||You are El Nino Administrator.|
|Dr. Don Chambers -- Second Keynote Address|
Item 4 16-MAR-1998 11:46 Phyllis Grifman (pgrifman)
Good morning. I'm pleased to introduce our second keynote speaker, Dr. Don Chambers, who will discuss using satellites to identify El Nino, and some of the predictive capabilities of the technology.
Dr. Chambers received his B.S. in 1990 and his Ph.D. in 1996, both in Aerospace Engineering at the University of Texas at Austin. He has worked at the Center for Space Research since 1991, where he
is now employed as a Research Associate.
His interests include the use of satellite altimeter measurements of sea level variation to study changes in the oceanic heat storage associated with El Nino. He contributes an analysis of sea level variations in the equatorial Pacific each month to the National Oceanic and Atmospheric Administration
Climate Diagnostic Bulletin, for use in climate prediction studies.
Please join me in welcoming Don this week. I'm looking forward to a very interesting presentation and to a great discussion!
Thank you, Phyllis. This week, I'll be continuing with several of the subjects Dr. Vasquez covered last week. Because of my background, I will place a little more emphasis on the satellite altimeter data.
One thing I hope you take from this presentation is how sea level and sea surface temperature give different but complementary knowledge about how El Nino evolves. By combining these two measurements, we can learn a lot more about El Nino than if we just had one set of data alone.
I'll also talk about some exciting new research that indicates that the Indian Ocean has a similar El Nino signal as that in the Pacific. This may explain previously unexplainable correlations in the monsoon rainfall and El Nino.
The presentation is in html format, derived from a powerpoint presentation. It can be reached at: sld001.htm
This may be a little slow to download and the graphics have been compressed considerably. If you have Powerpoint on your own computer, you might want to download the PC version Powerpoint file and view it directly. The file can be downloaded by clicking on the "i" in the control bar.
A Macintosh Powerpoint 4.0 file is also available via anonymous ftp from:
I hope enjoy the presentation and I look forward to your questions and comments.
There is also a Quicktime movie of sea-level variations of the 1997 El Nino which accompanies this presentation. Peter is working on making it available; hopefully it will be ready sometime this afternoon.
If you cannot see Quicktime videos, or would rather not wait, you can see an animated GIF version on my web-site:
If you are interested in getting a little more information about the TOPEX/Poseidon mission, or how altimetry works, check out this JPL web-site:
Thanks Don for your slide presentation. I am finding the topic of altimetry and El Nino quite fascinating. I have started to go through the slides and have studied the first 6 or 7 slides so far. I think it is easy, at first glance, for a novice to mistake the satellite sea level images for temperature images, so I'm learning to read the graphs carefully. I am looking forward to learning more and hope to spend more time tomorrow going on to the next slides.
By the way, when I went to see the animated gif, it crashed my system. Will try again later.
Thanks for a great presentation. I had not realized there were parallel phenomena occurring in the Indian Ocean during El Nino years. Has anything been noted in the Atlantic? If not, do you have any ideas why not?
Also, I have heard that one of the possible mechanisms for ending an El Nino is the reflection of Kelvin waves back across the Pacific towards the west. Have these waves been detected yet for the present El Nino conditions? And finally, how do you detect these waves?
One of the ways El Nino dissipates is via Rossby waves. These are similar to Kelvin waves, except they can exist at any latitude and they move westward. They can also extend north and south over a great deal of the basin.
Rossby waves can be created as a reflection of a Kelvin wave at the eastern boundary (South America), or because of wind cahnges when the Trade winds return to normal. And yes, it appears that Rossby waves are forming north and south of the equator in the last month. You can start to see them in the first two pictures in my presentation, as the positive sea-level anomalies north and south of the equator. At the equator, the sea level is lower. This is the signature of a Rossby wave. In the animation, you can also start to see westward movement. All of these observations point to strong Rossby waves and the beginning of the end for the present El Nino.
Concerning El Nino variability in other oceans. The discovery of the Indian Ocean El Nino is quite new (about 1993 to 1994). There is some evidence of interannual warming in the eastern Atlantic ocean in the Gulf of Guinea of the coast of Africa, but the surface warming is much smaller than in the Pacific and Indian Oceans, and I haven't seen any strong correlations between the Atlantic variability and the other oceans. There is not a large sea level signature either, which suggests that if a warming exists in the Atlantic, it is probably not occurring at deep levels.
Can you explain these Rossby and Kelvin waves. How do waves form, why are they waves, not just continuouls blocks of heated water.
What is the relation between Kelvin waves and degrees Kelvin?
Your second question is the easier to answer. The Kelvin wave andthe Kelvin temperature scale are both named after Lord Kelvin. He first observed and recorded the unique Kelvin wave in the Thames River in England. It was at first thought that for the wave to exist there had to be a solid boundary on one side (the right in the northern hemisphere, the left in the southern because of the Earth's rotation). It was later shown that the equator could act as a solid boundary, since the Coriolis force vanishes there.
I explain the creation of Kelvin waves somewhat in my presentation. Basically, they arise from the fact that the water does not have the same density throughout, but that it is stratified. Usually, light, warm water rests on top of heavy, colder water. Wind bursts cause a change in the general water flow. In the western Pacific, the eastwardly wind bursts tries to force lighter water into colder, deeper water. This creates an oscillation around the thermocline (the area where the water density changes the most rapidly), because the lighter water is more buoyant than the heavier water.
The oscillation moves away as either Kelvin waves or Rossby waves, depending on where the wind forcing occurs, its direction, and numerous other effects. Basically, a Kelvin wave is just one way the ocean responds to the external wind forcing. I can't really describe it any better than that without resorting to a lot of equations, and I'm sure you wouldn't want me to do that. :)
Also, there is heat advected eastward through the equatorial undercurrent. However, the speed of the undercurrent is at most 1 m/sec (an generally no more than 50 cm/sec). The Kelvin wave propagates eastward at 3 m/sec, so it's effects will reach the eastern Pacific 3 times faster than the current will. The Kelvin wave pushes down the thermocline and sets up the area for the El Nino warming. Also, there are other eastward currents associated with the Kelvin wave that carry warm water.
I hope this answer isn't too confusing.
thanks. I went back to the slides in question and re read them, which is a nice feature of this kind of presentation.
How far down does the wind then have this effect on the water, and how do we know it is the wind not something else?
This is a good question. We are pretty sure that it is wind forced and that it effects the water in the upper 300 meters or so because of numerical ocean models. Based on our understanding of the physics, we can model the ocean's response to any external forcing as a series of equations that can be solved on a computer. By including only wind forces, we can see what effect that this has on the ocean.
One of the results of early modeling efforts was the discovery that the winds will force these Kelvin waves along the equator, and this seemed to be linked to El Nino. Before altimetry or the TAO moored buoys, these waves were only observed in detail in these ocean models.
The following image is the most recent global sea-level anomaly map from TOPEX/Poseidon, after applying the precise satellite location. It was just finished today. The data is from February 17 to February 27, 1998.
so do I read that there is a ddiference of nearly 50 cm differnece in relative height from the north of australia to off the west of mexico?
Is that significant enough to affect the earth's spin or wobble?
I have to agree with Jorge. You all ask very good questions.
It would effect the Earth's spin if the sea-level change was due to a change in ocean mass. However, the biggest portion of the signal is a change in heat: the east warms and water expands, the west cools and water compresses. There is a much smaller effect due to changes in the ocean mass in the western Pacific, equivalent to less than a few cm in sea level.
However, there are detectable signals in the Earth's rotation rate that are tied to El Nino. These are mainly caused by changes in atmospheric mass connected with the Southern Oscillation. The atmospheric mass increases over the western Pacific during El Nino, and decreases in the central/eastern Pacific. This accounts for the majority of the "El Nino" variation in the Earth rotation.
We know this because we can measure the Earth rotation very well by bouncing lasers off of satellites and through VLBI (very-long-baseline-interferometry) studies. We can model the atmospheric mass veriations pretty well by putting surface atmospheric pressure measurements into a numerical model. This shows that most of the El Nino signal is caused by the redistribution of atmospheric mass. There are some small remaining signals, but it is not clear whether these are caused by errors in the model or oceanic mass changes. At least a small part of the remaining signal is probably due to oceanic mass movement; we just cannot be certain how much at the moment.
Dear Dr. Chambers,
What marvelous images! It takes a while to train the eye to pick up the patterns, but it's fascinating. I noticed in the November '97 T/P Sealevel image, the long margin of dark blue (lower sea levels) and dark red (high sea levels) around 5 degrees north.
What are the actual on-site sea conditions like when these side-by-side extremes exist? It seems to me that the mixing would cause great upwelling and downwelling areas. If this is true, is this a place where animals might thrive, even though it is so far off a coast?
Thanks for your excellent explanation of a very complex system.
I am a little confused and would like to understand better how the Ekman transport works in El Nino. The winds are moving west to east, so is it because of the Coriolis force that the movement is towards the Equator?
The signal you are seeing is one of the other sea level signatures that Jorge was talking about last week. A great deal of what you are seeing is due to a change in a surface current in this region: the North Equatorial Countercurrent (NECC). The current along the equator runs east to west, but there is a countercurrent just north of this that runs west to east. This is a surface current, so don't confuse it with the Equatorial undercurrent which is a deeper countercurrent along the equator.
What the map is showing is that the NECC has a much higher speed than normal. We can tell this because, in the northern hemisphere, higher sea level is to the right of a current and lower sea level is to the left. The NECC is normally much faster than normal during an El Nino. Along with the Equatorial undercurrent, it also helps advect warm water east.
There is undoubtedly some upwelling and downwelling where the shear (i.e., slope) is the largest. I don't know of any studies of animal populations in these regions, but that is not my area of interest.
The winds in the far western Pacific have blown from west to east during the present El Nino on several occassions, but a similar change in the Ekman transport can happen if the wind speed from the east to west (the normal direction) is much lower than normal. What causes the changes is the wind anomaly.
For a simple model, consider the winds blowing west to east for a thousand kilometers both north and south of the equator. Ekman found that the net transport was perpendicular and to the right of the wind north of the equator and perpendicular and to the left of the wind south of the equator. This is caused by the Coriolis force (due to the Earth's rotation).
Since the wind anomalies exist for several degrees latitude north and south, north of the equator there is going to be a general transport toward the equator (right of the wind). The same is going to happen south of the equator, although in this case the transport is to the left of the wind.
Don, One of the teachers has asked a question (College classroom, I think) about why El Nino has caused such heavy rains on the west coast of the U.S. and so little in the Western Pacific. I'm guessing that this is because the warmer ocean temperatures mean more evaporation in the Eastern Pacific, and hence more rain. But this gives the impression that rainfall (or moisture from oceanic-climatic processes) is finite. Can you clarify this for us?
Thanks for a great presentation. It took me a couple of days to get through it but it was excellent.
You're right that part of the reason there is more rainfall in the east than the western Pacific has to do with the water temperature. However, surface waters in the western Pacific as still quite warm. The other factors that need to be taken into account are the wind and the atmospheric pressure which drives the large-scale convection patterns.
During El Nino, there is a high pressure system over the western Pacific and a low pressure system over the central and eastern Pacific. Where pressure is high, little water vapor gets put into the atmosphere so it can not precipitate out. Where pressure is low, water can evaporate into the atmosphere and can thus precipitate out. This is shown in my slide presentation at sld009.htm and the following slide. The region with low pressure and rising water vapor is indicated by the clouds.
Since it is getting near the end of the week, I would like to thank Lynn, Tina, and Peter for inviting me to be part of this very interesting workshop. I'll still be around for the next few weeks, so if you have any more questions, please feel free to ask, either here or at my e-mail.
If we get any new TOPEX/Poseidon data before the workshop is over, I will try to post the latest image here. If you are curious as to the status of El Nino in the next few months, check out my web-site. It is updated monthly.
Thanks, Don for your participation. We have enjoyed your presentation.
As promised, below is the latest image from the TOPEX/Poseidon satellite, for the time-period from February 27 to March 9. Sea level is continuing to drop in the eastern equatorial Pacific. The areas of large positive sea level anomalies north and south of the equator are moving westward. These indicate Rossby waves, which are a part of the dynamical readjustment as El Nino dissipates.
Note that the sea level in the Indian Ocean is still very much higher than normal. It is unclear at the moment whether the warming there is dissipating or not.
February 27 to March 9, 1998