Download Waves are “disturbances”

Oceanography 10, T. James Noyes, El Camino College
5A-1
Waves
What are Waves?
Energy
is here.
Waves are “disturbances” in “mediums”
(substances or materials). There are many
different kinds of waves: sound waves, light
waves, seismic (earthquake) waves, and so
Energy is passed from domino to domino when they collide.
on. The energy of the disturbance gets passed The energy can move great distances, but the dominos
along by the medium – for example, by water hardly move at all.
molecules bumping into one another, like a row of dominos – and then returns to normal. The
key factor required to sustain waves in a medium is a “restoring force.” For ordinary ocean
waves, the restoring force is gravity, so technically ordinary ocean waves are called “surface
gravity waves.” Imagine a small hill of water. The higher pressure beneath the hill owing to the
weight of the water (the pull of gravity) pulls the hill of water downward and outward to the
sides (the wave moves). The ocean surface overshoots, though, going down too far, and the
ocean continues to bounce up and down for a while like a child on a swing who has to go back
and forth several times before coming to rest.
There are other restoring forces that can create waves. For example, surface tension – the
electrical attraction (bonds) between water molecules – also pulls the surface down or up. As
you might imagine, this becomes important on smaller scales. Scientists call small ripples in
water “capillary waves,” since surface tension also results in capillarity: water rises up into small
spaces like thin tubes, the bristles of a toothbrush, the holes in a paper towel, etc.
Oceanography 10, T. James Noyes, El Camino College
5A-2
Wave Orbitals, Wave Height, and Wavelength
As a wave goes by, the water molecules beneath the crest move in circle called a wave “orbital.”
Technically, they go a little faster at the top of their path than at the bottom, so it is not a perfect
circle and they move forward a little overall. The orbitals get distorted into ellipses (ovals) in
shallow water when they run into the bottom of the ocean.
Notice that the water moves in
Wave Orbitals
several small circles at different
depths, and that the circles near
the surface are larger. The size
of the circles is controlled by the
Ocean Bottom
height of the wave, the distance
from the bottom of the wave (the
trough) to the top of the wave (the crest). The depth that the
Measuring the height of a wave
wave reaches into the ocean with all those circles – called the
from the crest to the bottom of
“wave base” – is controlled by the wavelength, the distance
the ocean is one of the most
from the crest of one wave to the crest of the next. A wave
common mistakes on tests.
reaches down into the ocean a distance equal to about ½ of its
wavelength. Below the “wave base,” the water is practically motionless.
Crest
Water Level (feet)
4
Crest
3
Trough
Trough
Height
2
Wavelength
1
0
20
10
30
40
Horizontal Distance (feet)
50
60
70
The wave in the figure above has a wavelength of about 30 feet and a height of about 1.5 feet.
0
Crest
4
8
12
inches
1/2 Wavelength
Height
Trough
The wave has a height of a little over an inch, say 1 1/8 inches.
The wave has a wavelength of about 21 1/2 inches.
(The purple arrows show half a wavelength, so 10 3/4 inches has to be doubled.)
Depth
Oceanography 10, T. James Noyes, El Camino College
5A-3
Wave Interference
Unlike solid objects, waves do not bounce off one another when they come together. Instead the
actions of the water molecules “add together,” and the waves themselves momentarily combine
or “interfere.” If two crests come together, the sea surface gets even higher (you would see 1
higher crest). If two troughs come together, the sea surface gets even lower (you would see 1
deeper trough). These two cases are called “constructive inference,” because they add or build
upon one another. If a crest meets a trough, though, they cancel one another (the sea surface get
flatter: neither higher nor lower). This is called “destructive interference,” because the crest and
trough disappear or are “destroyed.”
2 crests
meet
2 troughs
meet
a crest meets a trough
The interference is lasts for just for a moment. After the collision, the waves keep moving
forward in the directions they were going originally with their original height.
1st
Wave crests move through one another.
After waves meet and "interfere," they are not destroyed.
2nd
Oceanography 10, T. James Noyes, El Camino College
5A-4
1st
2nd
Waves crests approach each other.
Wave crests meet and
"interfere" (add together).
3rd
4th
Wave crests continue in their original directions.
Experiment: Get a rectangular container with a flat bottom (e.g., a baking pan for brownies,
lasagna, etc.) and fill it with just barely enough water to cover the bottom. Now, lift up one
corner, and set it down quickly. Notice how this creates 2 groups of waves, one group bouncing
between both ends and the other bouncing between the sides. These waves go through one
another, momentarily adding or canceling, but they are not destroyed by their interaction. (It is
friction with the bottom of the pan that the eventually saps their energy.)
When you usually look at the ocean, it does not look as nice and regular as the waves that I show
in my diagram above: all crests do not have the same height; some crests grow and others sink;
some crests move faster than others; and so on. It appears to be a big mess, but oceanographers
Oceanography 10, T. James Noyes, El Camino College
5A-5
have learned than underneath this
complexity, the ocean is much simpler:
the complexity arises from several
different groups of waves (coming
from different directions) all arriving at
the same place at the same time and
momentarily “interfering.” Crests
appear to grow and shrink, seemingly
at random in some cases, due to crests
from different wave groups coming
together (growing), moving apart
(shrinking), and sometimes canceling
with a trough (disappear entirely).
When waves come together, they "interfere"
(adding in some places for a moment,
canceling in others) and then move on.
Wave interference is thought to be the cause of some “rogue waves,” huge wave crests that rise
up suddenly and do not seem to come from anywhere. The ocean may be pretty calm as far as
you can see and then suddenly a large wave appears next to a ship, possible knocking it over and
causing it to sink. Even the most experienced sailor cannot do anything to predict or avoid rogue
waves. The rogue wave is thought to be caused by a number of small wave crests from many
different directions all happening to arrive at the same place (next to the ship) at the same time
and momentarily interfering to
make one large crest for just
Troughs
long enough to swamp or knock
Adding
over the ship. The likelihood of
this happening is, of course,
Crests
one-in-a-million (or more!), but
Adding
given enough time, it happens
Crest & Trough
sooner or later. (I guess you
Cancel One Another
could say that people who get
The purple dashed line and the orange dotted line show independent waves
hit by a rogue wave “won the
that are moving through one another and "interfering." The solid blue line
lottery,” but in bad way…)
shows what the sea surface looks like for an instant owing to the
“interference” of the two waves.
There are other potential causes of rogue waves, particularly waves moving against a current.
Going against a current causes waves to grow larger, because it makes them slow down.
(The same thing causes waves to grow on a beach: see page 8.)
Oceanography 10, T. James Noyes, El Camino College
5A-6
Breaking Waves
Together, the wave height and wavelength can be
used to estimate the slope of the water’s surface,
called the “wave steepness 1.”
Wave
Steepness
Height
Wavelength
If the wave steepness grows
higher than 1/7th (0.14),
S te
ep n
the wave breaks. This is similar
ess
Height
to what happens when any object
is on a slope: if the slope is too
steep, then gravity overcomes the
Wavelength
friction between the object and the
surface, and the object slides down the slope. In this case, gravity overcomes the electrical
attraction (bonds) between the water molecules on the surface (the “surface tension”), and water
molecules start tumbling down the face of the wave.
Experiment: Place a small, paperback book (or similar object) on a larger, hardcover textbook
(or similar object like a large cutting board). Slowly raise one end of the textbook, tilting the
surface. You’ll notice that there is a certain angle at which the object starts to slide. It takes a
much larger angle to make objects with great natural friction (e.g., an eraser) to move.
Waves can break differently depending upon
Bubbles "Gently" Break,
how quickly they cross the 1/7th steepness
Spilling
threshold. There are many ways to describe
always close to 1/7
Breakers
breaking waves. I like to “break” them into 2
simple categories: spilling breakers and
plunging breakers. A plunging breaker is
Plunging
"Fast" Break,
when the top of the wave gets far ahead of the
Breakers
quickly go over 1/7
bottom of the wave, causing a “barrel” or arc;
this is the kind of wave preferred by surfers.
In this case, the wave gets steep very quickly; it rapidly crosses the 1/7th threshold. Since it takes
time for a wave to break, this kind of breaking waves gets much steeper than 1/7th before finally
breaking. A spilling breaker, on the other hand, gets steeper slowly. As its steepness barely
grows above 1/7th, water at the crest begins to slide (“spill”) down the front of the wave,
reducing the wave height and thus lowering the steepness back down to 1/7th. As the wave
approaches the shoreline a little more, it grows a little more, which causes it to break a little
more. In other words, the steepness “hovers” around 1/7th. In summary, a spilling breaker
breaks slowly and gently over a long time, while a plunging break breaks quickly (suddenly, all
at once).
1
Notice that this is actually a pretty bad estimate of the actual steepness of the front of the wave. It is off by at least
a factor of 2. However, as long as we all estimate the parameter in the same way, though, it still tells us when and if
a wave will break (just at a different value than the real slope). The beauty of this formula is that it is a lot simpler
than it “should be” (easier to memorize).
Oceanography 10, T. James Noyes, El Camino College
5A-7
Courtesy of PDPhoto.org (public domain)
Plunging Breaker
Spilling Breaker
People also describe “surging breakers.” In this case, the wave never really “breaks,” but just
flows up the shoreline. (This is what tsunami do.)
Wave Speed
Deep-water waves (waves that do not touch the bottom of the ocean) move faster than shallowwater waves (waves whose orbitals run into the bottom of the ocean, distorting them into ovals).
I like to say that the ocean bottom slows down a wave if it “feels the bottom.” (Technically, this
is not a frictional effect; it is related to keeping the water from completing the orbital quickly by
bending it off course.) The speed of a deep-water wave is controlled by its wavelength: the
longer the wavelength, the faster a deep-water wave moves.
Changes in Wave Height, Wavelength, and Wave Speed at the Shoreline
As a wave moves into shallow water, its orbitals “feel the bottom,” causing it to slow down.
The wave crests that are closer to the shore (“in front”) are in shallower water, so they are
moving slower than the wave crests farther out in the ocean (“behind”). This allows the wave
crests out in the ocean to get closer to the wave crests near the shore, reducing the wavelength
(the distance between the crests). This “squeezes” the water in-between the two wave crests
horizontally; the water cannot go down (the ocean is getting shallower as it approaches the
shore!), so it goes in the only direction it can: up! This is why waves grow larger at a beach.
Shorter Wavelength
Longer Wavelength
Beach
Slowest
Crest
"Squeezed"
Crest
Fastest
Crest
Oceanography 10, T. James Noyes, El Camino College
5A-8
I like to call the reduction in wavelength at the shoreline a “traffic jam” in the ocean. Imagine
that you are driving north on the 405 towards the junction with the 105 by LAX. (Big mistake.)
You begin with lots of distance between you and the car in front of you (unless you’re tailgating,
and thus endangering both you and the car in front of you), but as you come around the bend,
you’ll see a sea of red tail-lights in front of you: they’re all slowing down. Both you and the car
in front of you put on your brakes, but he saw them first so he starts stopping first. While you’re
going faster, you get closer to the in front of you, just like the wave crest “behind” gets closer to
the wave crest “in front” of it. In other words, the “wavelength” (the distance between you) gets
smaller.
The waves’ height, wavelength,
and speed change as they
approach the beach – but one
wave characteristic does not
change: wave period (frequency).
Here is another way to think about why waves grow at a beach: the front part of a wave crest is
in slightly shallower water than the back part of the wave crest, so it is always going a little
slower than the back part of the crest. As the back part of the crest catches up to the front part of
the crest, more and more of the water that was spread out over a wide area gets concentrated in a
narrower area. (See the picture above.) I always hesitate to use this explanation because many
students think that a wave crest that is “behind” another wave crest can actually “catch up” to the
wave crest that is “in front” of it. This cannot happen 2, because the wave crest that is “behind”
slows down more and more as it enters shallower water, so it can never actually “catch up” to the
wave crest “in front” of it. Instead, it is always “catching up,” but never can quite do so. 3 The
key thing to remember to avoid confusion is that the back part of a wave crest is “catching up” to
the front part of the same wave crest; two separate wave crests are not merging.
Here are a few other misconceptions: Waves do not grow at a beach because the bottom pushes
them up (as if they are hitting the bottom and bouncing upwards). Also, they are not gaining
energy as they grow. A scientist actually thinks of wave growth as an example of the
conservation of energy: the forward “motion” energy of the wave (kinetic energy) is being
converted into “gravitational potential energy” (it goes upward, fighting gravity), like a ball
being thrown upward loses speed (“motion”) as it goes upwards fighting gravity. However, as
you know, what goes up, must come down: the wave eventually becomes too steep and it gets
pulled down by gravity, causing it get all of its “motion” back.
2
In fact, one wave crest can catch up to another wave crest if waves with different wavelengths are arriving at a
beach at the same time. In shallow water the orbitals of the waves with a shorter wavelength do not “feel the
bottom” quite as much as the waves with a longer wavelength, so on a beach they move a little faster than the waves
with a longer wavelength! (The opposite is true in deeper water.) This allows a wave crest of the waves with a
shorter wavelength to catch up with a wave crest of the waves with a longer wavelength. Since their speeds are
pretty close, the crests can stay together for quite some time, leading to a significant increase in wave height due to
wave interference for much longer than a “moment.” This is why there are “sets” of waves with larger heights and
why some surfers have rules of thumb like “every seventh wave will be larger than the rest.”
3
This is like Zeno’s paradox in Greek philosophy.
Oceanography 10, T. James Noyes, El Camino College
5A-9
Wave Refraction
Most ocean waves are created by winds (often during storms). Winds can blow from any
direction, so at any given spot in the ocean, waves are typically coming from and going in a wide
variety of directions. However, if you think of waves at a beach, you know that they pretty much
go right towards the shoreline. In other words, they go towards you, not up or down the coast.
This means that as waves approach a shoreline, they turn or “bend” towards the shoreline, a
process we call “wave refraction.” (Not reflection: reflection is when a wave bounces off
something, like a wall.) The result is that wave crests tend to parallel or “match” the shape of the
shoreline as they break.
As waves come into to a beach, they bend to “match” the shape of the shoreline.
Mud
Begin to
fall behind
Grass
Beach
wa
ve
cr
es
t
fa
ste
r
slo
we
r
To understand why this happens, imagine a
line of soldiers marching towards some mud
at an angle. (Why march into the mud?
because Sarge told them to.) A soldier at one
end of the line will reach the mud first,
slowing him down, while the other soldiers
continue moving forward. Each soldier
spends a little more time on the grass than the
soldier to his left, so he moves farther than
his neighbor, getting ahead of him. The
result is that by the time the last soldier steps
into the mud, the line of soldiers has been
stretched out and now makes a new angle: the
line has “turned” or “bent.” The only
difference between this example and waves is
that they actually change direction, unlike the
line of soldiers. A better example is to imagine
pushing or driving a two-wheeled object like a
hand truck (“dolly”) or Segway into the mud.
The wheel in the mud would get stuck, but the
wheel on the grass would keep moving and turn
the object towards the mud.
Oceanography 10, T. James Noyes, El Camino College
As a wave crest approaches
the shoreline, typically one
end of the line is closer to
the shoreline than the other.
This end is in shallower
water, so it slows down
while the other end, in
deeper water, moves
forward faster. This swings
the wave crest towards the
shore, since the end in deep
water covers a larger
distance towards the shore
than the slower-moving end
in shallow water.
Faster
End
If the seafloor slope around an island is not
too steep, wave refraction can actually cause
waves to turn all the way around (“wrap
around” an island) and hit the opposite side.
However, waves tends to be weaker on the
side of the island facing away from the
original waves. Notice that in the soldier
example the line gets longer (“stretched”) as a
part of refraction. This means that the energy
in a wave is also spread over a larger area,
which reduces the height of the waves. In
other words, refraction often makes waves
smaller. However, if waves bend towards one
another and begin to come together, they
interfere, creating a higher wave crest. Thus,
wave refraction can also increase the height
of waves. This typically happens near the
shallow water of a “point” or “headland.”
The waves bend towards the headland, causing
them to come together, increase in height, and
pound the headland even more fiercely than the
rest of the coast. If waves completely “wrap”
around an island and come together on the
other side, they will become larger as well.
Typically, waves break before refracting
“completely” (becoming parallel to the shore or
perfectly matching the shape of the shoreline),
which allows them to push sand down the
coast, our next topic.
5A-10
Slower
End
Island
"Wave Shadow:"
Island has
blocked
the waves.
Headland
Farther from shore
= Deeper Water
= Faster
= Turns toward shore
Oceanography 10, T. James Noyes, El Camino College
5A-11
Longshore Transport of Sand by Waves
As a non-breaking wave goes by, the water beneath the wave moves in a circle. In shallow
water, though, the bottom gets in the way, distorting the circle into an ellipse (oval). At the very
bottom, the ellipse is completely squished, so that the water hardly moves up and down at all:
instead it goes “side-to-side” or “back-and-forth” as the wave goes by. This water motion pushes
the sand beneath it, causing the sand to wiggle back-and-forth as well. Even though the sand
moves, it does not go anywhere: like a child on a swing, it goes back-and-forth but does not
actually travel from place to place.
Waves often do not refract completely,
and come into the shoreline at a small
angle, allowing them to push sand up or
down the shoreline. In this case, they
are pushing sand to the left in the
picture.
Overall Sand Motion
A breaking wave 4, on the other hand, pushes sand
up the slope of the beach at an angle. The water
and sand then slide back down the beach slope
into the ocean, pulled down by gravity, but they
goes straight downhill, taking the fastest route
back into the ocean. Thus, they are not back
where they started. (This motion is often called
“zig-zag” motion.) Each breaking wave pushes
sand a little bit down the coast. This may seem
like a small effect, but waves endlessly pound the
shore, day after day, year after year, slowly
pushing sand down the coast: inch after inch
eventually becomes mile after mile.
4
Photograph courtesy
of Dr. Douglas Neves.
Motion of
the Sand
Beach
Direction of
Longshore
Transport
Break
ing W
ave C
Wave
rests
Direction
Non-breaking waves moving sand back-and-forth does cause the sand migrate or drift a little bit, but this is very
small and slow compared to longshore transport.
Oceanography 10, T. James Noyes, El Camino College
5A-12
Wave Period and Wave Frequency
Another way that scientists describe waves is to measure how quickly they cause the sea surface
to bounce up and down. One strategy for measuring this feature of waves is to watch a single
spot, and to count how long it takes for one wave crest to be replaced by the wave crest behind it.
(For example, watch a surfer or a bird bob down and then back up again.) This is called the
wave “period,” the period of time is takes for a wave crest to go by (travel the distance of 1
wavelength). Alternatively, you can measure the wave frequency, how often a wave comes by.
(For example, 6 waves pass by in a minute.) They are inversely proportional to one another:
frequency = 1/period, meaning that if one is high, the other is low; if waves pass often – high
frequency – then small amount of time between them – a short period of time. Note that if you
measure one, you can calculate the other.
Scientists measure wave period (or frequency) for several reasons. One reason is that wave
period and frequency never change 5 as waves travel, unlike wave height or wavelength.
Secondly, they are much easier to estimate from a distance than wave height or wavelength.
Finally, if you measure the wave period or frequency and know the depth of the water, then you
can calculate the waves’ wavelength and speed 6. (Pretty nice, huh?) The relationship between
wave period and wavelength is: the longer the period, the longer the wavelength. Think about it
for a moment: the larger the distance between the crests, the longer the time it takes for one crest
to replace the one in front of it.
Since long-period waves have a long wavelength as well, they are faster than short-period waves.
This means that if long-period and short-period waves start out together, they will eventually
separate from one another (“disperse”), because the long-period waves will get ahead of the
short-period waves.
5
Wavelength changes as waves approach a beach, because the waves are slowing down. The period does not
change at all; the wavelength does all the adjusting that is necessary. The period does change when the waves
break, but by this point it no longer matters: once they break, they are no longer waves.
6 2
ω = gk tanh(kh), where ω is the angular frequency, g is gravitational acceleration, k is the angular wavenumber
(wavelength), and h is the depth of the water. In deep water, it is simply ω2 = gk. In shallow water, it is ω2 = ghk2.
Oceanography 10, T. James Noyes, El Camino College
5A-13
Making Waves
Most ocean waves are created
by the wind blowing over the
surface of the ocean. The
largest waves are created by the
strong winds of storms. The
wind enhances small differences
in the surface of the ocean by
pushing the top of the waves
forward and making the waves
grow via the Bernoulli effect.
(Have you ever been standing
by the side of the road when a
fast-moving truck goes by and
felt “pulled” towards the truck?
This is owing to the “Bernoulli
Waves growing in a storm. National Oceanic and
Atmospheric Administration, Dept. of Commerce.
Effect.” The truck pushes air
out of the way quickly, and you get sucked in with the air moving in to replace it.
In the same way, a fast wind “sucks” the surface of the ocean upward.
Experiment: Take a piece of paper (lighter is better), and hold one end with both hands just
beneath your mouth. Now, blow across the top of the paper. Notice how the paper rises,
pushed upwards by the air beneath. The wind causes waves to grow in the same way.
Waves must go through a cycle of growth and breaking many times to become large (tall height
and long wavelength). As a small wave grows higher, it becomes too steep and breaks, causing
it to lose the height it gained. However, the breaking causes its wavelength to stretch out (gets
longer), so when it grows again, it can get taller before becoming too steep and breaking again.
The wavelength gets stretched again and again, and the wave grows again and again.
Remember: the wavelength of a wave affects its speed: the longer the wavelength, the faster the
speed. Eventually, the wave moves as fast as the wind, so the wind can no longer push it, and
the wave stops growing 7.
The largest waves are created by strong, steady winds that blow over a large area (called the
“fetch”). Since waves grow until they match the speed of the wind, strong (fast) winds make the
biggest waves. If the strong winds keep shifting – first making waves going one direction, then
another – then the wind will create small waves going in many directions, not large waves going
in one direction. It takes time for large waves to grow, so longer the winds blow in one
direction, the bigger the waves can become. Waves stop growing if there is no wind, so once
they leave the area where the wind is blowing (the “fetch”), they cannot grow any more. Winds
with a large fetch can help the waves grow for a long time before they leave the wind behind4.
7
In reality, winds fluctuate and create a wide variety of waves all at once, but the average wind speed provides a
good estimate of the longest-wavelength waves which in turn can have the largest height without breaking.
Oceanography 10, T. James Noyes, El Camino College
5A-14
As you can see from the plots below, winds over the ocean are strongest near the Poles, and
waves are also largest where the winds are strongest. Storms are common at these latitudes,
especially in the winter, though storms are more common in subtropics during the winter as well.
During the summertime, tropical storms are common near the Equator, and tropical storms that
grow into hurricanes can create huge waves. As you can see from these maps, though, tropical
oceans are much calmer than polar oceans most of the time. Thus, most of the large waves that
strike the coast come from the Poles.
Wind
Speed
Wave
Height
Courtesy of NASA/JPL, TOPEX/POSEIDON.
Oceanography 10, T. James Noyes, El Camino College
5A-15
Waves Across the Ocean
Once waves leave the “fetch,” they
lose very little energy until they break
along the shoreline 8. Like a row of
dominos, water molecules bump into
one another, passing the disturbance
and its energy from one to the next
quite efficiently. However, as the
wave moves away, it is spreading out
(like ripples on a pond), so the wave
Wave ripples move outward from a disturbance (like duck feet!).
height does decrease. (As you might
The ripples become smaller because the energy spreads out,
but the total amount of energy is the same; no energy is lost.
expect, waves from a distant storm are
Courtesy of Rennett Stowe (CC-BY-2.0).
smaller than those from a nearby
storm.) The reduced wave height does not mean that the waves are losing energy, though. They
have the same total amount of energy, but it is spread over a larger area. (So the waves from a
nearby storm are larger but affect a smaller section of the coast than waves from a far away
storm.)
The winds of a storm create waves with a variety of wavelengths and heights, but as they travel
across the ocean, they begin to sort themselves out by wavelength. The longer-wavelength
waves are faster, so they leave the shorter-wavelength waves behind. We call this “wave
dispersion” (disperse = too go apart, like the police telling a crowd to “disperse”). When all the
waves of different wavelength are all jumbled together, it can produce a very complex sea
surface (see the section on “wave interference”). Once waves separate (“disperse”), the sea
surface becomes more regular, resembling the nice smooth patterns in my side-view sketches of
waves. We call such nice, regular waves “swell.” (When jumbled together, we say that they are
“sea” waves.)
As waves move outward, a curious phenomenon can be observed in the wave groups. A crest
will emerge from the back of the group, move all the way to the front, and disappear. This keeps
happening again and again, with wave crests emerging at the back of the group and disappearing
at the front. Overall, this leads to a reduction in the group’s speed (the group speed is ½ of the
wave speed). The best – though confusing – way to understand this is that a wave group is
composed of several waves of slightly different wavelengths all mutually interfering. New wave
crests appear because the longer-wavelength waves are moving faster: instead of crests
overlapping with troughs (canceling both), the crests begin to overlap with crests and troughs
with troughs. The longer wavelength waves are moving slightly faster, leading to a wave crest
moving forward through the group. However, the interference at the front of the group causes
the waves to cancel out again as crests again line up with troughs, so the group does not move
forward as much an individual wave crest. Note that the largest wave crest is in the center of the
group. This is one reason surfers observe that every 3rd wave crest of a set (or every 7th wave
crest, or other rules I’ve heard them speak of) tends to be larger than the rest: two or more waves
are coming into the beach at the same time and interfering, growing, and breaking together.
8
unless wave interference makes them large enough to break,
or winds encountered in their journey add or remove energy
Oceanography 10, T. James Noyes, El Camino College
5A-16
The Importance of Waves in the Ocean
Waves have several important effects on the ocean, particularly breaking waves. Once a wave
breaks, the “wave motion” of the orbitals breaks down. Instead of moving in a circle, water
surges forward (just like water surges up the slope of a beach). Thus, wave breaking causes
ocean currents.
In addition, as water falls down the front of a breaking wave, it captures air. (In other words,
bubbles form: the white foam that you see on a wave crest and is left behind as it moves onward
towards the shore.) If these bubbles break underneath the surface of the water, then the ocean
water captures gases from the atmosphere 9 (they become dissolved gases). Similarly, breaking
waves disturb the surface of the ocean, sending a spray of water droplets into the air. The water
molecules, salts, and gases in the droplets now have a much easier time evaporating (there are
more directions in which to fly away), allowing them to enter the atmosphere. Salts, of course,
tend to settle back into the ocean or on land over time, but this is a major way in which the
oxygen made by ocean algae (like phytoplankton) enters the atmosphere for us to breathe!
(Thank you, waves!)
Finally, breaking waves and wave orbitals stir up the surface of the ocean (appropriately called
the “mixed layer”), making it fairly uniform in temperature, salinity, and other characteristics 10.
Their mixing brings up both sinking phytoplankton 11 and unused nutrients from down deep, and
sends down abundant oxygen from the surface.
Waves cannot reach down into the deep
ocean, and most deep-sea currents are
also weak. The small amount of
motion in the deep ocean is mainly
caused by swimming deep-sea animals!
9
Air molecules can also strike the surface of the ocean, or break free from the ocean surface, but this process is
much less likely (and thus slower) than the exchange of air molecules mediated by ocean waves.
10
The mixed layer is also “mixed” when surface water cools and sinks (higher density) and is replaced by the
somewhat warmer water from below, similar to the effect of wave orbitals.
11
Waves also tend to push phytoplankton down, but because phytoplankton tend to sink, there are more
phytoplankton who need to be brought up than there are floating near the surface, so overall waves tend to bring up
more phytoplankton than they push down. The same is true for nutrients.