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

Lecture 23.?El Ni?o

https://oyc.yale.edu/geology-and-geophysics/gg-140/lecture-23

El Ni?o and La Ni?a [00:00:00]

El Ni?o is a naturally occurring oscillation in the Pacific Ocean between two rather distinct states. El Ni?o 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.?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.

Now, one thing that’s curious about this is that these two states can exist without any external forcing.?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.

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.?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?the Western Pacific?is called the warm pool?because there we get the warmest ocean temperatures anywhere on the globe, at least during this phase of the El Ni?o, La Ni?a 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.?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 at the surface?makes it easier for nutrient rich water from below to mix up to the surface, in part due to coastal upwelling.?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 Ni?o situation. Now the word El Ni?o 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 Ni?o, the child, the Christ child.?

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, 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.

There are physical laws connecting all of these things together. But on this diagram, and in fact?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 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 Ni?o 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.

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.?So a year ago, there was a La Ni?a existing, because you can identify that immediately because of the colder than usual conditions in the sea surface temperature in the Eastern Tropical Pacific.?

Terminology [00:10:08]

So here's what you need to know for terminology. El Ni?o and La Ni?a, 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.

So on occasion you'll see plots of the Southern Oscillation Index, and that's the pressure symptom of this cycle. The Walker circulation?is 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 Ni?o. It's quite a remarkable set of instrumentation. Sea surface temperature we talked about before, and we've already defined thermocline and primary productivity.?

Symptoms of El Ni?o [00:11:33]

So here's a list of some of the symptoms then. So during El Ni?o 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 Ni?o, but occur in other parts of the world as well.?

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. 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 Ni?o 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 Ni?a, 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 Ni?o situation is when the thermocline is flatter across the Pacific Ocean. 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. 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 Ni?o, these are anomalies I believe, rather than absolute temperatures. 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.?

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 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 Ni?o weather patterns are for the winter season.

If you have an El Ni?o 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. 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 Ni?o 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.

ENSO Indices and Ocean Water Property Measurements [00:18:54]

One of the ways we monitor El Ni?o is to keep track of the ocean surface temperatures in these four kind of commonly defined areas. All the scientists who work on El Ni?o agree to monitor these four temperatures. Sea surface temperature in area one, two, three, and four. So you'll find plot, for example, of Ni?o four or Ni?o 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 Ni?o.

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.

So here's a couple of plots then. One is the ocean temperature departures for the combination of Ni?o 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. Tahiti is the Eastward-most station of the two, and Darwin is, of course, is in Northern Australia.?

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 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 Ni?o 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.

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 Ni?o 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.

Current ENSO Data [00:24:40]

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 Ni?a situation. Has it been a La Ni?a 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 Ni?a 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 Ni?o I would say. It never got strong enough or persisted long enough to have an El Ni?o. So it wouldn’t—19, sorry, 2010, 2011 would not show up as a El Ni?o period. And now we're solidly back into a La Ni?a situation with cold water in the Eastern Pacific, high productivity, and so on.

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 Ni?a 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 Ni?a situation.

Now what's going on beneath the surface in this month of 2011? 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 Ni?a situation that we saw here with colder conditions in the Eastern Tropical Pacific.

Usually when you start an El Ni?o 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 Ni?o period.?It comes back to the question of predictability. It's kind of random. And when you go into an El Ni?o, 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 Ni?o 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.

The terminology has changed a bit on this. Originally, there was normal and there was El Ni?o. 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 Ni?o, well it's going to be La Ni?a. 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 Ni?o,?normal if you're, say, within one standard deviation or so of this. And then La Ni?a you're down in the other extreme. So if you read the literature over the years, you see that terminology shift a little bit.?

Ice in the Climate System [00:39:45]

The subject which comes next in the course is ice.?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.?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.

Physical Properties of Ice [00:42:39]

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. 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.

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.?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.

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.

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.