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Chapter 7 — Transcript

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0:00Hi there, I'm Dr. Jeff Ashley and
0:02welcome to chapter 7 which deals with o
0:05ocean
0:08circulation. As you may recall in the
0:10last chapter we discussed what was going
0:12on in the atmosphere mainly that lower
0:15layer that we call the troposphere where
0:17our weather happens and we mentioned
0:19that there are certain cells both in the
0:21northern hemisphere and the southern
0:23hemis hemisphere and we went over what's
0:26driving that. Those were pretty much
0:28north south or south north movements in
0:30these circulation cells. We're going to
0:33see some similar cells that explain how
0:35the ocean at least the surface waters of
0:37the ocean move. So this chapter deals
0:41with ocean currents and most of the
0:43ocean currents that we'll be discussing
0:45are ocean currents that are moving that
0:47top layer of ocean. Okay, previous
0:51chapters we said that there's a top
0:53layer of ocean that is less dense
0:55because of density differences and
0:58there's a bottom layer that is
1:01sequestered or sits on bottom of that
1:04top layer and we call that the deep
1:06water. So imagine the surface waters and
1:09the deep waters and they are sort of
1:11disconnected because of the density
1:12differences density based on temperature
1:15and also density based on salinity.
1:18So with this in mind, this two-layering
1:20system, a lot of this chapter deals with
1:23the movement of that top layer, the
1:26surface of the ocean. And don't think
1:27it's just that little couple centimeters
1:29of the surface. It goes down meters to
1:31even a kilometer or more deep. We call
1:35these surface ocean currents. And it's
1:37just moving
1:39seawater. The transfer of heat from
1:41warmer to colder cooler areas happens
1:44because of this movement, as we're going
1:46to see in this chapter. And the movement
1:48is very similar to the major wind belts.
1:51So we talked about the westerlys and the
1:53trade winds in the last chapter. Those
1:55are the primary winds that are hitting
1:57the sea surface and enabling it to
2:00move. Ocean currents also affect coastal
2:03climates. As we mentioned before, I
2:05brought up the case of California and
2:07it's very temperate or nonvarying
2:10climate and that's due to the cold
2:12currents off that shore.
2:15We will also in this chapter discuss
2:17deep ocean currents. So rather than the
2:20surface currents, these are the deep
2:22ocean currents, cold, very dense water
2:24sitting on top of that less dense
2:26surface water. We'll get to that at the
2:28very end. Although it's important much
2:30of this chapter, as I said, are dealing
2:31with these surface currents. So again,
2:34breaking down the currents, we will
2:35primarily be discussing surface currents
2:38and they are primarily wind driven and
2:40they're primarily a horizontal motion.
2:43So imagine the two layering system of
2:45these deep ocean and the surface. Wind
2:48blows on top and it allows horizontal
2:52movement. Deep ocean currents are driven
2:54by densities uh differences caused by
2:58temperature and salinity as we saw in
3:00past chapters. However, it includes both
3:02horizontal movement and sometimes up and
3:05down or vertical
3:07movement. So how do you measure surface
3:10currents? Early oceanographers just used
3:13to throw bottles with notes in them,
3:15message in a bottle, and that's how they
3:16did it. And it worked. It allowed people
3:19to piece together how these surface
3:21currents were moving, in what direction,
3:23and even at what
3:25velocity. Modern-day oceanographers now
3:27use equipment such as this. This is a
3:30drift current meter, and it's deployed
3:33in the sea surface of the top layers. It
3:35has some buoyancy compartments that will
3:38keep it afloat. As currents pass by,
3:41this quadrant moves and sends the
3:44information on the velocity of the
3:46current and also its direction.
3:49Sometimes information on surface
3:50currents are garnered by mistakes. And
3:53your book is a gives a couple of these
3:55examples. One famous one was a cargo
3:58ship that was carrying Nike shoes. The
4:00cargo containers were displaced into the
4:03ocean, the surface of the ocean by a
4:06storm. And eventually those cargo
4:09containers opened up and released a
4:11whole bunch of Nike shoes. Well, people
4:13began to uh find these shoes on the
4:16shores of British Columbia and also
4:18Alaska and also down the Pacific North
4:21Coast to Oregon and even further. And
4:23this allowed oceanographers once people
4:25collected the shoes and identified them
4:28to begin to understand the surface
4:30currents in this area. This goes back to
4:331990. So information was still garnered
4:36from not so much a message in a bottle
4:38or a drift current meter, but by
4:40accidents like this. Your book also
4:43gives some other examples of these
4:44accidents. And one of them was uh
4:47Chinese toys, little plastic toys.
4:50Imagine rubber duckies or the like. And
4:52again, finding these things allowed
4:54people to piece together what these
4:56ocean currents were doing as far as
4:58their direction and also their
5:00velocities or the speed. So, a really
5:03great figure here. And remember that we
5:06said the sea surface has the ability to
5:08bulge at higher elevations or lower
5:11elevations than sea level. So, the red
5:13colors are areas that have a higher than
5:15normal sea level. So they're like the
5:18bulgy layers, couple
5:20centimeters, purple, magenta areas, and
5:23the green areas are a little lower than
5:25sea level. Placed on this map now is the
5:28white arrows. And the arrows indicate
5:31the flow of the currents, the surface
5:33currents, and also the velocity with
5:36longer arrows indicating the faster flow
5:39rates. So you can just begin to see what
5:41we're going to talk about. But a lot of
5:43the fast flowing movement of the surface
5:45waters is happening here around the
5:47equator. You can see these smaller
5:49vectors and the direction. So
5:52smaller, you can also see smaller arrows
5:55which depict a smaller velocity in the
5:57surface currents and their
6:00locations. Another way to measure
6:02currents and these are great ways to
6:05measure the deep sea current. So that's
6:07that bottom cold dense water and how
6:09that's moving either vertically as we
6:12said up and down or horizontally is
6:15using chemical
6:16tracers. So what are chemical tracers?
6:19They are humanly inputed tracers. Uh we
6:23know when they were formed and we can
6:25look at their radioactive decay or
6:27actual their presence and determine
6:30where they were deployed first of all
6:31and where they end up. So you can
6:33measure deep sea water uh for tridium or
6:36CFC's and measure their abundance and
6:39piece together the deep sea currents.
6:41Tridium is interesting because it was
6:43released uh unnaturally from nuclear
6:46bomb tests in the 1950s and60s in the
6:49Pacific Ocean. So it ended up in the
6:52ocean and now we can look at
6:53concentrations and infer ah we find a
6:56level of tridium in this area that must
6:58have originated from the source and we
7:00can determine the velocity and the
7:02movement of these deep sea currents
7:04equally. So chlorofluorocarbons were
7:06used as primarily propellants like in
7:09hairspray. They were banned and they are
7:12still globally ubiquitous found
7:14everywhere. These are the culprits of
7:16these ozone depletion. So primarily they
7:19were banned because of ozone depletion.
7:21It worked. We maintained some of the
7:23ozone layer that we were losing back in
7:25the 80s and 90s and they can be used as
7:28deep sea tracers as well to look at the
7:30movement of the deep sea ocean. Another
7:33way to do this and this is staggering
7:35when you look at the number of dots on
7:37this map. The number of dots represents
7:39the deployment of what we call Argo and
7:42Argo are floating and sometimes
7:44nonflating measuring devices. They're
7:47free drifting. They can have the ability
7:50to de go deep down uh to measure the
7:54subsurface currents as well and even a
7:56special type now that goes very deep
7:59down thousands of meters. So these
8:02primarily are floating measuring devices
8:05kind of like the current meter that we
8:07saw before. They are satellite
8:10connected. So they're dumping their data
8:12live and we get a really extremely
8:16robust system of measuring these
8:18currents throughout the
8:20world. This is a special kind of Argo.
8:24Um and Argo is just this device. Instead
8:27of measuring the sea surface
8:28temperatures, they are deployed deep
8:31down. So they descend at a cruising
8:32depth at around uh 10 cm per second.
8:35They can go into that cold and dense
8:38area, that deep current and they can
8:41then follow the direction and also
8:43measure the velocity of that moving
8:45water body very very deep down and then
8:48they pop up and then through satellites
8:52they can dump their data and we can
8:55infer a lot about the movement of that
8:56deep
8:57sea. Let's get back to surface currents
9:00because I said most of this chapter
9:02we're going to be discussing it. It's
9:04very important in determining a whole
9:06bunch of things as we'll see. But
9:08surface currents are really proposed by
9:10a frictional drag be between the wind
9:13and the ocean. Uh so it's primarily wind
9:16driven. How are we moving the surface of
9:18the currents? Just blow on your coffee.
9:20So you blow on your coffee and it moves
9:23the surface, right? Same thing's
9:25happening here. The wind is that blowing
9:27breath, right? And the ocean is your
9:29coffee. But primarily that's what we're
9:31going for. Although there's a whole
9:32bunch of things that sort of uh tweak
9:36that concept as we'll see. So it's
9:39primarily wind but there's other
9:41factors. You've got wind but then you
9:44have obstructions like continents. So
9:45we'll see how that infers the pathway
9:48and the velocity. Once a sea surface
9:51hits a current uh continent. How is it
9:53going to be moved in a direction and how
9:56is the velocity changing? Also, gravity
10:00to an extent. Water molecules are under
10:02the influence of gravity just like
10:04everything else. So, that has something
10:05to do with the movement as well.
10:07Frictional forces, we'll get to that in
10:09a minute. So, we'll see these frictional
10:11forces
10:12shortly. And as well, because these are
10:15large scale movements of matter, water,
10:19they are under the influence of the
10:20corololis effect. And just to remind you
10:23that coriololis is the deflection of
10:26large scale masses of stuff like water
10:28or air masses. And the rule, remember
10:30the rule, in the northern hemisphere,
10:32you're getting a deflection to the
10:34right. In the southern hemisphere, you
10:36get deflection to the left. And we're
10:38really going to see how that plays out
10:39in the ocean
10:41circulation. So, the movement of these
10:43surface currents is largely driven by
10:46wind and adding in a couple other
10:48factors as we'll see. But what happens
10:50is through wind we develop these things
10:54called gys. They are large circular
10:56loops of moving water. Let's go to a map
10:59and check them
11:00out. So here we have a map of the
11:03southern and the northern hemisphere,
11:05but we're zeroing in on the Atlantic
11:07Ocean. So the North Atlantic is here and
11:10the South Atlantic here. Look at the
11:14arrows that are purple in nature because
11:16they are showing the movement of the
11:18surface waters. And you can clearly see
11:21this clockwise rotation called a gy in
11:24the North Atlantic. And you can clearly
11:26see an
11:28anti-lockwise
11:29or counterclockwise movement in the
11:32southern hemisphere. These ocean surface
11:36currents, these gys are primarily wind
11:38driven. And remember the chapter when we
11:40discussed the prevailing westerlys and
11:43the trade winds either the northeast
11:45trade winds or the southeast trade winds
11:47and their movement and deflection to the
11:49right in the northern hemisphere and
11:51left in the southern hemisphere. They're
11:54primarily responsible for driving this
11:56circular motion of the surface
12:00currents. What we have in each one of
12:02these gys and these are just two
12:04examples of gys but there are more.
12:06We'll switch to the other ones in a
12:08moment. So think of these gys. Yes,
12:10they're represented as circles, but I
12:12like to think of them as boxes. Boxes
12:15with four sides. And those four sides
12:17represent different currents that
12:20represent the entire gy, but we label
12:22them differently. So here that northern
12:25current is called the northern boundary
12:28current. Here that current is the
12:31southern boundary current. Let's go back
12:33to this gy here. This is the western
12:36part of the Atlantic. So it's called a
12:38western boundary current and equally if
12:41you go to the other side right here this
12:43is an eastern boundary current and the
12:46one that is near the equator is called
12:48the equatorial current. Okay so there's
12:51the equatorial current in this gy
12:53equatorial current here western boundary
12:56current. We've got
12:58the southern boundary current and we've
13:01got the eastern current. So each one of
13:03these
13:04gys and we'll see more of these
13:06worldwide can be labeled in the sort of
13:09this box figure with the appropriate
13:13uh
13:14directional
13:15boundary. Okay. So here we see again
13:18worldwide we zoomed into the Atlantic in
13:22the northern hemisphere and the movement
13:24here of this gy counterclockwise in the
13:26southern hemisphere. But we can see
13:28these gys exist in this Pacific Ocean
13:31here. we see a large gy right. This
13:34would be the western boundary, the
13:36northern boundary, the eastern boundary,
13:38and the equatorial uh boundary. Here we
13:41have another gy again counterclockwise
13:43in the southern ocean. Here's the
13:45equatorial boundary, the western
13:47boundary because it's in the western
13:49Pacific, the southern boundary, and the
13:52eastern boundary here. So, as long as
13:55you know your north, south, east, and
13:56west, you've got the labeling systems of
13:58this four-part current system that makes
14:01up these gys. No, it's not like a box,
14:03but again, I like to remember it as such
14:05to remember the lab labels. So, eastern
14:09boundary, western boundary, northern,
14:11southern boundary, and equatorial those
14:13are sort of generic components of a gy.
14:17When we go to an actual map like this,
14:19you see they get special names. So we
14:21said this was the western boundary
14:23current in the uh northern hemisphere of
14:26the Pacific Ocean. That's called the
14:28Corisho uh current. Down here we have
14:32the California current. We spoke of this
14:33before. If you're going swimming in
14:35August off the co coast of California,
14:38it's going to be cold. It's a cold water
14:40current coming down from the northern
14:42part heading back to the equator. So on
14:44this map too, you see that red indicates
14:46these warm currents and the blue
14:49represents the colder currents. And it
14:51kind of makes sense if you're traveling
14:53on the equator, enriching that current
14:55in warm water. The Gulf Stream we know
14:58is a warm current heading up, but as it
15:00heads up, we cool these currents down,
15:03but they return on their eastern
15:05boundary towards the equator, but
15:06they're cold. And again, this was the
15:08moderation of the continental climate
15:11right offshore here, which I mentioned
15:12before as well. The Ecman spiral is one
15:15of the coolest things I think in uh this
15:17chapter. And in oceanography, surface
15:19currents move at an angle to the wind.
15:21So, if you go back to my little analogy
15:24of blow over your coffee to cool it, it
15:27looks like the coffee the surface of the
15:28coffee is moving in the direction of
15:30your blow. So, you would think if the
15:32wind's blowing, aren't the surfaces
15:34going to move in that direction? Ecman a
15:36scientist said no that they actually
15:39blow at an angle to the wind. The Ecman
15:41spiral describes the speed and also more
15:44importantly I think the direction of the
15:46seawater flow at different depths. So
15:49we'll take a look at a video because
15:50it's much better in action and in
15:52animation. But each successive layer
15:55moves increasingly to the right in the
15:57northern hemisphere, increasingly to the
16:00left in the southern hemisphere. Let's
16:02put this in motion. As wind moves across
16:05the surface of the ocean, friction
16:07between the moving air and the surface
16:08of the water causes the water to begin
16:11to move as well. This transfer of energy
16:13through friction is how wind causes
16:15surface currents. Once water at the
16:17surface begins to move, some of the
16:19energy gets transferred to deeper
16:21layers, allowing water movement to
16:22penetrate to depths of 50 to 100 m. The
16:26details of how water behaves as energy
16:28from wind moves from the surface to
16:30depth was first investigated by Walford
16:33Ecman in 1905. He was given the idea to
16:36investigate surface currents by the
16:37Arctic explorer Freechov Nansen. While
16:40on an expedition, Nanson noticed that
16:42windb blown sea ice did not move in the
16:45same direction as the wind. Instead, it
16:47moved 20 to 40° to the right of the
16:49wind. He correctly speculated that this
16:52was due to the influence of Earth's
16:54rotation. Ecman took Nansson's idea and
16:56built a mathematical model to explain
16:58it. Ecman's model treats water as a
17:00series of layers that move independently
17:03of each other. As each layer moves, the
17:06energy from that motion can transfer
17:08from layer to layer. Consider a small
17:11vertical column of water that starts at
17:13the surface and extends towards the
17:15bottom. As energy moves from the surface
17:17to deeper layers, two things happen.
17:20First, because some energy is lost in
17:22each transfer to a deeper layer, water
17:24speed diminishes quickly with depth.
17:26Second, as the movement is transferred
17:29deeper, Corola's effect deflects each
17:32layer to the right of the one above it.
17:34This creates a spiral pattern called an
17:36ecman spiral.
17:40One result of this deflection pattern is
17:42that at depth, a small amount of water
17:44is actually moving in the completely
17:46opposite direction than the wind that
17:48started the motion in the first place.
17:50Because water movement diminishes
17:52quickly with depth, Ecman showed that
17:54the net transfer of water is at a 90°
17:56angle to wind direction. Because this is
17:59all driven by the corololis effect, the
18:01deflection is 90° to the right in the
18:03northern hemisphere and 90° to the left
18:05in the southern hemisphere.
18:07This movement of water at right angles
18:09to the direction of the prevailing winds
18:11is called ecman transport. And
18:13consistent with Nansson's observations,
18:15the water right at the surface moves 20
18:18to 40° to the right or left of the wind
18:20direction. This variability is driven by
18:22differences in how long and how
18:24consistently the wind blows. The
18:26dynamics of ecman transport contribute
18:28to some important features in the ocean
18:30environment. In coastal areas where
18:32prevailing winds blow along the coast so
18:35that net water movement is offshore, the
18:37water pushed out to sea is replaced by
18:40deeper water causing upwelling. By the
18:43way, this is assuming you're in the
18:45northern hemisphere. So, if the wind,
18:47you're stationed here, and the wind is
18:48blowing towards you, that wind is going
18:51to slosh or move that surface water 90°
18:56to the right. So, it's like slloshing it
18:59away from the coastline as depicted
19:01here. That water is sloshed away and
19:03wants to be replaced. It's replaced by
19:06the cold, nutrient-rich bottom waters
19:08that come come up. So, this is called
19:10upwelling. We're going to get back to
19:12this concept more. The upwelling of this
19:15deeper water moves nutrients to the
19:17surface, making these regions of high
19:19productivity. Wind blowing in the
19:22opposite direction pushes surface water
19:24towards the coast. Again, the caveat is
19:26he should have mentioned we're assuming
19:28you're in the northern hemisphere. This
19:30would be opposite all opposite. It would
19:33move to the left 90° if you were in the
19:36southern hemisphere. That would cause
19:37upwelling if this were southern
19:38hemisphere. As water piles up at the
19:41shore, it is forced down creating
19:43downwelling. The concentration of
19:45relatively nutrient poor surface water
19:48makes these regions less productive.
19:50Ecman driven up and downwelling are not
19:52limited to coastal environments. In the
19:54open ocean, trade winds also cause Ecman
19:57transport. Along the equator, prevailing
19:59winds blow from east to west. This
20:01causes water to move away from the
20:03equator in both the northern and
20:05southern hemispheres, creating a region
20:07of diverging currents that lowers the
20:09water level right at the equator,
20:11causing deeper water to upwell. It's
20:14almost like we've heard the parting of
20:15the Red Sea along the equator because of
20:18the winds. They're slloshing water to
20:20the right 90 degrees and to the left 90
20:23degrees depending upon what hemisphere
20:25you're in shown nicely here. It's kind
20:27of like parting that water. That water
20:30those water molecules are going in
20:31either direction and want to be replaced
20:34with something replaced by the bottom
20:36waters. So we have upwelling. Upwelling
20:38does not have to occur at a coastline.
20:40It can happen here especially in the
20:42equatorial regions in mid latitudes
20:44around 30° north and south where
20:47prevailing winds transition from
20:48easterly to westerly water piles up as
20:51the easterly and westerly winds drive
20:53surface currents towards each other.
20:55This convergence causes downwelling just
20:57like along the coast. Upwelling along
20:59the equator brings nutrient-rich water
21:01to the surface stimulating primary
21:03productivity.
21:05the so again these are interesting
21:07because they're the convergence zones of
21:10wind right so that equatorial was the
21:12wind was blowing the uh in either
21:15direction leaving an opening right and
21:18that opening is replaced by upweld water
21:21but here in the depiction we have
21:23downwelling because your winds are
21:25coming towards each other piling stuff
21:28up and that water molecule likes to
21:31thanks to gravity head down so we've got
21:33downwelling
21:35piling up of water at mid latitudes
21:37contribute to the formation of gys
21:39that's so probably all new stuff to you
21:42incredibly important for surface water
21:45movement also explains upwelling and
21:47downwelling the ecman spiral so here we
21:50have the take-home messages we saw in
21:53that video the average movement of
21:54seawater under the influence is a
21:58deflection 90° to the right if you're in
22:01the northern hemisphere and 90° to the
22:03left in the southern hemisphere. That's
22:06the net transport. And again, if you're
22:08sitting on the sea surface here, are you
22:10moving 90°? No. We saw just as Nansen
22:13saw uh 20° to like 40° movement here of
22:17ice. Or if you're on your ship and the
22:19wind's blowing this way, you're going to
22:21be going that way if you're uh just free
22:25floating. So that last video alluded to
22:28the fact if you have this giring water,
22:31that motion actually piles up water
22:33molecules and raises the sea surface
22:36towards the middle of that gy. Those
22:39surface water molecules want to flow
22:42downhill because of gravity. Also, we
22:45get a deflection in the northern
22:47atmosphere to the right thanks to
22:48corololis. So we get a balance of
22:50downhill and to the right causing this
22:54geostrophic flow around a hill. This
22:57again is a complicated thing. So I think
22:59animation and a video is warranted here.
23:02Let's watch it. Water in the oceans is
23:04in constant motion. The major surface
23:06currents are driven by energy
23:08transferred from the wind by friction.
23:10The direction water travels is
23:12determined by how the transferred wind
23:14energy interacts with corololis
23:15deflection, other currents, and
23:17geological features such as continents
23:19and island arcs. A major feature of the
23:22current systems in all of Earth's large
23:24ocean basins is a central gy. These gys
23:29move water near the surface in large
23:31roughly circular patterns around the
23:33center of each ocean basin. Given the
23:35orientation of the wind in these areas
23:37relative to the direction the currents
23:39travel, it may look as if the wind
23:41directly creates the currents. But due
23:43to the influence of the earth's
23:45rotation, the process that forms gys is
23:48more complicated. This circulation
23:50pattern is an example of geostrophic
23:52flow which is a type of movement that
23:54occurs when the forces acting on objects
23:56are so weak relative to the influence of
23:58the rotation of the earth that corololis
24:01deflection is the factor that determines
24:03the direction of motion. To understand
24:05this, it helps to start with a more
24:07familiar situation. Consider a ball
24:09sitting at the top of an incin plane.
24:11The force of gravity pulling down on the
24:13ball will cause it to roll down the
24:15plane. In this situation, the force of
24:17gravity is strong and the motion of the
24:20ball is fast relative to other forces
24:22acting on the ball, allowing the ball to
24:24roll down the plane. As expected, the
24:26dynamics change if we alter the
24:28situation so that the slope of the plane
24:30is very shallow and the ball is tiny. As
24:32the slope of the plane and the mass of
24:34the ball decrease, the force of gravity
24:36acting on the ball becomes weaker. With
24:38a small enough ball and a shallow enough
24:41plane, Coriola's deflection becomes so
24:43influential that it overwhelms the
24:46orientation of gravity's pull down the
24:48slope and the ball will actually move
24:50across the plane instead of down it.
24:52This can create some seemingly
24:54counterintuitive behavior. If instead of
24:56a plane, the ball was on a small hill,
24:58it would roll around the hill instead of
25:00down it. This is geostrophic movement or
25:03as used in fluid dynamics, geostrophic
25:06flow. It is important to understand that
25:08for this type of motion to occur, the
25:10slope has to be extremely small and be
25:12extended over a very large area like for
25:14example a large portion of an ocean
25:16basin. While the ocean surface does look
25:19flat to the unaded eye, there are hills
25:21of water in the open ocean. These hills
25:23form at mid latitudes in both
25:25hemispheres where the low latitude
25:27easterly trade winds are replaced by
25:29westerlys at higher latitudes. The
25:31surface currents created by these winds
25:33are turned 90° by corololis deflection.
25:36The resulting ecman transport drives the
25:38formation of regions of convergence
25:41where water actually piles up forming
25:43small hills. These hills of water are
25:45only about a meter in height. But this
25:47change in elevation is enough to
25:50generate a difference in pressure across
25:51the basin with the elevated region in
25:53the center at a slightly higher pressure
25:55than the surrounding area. This creates
25:58a small pressure gradient across the
26:00basin. The pressure gradient acts like
26:02the hill in the example with the ball,
26:04generating a force that pushes water
26:06down the gradient away from the center
26:07of the hill. Since the elevation
26:09difference is small and is spread across
26:11a large distance, the pressure gradient
26:13force is weak, creating conditions for
26:16geostrophic flow to occur. So rather
26:18than moving down the gradient, water
26:20flows along lines of equal pressure
26:23around the hill instead of down it. To
26:26summarize, the formation of the large
26:28ocean gy starts with trade wind-driven
26:30Ecman transport piling water in the
26:33middle of the basins. This pile of water
26:35generates a pressure gradient that
26:37pushes the water back out away from the
26:39center of the pile. Since the pressure
26:41gradient is small and spread across a
26:42large distance, the flow down the
26:44pressure gradient is deflected by the
26:46rotation of the earth and the water
26:48actually flows around the pile instead
26:50of down it. This is a steadystate
26:52situation with energy from Ecman
26:54transport in balance with the force of
26:57pressure gradient pushing back out
26:59resulting in a stable circulation of
27:01water around the gy. Yeah, that's a lot.
27:04So, it brings in this issue that we saw
27:06with the Ecman spiral, some coriololis
27:08effect and it explains this gy
27:11formation. I just want to go back to
27:14this because we're going to see it. But
27:15if you remember the video here and
27:17you're like, "Oh, why aren't these
27:19perfectly ovalshaped? Why are the
27:22vectors on the western boundary seem to
27:25be clustering together rather than not
27:28clustering together?" Right? There's
27:30more space between these vectors on the
27:32eastern boundary currents. We're going
27:34to come back to that because of the
27:36geostrophic flow. we have a buildup on
27:39the western boundaries that these
27:41vectors are all sort of clustered
27:42together more. These are higher velocity
27:45surface currents. These are lower
27:48velocity meaning these western boundary
27:50currents move faster. These move slower.
27:53So if you put this into action like
27:55here's the Gulfream. Gulfream is a
27:58pretty fast flowing surface current.
28:01When you get over here and that eastern
28:03boundary current, that cold air or cold
28:05water coming back towards the equator,
28:07it's a slowm moving current. So, this is
28:10a great depiction and we'll see this in
28:12a moment. So, just looking at this, it
28:14is really complicated. So, here we have
28:16the link to the URL that that video was
28:20I just showed, but you might want to go
28:22back to that. And there's other
28:23resources as well. Don't get bogged down
28:25on all the minute details of this. I
28:28just want you to understand the large
28:30scale things that are driving
28:32this. Okay. So, I paused that video and
28:35I went back to notify you of that
28:37western buildup. The clustering of all
28:40those vectors means that the western
28:42currents heading to the north pole
28:44heading to the south pole are much
28:46faster. They're faster and faster and
28:49they're all clustered together. They're
28:51narrower. So, they're not broad
28:53currents, but they're narrower in
28:54compared to the eastern boundary
28:56currents. They're much deeper and since
28:59they're originating from the equatorial
29:01areas, they are warm water deliverers to
29:06the poles. So they're taking that warm
29:08water from the equatorial regions and
29:10moving it very quickly and deeply to the
29:13northern parts of the hemispheres either
29:16the north pole or the south pole.
29:17Counter to that we said wow look at the
29:19eastern boundary currents they are cold
29:22because they have just been towards the
29:24polar regions in both hemispheres. So
29:26they cool down. They're delivering cold,
29:28denser water via the uh surface
29:31currents. They're relatively slow in
29:34compared to the western boundary
29:35currents. They're shallower. They don't
29:38run as deep. And they're quite broad and
29:40wide. Warm ocean currents, warm air at
29:44the coast. And we have this example of
29:47the Gulf Stream that's delivering warm
29:50water, but that also influences our
29:53climate or daily weather conditions on
29:55the coastline. They deliver warm humid
29:57air. Humid climate is usually adjoining
30:01to the land
30:02mass. Cool ocean currents cool air to
30:05the coast. Think of California. Think of
30:07Britain, right? Those are cold eastern
30:09boundary currents that are offshore. You
30:12deliver cold water, but you also
30:14influence the air above. It's usually
30:16cool, dry air, and a drier climate to
30:18the adjoining land mass. So, ocean
30:21currents do inform or influence our
30:25climate. And and here we see it. We see
30:27the warm ocean currents that are moving
30:29from east to west, delivering warm
30:32equatorial water to the western current,
30:35the western boundary current, right?
30:37that then cools off and then you form
30:40your eastern boundary cool water coming
30:43back. So it's a circular motion of water
30:45being heated up cooling down almost like
30:48a convection cell that we saw in the
30:50atmosphere but here it's happening in
30:51the surface currents. Here we see
30:54something that we saw in that first
30:55video equatorial regions because of the
30:58wind going either direction they
31:01actually part some of that equatorial
31:03region. So water molecules are being
31:06pulled in either direction towards the
31:08poles and that leaves an absence of
31:10water. You can almost think of it as a
31:12little valley and then water wants to
31:14rush in to replace that. So you get
31:16upwelling of deeper bottom waters
31:18replacing that. Upwelling areas are
31:22usually nutrient-rich so they support a
31:24high biological productivity area.
31:28There are also areas that because of the
31:30winds pile up water into these little
31:33bumps and hills. Water then under the
31:37influence of gravity flows down. So
31:39that's called downwelling. Downwelling
31:42is not delivering any nutrients. It's
31:44just pushing that surface
31:45down. So we've already seen this issue
31:48of upwelling and downwelling. But I just
31:50want to make it really clear. In
31:51upwelling areas, we get a vertical
31:53movement of cold, very old,
31:56nutrient-rich water that's coming back
31:58up to the surface. Remember residence
32:00times, these are the water molecules
32:02that have been sequestered for thousands
32:05of years. But in the deep ocean, you can
32:08build up nutrients and then that cold
32:10rich water can come up in coastal areas
32:12or even non- coastal areas like the
32:14equatorial region. They support high
32:17biological productivity because there's
32:18an abundance of nutrients and then
32:21they're hitting the surface where
32:22there's uh sun. So sunlight plus
32:25nutrients. There we go. That's the
32:28recipe for high biological productivity.
32:30Which just means that phytolanton will
32:32be abundant. They have their food source
32:34and they have sunlight the energy
32:36source. Opposite that we saw that in
32:38some coastal areas or even open ocean,
32:41we could have downwelling this vertical
32:43movement of surface water. So, we saw
32:45this in the video, but always when you
32:47look at these images, make sure what
32:49hemisphere you're in because it makes a
32:51difference because you're going to use
32:52the Ecman spiral or Ecman transport
32:54theory to determine which way that
32:57surface water is moving. So, here,
32:59position yourself. Here's the coastline
33:01to your right. And here we are are on
33:03the west coast in the northern
33:05hemisphere. So, imagine northern
33:07California. That would be nice. And the
33:09wind is coming in this direction is
33:11blowing towards you. Remember, if the
33:13wind is coming in this direction, it's
33:15going to transport water 90° to the
33:18right. So, that's going to slosh water
33:21away from the coastline. Sloshing that
33:24water away from the coastline leaves
33:25kind of a water void. Think of it. It
33:27doesn't happen like movement of water.
33:29All doesn't rush away, but think of it
33:31as sloshing that water away. That water
33:34wants to be replaced. It's going to be
33:36replaced by cold, nutrient-rich bottom
33:38waters. This is coastal upwelling. We
33:41also have coastal downwelling. So here
33:43we are in the same coast, northern
33:45hemisphere, right? But the wind is
33:47coming in that direction. Remember that
33:49the Ecman spiral is going to shift the
33:52surface of the sea 90° to the right.
33:56That's going to slosh water in this case
33:59towards your coastline. It's going to
34:01build up that water. Water wants to go
34:03somewhere. It doesn't want to
34:04continually build up. It's going to be
34:06down well. So it's pushing those warmer
34:08surface waters down. Again, these maps
34:11can be tricky on a test. Sometimes I
34:14switch it from northern hemisphere to
34:16the southern hemisphere. I switch along
34:19switch the coastline as well. So, it
34:20gets tricky, but you just have to orient
34:22yourself what hemisphere I am. Remember
34:24the rules for the Ecman spiral and the
34:26Ecman transport and then take it from
34:28there. We seem to always forget about
34:31the Antarctic region. Here we have an
34:34Antarctic circulation. The surface
34:36waters are moving in a clockwise
34:40rotation around the Antarctic
34:43continent. So I just want to hit some of
34:46the interesting ones and this goes back
34:48to yeah chapter one when Benjamin
34:50Franklin realized that ships catching
34:52the Gulf Stream could reach Britain
34:54faster. So it's the best studied one. Uh
34:56you can have meanders or loops. This is
34:59a western boundary current. So it's
35:01warm. It's coming up from the equator.
35:03Water which has been warmed up pushing
35:05against the continent and driving it
35:07towards the northern part of the
35:09northern hemisphere here. But we saw
35:11this before in some of the LANCAT images
35:14just because you can get these weird
35:15undulations. You can encapsulate by eddy
35:19formation here, warm water eddies or
35:21even like close off cold water here and
35:24you get these cold
35:26rings. Here we have some LANCAT images.
35:28I showed this one before. Here's the
35:30warm Caribbean waters coming up. It's
35:33narrow. It's deep. It's fast. It's
35:34heading towards the North Atlantic,
35:36right? And that's going to form the
35:38northern boundary current when it
35:40reaches that area. And again, just an
35:42image that we showed before of the eddi
35:45formation happens all the time. And you
35:47can really see these eddies of warm and
35:48cold water. There's the cold water, cold
35:51water one. Okay, you've probably heard
35:54of El Nino. But before we get to what El
35:57Nino is, we need to know what a normal
36:00condition is. El Nino conditions are I
36:03would say abnormal, but they're the
36:05anti-normal condition. So normal
36:08conditions, we have air pressure across
36:10the equatorial Pacific being higher than
36:13the eastern Pacific. We have strong
36:15southeast trade winds. We have Pacific
36:17warm pools on the western side. We have
36:20a thermocline, remember that rapid
36:22change in temperature that's deeper on
36:24the western side. And we have upwelling
36:26off the coast of Peru. That's a lot, but
36:29those are the characteristics of the
36:30normal conditions. Let's look at a
36:33depiction of
36:36that. Okay, so here's our normal
36:39condition. Our normal condition is
36:41having what we call this walker
36:43circulation cell that's going from this
36:46is the Pacific going from the east
36:50towards the west side of the Pacific. So
36:52it's going to move that way. This is the
36:54northern hemisphere and a little bit of
36:56this other hemisphere here. So we've got
36:59this cell. The cell is unlike the
37:01atmospheric cells that were primarily
37:03going north south or south north that we
37:05talked about atmospherically the fereral
37:07the polar um cells that we talked about
37:11before. These are going in an east west
37:14direction. So we got this cell set up
37:16here. We have descending cold air that's
37:19creating a high pressure system here.
37:21The high pressure system likes to move
37:22to the low pressure system here. Notice
37:24the sea surfaces. Your wind is going
37:27this way and it's driving all
37:29that warm water over here. That warm
37:33water is concentrated here around
37:35Australia, Japan, Asia, continent right
37:38here. Okay. Warm uplifting air creates
37:42fair weather, rainy weather. High
37:44pressure systems creates nice dry
37:46conditions here. Look what's happening
37:48subsurface. This movement from high to
37:51low pressure, this green vector pushes
37:54all that warm water to the western side
37:56of the Pacific Ocean, leaving behind
38:00sort of absence of water on this side.
38:02It's actually piling up. You can look at
38:04elevation differences. You're piling up
38:06the water on this side of the Pacific,
38:08leaving a little void here. That void
38:10wants to replace itself with something.
38:12It's replaced by upwelling cold,
38:14nutrient-rich water. That's the normal
38:16system. So now that we know something
38:19about the normal condition, we're going
38:21to watch a video to just solidify our
38:23understanding of it. Let's look at at
38:25the abnormal phases. And one of the
38:27abnormal phases is called El Nino. It's
38:29a warmer phase. There's another one that
38:32we'll talk about called Leninia. It's a
38:34colder phase. But let's start with our
38:35normal conditions and see what happens.
38:38In El Nino, we have high pressure in the
38:40eastern Pacific and it weakens. Weaker
38:43trade winds. We get that warm pool
38:45migrating eastward. It was sloshed
38:47westward. Now it kind of migrates
38:50eastward changing the atmospheric
38:53conditions along the coast of the
38:55eastern the western um coastline of
38:58United States and South America. We have
39:01a destruction or a weakening of the
39:04thermaline. So it's deeper in the
39:06eastern Pacific. And we have
39:08downwelling. Downwelling means lower
39:10biological productivity. Corals are
39:13particularly sensitive to this warmer
39:15sea water. and you're going to do an
39:16activity looking at
39:19that. So here we have El Nino and look
39:22at the difference. It's almost like a
39:23reversal. Before we had a high pressure
39:25system here and a low pressure system
39:27here. Here it's reversed. High pressure
39:29system always moves towards the low
39:31pressure system. It's sloshing warmer
39:34water over here. This was colder water
39:36before in the normal phase. And because
39:39you're sloshing it over here, you're not
39:41creating a void and up uh upwelling will
39:44occur. you're actually just filling in
39:46more warmer water along the coastline.
39:49So you have a destruction or
39:51horizontalization of the thermocline. So
39:54go back and you're going to look at
39:55these two images and go, "Oh, I get the
39:57difference." Look at the weather. We
39:59said in a normal phase it was aid, dry,
40:01and sunny, right? Typical California or
40:04Mediterranean climate along these
40:06coasts, Central America and over here in
40:09South America. But now we get these
40:12under El Nino conditions, very wet,
40:14rainy, fair weather conditions. Okay, so
40:17I went back to this image which actually
40:19shows you the El Nino conditions and
40:21also the normal conditions. Let's take a
40:24look. Every few years, the El Nino
40:26phenomenon kicks into life in the
40:28Pacific Ocean around the equator. It can
40:31affect weather around the world,
40:33changing the odds of floods, drought,
40:36heat waves, and cold seasons for
40:38different regions. even raising global
40:41temperatures. But what is El Nino and
40:44how does it happen? Firstly, we need to
40:46know what's normally happening in the
40:48tropical Pacific. This vast stretch of
40:51ocean sees consistent winds called trade
40:54winds that blow from east to west. These
40:58winds push warm water near the surface
41:00in their direction of travel. So, the
41:02warm water piles up on the western side
41:05of the ocean around Asia and
41:07Australasia. On the other side of the
41:10ocean around South and Central America,
41:12as the warmer water gets pushed away
41:15from the coast, it's replaced by cold
41:17water which is pulled up from deeper
41:20down in the ocean, a process called
41:23upwelling. This creates a temperature
41:25difference across the tropical Pacific
41:28with warmer water piled up in the west
41:30and cooler water in the east. Warmer
41:33water adds extra heat to the air which
41:36causes the air to rise with more vigor.
41:38And it's this rising air that creates an
41:41area of more unsettled weather with more
41:43cloud and
41:44rainfall. That rising air in the west
41:48sets up atmospheric circulation across
41:51this part of the world with warm moist
41:53air rising on one side of the ocean and
41:56cooler drier air descending on the
41:58other. This circulation reinforces the
42:01easterly winds. So this part of the
42:04world sits in a self-perpetuating state
42:07until El Nino begins. If conditions are
42:11right, tropical Pacific weather systems
42:14or slow changes in the ocean around the
42:16equator can set off a chain of events
42:19which weaken or even reverse the usual
42:21trade winds. With weakened trade winds,
42:24there's less push of warm surface water
42:27to the western side of the ocean and
42:30less upwelling of cold water on the
42:33eastern side. This allows the usually
42:36colder parts of the ocean to warm,
42:38canceling out the normal temperature
42:40difference. Because the area of warmest
42:43water moves, so does the associated wet
42:46and unsettled weather. This changes
42:49rainfall patterns over the equatorial
42:51Pacific as well as the largecale wind
42:54patterns. It's this change in winds
42:57which has a knock-on effect changing
42:59temperature and rainfall in locations
43:02around the world. The main impacts are
43:05around the tropics where you see an
43:07increase in the risk of floods in Peru
43:10and droughts in Indonesia, India, and
43:12parts of Brazil. But virtually wherever
43:16you are in the world, El Nino has the
43:18potential to affect you directly via the
43:21weather or indirectly via socioeconomic
43:24impacts. There's another impact from El
43:27Nino which happens because of all the
43:29extra heat at the surface of the
43:31tropical Pacific. This releases vast
43:34amounts of energy into the atmosphere
43:36which can temporarily push up global
43:39temperatures. This is why El Nino years
43:42often feature among the warmest on
43:44record. Each El Nino event is different
43:48so the global impacts can change. You
43:51can find out more about the differing
43:53impacts of El Nino on our website. So I
43:56hope that video put my two figures and
43:59explanation and laundry list here into
44:01action and you understand it more. So
44:05really really important it is unperiodic
44:09or
44:11non-determinable presence El no events.
44:14So they sometimes can predict them by
44:16changes in sea surface temperature but
44:19they're erratic as we'll see
44:21momentarily. So I want to talk about the
44:23last one and this sort of looks like the
44:25normal condition map and it is. It's
44:28called leninia. It's called the cold
44:30phase and it's just a heightened normal
44:33phase where you get even more warm water
44:36slushed towards the western boundary of
44:38the Pacific Ocean. You get even more
44:41upwelling in the eastern portion here of
44:44the Pacific Ocean. So the western coasts
44:47of South America and North America. So
44:49it's a heightened normal phase here.
44:52Even more upwelling. So much colder
44:55temperatures here and uh but you still
44:58have this high pressure and uh
45:00Mediterranean climate here instead of
45:02wet conditions. So we can look at sea
45:04surface and in an activity you'll do you
45:07actually looking at coral reef impacts
45:09under an El Nino versus a normal phase.
45:14Um so sea surface temperatures again you
45:16can start to predict these and we have
45:18the ability to use satellite images to
45:20look at sea surface temperatures and
45:22either predict that an El Nino year is
45:24coming or a leninia event is coming as
45:26well. As I said these are irregular
45:29highly irregular highly unpredictable
45:31although scientists are getting better
45:33at using sea surface temperatures to
45:35look at small changes and to predict
45:37that larger changes. The phase usually
45:40lasts for 12 to 18 months and you can
45:42see the um the events here leading up to
45:45around uh 2020. So fairly recent
45:49data, the implications are global
45:52although we really concentrate on what's
45:54going to happen to the western United
45:56States weather under an El Nino. It's
45:58going to be much rainier. There's going
45:59to be mudslides. It's a worldwide event.
46:02And you can see here and read in your
46:04book that everything from m mar mar mar
46:06mar mar mar mar mar mar mar mar mar mar
46:06mar mar mar mar mar mar mar mar mar mar
46:06mar mar mar mar mar mar mar mar mar mar
46:06mar mar mar mar mar mar marine life can
46:07be affected coral reef damage you'll see
46:09that in the activity incidences of
46:11forest fires we've certainly seen that
46:13in California and British Columbia
46:16tropical storm events so this is not
46:18just a weather pattern and whether it's
46:21cool or warm weather off the coast of
46:23California or Central America it's it's
46:26globally influencing a lot of these
46:28things there is a video here and uh for
46:32the purposes of time I'm going to allow
46:34you to open up the PowerPoint and take a
46:36look at that but you can first
46:38understand these events normal El Nino
46:41and Leninia and then look at the
46:44effects I just want to kind of end here
46:47with some north pole and south pole
46:50implications of what's happening on the
46:52sea surface and the influence of
46:54currents um sea ice formation when ice
46:57forms under cold conditions it excludes
47:01the salt salt. So those ions in salty
47:03water are excluded. So as I said before
47:06that sea ice formation, if it happens
47:08slowly, it excludes the the salt and you
47:10can actually drink it because it's pure
47:12water. You get these needle-ike crystals
47:14becoming slush and slush becomes a
47:16discshaped pancake ice as it's called in
47:19calmer waters and then pancake ice
47:21coaleses to form ice flows. Here you can
47:24see some of the pancake ice name is
47:27apppropo here and some of these ice
47:31flows. The rate of formation depends
47:33obviously on the temperature. It's
47:35self-perpetuating and ice flows thicken
47:38and form thick pressure ridges and they
47:40actually sometimes cleave together and
47:43uh form these uplifting areas of sea
47:48ice. Iceberg formation. Icebergs can
47:50break off of glaciers. This happens a
47:52lot in Antarctica which is glacier
47:54covered. Uh we have floating bodies of
47:57ice and we uh quite different from sea
47:59ice. Active icebergs can cave off from
48:02western Greenland glaciers and they do
48:04Antarctica. They're carried by currents.
48:06So they're predictable uh with current
48:09satellite imagery of looking at the
48:12current and the direction of the sea
48:14surface currents. As I said, Antarctica,
48:17the glaciers are covering that
48:19continent, and the edges we've seen
48:21break off. They are sometimes enormous,
48:23like this image. And sometimes they are
48:25enormous. This is an aerial view of part
48:27of a large tabular Antarctic iceberg,
48:31and it is around the size of the state
48:34of Delaware sometimes. So, we've spent
48:37most of this video on surface currents.
48:41Don't forget those deep currents. They
48:43are below below the picnocline. So this
48:45rapid change in density and temperature
48:48it represents most of the ocean water
48:51because most of the ocean water is in
48:52the deep ocean. They're slow velocities.
48:55So they move very slowly. The movement
48:57is caused by differences in density.
48:59Like I said, temperature and salinity
49:02and as we know cooler seawater is denser
49:04and saltier seawater is denser. So we
49:06have this two-layer system of that cold
49:08salty seawater sitting upon the top
49:10surface waters. These two systems set
49:13off an enormous conveyor belt called the
49:16North Atlantic deep
49:19water. This is how surface currents end
49:22up subsiding and then becoming deep
49:25ocean currents. The cold surface water
49:28sinks at the polar regions. So in the
49:31red we have all the near surface or
49:33shallow currents and in the blue we have
49:35the deep cold currents happening. So
49:38much of this we were talking about
49:40what's driving these currents, the
49:42surface currents, but now we're going to
49:44take one last video look to determine
49:47this North Atlantic North Atlantic deep
49:50water circulation and also globally how
49:53this circulates. Let's take a look.
49:55Oxygen gets mixed into the ocean by the
49:58churning action of waves, currents, and
50:01tides on the surface. But it's the great
50:04ocean conveyor that takes oxygen to the
50:07ocean's deepest
50:09depths. The conveyor moves surface water
50:12warmed at the equator toward the poles.
50:16As the water cools, it becomes denser
50:18and sinks to the deep ocean, taking life
50:22sustaining oxygen with it.
50:26How do we turn off the conveyor? We
50:28simply remove the difference in
50:30temperature between the poles and the
50:32tropics. We warm the poles. We keep the
50:35tropics the same temperature. That
50:37conveyor which is driven by heat
50:40differences stops. When it stops, we
50:43lose oxygen on the bottom and we start
50:45the mechanism to mass extinction. Okay.
50:48So, I want to go back here. We've looked
50:50at these surface currents and these gys,
50:52right? The warm water cooling becoming
50:55warmer, right? Those were surface
50:56currents. What happens? And we're just
50:58going to take the example of the Gulf
51:00Stream taking all that warm, deep water
51:04up to the poles. Yes, we know it cools
51:07off. And part of it becomes the surface
51:09currents, the northern boundary and the
51:11eastern boundary returning to the
51:13equator. Those are surface currents. But
51:15part of what happens is when that cold
51:17water meets that colder pole
51:19environment, what happens? Well, you get
51:22sea ice. Sea ice formation enriches the
51:25bottom of that sea ice with even more
51:28salts. Those salts become dense. So as
51:31you exclude those salts from the ice
51:34pack, you enrich that water with saltier
51:37water. It becomes really, really dense.
51:39And now it flows to the bottom of the
51:42basin of the ocean, cold, nutrient-rich
51:46water. And as this video said, oh by the
51:48way, there's oxygen in there. So we can
51:50deliver oxygen globally through this
51:53network of deep water conveyor belts.
51:55It's a complicated system, but I just
51:57wanted to say we've studied most of this
51:59stuff in this chapter as a surface
52:01phenomenon, but those packets of water,
52:04which are surface currents, sometimes
52:06descend and become this complicated
52:09system of moving water beneath the sea
52:12surface in the deep ocean. So the deep
52:15ocean currents are cold. They're oxygen
52:17rich, as we saw in that video.
52:20Dissolved oxygen is really important for
52:22life and for some of the processes that
52:24are happening at the bottom of the
52:27ocean. Changes in the
52:29thermocline circulation can cause global
52:32climate change. And that video alluded
52:34to that. If we start to mess around with
52:37heightened increases of temperatures on
52:39Earth, we start to break down this
52:41conveyor belt that exists and imparts
52:44oxygen and nutrient-rich waters to the
52:46deep ocean.
52:48example here, warmer surface waters are
52:51less dense. They won't sink and their
52:54less oxygen will be in the deep ocean as
52:56a ramification of
52:59that. So, that concludes chapter 7. It
53:02was a whirlwind tour. Probably one of
53:04the most heady or meaty information-wise
53:08of the chapters we've hit so far with
53:10some really cool phenomenons that are
53:13happening to impart the direction and
53:15the speed of these surface currents and
53:18a little bit towards the end about the
53:20circulation in the deep ocean. I hope
53:23you enjoyed it. We'll see you in the
53:24next chapter.

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