal tides have per lunar day?
What is the tidal period for diurnal tides?
one high tide and one low tide per lunar day
tidal period= 24hr50min
How many tides do semidiurnal tides have per lunar day?
What is the tidal period for semidiurnal tides?
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
Explain the characteristics of mixed tides
- 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
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
- west coast of north america: mixed
- east coast of north america: semidiurnal
- east coast of europe: semidiurnal
- gulf of mexico: diurnal
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
true
(but typically, tidal pattern for a location stays the same throughout the year)
T/F
tidal patterns show clear switches b/w spring and neap tide cycles
true
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
- 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
The Bay of Fundy in Nova Scotia experiences very extreme tides. Give 3 reasons why this occurs.
- resonance: period of free oscillation of water in the bay is almost = to the tidal period. Results in constructive interference, amplifying tidal energy
- bay shape & bottom topography: bay becomes narrower and shallower toward the upper bay, forcing the water higher up on the shores
- bay orientation: bay curves to the right so the Coriolis effect in the NH adds to the extreme tidal range
