Climate and Climate Change - Salam Distinguished Lectures 2016 - Lecture 1 of 3

[00:00:00] Speaker ?: Thank you. Thank you. Thank you. [00:01:30] Speaker 1: Thank you. [00:02:00] Speaker ?: Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. [00:03:02] Speaker 2: Thank you. [00:03:32] Speaker ?: Thank you. Thank you. Thank you. Thank you. Thank you. [00:06:02] Speaker 3: Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. Thank you. [00:09:31] Speaker 4: Thank you. Thank you. Thank you. Thank you. Thank you. So just as an example of the level of scientific excellence we're going to have in front of us for the next three days. So, Brian, that experience showed us that Brian, I mean, he's not just a demon in the lecture theatre. taxman on Newsnight. I don't know if any of you have ever seen this. In England, we have real journalists, yeah, that chew up and they spit you out. None of this Berlusconi pre-prepared menu of questions. And so, Brian will tell you, he defended his corner extremely well. More recently, there was a BBC Radio 4 program where I believe you were up against Nigel Lawson on that occasion. And it's actually, it's quite a serious point. There were quite a number of complaints after the program where people were saying, well, you have an eminent scientist who represents the majority view of scientists on climate change against a non-expert ex-politician. Nigel Lawson was serving, as you probably know, in the cabinet of Margaret Thatcher. And the complaint against the BBC was upheld. But it's an important point of actually how science engages in the media, because at the end of the day, how the media portrays a scientific issue, especially one that has been as contentious as climate change, influences ultimately policy. You only have to look across the Atlantic at the moment at the, shall we say, the ongoing Republican Party discussions where you have the two leading candidates who are both basically climate change deniers, one of them calling it a hoax, just to make you realize how important, shall we say, the media portrayal of these issues are. So Brian, as I understand today, we'll be talking about the theory of weather and climate. Tomorrow, on climate change theory. But on Thursday, we'll be engaged in that topic of how climate interfaces with society and policy, I understand. So there are those three aspects. So make sure you're here for all three of those lectures to see the policy implementation. I'm very much looking forward to that. So just to finish off, as the director mentioned, and basically, Brian, of course, has had a number of accolades. He had the CBE and was knighted in 2007. So the wonderful thing about the internet now, as you get all the background stories, Paul Valdez, Professor Paul Valdez at Bristol, who was also at Reading when I was there, apparently gave some of the background. So Brian, he was saying that at the CBE award, accidentally turned his back on the Queen. So luckily for us, he's not active in the 16th century. Otherwise, we would have had our reserve speaker this week. And the other thing that Paul dished the dirt on was when receiving a gift from the Pope. Apparently, Brian said, that's very nice. But could I have another one, please? Because I have two children. So tip for Fernando, two of everything, two, okay? So anyway, but just to wrap up, I mean, my actual memory, because of course, it was me at the time was that it wasn't just a question of Brian having tea and cake with the Queen. It was also the fact that he would have tea and cake with everyone. So the thing that was amazing about Brian was despite his really heavy workload, all of the duties he's had to carry out, you've seen the list, and that just scratches the surface. He always had an open door. He was always keen to talk science with everybody from the undergraduate through to the PhD student, to all of his colleagues, and so was highly regarded and liked by his colleagues. So just to emphasise, especially to the diploma students, I mean, take the opportunity these three days to engage Brian, grabbing over the drinks reception afterwards, if you have a burning question about climate, and really take this opportunity because, you know, it's really an excellent opportunity. So I'm going to hand back to the director before he kicks me off, and thank you very much. [00:16:27] Speaker 3: Thank you very much, Edwin. Actually, I forgot to warn Edwin that we are being filmed. So, and it's actually a live stream. So, anyway. So before we start, before we start, I have to say some announcements. One is that the ICTPs and I'm distinguished lectures would not have been possible without the general support of the Kuwait Foundation for the Advancement of Science at CAFAS, so we are very thankful for their support. So, I would also like to invite all of you for some light refreshments after the lecture. And as it's traditional for the lectures, we ask everybody to go immediately after the question session for the refreshments, and except for the diploma students that we are asking to stay and then ask as many questions as they want to the lecturer. And with the promise that there will be some food remaining afterwards for them, okay. So that's something that's -- so -- and -- and as Edwin said, there's three lectures, and the three are independent, and the title of today's lecture is the Climate System and the Roles of the Atmosphere and the Ocean. And in summary, it's a great privilege to welcome Sir Brian Hoskins to the stage to give the first of these three lectures on climate change -- climate and climate change. [00:17:54] Speaker 1: Well, thank you very much, and thank you, Director, for your introduction there, Professor Covedo. It's a great honor to give this lecture in the memory of your great founder of ICTP. And with Adrian there, it's a warning to anyone. Always treat your students nicely because they might introduce you sometimes. So I did wonder what to give in these lectures, and I've -- in the end, as you've heard, I will be talking about the climate system. But in particular, I decided to concentrate on the two fluid elements of the climate system, the atmosphere and ocean, and compare and contrast those two. So that's going to be very much this lecture this time. And then tomorrow's is going to be getting on to the variability in the climate system and some of the phenomena there. And then perhaps the basic science of climate change, which will allow me time in the third lecture, then, to really get into the future, projections for the future, and, as Adrian suggested, the politics and what are we doing about it, the challenge to technology and the challenge to our society in general. So that's going to be number three. So if you don't like today's, it could be good tomorrow, so please come. Right, so that's what we're on today. And I'm going to start with a sort of basic thing of the climate system here. And it's a fairly usual sort of picture that shows various elements of the land surface, the ocean, the atmosphere, and a bit of radiation going between those, solar radiation coming down and the heat given off by the earth, and water running around, et cetera. But it's going to be these two elements, the ocean here and the moving ocean, and the atmosphere that moves as well. And they transport things around as they move. They're the two fluid elements of the system. And my background is as a mathematician in fluid mechanics in particular. So it's the study of the atmosphere in particular, but the ocean as well that's always fascinated me. And that's where I'll spend my time. But just to put one further slide as sort of a background to some of the things we'll think about. And I'm not going to deal with this in detail at all at the moment. But just to say there's a solar radiation coming in. And a little is absorbed in the atmosphere, about 20% of it. But most or half of it gets through to the earth's surface. And that's going to be important later on. The atmosphere is mostly transparent to the solar radiation. And it reaches the surface of the earth where it either is absorbed by the ocean or it's absorbed by the land surface. And both of those will then tend to warm up because of the energy absorbed. And then the heat energy given off, the long wave radiation given off, is to space. Mainly comes from the atmosphere maybe five, six, seven kilometers up. Something like that. So it isn't from the surface. It's from higher in the atmosphere. And that will be important again when we're looking at the properties of the atmosphere. But that will be sufficient for the moment on that. So I really am going to go right back to square one about what is the atmosphere and what is the ocean. And well, the atmosphere is some well mixed gases. At least they're well mixed in the bit of the atmosphere I want to deal with which is below 50 kilometers or so. And then there's water vapor. And water vapor is clearly a very variable part of the atmosphere. And an extremely important ingredient in the atmosphere and in the climate system. And then there's other things that we'll talk about later. Carbon dioxide, et cetera. So other ingredients. Now the ocean is mainly water. But it also has very importantly various salts. And the proportions of those are about the same. So we can just talk about the salt content of the ocean. And then there's some trace elements in the ocean as well. The depth of the atmosphere, if it was all a sort of near surface pressure. The depth of the atmosphere is about 10 kilometers really. So you can think of that as an e-folding depth of the atmosphere. The density decreasing over 10 or 11 kilometers by a factor of e. And obviously it becomes more and more tenuous as it goes out. But basically it's about 10 kilometers of atmosphere we're thinking of. And the ocean, about four kilometers if you go over the deep ocean. Sometimes it's up to six, but an average would be four kilometers. So not very different numbers there. But clearly very different when we start thinking about the density. So the density of the atmosphere near the surface is about 1.2 in MKS units. But then decreasing again exponentially as you go up on a scale of about 8 or 10 kilometers again. But the ocean density is about 1,000. So we have a ratio of about 1,000 in the density. And that's clearly going to have implications later on. So let's just think of the mass of the two systems. The mass of the atmosphere, then 10 kilometers of atmosphere density, about 1. Then it's about 10 cubed kilograms per square meter. Whereas the density of the ocean is 1,000 times greater. So that's just the top 10 meters of the ocean weighs the same as the atmosphere. So the ocean is about four kilometers deep. So we're thinking of sort of 4,400 times the mass of the ocean as we are of the atmosphere above it. So very different masses of the two systems. Does that mean they're totally different? Well, we'll find out there are a lot of shared properties as well. Just continuing with some basics on the thermodynamics now. Then the specific heat of the ocean is comparable with that of the atmosphere about four times greater. So if we look at something, suppose we add some heat to the system, each system then. A depth h, we have rho times the depth of the system we're thinking of. Cp delta t would be equal to the amount of heat added. So if we think of the thermal inertia of the two systems. Because rho is a thousand times larger and Cp is four times larger in the ocean. There's a huge thermal inertia of the ocean compared with the atmosphere. And it means that the top two and a half meters has the same sort of thermal inertia as the atmosphere. So the top two and a half meters of the ocean, you add a certain amount of heat to that. Then it would warm up by the same amount as the whole atmosphere above it if you put that heat there. And the thermal capacity of the ocean then is if you have 100 watts per square meter added to the top 200 meters of the ocean. Which is the typical bit which goes into. Then in order to raise the temperature by one degree it would have to last for a whole season to change the temperature by one degree. So just 200 meters of ocean, 100 watts per square meter coming down on it. And a whole season later you might increase the temperature by one degree. Whereas the atmosphere changes on a diurnal timescale almost. So the thermal inertia is very different to the two systems. The variations in density then, the atmosphere clearly right up where it's become extremely tenuous. The density is very small through to about 1.3 at the maximum at the surface. And it's a perfect gas to a good approximation. Water vapor, the variable content of water vapor impacts on the density a little. But to the first order we can neglect that and think of it as a perfect gas. So we have a perfect gas with the variation of density and pressure going through from the surface value through to about zero. Whereas the ocean then the variation in density is much less than... And it's around 1,000 and say between 1,020 and 1,040. But most of the ocean is over a much smaller density range than that. And usually when talking about the ocean people drop the 1,000 and they just say just put the 20 to 40 of the density. So there's no exact equation of state. But you can then determine from laboratory how a change in temperature affects the density. How a change in salinity or a change in pressure. All of those can change. Only the change in pressure is because the huge increase in pressure as you go down in the ocean. And that does have an effect on the density. But near the surface then we're thinking of changes in temperature and changes in salinity affecting the density of the ocean. And it depends where you are. At the sort of temperatures in the tropics then it's the temperature, the variations in temperature that affect the density of the ocean. Whereas in the polar regions where it's cold and much near a freezing point then it's the salinity that really matters. And we'll see again the importance of some of these things later on. Now I've already mentioned that the atmosphere is only a small part, about 20% of the solar radiation absorbed in the atmosphere. Whereas about 50% is absorbed in the surface. And then if we look at where the long wave, the heat loss, then it peaks at somewhere around 6 kilometers. So that tells you quite a bit about the atmosphere. Essentially it's being heated from below. And then it's losing heat from higher up. That means it's going to be a very turbulent sort of medium. And that's the weather, such as what happened earlier this afternoon. That's the sort of atmosphere we have where it's bubbling up now and then. And essentially heated from below. And I'm going to contrast that then with the ocean. Where the ocean then, the solar radiation comes down at least half of it through the atmosphere. And reaches the surface. And then is absorbed in the top 10 meters of the ocean. In fact, more than half of it is absorbed in the top meter of the ocean. So the ocean is very much heated from the top, unlike the atmosphere. And its long wave loss is actually from the top few millimeters. So you have a system which is very different. Where all the heating and cooling is happening from the top. And it means it's a very stable system in comparison with the atmosphere. So the bottom water is the ocean. The typical time scale for the bottom waters of the ocean to be refreshed is a thousand years or so. Whereas the troposphere, the bit of the atmosphere below say 10 to 15 kilometers. That's being refreshed all the time turning over on sort of diurnal weekly sort of timescales. So we have very different systems. The atmosphere bubbling away like this. And the ocean very stable and with huge length of time to really overturn the system. I'm sorry, I apologize these slides. I hope you can actually see enough of these anyway. So what I'm doing here is contrasting then the vertical structure of the two systems. So let's start with the ocean here. And this is density, or it's actually a potential density. And this is the equatorial region here. This is the South Pole on this side and that's Antarctica. And this is down to one kilometer and then that's down to five kilometers down here. And then this is the North Polar region here. And the equatorial region here. So what we can see then is this very warm bath, which is the tropical ocean. Remember all the heat is absorbed here. And this is this very stable system with it sitting on the top here. And underneath that, even in the equatorial region, it gets cold and dense pretty quickly as you go down. If you go to higher latitude then it's clearly cooler and denser. And then the change in density with depth, particularly in this northern region here. So there are some regions where the density is not very different as a function of height. But only in very particular regions is that the case. So we have this stable bath here. And its typical depth is the order of a couple of hundred meters or so. So we get this warm water sitting on the top. And then the body of the ocean underneath is cold and dense. So that's this very stable system here. And the atmosphere, I'm showing temperature here. Maybe I shouldn't have done, but we do have the warmth here. But this is the equatorial region and this is the warm region here. The temperature decreases with height. But actually that's only because this air, if you take it up, then it will expand as it goes to lower pressure. And it does work and it cools. So although this air looks warmer than the air up here, actually if you took it up there, it would actually be cooler. And it would sink back again. So the limit of how warm this gets is actually just that limit of convective instability essentially. So otherwise this would be even warmer here than it is. But again, we have something similar to this but upside down and the temperature gradient again. We do have very different behavior up here. Having got colder the whole way, the atmosphere then reaches a minimum temperature and then starts to warm again. And that's because of the absorption of solar radiation by ozone. So ozone, I said a small amount of the solar beam is absorbed. And in particular, the stratosphere here is a system where it's actually heated above by the absorption of ozone. So that's more similar to the ocean. Very stable. Very stable. And so there's a very stable layer above here which is to do with the ozone heating. But this bit down here is where we live and where our weather occurs. So now let's just look at the amount of energy actually absorbed by the ocean-Earth atmosphere system. And I'm showing here, this is the South Pole here, North Pole here. So in December, in the austral summer then, the solar radiation actually received by the system is pretty constant across the whole hemisphere. And then dives down in the winter hemisphere to near zero essentially in the polar night here. And the opposite is there in June where it's almost constant around the northern hemisphere. But then dives to zero in the winter hemisphere again. But averaging over the whole year, then this is the annual amount of solar radiation received and absorbed by the system. Not zero at the poles because of the tilt of the axis and the seasons we have. So that's what we have. But the Earth is, our climate system is very clever at making the polar regions much warmer than they should be. And the tropical region cooler than it should be. And this is the actual heat given off by the climate system. So at the top of the atmosphere, that's the heat escaping. And this does not vary as much with latitude as the solar radiation received. And that's because of our climate system. So in order to do this then, there's more absorbed shortwave radiation here than there is outgoing longwave radiation. And the opposite of the polar regions. So essentially this energy here is transported by our climate system into the polar regions on either side. And that's our climate system in action. And we'll look at quite a bit of how that actually occurs. So this is the transport of energy then from the equatorial region towards the North Pole in this region here. That's the total here. And towards the South Pole here. So almost from the equator then you get this transport. And the transport is a maximum somewhere around the middle latitude region. Also in this slide you'll see how much happens of that occurs in the atmosphere. And how much occurs in the ocean. And that's going to be then part of the story of the atmosphere and ocean moving then. And how they move properties around. So the atmosphere transports the major part of this in most latitudes. And we'll maybe look at some of how that is achieved. But then also in the more equatorial regions, the ocean is as important, if not more important, in the deep equatorial region. So the ocean plays its role there. And in fact into the northern latitudes plays a role of moving heat right into the northern polar region. So both systems were involved. But the equatorial ocean is particularly important. The tropical ocean is particularly important in moving energy around. So this is how the energy is transported in the equatorial region. And I know there's various levels of what people know about the atmosphere and ocean here. So I'll go fairly slowly and I hope that I'll carry along with me. So again North Pole over here, South Pole over here, this is the atmosphere. I'm showing pressure here from a thousand millibars near the surface up to zero here. But this is essentially the top of this troposphere, the weather region of the atmosphere somewhere around here. And if you look in averaging along a line of latitude, then we find that there is these giant cells here with rising motion in the equatorial region where there is the deep convection going on. And descent on either side in the subtropics where there is the desert regions. So we have these overturning circulations like this. And those transport energy. But they do it in a rather complex way. Because of the compressibility of the atmosphere, the actual energy transport by these so-called Hadley cells is the amount of mass involved in these. But then it's the difference between the temperature in these two branches here. Well you can see this seems to be working in the wrong direction. And it's taking warm air towards the equatorial region here rather than away. And then up here where it's cold, it's taking it back again. So it seems to be acting in the wrong direction. And another part of it is the moisture, the latent heat. And again, it's near the surface here. It's taking lots of moisture towards the equator where it's rained out. So again, it's transporting energy in the wrong direction. And the only reason that these actually do transport energy the right way is this term here to do with the potential energy. That the air being exported from the equatorial region has a much higher value of the potential energy than the energy being taken in. So this one is plus and these two are minus, and this one wins. But it's a bit of a competition. Whereas what goes on in the ocean is much simpler, which is why the ocean does more in the tropical regions. Underneath this, and we'll see the reasons for this in a moment, there's a cell in the ocean where wherever there's motion in the atmosphere towards the equator, the ocean does the opposite and takes mass in the opposite direction. And we'll see in a moment, actually, those amounts of mass are almost equal. So if the atmosphere is going that way with a certain mass, the ocean has to go take mass in the other direction. So the ocean does a sort of cell like this in the opposite direction, which is only a couple of hundred meters deep, unlike the atmosphere. And it's really quite efficient because you only have to think of the delta T involved in these. And this is warm water going away from the equator. It's being cooled, and this is cooler water coming back to the equator. So this is how the ocean does its transport in the equatorial region by, again, a similar cell underneath these Hadley cells in the atmosphere. And you notice these are actually turning around the same way, but this mass transport is in the opposite direction. So the transports there, this is how the transport of energy is occurring in the equatorial region. But we're now going to look a little bit more at the transport in the ocean to see what's happening in the different ocean basins. And let's look at the obvious ones first here. The axis has changed here. So these are smaller values than in the previous things. I should have said in the previous, we're talking about petawatts here. I mean, this is huge transports that are taking place. So this is 6 petawatts, 10 to the 15th watts we're talking about. So there's a lot of energy being transported. And here the maxima is somewhere around 2 petawatts. And you can see that if we take the Pacific Ocean, then we just have these cells here with the ocean transporting energy away from the equatorial region in both hemispheres. And the Indian Ocean doesn't have much in the northern hemisphere, but it does its bit in the southern hemisphere. It's the Atlantic which is really very different. The Atlantic takes energy from the southern hemisphere and moves it right into the high latitudes of the northern hemisphere. So there's a very different behaviour in the Atlantic Ocean, one of the most peculiar things in the climate system. So we really, actually, that is why we can have nice time in Europe. And northern Europe, the latitude of the UK, 52 degrees. If you go on the other side of North America, then you'll be well into Alaska by these sorts of latitudes. So we do have a lot of heat transport by the Atlantic Ocean right from the southern hemisphere into the northern hemisphere. And it's very good for us that we do. So this actually occurs then in a transport where the warm waters are being transported right from the southern hemisphere, right across into the northern regions. And then, as I showed in the earlier picture, this is one region where the water can actually, on occasion, descend to depth and then return, the colder water can return. So there's a giant overturning circulation here. This is quite a subtle thing in that this water is being cooled as it comes forward, and that makes it more dense. And it's a question then of the rainfall there, whether it can actually stay fresh enough to actually stay near the surface and then just be cooled enough to descend. So the water has to be dense enough to descend, but not earlier on. And there's quite a balance going on there. But this is the circulation that occurs in the Atlantic Ocean, not in the Pacific or the Indian Ocean. And it's often referred to as the Atlantic meridian overturning circulation or sometimes the thermohaline circulation, which actually says it's the temperature and the salt content that allows this to actually occur, and the water to become dense enough to actually descend. Now I'm going to get into just a little bit of dynamics here, where we talk about some of the motion that takes place on our planet and that it can occur in the ocean and the atmosphere. So the first thing is that we live on a rapidly rotating planet. If you're on the equator and you think of the speed that you're travelling around, it's about 465 metres per second. Really rather rapid. Whereas the typical wind that we think of is maybe 5, 10 metres per second. Even higher in the atmosphere, 40 metres per second would be a strong, strong wind. So to a first approximation then, the atmosphere and ocean are rotating around with the Earth. And so this rapid rotation of the planet and the fact that the two systems are just about moving with that then dominates the sort of motions that are possible. And in particular then, we tend to get the so-called geostrophic motion, where if you have low pressure over here, there's a pressure gradient force moving in this direction. But because of the rotation of the Earth, if we're looking relative to the Earth, then the air tends to feel the force, the Coriolis force, moving it to the right in the northern hemisphere provided. So you can get the flow of the air in this direction, the atmosphere travelling in this direction, the wind, in balance, where the pressure gradient force and the Coriolis force are in balance. And the strength of the wind then will be rho times V is 1 over F dp by dx. I thought I put what F was, but it seems to have disappeared. So F is the so-called Coriolis force, which is twice the rotation rate of the planet times the sign of the latitude. So F goes to zero at the equator. And that means from this sort of balance that in the equatorial region, the pressure gradients are going to be very small as F goes small. So you don't get much of a pressure. The barometer is not very useful as a weather tool if you're in the equatorial region. You'll see the diurnal tide more than anything else. It's only when you get away from there that the pressure is a good indication of what's going on in the atmosphere. So this balance is the dominant balance in the atmosphere and ocean, this so-called geostrophic balance. Now if you take this and you integrate around a latitude circle, then this would integrate to zero. And so that means there's no net northward motion of the atmosphere in terms of the geostrophic motion. So there is, I've shown you that in the Hadley cell, but it's a departure from this geostrophic motion. Whereas that's not true in the ocean because we have ocean basins. So these ocean basins, you can get the surface of the ocean tilted and there can be a difference in pressure between east and west. And so there can be a net northward geostrophic mass flux in an ocean. So you can get something like that thermohaline circulation, can be a geostrophic flow going poleward. The only bit of the ocean that really is not confined is the southern ocean. And we'll see that that does have implications for that. So a little bit more detail about the atmosphere if we average along a latitude circle. So longitudely average, that means average around a latitude circle. I've already shown you these Hadley cells. We're looking at the average around the whole year. So you see the annual average and these are what the Hadley cells look like. In the individual seasons they would be dominated by one or other of these cells. And before we looked at temperature, this now is potential temperature, which is the temperature the air would have if you took it from a standard pressure, where it is to a standard pressure at the surface. So this really shows that the air is potentially warmer as you go up and it's colder as you go towards the poles. Now I've shown this because they're associated with this geostrophic balance. Because you also have hydrostatic balance, that means there is a balance between the vertical shear in the wind and the temperature gradient. So wherever you have a latitudinal temperature gradient, you must also have a westerly wind that increases with height. So as it gets colder going towards the pole, that means the westerly wind increases with height. So in the tropics we have low-level easterlies. In the middle latitudes we have low-level westerlies. But these increase as you go upwards to the so-called jet streams in the subtropics, somewhere around 40 meters per second or so on average. So that's where the easterlies decrease as we go up. So that's the zonally, the longitudinally, however you want to refer to it, along around a latitude circle. Now I'm going to look at a few more properties of the two systems. I'm going to look at the near-surface wind in the atmosphere and then the near-surface currents in the ocean. And let's start with the atmosphere. We've seen in the average along a latitude circle that the air goes towards the equator. And if you look in latitude-longitude, then this is what is the Hadley cell, the trade winds in the two hemispheres blowing towards the equator, but again deflected and actually then being from northeast or southeast on the other side of the equator. So this is the two branches of the Hadley cell, the trade winds. Here we have the middle latitude westerlies over the southern ocean here, and then in the two ocean basins in the northern hemisphere, the westerly winds again. So that's the sort of simple picture of how the atmosphere is constructed there. And perhaps at some point we may see if we can see why those winds are like they are there. At least I'll give some information about that. This is not a very good picture of the ocean that I took off the web here. I'm sorry I didn't find a better one. But to look at ocean currents, let's start at the southern ocean to start with. We've got the westerly winds over the southern ocean, and in fact the southern ocean has a strong current around the latitude circle, and that's the region that isn't confined in longitude. But if we take particularly the northern ocean basins here, the Atlantic and then the Pacific is split in two here, then there's these so-called gyres here where there's a very strong current along the western boundary of each basin, the Gulf Stream here, the Kurashio here, and then this goes into the ocean basin. There's weak return currents in the body of the ocean. And so there's a gyre here, but a very strong current on the ocean basins. And there's lots of detailed structure in the tropics as well that I haven't got time to go into. But I thought I would give just a little bit about about some of the transports by the winds that we've looked at and the currents we've talked at. The first thing to note is that the people who deal, meteorologists and oceanographers, have made sure that they cannot communicate with one another because meteorologists talk about a westerly wind. So they're always saying, "Where does the wind come from?" because that, to some extent, defines the properties. So there's a warm westerly wind coming or there's a warm southern wind coming into Trieste, whereas the oceanographers are more forward-looking and they say, "Where is it going?" and so they talk about an easterly current. So this is one of these things erected to make sure, as I say, that we can never really talk to one another. But if we look at the numbers, we find that a typical wind in the atmosphere might be ten metres per second and a typical current in the ocean might be about a centimetre per second. So there's about a ratio of 1,000 between those. But remember, there's a ratio of 1,000 and the density is the other way. So the actual mass transport is rather similar. So I've sort of mentioned that already, that if we start multiplying the numbers together, they do work out quite similar. So if we actually then try and do some sums where we look at the actual mass flux by certain jets or currents, so we're taking the density, the velocity, normal to a section, and then the area of that section. Then we can look at the actual mass transport by some of the major currents and I'm just going to take a couple here, I think. So this is what the Gulf Stream looks like if you look across it near Florida. So you get this very strong current and we're talking about more than a metre per second up here, maybe two or three metres per second near the surface, but extending through to a one kilometre in depth. So this and then we have maybe 20, 30, 50 kilometres in width, something, a few tens of kilometres in width. That's what it's like near Cape Hatteras. And this we've already looked at, these westerly winds here. So if we compare the numbers for those two, I won't perhaps take this in gory detail, but let's take this Gulf Stream mass flux density, maybe a thousand, and the wind speed will take a particular one metre per second, one kilometre depth, and maybe a hundred kilometres as the whole width across this thing here. And if you multiply those two together, you get 10 to 11 kilograms per second of water going down the Gulf Stream. There's a lot of water. Now, again, oceanographers have made a unit here, which is again to keep the rest of us so we don't know what they're doing. And this is called the sphere drop. And that's 10 to the ninth kilograms per second. In terms of water, it's actually a million cubic metres per second. But I want to compare with the atmosphere with mass flux, so I'm going to stick to this here as the definition. So this is a hundred sphere drops. If we do the same for the atmosphere, I've just put in a few numbers here, you come up almost identical mass flux. So these westerly jets here are taking about the same mass as the Gulf Stream is taking. So the numbers in the two systems are really very comparable. And I've already said, too, for the Hadley cell and the motion underneath that they're very similar. Now, I'm going to look at one aspect and say that this isn't chance, it has to be. And I've talked about the geostrophic motion in the free atmosphere. But what happens near the surface where there's drag? There's a surface stress and drag between the atmosphere and the ocean or between the atmosphere and the land surface, it could be. But between the atmosphere and the ocean is what I'm going to talk about. So if we have a wind in this direction, the drag is going to be in the opposite direction. So we've now got three forces. We've got the pressure gradient force, the Coriolis force at right angles to the wind, and we've got a drag here. So to get those three forces in balance, the wind has got to turn and be reduced in magnitude and turned. And so the wind is turned towards the low pressure in order to balance those three forces. So that has some implications. If we have a cyclonic circulation, a low pressure, a cyclonic circulation around that, then the wind is going to be turned towards that low pressure. So we're going to get the wind coming in from all sides into that low pressure, and there will be ascent above it. However, then the opposite occurs in the ocean because this is an internal force. This is just friction between the atmosphere and the ocean. An internal surface can't force the fluid to go off in a direction. So if that internal stresses are going to send the atmosphere in one direction, then the ocean's got to be going in the opposite direction. So wherever the mass flux is in like this, the ocean must be sending a mass flux in the opposite direction, so that the net mass flux associated with this internal friction is zero. As I say, you can't have an internal bit of friction in a fluid driving it all in some direction. So an opposite mass flux must occur in the ocean, and that's what we've already seen in these overturning circulations, the Hadley cell. And it means if there's motion outwards here, there's got the upward motion in the ocean. So there's upward motion in the atmosphere and upward motion in the ocean underneath. One place this occurs is in a tropical cyclone. If you get a tropical cyclone going over the ocean, it induces this upward motion, and you get colder water coming from underneath. So if the tropical cyclone stays where it was, it would lose the warm ocean underneath, and it would soon die. So a tropical cyclone as it moves over the ocean leaves a wake of cold water, but it's always feeding off warm water ahead. I'm going to skip over this fairly quickly, so apologies if this is too quick, but I don't want to spend too long. But I do want to explain what this means for the ocean. Given we have westerly winds in middle latitudes and easterly winds in the subtropics, then this turning, this mass flux across the direction of the wind means that the ocean must tend to be converging in this region here, and there will be descent. Just as in the previous cyclonic situation there was ascent in the ocean, here there is an anticyclone, anticyclonic motion in the atmosphere, and there will be descent in the ocean. And the balance of vorticity then, so let me see if I can wave my arms to persuade you that this is what happens. So if you, we have our Earth rotating like this, okay, so if you have a column of fluid underneath this, it's rotating because of the rotation of the Earth. If you squash it, it's going to get broader. So conservation of angular momentum will set, it spins less fast, okay. And that means relative to the rotating Earth, it's going to be an anticyclone. So it's going to tend to want to spin down. And the only way you can get a balance in the ocean is by getting water from further north where it's spinning more rapidly. So the ocean responds by bringing water from further north, which is more rapidly rotating, and then it's shrunk, so it spins down. So the balance, which is called, again, it's the same man's sphere drop, which was that unit before. So there's a balance here, and on one version of this I put what beta was, but it's two omega times the cosine of the latitude. But anyway, this is the balance, but you need to worry about that. But what it says, essentially, and the oceanographer is always looking for this, that over the body of the ocean, the motion, the north-south motion is like the curl of the wind stress. And in a situation like this, there will be a southerly motion. So this is what's going on over the body of the ocean there. So where we get the atmosphere with this anticyclonic sense in the ocean, that drives southward motion over the body of the ocean, and the Gulf Stream and Kuroshiro are the return current to replace that. So of the body of the ocean, the water is going south, and you can't deplete the ocean here, so it's got to return somewhere, and it returns by the coast in a strong current. So that's the wind-driven ocean circulation we're talking about there. I'm just about reaching the end now. I think there's a few more bits I wanted to put in terms of the behaviour of the atmosphere and ocean linking with that and the other. And first, here I want to look at the salinity of the ocean. It's a thousand and something, and the question is what the something is. Well, if we look in the Atlantic, it's 1,037.2 here in the subtropical oceans, 1,037 here. These are the largest values near the surface. In the Pacific, they're large but not quite as large. So these are the regions of high salinity. And then further north, you get really low salinity in the polar regions. And in the Atlantic, the values are quite high right through to the northern latitudes, right until you get into the Arctic. And again, that's because the water is coming forward here and that this water is really quite dense and is able to sink because it's maintained its high salinity. So the Atlantic is the ocean of high salinity. But we do have these subtropical regions with very high salinity. And if we look at the exchange of water between the atmosphere and the ocean, this is the evaporation from the ocean. That's taking water up. And the precipitation, the rainfall, the atmosphere is taking water down. So the precipitation winds in the tropics, the deep tropics, and so you're adding water to the system there. And then we have the subtropical regions where there's very little rainfall and a lot of evaporation in the trade winds. And that's the regions of high salinity then, where there's a lot of evaporation going on. So the fresh water is being lost and then the ocean becomes more saline in these regions. So the exchange of water explains this pattern of overall salinity. In higher latitudes, again, the rainfall winds and the salinity is lower. But there's an interesting balance here in terms of the two oceans. The Atlantic, the evaporation and precipitation are nearly in balance till you reach the UK. In fact, you have to go further poleward. So it's the difference between the Atlantic and Pacific in terms of the storm track that we'll come to tomorrow, which actually enables the Atlantic Ocean to stay more saline until it gets a long way poleward. The other last property I was going to be looking at to talk about the exchange between the two systems is the exchange of heat. And this is where this is, the yellows and oranges, then this is heat being given to the atmosphere. And where it's blue, it's heat being given to the ocean. The ocean is very, very cold in these regions here and is taking heat from the atmosphere. But in these coastal current regions, then the atmosphere is being strongly heated by the ocean. And again, so it shows the importance of that whole gyre situation and the two systems interacting. In particular, in the wintertime, the cold air off the continent can come over the very warm ocean in this region here. And there's very high sensible and latent heat given to the atmosphere. In the regions of Africa and of North America here and South America, then there's strong cooling of the atmosphere by the very cold ocean underneath. So there's some major properties there which we talked about. And I hope I've managed to give you some idea of the two systems. I've tried to do it in a way which is not conventional, where I've taken the two systems and talked about the similarities and differences of the ocean and atmosphere. And how they exchange properties, how the one drives the other, how the other drives the one. And really we have two linked fluid systems, very different in density, but sharing many common features. Now, tomorrow I will proceed on to discuss some of the seasonal variability, seasonal variation in the systems, in particular in the atmosphere. Because the ocean, with its strong thermal capacity, doesn't change much from one season to the other. But the atmosphere really does. And we'll look at the seasonal behaviour of the atmosphere. And then we'll look at some of the phenomena that give us more like the weather that we actually experience. And as we said before, climate change, I'll try and start the science of that tomorrow, which will allow me time to really get into the challenge to society and to politics and everything else of climate change in the last one. Thank you very much. [01:06:19] Speaker 3: Thank you very much, Brian. It was a wonderful lecture. Questions? Yes? Very, very nice talk. [01:06:26] Speaker ?: Thank you. [01:06:27] Speaker 5: Now, I saw your numbers and your calculation in which you showed that the mass transport with the westerly winds is perfectly balanced by the easterly currents, adding up to about 100 spart drops. Now, knowing that the specific heat, as you also showed, of the atmosphere and the one of the waters of the ocean, is different. There's almost a factor of four. Isn't there a corollary of your demonstration that these westerly and easterly movements are isothermal? [01:07:11] Speaker 1: To really look at the transports of heat, we have to look at the oceanic gyre. Let me go back to that picture. Did I miss it? Yes, sorry. I didn't realise it was so far back when I started this. Right. Really, to look at the net transport, this water mostly stays near the surface, in the wind-driven. And really, to look at the net heat transport, we should be comparing the temperature of this branch with the temperature of the returning branches, to look at the net transport. Now, this is cooler than this, but not by a huge amount. So the net transport is not perhaps as great as one might have thought. Now, the westerly winds are charging round and round, but really, they're not doing that much heat transport. They're just going round and round. For the atmosphere to do... We've talked about the overturning circulations in the tropics doing the heat transport. I wondered whether to put in a picture here, but let me say it now. The atmosphere does its polar heat transport away from the tropics by taking warm air towards the pole and taking cold air back towards the equator. And that's what happens in weather systems. Perhaps we'll see that tomorrow. And there again, you're comparing the... comparing the temperatures of those two. But a lot of it happens in the transient weather systems, rather than in the fixed gyre, like these things of the ocean. So here we have, in the Atlantic, it's complex, but some of this, the heat transport is by this overturning circulation, where you're comparing the temperature of the top ocean with that down maybe one and a half kilometers depth. But then the gyre itself doesn't really transport that much because the difference in temperature is not that great. I was conscious putting this together. I was going to have to skate over so many things, so thank you for asking. [01:09:35] Speaker 3: Any questions? [01:09:45] Speaker 2: When you talked about the thermal hairline circulation or the Gulf Stream or AMOC, you emphasized, so this could be relevant for the warmth or for the moderate climate we have in Europe. But there's also circulation and the thermal inertia of the ocean from winter to summer and from summer to winter. I wonder if you could elaborate on this. What is the relative importance? [01:10:11] Speaker 1: Well, I would say that the ocean is warm, the Atlantic Ocean is warm in the higher latitudes of the northern hemisphere because of the overturning circulation. However, then the thermal inertia means that that stays warm in the winter when you think it might be losing a lot of heat to the atmosphere. It is losing heat to the atmosphere. It makes a lot of effect on the atmosphere. It makes rather little impact on the ocean. So I think rather than being competing things, I think the overturning circulation enables the warmth of the North Atlantic here and then the thermal inertia of the ocean. So the current enables that to be very warm. The thermal inertia of the ocean says it's not going to cool off much in the winter. So it enables then the atmosphere to be given a lot of heat. I myself have actually said I don't go along with my oceanographer's story too much. And if you're not careful, the oceanographers start thinking that theirs is the only role in the climate system. And actually then a lot of the warmth in Western Europe is associated with the heat given to the atmosphere over this region, the Gulf Stream. So it's actually not necessarily the warmth of this ocean. It's actually the warmth over here given to the atmosphere. So it isn't just that we have the Gulf Stream lapping our shores. We don't. There's many seaside resorts in my country that try and say the Gulf Stream is turning them into somewhere where palm trees will grow. It's not. The Gulf Stream does not reach the UK. However, this overturning circulation does keep the Atlantic pretty warm. And the heat in the whole Atlantic system is very important then for keeping Western Europe warm. [01:12:19] Speaker 4: Just to follow up that theme, I was wondering if you could perhaps give some more detail on the deep water formation, especially in terms of this. You talked a little bit about the cooling and the salinity opposing each other. But there's been lots of talk of the, shall we say, perhaps the on-off nature. So I was wondering if you could perhaps give a little bit more detail on the deep water formation and its role on the... [01:12:47] Speaker 1: I'll give my two-minute version of the deep water formation then. So the really deep, the deepest part of the ocean is actually associated with descending motion just off Antarctica in the deep Southern Ocean here. And that sort of fills the bottom layers of the ocean. But the overturning circulation here, it's a battle. If that water became too fresh, the Atlantic is very saline, I've said that. And that tends to make it more dense, so it could actually descend. But if it became too fresh in this region here, then it would not be able to descend. It would be too light. It couldn't descend to the bottom. So too much melting of ice around here could do it. And one of the theories about some of the changes in climate in the past is as the ice... the last glacial maximum finished and the ice sheets tended to retreat. The melting of those ice sheets, if that happened in a sudden rush at some time, it could make the Northern Atlantic Ocean quite fresh and unable to do this sinking motion. So it would no longer be able to draw the warm water into this region. So one of the theories about times at which the European region, but the whole Atlantic, has become... gone back into the glacial period is that the overturning circulation switched off. So there is a sensitivity to too much fresh water being put into this region, and that could switch the circulation off. And one of the interesting climate change is just how stable this circulation is. And nearly all the models of a warming climate suggest it will slow down. But there is absolutely no agreement about how much it will slow down. If it slowed down a lot, that could really make the Northern Atlantic region perhaps the only region of the world that didn't warm up. While the rest of the world warmed up, we wouldn't. But most of the suggestion is it will slow down, but not completely switch off. But it is a sensitive, but the most sensitivity is probably when we have a lot of ice. Because then when you melt those huge ice sheets, then you have a lot of fresh water available to switch off. So it appears that the Earth's climate is the thermohaline circulation, does show a lot of variability when we have a lot of ice that can be melted. But when there isn't so much ice, as in the present time, it's more stable. However, as we warm the climate system more, it could become less strong anyway. [01:15:39] Speaker 6: Sorry, just to keep going on that topic. I seem to remember that it was said, you mentioned a thousand years for the thermohaline circulation. If that's the case... [01:16:00] Speaker 1: No, the bottom was the ocean, which is the descent of Antarctica. That's the thousand-year time scale. This would be a few hundred years, though, this one. [01:16:11] Speaker 6: Still... Long time. Yeah. Given that time scale, how is it possible to relate that to the changes in salinity you mentioned because of fresh water and therefore comment accurately on the change in variability? [01:16:28] Speaker 1: The many hundred-year time scale is the time scale to do that. However, if this water is no longer able to sink, then you immediately would stop this whole branch of the water coming... I can't remember which way around it was this way, was it? The water coming from the southern hemisphere right up to the northern hemisphere. It can't do that if it's no longer able to sink. So you would switch off that surface warm water moving towards the northern pole. And the time scale for this is different. That's the time scale to take a bit of water from here to here to here to here to here to here. Whereas if you're just saying how much warmth are we moving this way, that's a shorter time scale to change. No. No. The inertia is negligible. [01:17:24] Speaker 3: More questions? I have a small complaint. In your salinity maps? Yes. Can you show them at some point? [01:17:36] Speaker 1: No. If you're going to complain, I'm not going to show them. [01:17:40] Speaker ?: Very good exercise for my thumb anyway. Yes? Yes. [01:17:44] Speaker 1: Somehow Central America disappeared. [01:17:47] Speaker 3: I just to joke to my European friends that if you take out Central America, then the Gulf Stream will not work well and then the climate in Europe will be much worse than now. Is that correct? Yes. [01:18:08] Speaker 1: There's actually, I mean it does raise, thank you for saying it. Yes, I'm sorry. I do apologize for that. I'll blame Dennis Hartman for that, I think. So, there has been a lot of the, how, how does the Atlantic able to sustain a much higher salinity than the Pacific? And one of the theories was that the, the, the, the high, that if we didn't have, well certainly if we didn't have Central America, it would be, we'd have one ocean basin. But how important is it, is it just how high the topography is in this region for transporting water across there. And the, the, some asymmetries that we'll talk about next time, for instance, that the, you can see the, the lowest salinity water here is north of the equator. There's some asymmetries here and how much is that associated with properties of, of the continents here of Central America. Some interesting aspects that I, I don't think, certainly I haven't heard bottomed out. So, we'll still give Central America some importance here, even though it's disappeared. [01:19:26] Speaker 3: Thank you very much, Brian. Thank you. So, okay, so as announced before, so everybody can go for the refreshments, hopefully they are there. there, and the students please just