GG 140: The Atmosphere, the Ocean, and Environmental Change
|Transcript||Audio||Low Bandwidth Video||High Bandwidth Video|
The Atmosphere, the Ocean, and Environmental Change
GG 140 - Lecture 23 - El Niño
Chapter 1: El Niño and La Niña [00:00:00]
Professor Ron Smith: Well we’re going to talk about El Niño today. I’ve got a PowerPoint presentation, but before I get into that, I wanted to give a bunch of definitions. So El Niño is a naturally occurring oscillation in the Pacific Ocean between two rather distinct states. El Niño was defined about 30 or 40 years ago as a state that the ocean went into occasionally. Originally it was defined as a condition of having warmer than usual waters in the Eastern Tropical Pacific, and low biological productivity. Those two things were the original definition.
As we began to study it, we realized that there are many other things going on involving trade wind strength, Walker circulation, precipitation patterns. So now it is–its definition has broadened to include a number of related symptoms, and I’ll be discussing those in the PowerPoint presentation. Let me go through these two diagrams to kind of get it started.
Now, one thing that’s curious about this is that these two states can exist without any external forcing. It’s like one day you wake up in the morning and you feel great, another morning you wake up in the morning you feel crummy, and there’s nothing external necessarily that’s driving this. It’s just kind of the way it is. Well El Niño’s not exactly like that. There’s a lot of physics involved in it.
But it doesn’t seem to be driven by anything external. It’s not the Sun, it’s not the tilt of the Earth, it’s not some kind of change occurring by human-induced changes. It just seems to have periods of several months when it does one thing and periods of several months when it does something else. And this will kind of get you started on the discussion.
So I’ve got two identical cross-sections here. They run West to East through the Equatorial Pacific Ocean. So there’s Asia on this side, the Andes in South America on this side. The International Date Line would be somewhere in the middle–that’d be 180 degrees West or East longitude. And most of the time you’ve got a situation that looks something like this. You’ve got very warm water in the Western Pacific, warm air as well, and because of that you’ve got a lot of convective precipitation.
Sometimes this part of the ocean is called the warm pool. If you say a warm pool to an oceanographer or to an atmospheric scientist, they know that you’re referring, or they suspect you’re referring to the Western Pacific because there we get the warmest ocean temperatures anywhere on the globe, at least during this phase of the El Niño, La Niña cycle. So this is a warm pool in the Western Equatorial and Tropical Pacific. Lower atmospheric pressure because the air is warmer, and because the air is warm it’s less dense, so the pressure underneath, warmer air is usually lower pressure.
During those periods of time you have strong Easterly trade winds, so I’ve drawn them this way. The trade winds return aloft in what’s called the Walker circulation. So remember, the Hadley cell you would see if I sliced it this way, across the Equator, here I’m slicing it along the Equator. So we don’t see the Hadley circulation, but we do see this Walker circulation. It returns the low level Easterlies in upper level Westerlies.
In the Eastern Tropical Pacific you have generally higher air pressure. Cooler ocean waters, and the cooler waters makes it, at the surface, makes it easier for nutrient rich water from below to mix up to the surface, in part due to coastal upwelling that we spoke about last time. And as a result of that, you have generally a high biological productivity during this phase of the cycle.
So let’s say then we go into an El Niño situation. Now the word El Niño comes from the fact that when this switchover occurs, it usually occurs in November, December or January, not too far from the Christmas season. The countries along the West Coast of South America are generally Catholic in their religion, and the Christ child is worshipped. And the fact that this switchover, when it occurs, usually occurred within a few weeks of Christmastime meant that it was given the name El Niño, the child, the Christ child. That’s where the term comes from.
So it has kind of everything reversed. You’ve got weaker trade winds, cooler than usual ocean temperatures and air temperatures, higher air pressures, the Walker circulation is weakened–I haven’t drawn it here–lower pressure than usual in the East side of the Pacific. Along with precipitation occurring along the coast of South America near the Equator. With the warm water here, you have strongly stabilized the ocean. Warm water floating above cold water. The nutrients are still down there below, but they can’t mix to the surface, so you get a low biological productivity in that part of the cycle.
Any questions on that? Yes.
Student: Why does the Walker Circulation weaken?
Professor Ron Smith: Well because it’s part of this–—the air that goes Westward in the trade wind has to move back Eastward. So that whole circulation is called the Walker circ–—it’s just closing the cycle on the strong Easterly trade winds. But remember, when air in the Walker circulation reaches this point and then tries to descend to match up with this, that descending air is going to prevent clouds from forming here, and that’s consistent with what I’ve drawn.
And here, when this air has to ascend to then form the rest of the Walker circulation, the ascent occurs in these convective clouds. So that’s consistent as well. Then when I’ve weakened the Walker circulation, that’s consistent with the way I’ve drawn the clouds there as well.
So there are physical laws connecting all of these things together. But on this diagram, and in fact, in most of what I’ll be saying today, there’s going to be very little explanation about why the system would switch back and forth. So I’m mostly going to be talking about what we observe the atmosphere-ocean to do in this part of the world, how the various symptoms tie together physically.
But the scientific community does not have a clear understanding of what would make this system suddenly switch from one phase to the other. So I’m not going to spend a lot of time dealing with that. It’s kind of one of the remaining unknowns. It’s not that there hasn’t been work on it, it’s not there hasn’t been theories proposed, but I would say it’s not a solved problem in terms of what causes the system to cycle back and forth like this.
So I’ll get in now to the way some of these things look. And stop me if you have questions.
So it’s a natural oscillation in the air-sea state of the Tropical Pacific Ocean. I’m going to start out with a lot of attention paid to this region. We described it last time as a region of coastal upwelling. And that, indeed, is where the studies of El Niño began. That’s where it was first identified, and that’s where a number of the early definitions come from is the variability along the Peruvian Coast there.
And to remind you, a typical–well, this is not necessarily typical, this is a temperature anomaly from a year ago, just about a year ago today–these are what the sea surface temperature anomalies look like. The blue means cooler, so you’ve got a cooler water than usual in the Eastern Tropical Pacific a year ago. And which side of the cycle? Is that El Niño or La Niña?
Student: La Niña.
Professor Ron Smith: La Niña, right. So a year ago, there was a La Niña existing, because you can identify that immediately because of the colder than usual conditions in the sea surface temperature in the Eastern Tropical Pacific. So a year ago there was La Niña, and near the end of the presentation I’ll bring that up to date, we’ll see what state the Tropical Pacific has today.
Chapter 2: Terminology [00:10:08]
So here’s what you need to know for terminology. I wasn’t able to get the umlauts over the n’s there. But El Niño and La Niña, which are the phases of this oscillation, the Southern Oscillation Index is defined as the pressure difference between Darwin in Australia, and Tahiti in the Eastern Central Pacific Ocean, and that is a measure of this pressure oscillation that I told you about.
So on occasion you’ll see plots of the Southern Oscillation Index, and that’s the pressure symptom of this cycle. The Walker circulation I’ve defined. It’s a circulation that occurs more or less in the plane of the Equator, taking some of the trade wind flow and returning it back to the East aloft in the upper troposphere.
In a few minutes I’ll show you the TOA array, which we now use to monitor El Niño. It’s quite a remarkable set of instrumentation. Sea surface temperature we talked about before, and we’ve already defined thermocline and primary productivity. I’ll be using those terms freely as we go through the discussion today. So be sure you’re clear on all of those terminologies.
Chapter 3: Symptoms of El Niño [00:11:33]
So here’s a list of some of the symptoms then. So during El Niño you’ve got reduced biological productivity in the Eastern Pacific. Warm water in the Eastern Pacific. Weak or reversed trade winds. Lower pressure in the Eastern Pacific. Rain in the East, and as I’ll show you later on, there are some distant climate anomalies that are connected with El Niño, but occur in other parts of the world as well. I’ll get to that later in the lecture.
So here’s kind of where it began. As you saw from the chlorophyll map, it’s a very highly productive region along the Peruvian Coast because of the cold water being brought up in the Humboldt current or the Peru current, generally destabilizing that ocean. And then the winds, the trade winds, with the Ekman layer pumping water offshore causing coastal upwelling bringing nutrient rich waters to the surface.
Well given that then, and here’s an example of a Peruvian fishing boat catching anchovies. Given that understanding, what would cause such a phenomenon where the anchovy catch increased from the ’50s up to the ’60s, and then in the early ’70s, especially in 1973, it crashed. Well this is a buildup of the fishing fleet. More and more—more and more fish, and people, and ships involved in fishing.
What would cause a sudden drop? Two possibilities. It could be overfishing. But it turns out later on this rebounds again. So it turns out it’s not overfishing. It was some change in the natural condition of the Eastern Tropical Pacific, and that was the onset of one of the strongest El Niño situations.
So it was first defined as this drop in biological productivity, which we know has something to do with coastal upwelling, the stability of the ocean thermocline, and we’ll follow that train of cause and effect as we go.
Here’s the way it’s defined today. Largely in terms of the surface temperature of the Pacific Ocean, and the depth of the thermocline. So during La Niña, you have generally a steeply inclined thermocline, deeper in the Western Pacific with lots of warm water then. But shallower in the Eastern Pacific, which means the cold water is brought up very close to the surface, even to the surface.
The normal condition is halfway in between, and then the El Niño situation is when the thermocline is flatter across the Pacific Ocean. Remember the Date Line is in here, so this is 120 East and this is 80 degrees West. The Date Line is somewhere in there. But this means you’ve got a big deep layer of warm water near the Eastern–in the Eastern part of the Tropical Pacific preventing efficient mixing of nutrient rich waters to the surface. So that’s the way we envision this today after 30 or 40 years of study.
And the Walker circulation looks something like this. Now, in the normal circulation you get strong trade winds, rising motion over the warm pool, and then a return of air aloft towards the East and then sinking. During the Walker circulation that is weaker and breaks up and you get rising motion in the East. And this may exaggerate it. This shows an actual reverse trade winds. That isn’t always the case. But at least they are weaker than they would be during the typical situation. So weakened trade winds, and low pressure and rising motion in the Eastern Tropical Pacific.
The ocean surface temperatures differ dramatically. Here’s an example of a map derived by satellite using infrared radiation emitted from the sea surface to determine its temperature. During El Niño, and indeed, these are anomalies I believe, rather than absolute temperatures. Yeah, but they show this warm anomaly as much as three or four degrees warmer than normal for sea surface temperature in this coastal region, and then extending out along the Equator in the Eastern Tropical Pacific. So the temperature differences are really substantial. Yes?
Student: What’s the difference between an anomaly and a normal temperature map?
Professor Ron Smith: Yeah. So what they do is subtract–for every pixel, they subtract off a long-term average sea surface temperature. So what you end up with is–of course, when you plot, then, normal conditions, it’s a very small variation because you subtracted off most of the normal considerations, most of the normal pattern.
So what’s left allows you to amplify these changes. So you just subtract the normal–I think some of the plots you’re working with in lab this week, some of that data was given to you in terms of anomaly, rather than the actual temperature. So it’s a common trick played by climatologists to bring out the differences between the current state and the normal state.
It doesn’t mean, for example, that these are the warmest temperatures in the ocean. It might still be slightly warmer over here, but because that’s normally warm that doesn’t show up in this anomaly map.
I mentioned global impacts. This is not completely understood either, but people have–once they understood this cycle of changing conditions in the Tropical Pacific, they tried to then correlate it to climate in other parts of the world. There’s been a few rather well-known papers been published on this the last 5 or 10 years. For example, here’s what we think the El Niño weather patterns are for the winter season.
If you have an El Niño in the particular year in the winter season, Northern Hemisphere winter, December, you should expect to get generally, drier conditions in through here, warmer conditions there. Well that’s part of the definition, I have that in my diagram. But elsewhere you can get anomalies as well. For example, even here in New England, you might have a warmer than typical winter because of some kind of remote effect of this El Niño cycle. In the Pacific Northwest you’d have warmer conditions, in the Southern part of the US you’d have drier conditions. In the summertime the relationships work a little bit different, but they’re shown here.
Chapter 4: ENSO Indices and Ocean Water Property Measurements [00:18:54]
One of the ways we monitor El Niño is to keep track of the ocean surface temperatures in these four kind of commonly defined areas. All the scientists who work on El Niño agree to monitor these four temperatures. Sea surface temperature in area one, two, three, and four. So you’ll find plot, for example, of Niño four or Niño three, a temperature plotted as a function of time. It’s right on the Equator. Usually it goes 5 degrees North to 5 degrees South, and that’s kind of a commonly agreed upon way to monitor El Niño.
And here’s how it’s done. So there’s a rather remarkable array of buoys that was put in a decade or so ago across the Equator in the Pacific. Many of these have thermistor chains that go down into the ocean a couple hundred meters, so you can monitor not only conditions at the surface, but subsurface temperatures as well. That’s where that diagram about the depth of the thermocline came from, looking at the thermistor chain data dangling beneath these buoys.
But you can log in, you can just go to their website at anytime day or night and get current data for what the ocean is doing in this part of the world. So it makes it a very–if you’ve got a new theory, if you’ve got a better theory than anyone else, you can test it immediately because this data is available, not only historically going back 10 years or so, but it’s available day by day, so you can test new theories and maybe make predictions. So it’s become a very kind of open-ended egalitarian subject where anybody with a better idea can put it forward because the data is there for anybody to work on.
So here’s a couple of plots then. One is the ocean temperature departures for the combination of Niño three and four. And then below that is the SOI, the Southern Oscillation Index, which is defined as the pressure difference, it’s Tahiti minus Darwin. Right? Tahiti is the Eastward-most station of the two, and Darwin is, of course, is in Northern Australia. So be sure you get the sign of this right.
So, you know, you wouldn’t expect to find such a tight relationship between these two rather different quantities. One is a sea surface temperature, one is an atmospheric pressure. However there is a relationship between the two, and I think–let’s see which way it works. So when the Eastern Tropical Pacific is warmer, that means Tahiti has a–let’s see, which way does that work. By the way, this is plotted not in pressure units, but in units of standard deviations.
So there’s a normal fluctuation to this pressure difference. You compute the standard deviation and then you mark whether you are close to normal relative to one standard deviation, or one or two or three standard deviations away from normal. So that’s the way that is. That standard deviation, again, I think is the quantity you’re dealing with in lab this week.
So generally, when you have warmer conditions you have lower pressure in Tahiti relative to Darwin. That would make sense because if there’s warmer conditions in the East, which would be this condition, with warmer air aloft hydrostatically, that would mean lower atmospheric pressure beneath it. So there is a physical law connecting these two, but still, it’s a bit interesting that it comes out to be such a nice relationship. So where are the El Niño periods then?
Well one of the most dramatic ones was in 1973, and that was the drop I showed you in the ocean productivity in that region. There have been some other since then, especially 1983, ‘84, ‘87, ‘88, ‘92, and then one in ‘90–I guess that would be ‘98. And you can spot them either from the ocean temperature departures or from that pressure difference, East-West, across the Pacific Ocean.
Questions on this?
So how often does it occur? Well it’s not periodic. It’s not as if you can find an equal spacing. If you had to make a guess, maybe you would say that it’s about every, what, five to seven years, something like that. But it’s not periodic, it’s not predictable, and, you know the first person that comes up with a really accurate way to predict El Niño will be rich and famous, because it’s an unsolved geophysical problem, but it also has big implications for the way people live, for agriculture, for fishing, and so on. So it’s a big question as to what causes these changes and how to predict them in the future.
No questions on that?
Chapter 5: Current ENSO Data [00:24:40]
So where are we today? This is from a week or so ago in 2011. Here’s a sea surface temperature map, and a sea surface temperature anomaly. So we’ll get to address that question of map versus anomaly. So there’s the warm pool, and the temperature is given here so there’s actually temperatures higher than 28 degrees Celsius in the warm pool.
Generally, we’ve got pretty cold conditions it looks like here, but what’s the anomaly? Well the anomaly is cold too. So we are in another La Niña situation. Has it been a La Niña ever since last year at this time?
Well here’s a plot of Nino one, two, three, four, and then just Nino four, and look what’s happened. We were in a La Niña a year ago. We came out of it during the summer. These are anomalies plotted, SST anomalies for these regions we defined in here. And now we’ve slid back into it. So it never was really an El Niño I would say. It never got strong enough or persisted long enough to have an El Niño. So it wouldn’t—19, sorry, 2010, 2011 would not show up as a El Niño period. And now we’re solidly back into a La Niña situation with cold water in the Eastern Pacific, high productivity, and so on.
Questions on how to read these diagrams?
So I want you to be able to look at a map like this and tell what is the state of the Pacific Ocean. Yeah, Jordan.
Student: Is the one and two referring to, like, where the temperature is–?
Professor Ron Smith: They just sum the two together. They take the–let’s go back to that. They often don’t want to distinguish between the two, so they’ll just sum these two together. And sometimes they do that with three and four, they sum the two together. It all depends what kind of detail you want to have in your SST description.
Student: I’m sorry, was that 3.4 that, like dotted red and green thing?
Professor Ron Smith: Where am I looking here?
Student: On the previous slide that you were just on. Is that 3.4, like the red and green overlap area?
Professor Ron Smith: Yeah…maybe that’s right. Maybe that’s 3.–you’re quite right. That’s not the average of the two. That’s actually four plus that little bit of three. That’s correct. Thanks for spotting that. I was wrong on that. Melanie, is that right?
Student: It’s half of four and half of three.
Professor Ron Smith: Oh, it’s half of the two. Sorry. So 3.4 is that part of four and that part of three. Melanie’s our El Niño expert. So she’ll tell me afterwards all the things I said wrong today.
Let’s see. OK now, here’s another way to look at it. I think I’ve shown you diagram like this before. It’s called a Hovmuller diagram. You take data along the Equator, and this is a longitude scale where the Date Line is right there. And then you plot sea surface temperature–well this is wind in this case–trade winds, as a function of time. So you get a time-distance diagram. So at each time you can see what the strength of the trade winds were across the Pacific Ocean.
So these are the trade wind strengths between 5 North and 5 South. They are negative because a positive velocity would be towards the East, and as you know, trade winds blow towards the West. So generally you’ve got trade winds all through this part of the Central Pacific, and these are the U anomalies where you subtract off the normal.
And indeed, what we’re finding there is that we actually have a bit of a reverse anomaly in the Eastern Pacific during the last year or so. And that is–that’s connected with the La Niña situation–you look at my diagram over there. So it’s not that the trade winds reversed, but they’re weaker than normal, at least in the Eastern Tropical Pacific, and that gave rise to the–or that was consistent with the La Niña situation.
Now what’s going on beneath the surface in this month of 2011? Again, trying to understand how to understand these maps. Here is the temperature anomaly in degrees Celsius for a few days ago. From the TAO array. And there is warmer than usual ocean water beneath the surface, but not much of an anomaly at the surface in the Western Tropical Pacific. However, in the Eastern Tropical Pacific, cold beneath the surface, but also at the surface. And we see it here expressed in a vertical section where the thermocline is there allowing that cold water to come up to the surface. And that, again, is consistent with the La Niña situation that we saw here with colder conditions in the Eastern Tropical Pacific.
So I think that is everything about El Niño. And we can stop and discuss this for a few minutes. Are there some questions about El Niño? Yeah.
Student: So when you say it’s an El Niño time, you’re not talking about the whole year, you’re talking about-it would be kind of a specific period of time?Does it change during–?
Professor Ron Smith: Yeah, let me be clear on that. So usually when you start an El Niño it starts around Christmastime, but then it could last a few months or even a year. So we would refer to that full period of time, as long as it persists, as the El Niño period. I think I showed you a case where it lasted longer than–—for example, well that one there was really, it extended about a year and a half. Whereas that one was really gone in less than one year. And this one you had kind of a weak one, but it persisted actually for several years.
So this comes back to the question of predictability. It’s kind of random. And when you go into an El Niño, you’re not sure if you’re going to come right back out of it. It could kind of diddle along for a while in a weak El Niño and last a bit longer. But I’d say more times than not, it’s over in about a year. You come out of that thing about 12 months after you went into it.
Does that help, Rachel? Other questions? Yes, Victoria.
Student: If it’s not an El Niño, is it always an El Niño—La Niña?
Professor Ron Smith: Well so this is confusing. The terminology has changed a bit on this. Originally, there was normal and there was El Niño. Those were the two things we heard scientists talking about. Some clever individual decided that they should think of this as really a yin/yang situation where it was one thing or another, rather than normal. So if you’re going to come up with something that’s the opposite of El Niño, well it’s going to be La Niña. I don’t like it myself, but now it’s already embedded in the literature as being kind of the opposite extreme with a range of normal in between.
So current terminology would be El Niño–well, it’s here– El Niño, normal if you’re, say, within one standard deviation or so of this. And then La Niña you’re down in the other extreme. So if you read the literature over the years, you see that terminology shift a little bit. But don’t be confused about it. In the old days, everything like this was normal and that was El Niño. Today it’s El Niño, a range of normal, and then La Niña out the other side. Yes.
Student: When you showed us the trade winds diagram and the anomalies, didn’t you say that when they had strong winds it was an El Niño? But what you drew is–?
Professor Ron Smith: Let’s go back and check that because I may have misspoken. So what I’m claiming over here is that the La Niña, this normal situation with colder water, has a stronger trade winds. And–yeah, maybe that was opposite–let’s go back and look at that diagram and see if I have that right. Thanks for pointing that out. Let’s see, that diagram was–yeah. My computer has locked here. Here, was it? Yeah.
Yeah, so it may be–I put the emphasis on the Eastern part, maybe I shouldn’t have done that. We expect that during a La Niña situation we have strong trade winds. And that would probably correspond to this. This is the anomaly. So over the Central Pacific–not the Eastern Pacific–over the Central Pacific we had stronger than usual trade winds.
Student: So that’s stronger, the negative?
Professor Ron Smith: Negative would be–remember, trade winds are from East to West. So when you see a negative anomaly in a region which is normally negative, that means it’s a stronger Easterly wind.
Student: Is it measuring wind speed, or–?
Professor Ron Smith: No, it’s not—it’s U, it’s the East-West components of the wind. Remember when you did the balloon lab and you plotted up the data from your pilot balloon, you plotted U, which was positive towards, with the wind toward the East, negative towards the West. It’s that convention that we’re using here. You can be sure of that because this is the normal situation and those numbers are negative, indicating that that’s an Easterly wind blowing towards the West. That’s the negative sense of this quantity U that I’m discussing. Yes.
Student: Where are the hectoPascals coming from? Is that what hPa stands for?
Professor Ron Smith: Yeah, that’s a pressure unit. They’re referring to the level at which this is measured. So this is at the—this is not at the sea surface. This is the 850 hectoPascal level, which is about a kilometer and a half above the ocean surface. Remember again, when you’re doing the balloon lab and you plotted things with altitude, very often atmospheric scientists use pressure as their indication of how high you are in the atmosphere. So this is data not at the sea surface, but about 1,500 meters above the sea surface, at the 850 hectoPascal level. Good question. Yeah.
Student: What is this map called again?
Professor Ron Smith: Well it’s called a Hovmuller diagram. H-O-V-M-U-L-L-E-R. Hovmuller diagram. And it’s used commonly in atmospheric and oceanic sciences. Whenever you want to see how data along a line varies with time. So it wouldn’t always have to be longitude. You could construct some other line. But in this case, they’ve chosen East-West as their linear dimension, and then time going up as their time dimension.
Other questions on this? Yes.
Student: Would you go over why it’s negative?
Professor Ron Smith: Why these numbers are negative? The convention is for this quantity U, it’s positive when the wind blows toward the East, and negative when it blows towards the West. You do see a little bit of positive wind near the edges. But over the vast majority of the Central Tropical Pacific you have the trade winds, and that is a wind that blows from East to West. We call them Easterlies because of where they’re blowing from, and with this U convention that’s a negative on the sign convention.
I’m glad you asked that because this is confusing with those signs there. Other questions on these diagrams?
Student: And the one to the right is during an El Niño?
Professor Ron Smith: Well the one on the right is the anomaly. So what they’ve done is taken this data and subtracted off the normal, the long-term average, to get an anomaly. So when you find a positive anomaly, that would be a weaker trade wind. It’s only two meters per second where this was six and eight. So what that’s going to do is basically drop the strength of the trade winds by a couple of meters per second, but keep them going towards the West.
Student: So those values are the actual speed or are they the difference from the average?
Professor Ron Smith: They are the difference, they are the actual minus the average. I haven’t shown you the average here, but these numbers are this number minus whatever is the normal wind at that—at each location.
Anything else on El Niño?
Chapter 6: Ice in the Climate System [00:39:45]
So I think we have a few minutes to start the next subject. Let me find this–We won’t get very far into this. The subject which comes next in the course is ice. So it’s a little odd because we’re jumping from El Niño, which is primarily a Tropical phenomenon, to the study of ice in the climate system, which is primarily a high latitude subject. But they’re both very important in the way the atmosphere and the ocean work to form our climate.
I wanted to first just run through a set of definitions for the different types of ice that we’ll be interested in. Sea ice is frozen seawater, where you take seawater, you blow cold wind over it, you draw the heat out of it, you bring it down to the freezing point, and then eventually you freeze it. That is sea ice.
Ice sheets, like Greenland and Antarctica, are large plateaus of ice formed from compacted snow. Snow that fell month after month, year after year, piled up, squeezed, compressed to form ice. So the origins of these two things are almost completely different. In one case you freeze seawater, the other case you just compact snow.
Glaciers are streams of moving ice. They’re moving under the influence of gravity. So normally if you’ve got a snow fall on the top of a mountain that builds up, after a while it gets deep enough, it begins to be pulled gravitational down the slopes of the mountain.
Ice shelves are fixed, floating ice sheets. They’re basically ice that’s come off an ice sheet, flowed down to sea level in a glacier, and then has spread out over the ocean surface still fixed to the land. I’ll show you diagrams of all of these.
Icebergs, I’m sure you know that one. Those are chunks of drifting, floating ice broken off from glaciers or ice shelves. And then, of course, on land, you can have permanent or seasonal snow fields, just areas that have received either just snow for that winter and it’s going to melt off the next summer, or snow that is able to persist over the summer season and still be there the next year would be called a permanent snow field.
Chapter 7: Physical Properties of Ice [00:42:39]
So these are important terms in the descriptions I’ll be going through mostly next time. These are equally important. These are the physical properties of ice that we’ll need to know in order to understand the way ice works in the climate system. First of all, there’s this latent heat of melting or freezing. You can look it up, you can Google it. The value is 334,000 Joules per kilogram.
Every time you take water and freeze it, you have to remove that much heat. And every time you take ice and melt it, you have to put that same amount of heat back in. So it’s a reversible process. That number applies to either melting or freezing. And you can do that in a variety of ways. You can radiate that heat away, you can have cold winds or warm winds provide that heat, or remove that heat.
But in any case, whenever you’re melting or creating ice, you’ve got to deal with that amount of heat. It’s a large number. It’s not as large as the latent heat of condensation. Remember, the latent heat of condensation was many times that number. You might want to look back and see what that number was. This is still big, but it’s not as big as a number needed to condense water or to evaporate water.
I want to remind you also that while the freezing point of fresh water is zero degrees Celsius–in fact, that’s the basis on which the Celsius scale is defined–for salt water with a salinity of approximately 35 parts per thousand, that freezing point is depressed to about minus two Celsius. You’d have to cool the water a bit colder if it’s sea water before it’ll start to freeze.
This third bullet, I’m sure you know this, but I want to impress on you how remarkable it is. Most objects, most materials, when you freeze them from the liquid they reduce their volume. That is, they become more dense. Almost every subject known to man does that. Water is one of the very few exceptions that when you freeze it, it actually expands. I think the only other one that I know that does this is bismuth. Bismuth has that same property of water as when you condense it from the liquid to the solid it actually expands.
Now you know this, because if you leave a bottle of water out on a cold night and it freezes, it’ll expand and break the jar. So you know it’s expanding. You also know it because when you drop an ice cube into a glass of water, it doesn’t sink to the bottom, it floats on the top.
You’re so accustomed to seeing that, but I want you to kind of marvel at that the next time you see it. That the solid is actually less dense then the liquid. It’s a very rare circumstance, it’s a very rare situation. But it’s very controlling in the way ice works in the climate system. The fact that it floats on top of the ocean, for example, rather than sinking to the bottom. It changes everything in terms of the way the climate system works.
So a typical density for ice is about 917 kilograms per cubic meter. Typical density for fresh water is about 1,000. Typical density for sea water’s about 1,025. So this is less dense than both of those, and would float either in fresh water or in seawater.
Any questions on that?
This is an interesting one. If you’ve got a bucket of seawater and you freeze it, or start to freeze it, as the water freezes, it’ll expel most of the salt. So that if you take a chunk of that ice that you formed and melt it, the salinity is going to be far less than the salt water it was made from.
I’ll give you an example. If you start with a bucket of ocean water with a salinity of say, 35 parts per thousand, and you froze it very slowly, you could actually generate ice that when melted had a salinity of perhaps only 5 parts per thousand. It really does expel most of the salt when you freeze that water. And that plays a big role, as you might imagine, in the way the Arctic Ocean works.
When you form sea ice, the water beneath it gets saltier because you’ve expelled that salt into the ocean. The Inuit people know this. When they’re out travelling over the sea ice hunting, if they need water, well, you could dig a hole through the ice and get sea water to drink, but humans cannot live on seawater, it’s too salty.
But they could knock off a chunk of–of course, the best thing would be if there’s snow. If there’s snow on top of the ice, gather that up and melt it. That’ll be fresh water. But if there’s no snow on the ice, take a chunk of the sea ice and melt it down and you could live on that. It’s not very tasty, it’s brackish water, but you could live on that because it’s enough fresher than the ocean water was, that the human can exist on that particular kind of brackish water. So this is important.
For climate purposes it’s important that ice and snow is highly reflective of sunlight. The typical albedo for ocean water is something less than 0.1. It absorbs more than 90% of the light that hits it. On land, a more typical albedo is something like .2, 0.2. But yet for ice and snow, the albedo is more like 0.8. In other words, something like 80% of the radiation that hits it is reflected back to space. And that, as you can imagine, albedo is playing such an important role in climate, that’s an important role for ice controlling the albedo of the surface.
And the last one I want to mention is that ice, while we think of it as something brittle, when you take a chunk of ice and hit it with a hammer, it will shatter. When it’s under high enough pressure it’ll flow like a liquid, like a viscous liquid. And we’ll talk about that when it comes to how glaciers and ice sheets move.
So review these and we’ll start on the description of ice and the climate system next time.
[end of transcript]Back to Top
|mp3||mov [100MB]||mov [500MB]|