GG 140: The Atmosphere, the Ocean, and Environmental Change

Lecture 22

 - Ocean Currents and Productivity


Ocean currents are generally divided into two categories: thermohaline currents and wind driven currents. Both types of currents are forced remotely rather than locally. Wind driven currents are initially forced by the wind stress causing water to pile up in certain locations. This produces a pressure gradient, which is then balanced by the Coriolis force and geostrophic currents develop. The gyre circulations found in the Atlantic and Pacific Oceans are wind driven currents. There is a connection between the physics of these currents and the biological productivity in the ocean. For example, productivity is greatest in areas of equatorial and coastal upwelling as nutrient rich deep water is brought to the sunlit surface.

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The Atmosphere, the Ocean, and Environmental Change

GG 140 - Lecture 22 - Ocean Currents and Productivity

Chapter 1: Ocean Currents [00:00:00]

Professor Ron Smith: We are right in the middle of a section on oceanography. We started about a week ago. And we started by talking about ocean basins. We talked about the salt in the sea, where that comes from, how it’s distributed.

And now we’re getting into ocean currents. And I began this subject last time, but I wanted–before I get into the subject again, I wanted to tell you what we’re trying to do, in terms of the way we organize—the way oceanographers organize their thinking about ocean currents. For the most part, we try to break them down into two broad categories–thermohaline and wind driven.

And you could define them, I guess, in the simplest way. Thermohaline would be all currents that are driven by the density differences in the ocean. Thermohaline is the name for that, because temperature and salt determine the density. And then gravity acts on those density differences to produce so-called thermohaline circulations.

And we can’t really do this experiment, but at least in the back of our minds, we can imagine that if we turned off all the forcing at the ocean surface that related to temperature and salt–for example, we shut off all the heat fluxes in and out of the ocean, and we shut off all the fresh water fluxes in and out of the ocean, the thermohaline circulation should cease. So at least that would be their definition. If we turned off that kind of forcing, then those circulations would stop.

Wind driven is driven by the wind stresses at the surface of the ocean. And there again, a way to define that, or perhaps an imaginary one would be, if you could magically turn off all the winds, then perhaps not at that instant, but over some days or weeks, all the wind driven currents in the oceans would cease.

Now, the thing to remember, though, about both of these types of currents, they’re not driven locally by those forces. In other words, you could have wind blowing from west to east, and have, at that same location, wind driven currents in the opposite direction.

For example, these large gyres are driven by the mean wind stresses, westerlies in mid-latitudes, easterlies in low latitudes. And there well may be, in fact, there are many places in the ocean where the local wind driven currents are opposite to the local winds. So don’t get confused by that. They’re still wind driven currents. If you turned off all the wind stresses, those currents would cease over time.

Now, the problem with this nice, clean categorization is that there are some ocean currents that seem to be driven by a mix of the two things. For example, the Gulf Stream, which I’ll be talking about today, is generally considered by oceanographers to be a wind driven current.

On the other hand, it carries warm water northward, some of which cools and sinks, and returns underneath in the deep ocean, towards the south as a colder current. So at that point, it seems to be more of a thermohaline current. So some fraction of the Gulf Stream, quote, “wind driven,” returns southward as a thermohaline current.

So there is an example where we can’t quite make that distinction as cleanly as we might like. We just have to live with that, though. And as our models of ocean currents become more sophisticated, we can try to apply those definitions more and more securely to understanding these differences. Any questions on that?

So I’ll just back up for a moment or two here, and remind you what we talked about with respect to thermohaline circulations. They’re driven by density. Typically, they tend to be slow currents, probably no more than maybe a couple of centimeters per second. Like about this. Maybe even slower in the ocean. So they’re quite slow. They do, however, extend, in some cases, right down to the bottom of the ocean.

As opposed to wind driven currents, which are usually in the upper kilometer, so the upper fifth or so, of the ocean column. They’re responsible for the water mass distributions. We talked about the ones in the Atlantic Ocean. That conveyor belt circulation between the Pacific and the Atlantic Ocean. And these estuaries, and inverse estuary circulations are all thermohaline circulations. We talked about that.

Stop me if you have any questions on these sections. We went over them before. That was a north-south section for temperature in the Atlantic, for salinity in the Atlantic.

Oh, I didn’t mention one thing that probably should be discussed. There was this blob of salty water–you’d expect the water in the tropics and subtropics to be salty, because there’s an excess of evaporation at the surface of the ocean. But what explains this blob of salty water is actually that outflow from the Mediterranean Sea.

That salty outflow from the Mediterranean Sea then moves westward across the Atlantic Ocean, and catches us in this north-south section. So this is–if that’s north and that’s south, then that’s west and that’s east. This is the Mediterranean water coming into this section from the east as an outflow from the Mediterranean.

So it flows over the sill there in the Straits of Gibraltar, flows down till it finds it’s appropriate density level, where the water above it is less dense, the water below it is more dense. And then it floats horizontally as a tongue of salty water, finally reaching that section. It’s part of the reason why the North Atlantic deep water is generally salty, because the Mediterranean water gets entrained into that North Atlantic deep water.

We talked about the oxygen. And this was the summary of the–in a north-south section of the Atlantic Ocean thermohaline circulations. Any questions on this? So this is the Antarctic intermediate water, the North Atlantic deep water, and the Antarctic bottom water. We talked about this.

Chapter 2: Wind Driven Currents [00:07:37]

OK. Now, I got into wind driven currents a bit. I want to review that as well. The chain of events, as we understand it today, is that the wind stress produces a shallow transport of water in the layer that feels that wind stress directly. That’s a pretty shallow layer, probably only 100 meters deep. And in that layer, the water gets pushed off to the right of the wind stress, in the Northern Hemisphere.

That Ekman layer transport piles up water. For example, in the middle of the North Atlantic gyre, there’s this piling up of Ekman layer drift. And that, in turn, that slight non-uniformity in the height of the ocean surface, generates pressure gradients beneath–horizontal pressure gradients–that then generate geostrophic ocean currents.

I mentioned that we can now measure this, and I’ll show you that plot again in just a moment. But it dawned on me that I forgot to mention that there were clues to this, even in the early days. For example, the center of the North Atlantic gyre is referred to as the Sargasso Sea. Does anyone know where that term, Sargasso Sea, comes from? It’s an old term. It goes back several hundred years.

If you take a ship across the Atlantic, through the middle of this gyre, when you get into that region of the ocean, you begin to see floating pockets of plant material. It’s mostly sargassum weed. It’s floating there. And the reason why it’s collected there is because of the Ekman currents pushing it there. So we had some clue that that was a region of convergence, even before we could measure that sea surface height anomaly from satellites.

If you dumped in–and now that there’s more pollution in the ocean, people have dumped loads of trash into the ocean that floats, some of that is found in the Sargasso Sea as well. It’s just an area where water tends to collect, and then, of course, sink eventually. But in the collecting, all the things that float remain kind of gathered there in the middle of the ocean.

We’ve also found these gatherings in the middle of the other ocean gyres. So there’s evidence that ocean water does collect there, and the things that float are still found there. So the fact that it’s called the Sargasso Sea gives another clue to this sequence of events.

So there’s the winds, and there’s the ocean topography, as measured from space. With units in centimeters, typically it’s only a topography of about like this. You would never notice it if you were sailing through there. But the water is slightly higher there than the geoid of the earth, there’s a surface you can imagine, nearly a sphere, which we consider to be the level surface. And this rises slightly above that level surface. And then, in turn, gives rise to the pressure gradients underneath that.

And these are the wind driven currents. I don’t want you to memorize all these names, except some of them probably should be known, and some of them are quite obvious, that it’s hard, once you’ve seen them, not to know them. For example, the Brazil Current runs down the coast of Brazil, the Peru Current runs up the coast of Peru, the California Current, the Alaska Current, the West Australian Current, the East Australian current. Pretty hard to forget those once you’ve seen them.

The Gulf Stream is part of the North Atlantic gyre. The word gyre is spelled G-Y-R-E, and it refers to these circular current systems. Forced to be circular because the continents prevent them from going all the way around the earth. Only in the southern ocean, where you get the Antarctic Circumpolar Current, do you get a current going all the way around the ocean. Otherwise, the continents break up these wind driven currents into gyres.

The North Atlantic Gyre consists of the Gulf Stream, the North Atlantic Drift, the Canary Current, and the North Equatorial Current. Some of the Gulf Stream, however, breaks off and becomes the North Atlantic Drift, which eventually goes into the Arctic Ocean.

You can have reverse currents in the very high latitudes, like the Alaska Current. Notice that goes opposite to the main North Pacific Gyre. And the Labrador Current, in a sense, goes a little bit opposite to the North Atlantic Gyre, as well.

The equivalent to the Gulf Stream in the Pacific Ocean is the Kuroshio Current. And there’s a lot of studies kind of comparing those two currents, because they play the equivalent role in the two large Northern Hemisphere basins. They’re both warm currents. And you can find equivalent things in the Southern Hemisphere as well. The three warm currents would be the Brazil Current, the Agulhas Current, and the East Australian Current. Each returning equatorially warmed water down into the high latitude colder areas.

Near the equator, things can get pretty complicated. You can have an equatorial counter current. And notice that this would be the region of the trade winds. Blowing from east to west. And yet, there’s the counter current, a wind driven current, I propose to you. And yet it’s running opposite to the local wind direction. So that’s one of those confusing points where the winds don’t always seem to be pushing the currents around locally.

But then again, that has to do with this indirect effect. It’s not just the wind blowing on the water to make it move. You’ve got that chain of events, the Ekman layer, the piling up of the water, and then the geostrophic currents. It’s not a direct, air pushes water, water moves scenario. It’s a little more complicated than that.

Another interesting fact, which is why we have to have this slightly more complex theory, is that these ocean currents, for the most part, do extend about a kilometer down into the ocean. They don’t extend to the bottom. But they extend about a kilometer down, and that’s far deeper than the wind stress is felt directly.

So in order to explain that, we have to have this more complicated theory. Then it’s the pressure gradients which extend down to about a kilometer below the surface that are generating these so-called wind driven currents.

When I say they’re geostrophically driven, or geostrophically balanced, I mean they have a balance between pressure gradient force and Coriolis force. Where does wind appear then in that? If it’s Coriolis force and pressure gradient force, does that mean wind is not active? No. Wind was important in setting up the pressure gradient force. So wind is still in there, even though it doesn’t appear directly in that force balance.

Any questions on this current map of the world? OK.

Chapter 3: Transport of Water in Ocean Currents [00:15:43]

Now, we try to estimate how much water is moving in each of these currents. Now, in some cases, it’s easy to do. For example, where the Antarctic Circumpolar Current passes through the Drake Passage, between the Antarctic peninsula and the southern tip of South America, that’s a well-defined channel, if you like. And so, we can measure with ships and so on, and current meters, we can measure how much water flows between there. And that would be a local measurement of the Antarctic Circumpolar Current.

In other places, for example, the Gulf Stream, where do you put the outer edge? OK, it does weaken as you go out, as I’ll show you in a minute. But it’s still a little bit ambiguous about where you mark the outer edge of the Gulf Stream. Nonetheless, we have come up with these estimates for–this isn’t inclusive, but it’s a few examples of estimates of how much discharge are in some of the major ocean currents around the world.

And we’re using the unit of Sverdrup here, which is named after a famous oceanographer. It’s not really an SI unit, but it’s closely related to one. It’s simply one million cubic meters per second. So it’s a volumetric discharge rate, just like we’ve been using in rivers. It’s a million cubic meters per second.

And one way to remember this, that if you add up all the discharge of rivers in the world, all the freshwater rivers dumping water into the ocean, the number is approximately one Sverdrup. It just turns out, by accident, to be that way. So it’s easy to remember. Summing up the Amazon, the Mississippi, all the major rivers, you get a number something like this. It’s a good comparison point for then looking at the discharge in the ocean currents.

The Florida Current, which is where the Gulf Stream is passing up along the coast of Florida, it’s only 30 Sverdrups there. But a few hundred miles north of there, where it’s just leaving Cape Hatteras and heading eastward, If you do a north-south section through it, it’s grown to 150 Sverdrups. One of the strongest currents in the world.

The Labrador Current, that cold current coming down through the Gulf of Maine and reaching, in some cases, even Massachusetts, has about eight Sverdrups. That Antarctic Circumpolar Current that I mentioned, at least where it passes through the Drake Passage, is 110 Sverdrups.

And the Kuroshio Current is 50 Sverdrups. Somewhere between these two estimates for the Gulf Stream. So these are just round numbers, but they’ll give you some sense for how much water is moving around in these ocean currents.

Chapter 4: Atlantic Ocean Circulation [00:18:49]

So let’s focus on the Atlantic Ocean for a few minutes. It’s complicated. The Antarctic Circumpolar Current flows along at the southern edge of it. Some cold water gets peeled off and put northwards into the Benguela Current. And we’ve talked about the role that this place plays in keeping the south equatorial Atlantic rather cool.

Very few hurricanes form in here. And the ITCZ never really shifts south of the equator in this ocean. And that’s because of the cold water being brought up by the Benguela Current. There is a South Pacific—sorry a South Atlantic Gyre. And it consists of the South Equatorial Current, the Brazil Current, the South Atlantic Current, and then the return in the Benguela Current.

The equatorial region. The equator is here. It’s a little complicated. There are some counter-currents. So let’s shift up into the Northern Hemisphere, where I’ll spend most of my time here.

And that is, you’ve got another gyre, a subtropical gyre, consisting of the Gulf Stream. Some would call this the Gulf Stream extension, or the North Atlantic Current. It returns in the Canary Current and then the North Equatorial Current there. Some of this enters into the Caribbean Sea. Then between Mexico and Cuba, it flows up into the Gulf of Mexico. Then comes out right along the coast of Florida. And that’s the number I quoted you for the Florida Current. But then additional water joins it, and by the time it gets out here, it has more total transport than it had as it moved up along the coast of Florida.

So that’s–oh, then you’ve got the Labrador Current up here. Water coming down out of the Arctic Ocean moves along the east coast of Greenland, back up along the west coast and Baffin Bay, and then becomes the Labrador Current, then comes down here. And that’s a cold current.

So the Gulf Stream was discovered, well it was known, in a way, from the earliest sailing expeditions across the Atlantic Ocean. But actually, it was Ben Franklin who did the first scientific study of it. By looking at ship’s logs. He, himself, took that trip a couple of times when he was posted as being ambassador to the French court. And he would take observations as he traveled across.

And this is his map from approximately 1769. We’ll compare that with what we know today about it. But he’s got the basic idea right. It comes up along the coast of Florida to the Cape Hatteras, and then seems to leave the coast and move out towards the east. And doesn’t come that close to us here in Connecticut, we’re right there. He doesn’t show anything splitting off to go up north, the way the previous diagram did.

Here’s a satellite image. One really neat way to look at this is just to take an infrared, a thermal infrared image of the ocean. Play some tricks to get rid of the clouds, and then interpret the intensity of that radiation as the temperature of the emitting object. Remember the Stefan-Boltzmann law–the warmer an object is, the more it radiates.

So that’s what’s done here. A color code has been applied. The red is areas that are emitting strongly in the thermal infrared. The blue are areas that are emitting less strongly. And then that’s directly related to the temperature of the sea surface. So this is sea surface temperature.

Warm water in the tropics. And then this little stream of very warm water coming up here. It’s getting mixed in a little bit, but it continues as a warm water current, until it leaves Cape Hatteras. We see it continuing, but as soon as it leaves, it begins to meander. And you form eddies on both sides. Cold eddies poking through from west to east. Warm eddies from east to west. And it begins to mix in.

However, you can still see remnants of it further across the ocean. And it still has a pretty good speed there, but it’s losing some of its thermal characteristic as it mixes in with the environment. So that’s a good view. But remember, this is going to be changing with time.

So if you were to take another image a week or so later–if you took it just a day or so later, it would look a lot like this. But if you took—if you waited a week and took another image of it, those eddies would have moved around quite a bit, and new ones would have formed. And it would look quite different.

Here’s a zoom in on another date. Again, the trick is the same. We’re looking at sea surface temperature derived from emitted long wave radiation. And here’s Delaware Bay, Long Island Sound, and there’s just a little bit of the tip of Cape Hatteras. So you see the warm water coming up here. This may be a little bit of the Labrador Current coming down in here. And then you’ve got the mid-ocean, the Sargasso Sea out here in the middle.

A quote from one of the first books to lay down a description of this, was a book by Matthew Fontaine Maury in 1855. Where he was working for the Navy and he put together–gathered together information from ships’ logs. From all the US Navy ships. And wrote this rather remarkable book called The Physical Geography of the Sea.

And here’s a quote from that book. “No feature of the Gulf Stream excites remarks among seamen more frequently than the sharpness of its edges, particularly along its inner borders.” He’s referring to this edge here. When you sail across that, you really notice that boundary. In part, because of what he says.

The color of the water changes from the bright indigo of the tropical to the dirty green of the litoral. So you’re getting–this water is basically a clear tropical light blue, but the water back here is rather dirty and green. And you see that boundary very, very sharply as you sail across it.

There are other things, though, that a sailor would notice as well. There tends to be cumulus clouds often over the warm Gulf Stream, but not over the cold water. That’s to be expected, because the warm water would destabilize the atmospheric boundary layer, and cause thermal convection. So when you’re approaching that, you can often see it at quite some distance, because you see cumulus clouds are building up over the Gulf Stream Questions?

Now, what about this Gulf of Mexico thing, that’s rather interesting. It’s called the Loop Current. Remember, so the–as part of the North Atlantic Gyre, you’ve got water pushing into the Caribbean Sea, then coming up into the Gulf of Mexico in a well-defined current. But that current is not steady. It changes from week to week, and it goes through a cycle. A rather interesting cycle.

For a few weeks, it’ll seem to turn directly eastward and end up in the Florida Current. Then that loop seems to elongate. Then that will break off, it will form a clockwise eddy, which will then drift off. And the shortcut is then reestablished. It’s more like this. That eddy will eventually drift into the western part of the Gulf of Mexico, and the cycle will then continue. So it’s an unsteady current going through this cyclic behavior of taking a shortcut, extending in a long path, and then breaking off into a new eddy.

And you can see that in satellite images. And here’s a sequence of satellite images. And the dates are given here. They’re from 1998. But there’s the loop. And then as it drifts westward, it weakens. The new direct link is established there to get water into the Florida current. So depending when you’re down there, you’re going to find either a lot of warm water here or not, depending where you are in the phase of this cyclic alteration.

Chapter 5: Pacific Ocean Circulation [00:28:06]

OK. Well, I’m not going to go into the Pacific Ocean currents nearly as deeply. If you want to take a course in this, you could take a course in physical oceanography and learn about each of these ocean basins in a more complete way. But I will say that they–there are certain parallels.

You’ve got a North Pacific Gyre, just like you had a North Atlantic Gyre. A South Pacific Gyre, just as you had a South Atlanta Gyre. Here, the current is the Peru Current, sometimes called the Humboldt Current. And then the South Equatorial Current, the East Australian Current, and then it returns here. The one surprise, perhaps, is this Alaska Current.

So you get the current coming across here from the Kuroshio, that then splits. Some of it comes down the California coast, and some of it returns in the other direction, back across the northern stretches of the Pacific, and returns into the Kuroshio that way.

Chapter 6: Southern Ocean Circulation [00:29:13]

The South–the Southern Ocean has its own current system, that’s the Antarctic Circumpolar Current, sometimes called the west wind drift. It pretty much goes all the way around Antarctica.

Chapter 7: Arctic Ocean Circulation [00:29:31]

And in the Arctic Ocean, you’ve got a well-defined gyre, called the Beaufort Gyre. And then, as part of that, or consistent with that, is a current that runs kind of across the basin, from one side to the other, called the Transpolar Drift.

Now, there is water leaking in and leaking out of the Arctic Ocean. I’ve already mentioned that the Gulf Stream extension will come up here. And water then will enter the ocean—the Arctic Ocean, stay there for several years, presumably. And then some of it will leak out on the East Greenland Current and come out that way. There’s also some exchange between Alaska and the Soviet Union. There’s some exchange of water there as well. Between the Pacific Ocean and the Arctic Ocean.

One the interesting stories about this Transpolar Drift is the trick that Fridtjof Nansen tried. Now, you already know that name, because he invented the Nansen bottle. But this was really his claim to fame. He wanted to be the first one to reach the North Pole. And he figured, why should he have to work at it? Why shouldn’t he let nature do the work for him?

He already had some evidence that Transpolar Current existed. So he said, if I can just get my ship locked into the ice about here, it should carry me close to the Arctic, to the North Pole. And he did that. He left the ship here and tried to do the rest of it by land–not by land, but over the ice. Couldn’t make it, and ended up having to travel on the ice all the way back to here, where he finally got back to civilization. While the ship continued to move like this.

So it was a good idea, but it didn’t quite get him close enough to allow him to reach the North Pole in that way. But it’s a great adventure book. If you want to read about one of the great adventure stories in human history, read about Nansen’s attempt to get to the North Pole using that Tranpolar Drift. Questions here?

Chapter 8: Primary Productivity in the Ocean [00:31:45]

OK. Switching gears slightly, we’re interested in the relationship between physical oceanography, that we’ve been talking about so far, and biological oceanography. In particular, what is it that controls productivity in the world ocean? By this I mean biological productivity.

Now the way a biologist approaches this, is first understand the food chain. What feeds on what? And I’ll show you that in just a minute. But the most basic level, where the food chain begins, is called primary productivity. And it’s usually referring to plant life in the ocean.

Plant life requires sunlight and nutrients. You need to get those two things together in order to get the primary productivity. And then the rest of the food chain builds on that. And as I’ll be showing you, there are vast parts of the ocean that are devoid of any primary productivity. And for that reason, largely devoid of any biological activity at all.

But there are other regions in which biological activity is very, very active. And if I had to categorize them, and I’m going to try to categorize them, I would put them into those four categories. There’s a lot of primary productivity in high latitudes, in regions of coastal upwelling, in regions of equatorial upwelling, and in regions of river outflow. And for the most part, the rest of the ocean is very limited, if not completely devoid of primary productivity.

So in the next few minutes, we’re going to talk about this kind of way of understanding productivity in the oceans. So here’s the typical food chain. You start with single-celled life that are plants. And they require sunlight and nutrients, especially phosphorus and nitrogen. It’s a carbon-based life, which there’s plenty of carbon available. So it usually depends on the phosphorus and the nitrogen that allows that to get started.

These organisms get eaten by small animals, so-called zooplankton. These are so-called phytoplankton. They get eaten by zooplankton. And then, in turn, eaten by small fish, larger fish, larger fish, and so on. So that’s the base of the food chain. That’s the primary productivity there.

Now, here’s the great catch 22 for ocean productivity. You need light from the sun. But light is absorbed rather quickly as you go down into the ocean. The region that has lots of light in the ocean, the upper 50 or 100 meters is referred to as the euphotic zone. E-U-P-H-O-T-I-C. The euphotic zone. There’s a layer beneath that that gets a little bit of light. And then, however, most, the vast majority of the ocean depth is dark. Because the sunlight doesn’t penetrate that far. That’s the aphotic zone. Aphotic meaning no light.

On the other hand, if you look at the concentration of nutrients in the ocean–and here, it’s plotted versus depth in kilometers. Sea surface, one kilometer, two kilometers. Most of the nutrients, phosphorus and nitrogen, are in the lower parts of the ocean.

And in the upper parts of the ocean, where you have the light, you have depleted nutrients. So here’s that catch 22. Where you’ve got light, you don’t have the nutrients. Where you’ve got nutrients, you don’t have the light. That’s the great brake upon ocean productivity.

To get ocean productivity, you’ve got to violate this general anti-correlation, and get the two together. You can’t get the sunlight deeper, so the only way to do that is to get some of these nutrients up to the surface. And that’ll be the focus of our discussion for primary productivity. How can we get some of these nutrients up to the surface?

Now, why is it like this? Well, it’s because you’ve had some blooms of phytoplankton. They’ve depleted the nutrients. And then their bodies will fall gravitationally into the lower ocean, and then rot. And the nutrients are released again, but because of gravity, they’re released in the lower parts of the ocean. And once that has happened, then you have that depletion. And the, at least in this simple set of plots, the productivity would shut down. You need to do something to reestablish concentrations of nutrients in the surface waters of the ocean in order to get the primary productivity reestablished.

This is one of my favorite diagrams. It comes from a satellite in orbit called SeaWiFS, which looks down at the surface of the ocean. And by using two channels, two reflected channels, it maps out the concentration of chlorophyll–chlorophyll a in this case–in the surface waters of the ocean. So in a way, it’s detecting those phytoplankton directly, because they have chlorophyll in their bodies.

And so, here we have a global map of the primary productivity in the ocean. It’s a remarkable thing. And the scale here, it’s a log scale, the units are milligrams of chlorophyll per cubic meter of ocean water. Going from 60 units on down to 0.01 on a log scale. So the purple areas are biological deserts. There’s really very little primary productivity going on in these areas.

We can imagine why that is, because remember, that warm water lens extends from there to there. And so, you’ve got this layer of warm water floating on the sea, very strong thermocline beneath it, which, just like an inversion in the atmosphere, makes it very difficult to mix water vertically. So while you probably have a lot of nutrients just a few hundred meters down, because of that static stability in the thermocline, you can’t mix it to the surface.

So let’s talk about the high latitudes. I’ll go through and check off the four items I had there. High latitudes are generally high productivity. Well, that’s because that warm water lens doesn’t extend to high latitudes. You’ve got cold water at the surface. You’ve got cold water at depth. You don’t have a strong thermocline in the high latitudes. You do have, also, strong storms in the atmosphere, putting winds on the surface of the ocean, tending to mix things.

So it’s pretty easy to get water to mix up and down when you’re in the high latitudes. And that’s why a lot of the major world fisheries are found in high latitudes, because you can get primary productivity, because you can mix up the nutrient-rich waters into the euphotic zone and get those phytoplankton growing.

A second example would be–let’s see, go back to the order that I had them here. Coastal upwelling. There’s a lot of upwelling, a lot of productivity, for example, along the Peru coast. And that, we believe, is due to coastal upwelling. Basically, you have winds putting a wind stress on the ocean. And the Ekman drift that responds from that is pushing water offshore. So if you’re pushing surface waters away from the shore, then that must be replaced by waters coming up from the deep.

So any time you’re–for example, if the wind was like this, the Ekman drift would be perpendicular to that, pushing water offshore and allowing nutrient-rich deep waters to come up to the surface, giving you high productivity along that coastline. And you find at other places as well. Coastlines tend to be highly productive because of the upwelling and the mixing that can occur there.

Equatorial upwelling. I think this is the most remarkable aspect of this diagram, is this little line. And some other productive areas around it, but especially that little line right at the geographical equator. That’s not at the ITCZ. That’s not approximately at the equator. That is at zero degrees latitude. There’s a highly productive region right at zero degrees latitude. And why would that be? Why would there be a highly productive region right at zero degrees latitude?


Student: A lot of sunlight?

Professor Ron Smith: A lot of sunlight. But you know, the sun–remember, because of the tilt of the earth, there are seasons of the year when you get more sunlight here than there. And more sunlight here than there, in Southern Hemisphere summer. And really, that begs the question of why it’s so narrow. Really, why is it right at the, right at the geographical equator? What happens, what’s special about the geographical equator? Yeah?

Student: There’s no Coriolis Force?

Professor Ron Smith: No Coriolis force.

And along with that, it’s where the Coriolis force switches sign, from being to the right, to the left. That’s what’s unique about the equator. And I think I have a diagram for that. In fact, I have two diagrams. So, for coastal upwelling, if that’s the continent and the wind is like this, it pushes the Ekman drift offshore, it must be replaced by upwelling. Here’s the diagram for the equatorial upwelling.

Let’s say the ITCZ is located somewhere north of the equator. So the winds crossing the equator are the southeast trade winds. The southeast trade winds. So the wind stress is along the blue arrow. When you’re in the Southern Hemisphere, the Ekman drift is to the left of the wind stress. When you’re in the Northern Hemisphere, the Ekman drift is to the right of the wind stress.

So now, draw a section like this. That is pulling water northwards away from the equator. This is pulling water southwards away from the equator. That water must then be replaced by upwelling. So imagine you’re in a bathtub, you put your hands like this, you push water away. Well, that water’s going to be replaced from beneath. And that’s what’s happening right along the equator to give equatorial upwelling.

Any questions on that general point? Did I cover them all, by the way? Let’s see here. River outflow. Well, if you look at, for example, where the Amazon River comes in, where the Mississippi comes in, where some of the other major rivers come in, you see little red regions. That’s because nutrients are brought in from the land. The rain has fallen on the land, moved through the soil, picked up nutrients. And then, where that passes out into the ocean, you’ve brought nutrients into the euphotic zone in that way.

So that pretty much covers it. Why you have high productivity regions in some areas, and low productivity regions in other areas. Any questions on this? This, of course, controls where are the major fisheries.

Now, it’s a little sad to notice that we pretty much exploited the world’s ocean productivity. In the blue is the total marine fish catch from 1950, in units of millions of metric tons. And it’s been increasing from the 1950s. But then, about 1990 onwards, it’s pretty much plateaued. In other words, we’re taking about as much edible biomass out of the oceans as the oceans can produce. So the human population has grown to the state that it’s pretty much fully utilizing everything that the ocean is producing in these high productivity zones.

I wanted to just link this curious pattern of productivity to one famous book, and that’s Moby Dick. You probably have all read that. And Captain Ahab was hunting for a sperm whale. Sperm whales are found in all oceans of the world. But occasionally, the males will go down and join the females on the breeding grounds close to the equator. And when you read that book, you may have wondered, what’s special about the equator? Well, now you know. It’s this equatorial upwelling that gives rise to a high productivity region right on the geographical equator.

And then if you read a quote from the book, “You meet them on the line.” Right? By “on the line,” Herman Melville was referring to the equator. “Just having returned, perhaps, from spending the summer in the northern seas, cheating summer of all its unpleasant weariness and warmth. By the time the have lounged up and down the promenade…”

OK. I won’t go through this in detail, but the point is, that it was known by the whalers also that you had highly productive regions in the high latitudes. But also, an additional high productive region right along the equator. They didn’t know it was because of a shift in the Ekman layer, but we know that now, that you have a zone there of high productivity. Questions on that?

OK. Well, we finished up the basic section on oceanography. Next, on Wednesday, I’m going to be talking about El Nino, which you can read about in the book. And we’ll talk about this particular interaction between the atmosphere and the ocean.

[end of transcript]

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