Q. What is known about El Niño vs. what is hypothesized?
A. I think that one of the questions is regarding the onset of ENSO and what triggers ENSO actually? You asked me what magnitude of temperature change in the eastern Pacific is needed to trigger an El Niño?
But first there is no general agreement on what triggers an El Niño. Often what we have looked for as a first sign of El Niño onset is a swing in the western area, the westerly wind burst. These westerly wind bursts are particularly difficult to predict. This area of knowing the onset of El Niño is a very challenging area. This is something that is not very well known. This may change from decade to decade. There are different ways that El Niño can start. There is no real agreement on what causes the onset.
Westerly wind bursts are a way of looking at the onset of El Niño, but also you have the question of what causes these westerly wind bursts. One cause is to look at what we call the intra seasonal oscillation, a gigantic wave in the atmosphere that goes around the world and causes the strong westerly winds in the west Pacific, and this can trigger some waves in the ocean that start to warm the eastern part of the ocean. These mechanisms, it’s not easy to talk about them in a regular conversation like this. We need to go into what is the mean state. What are the mechanisms for these wind events to trigger some warming in the east like the wave propagation in the ocean, and so on. These are things that are not easy to explain in a normal conversation. You need to know concepts, and not necessarily go far into the mathematics, but the mean structure of the distribution of SST, how these things are in their normal state, how they can be altered, disrupted.
Q. Do we have a clear idea of what the normal state is?
A. Yes, yes, we do. We have a clear idea of it. One thing that is very important to know about the temperature distribution is that you have warm water in the western Pacific around 28 to 30 degrees celcius, and it’s west of 180 east, the dateline. East of the dateline roughly, you have the cold tongue that goes along the coast of South America and along the equator. The temperature of the water is about 26 to 20 degrees celcius. Twenty is along the coast of Peru. The 26 is at the western edge of the cold tongue. The edge of the warm pool is actually 28 degrees.
This is essential, because this distribution drives or is driven by the atmosphere. So you have for the mean state an interplay between ocean and atmosphere. You have the contrast in temperature that drives the wind at the surface which in turn drives the …
The mean state is called the Walker circulation. And you have the trade winds that go from the east to the west. These trade winds are maintained by the temperature differences between the west and the east. In the equatorial regions, the wind tends to go from cold water to warm water.
You have asked me about what the normal water temperature is and what is the El Niño temperature? Also is the more relevant variable the difference in temperature between the eastern and western tropical Pacific. These are very good questions. The normal distribution is what I just explained. When you have a fully developed El Niño, you have a rise of about 5 degrees C. in the cold tongue. So this is a completed disruption of this distribution because you have very warm water in the cold tongue.
What is important, this difference of this temperature between the east and the west. But what is also important is the temperature itself. Especially when it gets close to 28 degrees celcius, this is when deep convection starts to occur in the lower layer of the atmosphere. The deep convection is what happens when you have warm water, and this warm water has the tendency to evaporate at a higher rate, and this evaporation causes the air to rise, and this rising air starts to transform into water again. But that is high altitudes. This effect is enhanced all of the time, so that is called deep convection. You have these huge cloud towers developing over warm water. Water with a temperature higher than 28. This deep convection causes the surface circulation to change.
In the mean state in the western part over the warm pool, you have temperatures greater than the 28. You have this deep convection occurring over time, and this is the mean state. During an El Niño, the warm pool moves towards the east. So this deep convection is following the warm water, and it disrupts in these areas the long- term weather. You start to have rainfall in areas that are usually dry.
Q. Do we know that the currents start heading back east?
A. All of these variables, wind, sea surface temperature (SST), surface currents during the development of El Niño change. The SST changes because the processes that control the SST are changing. For example in the cold tongue, I would say between the warm pool and the cold pool, there are processes that reverse sign. The cold tongue temperature is maintained in the cold state because of oceanic processes that cools down the surface of the ocean. These processes are upwelling, or rise of the cool water from deeper levels to the surface. And you have heat transport through advection from the east to the west, so it is cooled down by the surface currents, which usually go westward. These processes maintain the cold tongue against processes that try to warm the surface. This is the incoming flux from above. Mostly from the shortwave of the sun. When you shut down these cooling processes, then the cold tongue starts to heat up.
This is one aspect of it. Another aspect is the warm water that is accumulating in the warm pool tends to shift to the east. It is also an advection process. It’s like you have a border between warm and cold water that is pushed to the east. And this is due to the type of wind. The winds are first westerly wind bursts. When these wind bursts are efficient enough, they always tend to push the easterly edge of the warm pool to the east. And because of that, they can reinforce themselves. Surface winds are occurring more often again and again, and you have a complete disruption of the trade winds. This way you arrive at a fully developed El Niño.
Q. In the work you are doing, you are trying to trace the extent at which the surface heating in the eastern Pacific is due to advection vs. flux?
Q. With advection, that assumes there are surface currents carrying water over. Are you able to trace these currents using the OSCAR data?
Q. And in terms of the flux, where are those data coming from?
A. We are now using flux data from the satellites. This is not what we are developing ourselves, but we have a collaborator at Woods Hole who is working on deriving heat fluxes from satellite data. Heat flux includes solar radiation with short wave and long wave range. You have the evaporation and precipitation flux. This is related to latent heat flux. When water evaporates, energy is taken from the system. When it condensates, it’s the reverse. It releases latent heat. The sensible heat is a fourth one. It’s when you have the wind blowing over the surface, and this wind is going to take with it some heat. Like when you blow on your coffee to cool it down. These are the four heat fluxes that are needed to estimate the net heat flux on the surface. In total, this net heat flux is positive, the largest positive part of it is the incoming shortwave sun radiation.
In other circumstances, latent heat flux can be an important process in the interaction of ocean and atmosphere. I’m not an expert on this, but it doesn’t happen in the Pacific. It may happen in the Atlantic.
Q. So it doesn’t account for the change in temperature?
A. No. It accounts for it, but it is not the preeminent process.
Q. How do you separate out the flux from the advection?
A. These are two processes that we can quantify. For advection, we have the sea surface temperature distribution, and we have this as a function of time, so we have the distribution as a function of space, and time, and this is updated every week. We have from 1992 until now. And that is a long time. We can start doing statistics on that. So we have temperature distribution, and we have surface currents. So we are able to estimate the advection.
Another important things is that doing this we assume that sea surface temperature is equal to the mixed layer temperature. In the ocean you have a mixed layer where the temperature is relatively uniform when you go to deeper levels. So you assume that there is some mixing going on until you get to a certain level where the temperature starts to cool down as you go deeper. We assume that what is measured by this satellite is related to the mixed layer. So an important variable regarding this is the mixed layer depth. So it’s the depth that we can assume the mixed layer temperature is relatively close to the sea surface temperature.
What we are looking at is the heat balance equation. It’s an equation that informs the temperature. All the processes that we try to estimate, try to infer. A very important one, like in the coastal areas, you have the upwelling heat transfer. This is a very difficult term to estimate. One of your questions is what is one of the biggest challenges. I would say that the biggest challenges are to estimate all of the terms of this heat budget with enough accuracy.
Q. How do you measure that?
A. Upwelling is related to the convergence and divergence of surface currents. When you have the divergence of the currents, such as when the water at the surface goes away from a certain location, as it happens along the equator, the water goes north and south of the equator, so you have a divergence, and in order to replace the water that is going away, you have to have water from under. This is what’s called upwelling. So from estimating divergence and convergence, you can estimate upwelling. But currents are only known with a certain accuracy. These are challenges mostly for people who use observations. The challenges are errors in the observations. And when you deal with a heat budget, how you are able to close the heat budget, complete it.
Q. So you have the heat flux variables, and the advection variables, and that doesn’t quite do it, so you add the upwelling.
A. Well once you have these terms, you can already start to study things and build an analysis. For example, we are interested in looking at what causes the sea surface temperature changes. We calculate the temperature tendency, the rate of change with time of these temperatures. And it’s in the equation. So this term is equal to all of the processes that tend to change with sea surface temperature. So we can start to calculate some statistical coefficients and plot these terms and see how things are going. And we can see that there are interesting reasons that we can derive from this even if we don’t close the heat budget.
Q. So you are looking at the changes in variables as you go into an El Niño. Is there anything you want to add?
A. You said you were not entirely clear on all the data sources.
Q. What is the source of the data? There are satellites. Are any data coming from buoys?
A. Yes. In situ data, especially for surface currents comes from drifting buoy data. You have drifting buoys disseminated all over the world that are tracked by satellite, so you can estimate the speed. So you have a whole set of data out everywhere. It is very important for us in order to make sure that our OSCAR processing system and model are doing a good job. So we are using the data especially to compare, to validate the OSCAR calculations.
I think that we are not satisfied, we are never satisfied. But we are reasonably satisfied that we have a relatively good comparison resource. It’s very interesting to see how on one side, what we do with OSCAR is we take satellite data, and we use them the best we can in the diagnostic model in order to calculate surface currents. And on the other side, you have these completely independent data, the drifting buoy data, which are taken from these things, are in the ocean itself. And when we get results where you have both data sets and we see that we have a good regression line, we see that the comparisons are good. We are looking at the same ocean. It’s satisfying.
Q. Are there a lot of these buoys out there, or are there still big holes.
A. Yes. There are a lot of these buoys. They started to be launched at the end of the 70s. They have been launched since then. On our OSCAR website, we provide to users maps where people can directly see the comparison between OSCAR vectors and buoy vectors. I really like these maps. You can see that there are a lot of differences. It’s not perfect. But it’s also very good.
Q. How far along are you in calculation the advection vs. heat flux?
A. As far as we can go right now I think …well it’s done. We just have to work on the analysis. Perhaps in the future, we’ll find something that will improve the OSCAR currents to accuracy much higher than it is now. But there is a point where we have to stop.
Right now I’m kind of between applied science, which is about estimating surface currents for a large branch of use, applications, and basic research, which is about investigating processes during El Niño and different disturbances in the tropics. So at some point I have to stop trying to refine these surface currents and start to go into deeper analysis to build a story. To write.
Q. Go more into examining the heat budgets? You have all of the data now. You just need to put it together and see what it means?
A. Exactly. It’s similar for a lot of people, most researchers. To find when it is time to stop. For me it’s quite possible that this is the end of this project.
I think that one thing that can be important. El Niño is the most important climate disturbance on the short-term range. It’s where the signal is the most important. Its mathematical explanation for El Niño can be embedded in the interaction between ocean and atmosphere and may require a lot of mathematical insights. I think that what I was talking about are much more about processes and trying to build on a story about these processes. But any people who are looking at the mathematical behavior of a model that reproduces El Niño. This mathematical behavior cannot really be explained in normal words everything that can be said about El Niño is a simplification of what is really happening. For the students, people who are hearing about the outreach, looking at outreach pages, it’s something to say that this is not as simple as it looks like. It’s required to go deep into the mathematics, into the complexity of the processes, the way the processes act one upon the other. It’s not easy to go deep into the understanding of this phenomenon just by using simple words. You can have some general view of what this is about.