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0:01
I'm going to talk about the role of the oceans in climate. This is a beautiful image of clouds over the Pacific. And it just reminds us that there's a lot of ocean on the planet. 70% percent of the ocean-- of the planet is covered by the ocean. And the ocean's important in climate for at least three reasons. The first is that it's a big buffer of climate change. So it's exchanging heat and carbon with the atmosphere. But he's got a huge heat capacity. And so it's buffering climate change.
0:37
If it weren't for the ocean, we'd be warming up a lot faster than we are at the moment. The second thing is that's water can, of course, change state and become ice. Ice is very reflective. And so it plays a big role in the planetary albedo. Which is regulating the global temperatures of the Earth. And the third thing is that the oceans are fluid. It moves around and its carrying properties around the globe. And, particularly, carrying heat around the globe. Now one of the things you can notice
1:16
here is that there is band of cloud north of the equator. This is known as the Intertropical Convergence Zone. It's the place where we get a maximum in tropical precipitation. David McGee showed us a picture of this. And it's actually north of the equator in the annual mean. And then the ocean plays an important role in setting this interhemispheric asymmetry in climate. So I'm going to spend a bit of time talking about the way the ocean sets up regional large-scale patterns of climate.
1:57
So I'm going to start by talking about the oceans and asymmetry of climate. Noting the northern hemisphere is warmer than the southern hemisphere in the current climate. So this shows the asymmetric temperature distribution. This is the component of temperature which is asymmetric about the equator based on observations. So this is the equator. And we're going up in the atmosphere here. This is like 10, 15 miles up in the atmosphere. This is at the surface. This is the atmosphere.
2:36
And this is the ocean. So you can see that the northern hemisphere troposphere is 2 or 3 degrees warmer than the southern hemisphere troposphere. And the northern hemisphere ocean is warmer-- slightly warmer-- than the southern hemisphere ocean. And what we think is happening-- what we know is happening-- is that the ocean is actually carrying heat from the southern hemisphere and moving into the northern hemisphere. So that warms up the northern hemisphere troposphere.
3:09
So the ocean carries heat across the equator, warming the northern hemisphere.
3:18
Another big asymmetry of climate is that there's much more ice over Antarctica than there is over the Arctic. So there's 2 miles of ice around Antarctica. Whereas, the Arctic ice is much less extensive, and we think might be quite vulnerable as climate changes. I think we should worry about Antarctica losing all its ice. It's not going to happen anytime soon. Big chunks of ice might break off and drop into the ocean. But it's going to be there. Now, one of the roles that the ocean plays
3:55
is that it thermally isolates Antarctica from the rest of the climate system. But because of it there's a circumpolar current moving around the southern hemisphere through Drake Passage. And that thermally isolates the Antarctic, enabling ice to build up. So that's another important role that the ocean plays. Third aspect is the Intertropical Convergence Zone. This band of excess precipitat-- this band of precipitation is slightly north of the equator. This horizontal line is the equator.
4:36
And this is the annual mean precipitation in millimeters per day. It's slightly north of the equator. So the axis of symmetry of climate is not at the equator. It's slightly north. And if we go into the atmosphere here. This is the equator moving up into the atmosphere. These are the Hadley cells. And this rising air in this convergence zone, which leads to all the precipitation. And the Hadley cell is no-- the rising is north of the equator. And it moves southwards.
5:12
And this cell actually carries energy from the northern hemisphere into the southern hemisphere. So the atmosphere is trying to equilibrate it's hemispheric temperatures. And it does that by shifting the axis from the equator north.
5:32
Another important asymmetry is that the Atlantic sector is warmer and saltier than the Pacific. So here, this is the surface salinity. A departure from the zonal mean. And the yellow areas are places where-- tell us that the Atlantic is saltier than the Pacific. And this has a really important part of-- this is a really important part of today's climate. Because salty water is heavier, more dense, than fresh water. And so you get, in the ocean, a big convective overturning cell.
6:10
Which we know it, the ocean conveyor belt. In which we get sinking. This is a section through the ocean. This is the equator north-south. This is the top few kilometers of the ocean. We get a great big overturning cell emanating from the North Atlantic, which carries warm water across the equator. And we get cooling here, and it returns to the depth. This carries energy from the southern hemisphere into the northern hemisphere. And plays a role in this asymmetry
6:50
So these gross large-scale interhemispheric and zonal asymmetries of climate. The ocean is playing a critical role and in these patterns. So what I want to do is-- you may think this is a bit of a bizarre thing to do. But I am an oceanographer. So what I'm going to do is actually try to build a cartoon of the climate system. I'm going to try and build a system which starts very simply. And I put in a little bit more complexity until I get something that looks quite a bit like the real climate.
7:26
And I'm going to start off but just having-- thinking about a world in which there were no land at all. So I'm going to look at the climates of water worlds. And what would the climate of the Earth be like if there were no land around? So here is this world. There's no land. There's an atmosphere, an ocean, the possibility of ice. But no land. And that's a simple system that you can think about. But one of the things that we see-- one of the things about the ocean is that it's in ocean basins.
8:08
So it's constrained, geometrically, by coasts. So the ocean can't move in arbitrary paths. There are geometrical and rotational constraints on ocean circulation. Rotation meaning earth's rotation. So we can put this kind of information into this system by introducing barriers. So I'm going to consider an aquaplanet which is a pure aquaplanet. It's an ocean which is a few kilometers deep, with a flat bottom and no land. And then I can introduce a ridge.
8:45
Which is a barrier going from the North Pole to the South Pole. Look to see how the climate changes if I stick a ridge into the system. And then I can open up this is-- I can open up a Drake Passage by putting a gap in the barrier. Or I can have-- this is my most realistic simulation. I have a double Drake, which is-- I actually get something that looks quite a lot like the real world. I have a double Drake, which is a small basin, i.e. the Atlantic. And a large basin, which is the Pacific.
9:24
So I'm going to do this numerically. I'm going to explore this with a series of numerical simulations of these highly idealized water worlds. And see if I can build up something that looks a bit like the real world. So here's my aquaplanet simulation. On the left we have the temperature at 500 millibars in the atmosphere, which is about 5 miles up. The red areas are warm, the cold-- the blue areas, cold. These are our weather systems and synoptic systems in the numerical simulation.
9:59
Warm air tends to be moister than cold air, so the whites are the humidity and cloudiness in the atmosphere. So we have more moisture in the tropics, less over the poles. And we integrate the system out for 5,000 years. It's blowing over the ocean and the ocean's moving around. And we're getting a coupled simulation. So this is day 300. We have to integrate a long time because the ocean's a bit like an elephant. It kind of remembers things for a long time.
10:34
It takes thousands of years to mix itself up. So in the pure aqua planet, if we do this simulation. We end up with two ice caps. One over the northern hemisphere and one over the southern hemisphere. This is sea surface temperature in the climatology of this simulation. I've got an ice cap here and an ice cap there on my pure aquaplanet. And the reason for this is that the ocean-- because the Earth's spinning around, there are angular momentum constraints on it.
11:08
So the ocean kind of moves along in zonal currents. And it can't get energy to the poles very easily. And so we get these big ice caps. But now, if I put a ridge in the system that connects one pole to the other, what do you think happens? The ridge is an assist, and it enables the ocean to move energy to the poles. And so what happens is that we get no ice caps in this system. So the ice goes away in this system in which we have a barrier going one pole to the other.
11:51
Well, what happens in the Drake simulation? Can you imagine what might happen? You get an Antarctic ice cap, and no Arctic ice cap. So in this way, we're able to build up this very important interhemispheric asymmetry in climate. I.e, the southern hemisphere it colder than the northern hemisphere. And there's a great big ice cap over Antarctica and no ice cap in this particular simulation with the Drake. You can go further and look, again, at the Drake minus Ridge to get the-- and look
12:32
at interhemispheric asymmetry in temperature. And this is the asymmetric temperature distribution. It's warm in the northern hemisphere and cold in the southern hemisphere. And what the ocean's doing in this Drake minus Ridge is it's carrying energy. The ocean is carrying energy from the southern hemisphere into the northern hemisphere. And it's actually carrying it up a gradient. It's carrying it from cold places to warm places. The reason they can do this is that it's mechanically forced.
13:05
It's not working like a heat engine. It's mechanically forced, and so it can actually carry energy up the heat-- up the temperature gradient. And the atmosphere is moving energy from the northern hemisphere into the southern hemisphere. And it does that by shifting the ITCZ north of the equator. I don't have time to show you this. But the Intertropical Convergence Zone, this band of cloud and excess precipitation, is actually north of the equator in this model.
13:37
And if I compare that with the-- if I compare that with the observations, there are some broad similarities between the two.
13:48
So now the high end of this hierarchy of models is this double Drake. So now I do the same kind of calculation, but I have a small basin and a large basin. And so what happens now is very interesting. So what happens is-- so this is the moisture field in this simulation. And what happens is that the ocean tickles the atmosphere. So you get more weather systems being formed over the small basin. Because there are sea surface temperature normalities associated with the boundary currents.
14:30
So the storm track in the atmosphere gets organized. You get evaporation of water from the ocean in the small basin. And it carries it zonally across and dumps it as water over the large basin. So the large basin gets fresh, and the small basin gets salty. And what we then see is that the salty water is denser than freshwater. And so you get convection in the ocean. And an overturning cell, this conveyor belt, which ends up being confined to the small basin.
15:09
And so you get something that looks really quite a lot like the-- mean if you kind of take a bird's eye view of this. It looks quite a bit like the present climate.
15:22
And you can go further and look at the heat and fresh water transporting in the system. So this is the meridional energy transport as a function of latitude. This is the equator moving northwards. This is in the small basin. So this is the transport of energy across the equator from the South to North, which is in petawatts. This is an order 10 to the 15 watts per meter sq-- watts going across the equator. This is the large basin, this is the freshwater transport.
15:56
I don't have time to go into these details. But now, if I compare this to the observations. This is the actual observed transport from data, or inferred from observations. There's a striking similarity between the two. Which tells us that, really-- the land doesn't really matter in this regard. Actually, does matter to some degree. But we've been able to, in a cartoon-like fashion, we can construct a world that looks a little bit like our present, by just having a few sticks in the ocean.
16:34
Now, this aquaplanet exhibits multiple equilibria. This is a really fascinating thing. And if it gets back to some of the work the David McGee was reporting on. That the Earth has been in many different states. It's been in hot houses, it's been in glacial-- we've been in glacial climates. We've been, maybe, even in a snowball 800 million years ago. And the remarkable thing is that both the ridge world and the aquaplanet exhibit multiple equilibria.
17:07
In the sense that if we use the same forcing, the model, we can maintain three different climates states. One which is warm. One which is a glacial state. And a snowball, where the whole planet freezes over. And these are multiple equilibria, in the sense that you've got the ball rolling around a corrugated surface and there are equilibrium states. So one of the questions that one might ask is, could Earth flip into another state? Are multiple equilibria a way of thinking about past climates?
17:55
Have I got zero minutes? So good, conclusions.
18:04
Studying the climate of these aquaplanets is fun. I've become obsessed by it for-- and I'm now reluctant to put land into the system. [LAUGHTER]
18:19
It informs us about the elemental role of the ocean in climate. So we can think of this progression as a cartoon of the climate. If I wanted to draw a cartoon, what would be essential things be about the present climate? So it would be interhemispheric asymmetries, geometrical ones, and zonal asymmetries. We can also think about it as an evolution over geological time. As the plates move around the world, we move into-- the tectonics drives the land into this evolution.
19:00
There are many, many unanswered questions. Do multiple equilibria of Earth's climates exist? They do in this model. If so, how stable are they? And we could also use these kind of calculations as a framework for thinking about the dynamic, say, of exoplanets. We've been working with Sarah on these matters. So and I'm going to finish off by thanking lots of people, who over the years have been working on these kind of problems. This is a very high resolution simulation of an aquaplanet.
19:35
So this is what the ocean would look like if North America were shrunk to a-- I've to stop. [LAUGHTER] Thank you, very much. You thank you. [APPLAUSE]
19:52
Feels like an AGU meeting in the [INAUDIBLE] here. And this is exactly why the geologists and the oceanographers love each other in our department. Looking at the rocks. But your sticks, of course, had a confidence. And they move all about. So it's all about land area. Anyway, one question and then we have to move on. Victor Starr's main contribution to the dynamics of the climate was that most of the interesting energy motions are upgradient. He summarized that in the book he
20:24
wrote on the physics of negative viscosity phenomenon. Yes. You said that you had to put in a ridge in order to get upgradient transports. But you don't. What Victor Starr said is you can get those anyway. Now, the ridge may help it, but it isn't necessary. OK, so the question was can-- momentum is transferred upgradient. Victor Starr was talking about momentum. I'm talking about heat. The atmosphere takes heat downgradient. It's thermally driven.
20:58
And they can do-- it can transfer momentum upgradient. And Victor Starr realized that. Not always, but Victor Starr understood that. The ocean can actually transport heat upgradient, because it's mechanically forced. That's a different thing.
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