Introduction
Hello my name is Jorge Vazquez. In my 16 years at the Jet Propulsion Laboratory, I have had the pleasure of seeing a new science emerge. That science is satellite oceanography, and altimetry has been a major component of the evolving new science. I would first like to give a brief history of how altimetry has changed in its 25 year history.
An altimeter is basically a microwave radar pulse that is sent from the orbiting satellite (space station?), bounces off the earth’s surface, and returns to the orbiting spacecraft. (see Figure 1). The travel time of this pulse (the time it takes to get to the surface of the Earth and return to the satellite) is then recorded. This is the same principle that allows an individual looking at a radar screen to determine how far an object is. Doppler radar is used to detect where rain is falling. Thus the distance is related to the time the pulse has traveled. The farther away an object is the longer it takes that pulse to return. Thus as the ground to spacecraft distance changes so does the travel time. This is the same concept used in certain airplane altimeters and radars. Thus the physics of altimetric measurements is nothing new, but its application to oceanography began a revolution and oceanography has never been the same.
First we need to find out what is it that an altimeter measures. Basically the height of the ocean. There are many reasons why the height of the ocean is not the level (see figure with TOPEX image). There are seamounts and trenches which change the gravitational pull on the ocean surface and thus the height. There is the moon and sun pulling on the ocean surface that causes the tides. There are tremendous ocean currents, such as the Gulf Stream, which can cause a 1 meter (yard) change in the height of the ocean between the US East Coast and Bermuda. There are winds which cause waves and also force ocean currents. There is the sun heating the ocean, which causes it to expand. There are atmospheric storms which change the pressure on the ocean surface. All of these things change the ocean surface by a different amount. By far the gravitational pull is the greatest amount (100 meters). However, the heating of the ocean surface (30-100 centimeters, 10 to 40 inches) and how the atmosphere and ocean react to that was for the first time measured by TOPEX/Poseidon. It was these measurements that allowed us for the first time to observe El Niño. Although this might seem a small amount, the climatic changes worldwide can be catastrophic.
Skylab
The first experimental altimeter was flown on Skylab. its use, however for oceanography was very limited because it had an accuracy on the order of 100 meters (yards). Remember that, in general, the waves in the ocean are only a few meters high, and the now world famous El Niño changes the level of the ocean by only 30 centimeters (10 inches). One can immediately see that the Skylab altimeter was more a “proof of concept” instrument than the detector of El Niño we see today.
Seasat
The first true oceanographic altimetric mission occurred with Seasat in 1978. Unfortunately the mission only lasted for 3 months because of a catastrophic electrical failure. The accuracy of the altimeter on Seasat was shown to be on the order of several meters and with it came the birth of the new science of satellite oceanography.
The accuracy of the Seasat mission allowed for the first mapping of the Ocean’s sea surface, which showed the large mid-atlantic ridges and trenches of the ocean bathymetry. Remember these changes can be around 100 meters. See the figure below:
In scientific terminology it was able to detect global features associated with the bottom of the ocean. For example the ocean surface over the mid-atlantic range (undersea mountain chain in the middle of the Atlantic) rises several meters due to the extra mass of the earth’s surface at that point. In the figure above highs are marked by red to white and lows are blue. Thus Seasat was good for detecting changes in the depth (because of mountains and trenches) of the ocean that effect the ocean surface, but could not detect how the major ocean currents change over time or El Niño.
The reason for this is the following. If you draw a straight line from a point on the surface of the earth to the center, there is more mass (in an average sense along that line) if it goes through a mountain than say at death valley, which is below sea level. This is because the earth’s surface has bumps, hills, valleys all of which change the pull of gravity. The ocean surface changes it shapes based on changes in the Earth’s mass and thus the pull of gravity. . That is why one sees a trough in the ocean surface over trenches and high’s over mountains. These changes due to the “geoid” can amount to several meters and even 100 meters. These were the changes seen by Seasat. Over a large seamount the direction of the pull of gravity is always towards the seamount (more mass). This has a tendency to push up the ocean surface at the location of the seamount. The opposite is true over a trench. Let us do a thought experiment. Think of a balloon that is filled up pretty close to the max with air (a fluid like the ocean). Now remember that gravity is a force so let us substitute the force of gravity pushing in on the ocean surface (balloon) with your hands pushing in on the sides. If you push in on both sides of the balloon you are basically doing the same thing the seamount does, ie cause the force vectors to push in on the sides of a liquid. Assuming you don’t push too hard to burst the balloon what happens? It expands in the middle upward. The effect of a trench would be if you sit on the balloon and cause the force vectors to point outward. The balloon expands but sideways not upwards. Also now there will be a trough where you are pushing down on the balloon. Of course the analogy is simplistic, but it does show how the direction in which a force is applied to a surface filled with a fluid can cause the surface to either be pushed upwards or downwards. The scales are also important. In the ocean seamounts that can be kilometers in height cause changes in the ocean surface of 100 meters. This is the single largest factor affecting the change of sea level. Remember though that the Earth’s gravitational field doesn’t change with time (well at least on the time scales we are looking at). So if a satellite goes over the same point on the Earth at given intervals of time, any change in the level of the ocean cannot be because of the gravitational pull. That is seamounts and trenches might change on geologic time scales but not on scales of El Niño. Any change must be due to other forces. This is actually great news, because by passing a satellite over the same point, all we need to do is look at the differences as it passes over the same point. Such a difference can’t be because of the pull of gravity. Forgive me for giving you a little history of altimetry and satellite oceanography, but I want you to understand the amazing engineering and scientific accomplishment of TOPEX/Poseidon. As an oceanographer who grew up during this time I feel it is just as impressive as Mars pathfinder.
TOPEX/Poseidon
For the first time with TOPEX/Poseidon we were able to see how the ocean changes from one year to the next. Earlier missions, such as GEOSAT, a Navy altimeter, had allowed one to detect the motion of large ocean currents, such as the Gulf Stream, from one week to the next. However, it was still not possible to see El Niño. Thus TOPEX/Poseidon was exactly what was needed to study El Niño and climate change. With the advent of the 1997, 1998 El Niño, TOPEX/Poseidon allowed for the first time global observations of how sea level was changing in the Pacific. Remember how far we have come. With Seasat we could basically only see changes on the order of several meters (yards). With TOPEX/Poseidon we can now see changes of only about 2 centimeters (1 inch). There are many reasons for this improvement. The foremost is in knowing the orbit, or more simply put, where exactly the satellite is. Look at Figure 2 carefully and one can see why without knowing the orbit the measurement of sea level change is impossible. The reason for this was using very accurate laser beams to detect the where the satellite was and the global positioning system. This accuracy allowed the National Oceanic and Atmospheric Administration to forecast the event and give time to prepare for its effects. These effects are different worldwide and not always consistent. In California we generally get more rain which can have devastating effects. For the first time changes in sea level could be used to predict:
1) Fisheries
2) Coral reef-A major study is underway to detect how the bleaching (due to warm sea surface temperatures and higher than normal sea level) of coral reefs is related to climate change.
3) Regional applications-A major application of altimetry has been in the detection of ocean eddies. One example is the loop current that flows northward from Cuba. At times this current sheds an eddy (think of it as an ocean storm or circular moving body of water with a diameter of 100-200 kilometers (80 to 121 miles) which travels into the Gulf of Mexico. The image below shows this loop current as it begins to shed an eddy into the Gulf of Mexico. Notice the red circular field Northwest of Cuba and West of Florida.
