Oceanography 10, T. James Noyes, El Camino College 5A-1. Waves

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1 Oceanography 10, T. James Noyes, El Camino College 5A-1 What are Waves? Waves are disturbances in mediums (substances or materials). There are many different kinds of waves: sound waves, light waves, seismic (earthquake) waves, and so on. The energy of the disturbance gets passed along by the medium for example, by water Waves Energy is here. Energy is passed from domino to domino when they collide. The energy can move great distances, but the dominos 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.

2 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 several small circles at different depths, and that the circles near the surface are larger. The size of the circles is controlled by the height of the wave, the distance from the bottom of the wave (the Ocean Bottom Wave Orbitals trough) to the top of the wave (the crest). The depth that the wave reaches into the ocean with all those circles called the wave base is controlled by the wavelength, the distance from the crest of one wave to the crest of the next. A wave reaches down into the ocean a distance equal to about ½ of its wavelength. Below the wave base, the water is practically motionless. 4 Crest Crest Measuring the height of a wave from the crest to the bottom of the ocean is one of the most common mistakes on tests. Water Level (feet) Height Trough Wavelength Trough Horizontal Distance (feet) The wave in the figure above has a wavelength of about 30 feet and a height of about 1.5 feet Crest inches 1/2 Wavelength Height Trough Depth 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.)

3 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. 1 st 2 nd Wave crests move through one another. After waves meet and "interfere," they are not destroyed.

4 Oceanography 10, T. James Noyes, El Camino College 5A-4 1 st 2 nd Waves crests approach each other. Wave crests meet and "interfere" (add together). 3 rd 4 th 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

5 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 long enough to swamp or knock over the ship. The likelihood of this happening is, of course, one-in-a-million (or more!), but given enough time, it happens sooner or later. (I guess you could say that people who get hit by a rogue wave won the lottery, but in bad way ) Crests Adding Troughs Adding Crest & Trough Cancel One Another The purple dashed line and the orange dotted line show independent waves that are moving through one another and "interfering." The solid blue line 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.)

6 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 1/7 th (0.14), the wave breaks. This is similar 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. Steepness 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 how quickly they cross the 1/7 th steepness threshold. There are many ways to describe breaking waves. I like to break them into 2 simple categories: spilling breakers and plunging breakers. A plunging breaker is when the top of the wave gets far ahead of the bottom of the wave, causing a barrel or arc; this is the kind of wave preferred by surfers. Spilling Breakers Plunging Breakers Bubbles "Gently" Break, always close to 1/7 "Fast" Break, quickly go over 1/7 In this case, the wave gets steep very quickly; it rapidly crosses the 1/7 th threshold. Since it takes time for a wave to break, this kind of breaking waves gets much steeper than 1/7 th before finally breaking. A spilling breaker, on the other hand, gets steeper slowly. As its steepness barely grows above 1/7 th, 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 1/7 th. 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 1/7 th. 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).

7 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

8 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.

9 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. 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. Mud slower Begin to fall behind wave crest Beach faster Grass

10 Oceanography 10, T. James Noyes, El Camino College 5A-10 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 Slower 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. Island Headland "Wave Shadow:" Island has blocked the waves. Farther from shore = Deeper Water = Faster = Turns toward shore

11 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 Photograph courtesy of Dr. Douglas Neves. 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. Motion of the Sand Wave Direction Beach Breaking Wave Crests Direction of Longshore Transport 4 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.

12 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 ω 2 = 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 ω 2 = gk. In shallow water, it is ω 2 = ghk 2.

13 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 Effect. The truck pushes air Waves growing in a storm. National Oceanic and Atmospheric Administration, Dept. of Commerce. 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 behind 4. 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.

14 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.

15 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 height does decrease. (As you might expect, waves from a distant storm are smaller than those from a nearby Wave ripples move outward from a disturbance (like duck feet!). The ripples become smaller because the energy spreads out, but the total amount of energy is the same; no energy is lost. Courtesy of Rennett Stowe (CC-BY-2.0). 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 3 rd wave crest of a set (or every 7 th 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

16 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.