zondag 14 april 2013

My first 'real' scientific report: Rossby wave observations in the Atlantic Ocean

My background is not entirely scientific (maybe that is a good thing, I don't know). I've done my bachelor and masters at the faculty of Aerospace Engineering of TUDelft, which is a Technical University. The biggest part of the education is focused on engineering problems, team work and just building cool stuff. Which is great, but completely different from scientific research (we could debate this, but that is for another blog post (email me ;))). So what decided me to dedicate my life (well for now, I can't look in the future) to science.

In our last year of bachelor, we have an end project called the Design Synthesis Exercise. Ten students, ten weeks, locked up in very small room, creating, designing and engineering. Our assignment was to design a mission to Ganymede to find evidence for a sub-surface ocean. Cooooool!!! After a lot of work and fun, we came up with the Marius Space mission (Simon Marius (No not Galileo) was the second man (yes the first one was Galileo) that observed the Jovian moons, and gave them their current names (Galileo's were stupid)). One of the our team members had just followed a movie-making course, so you can see our end-result in a well designed Hollywood trailer (There is a rocket coming up from the ground, which is kinda cool. And you can see me nodding in-time, with a terrible haircut). Oh, the design looks a lot like the current Juice mission. We gave ESA our design, and they were well, flabbergasted (is this how you spell it) and used it...

Back to reality. During this assignment the supervisor of this project inspired me in looking at the science of the mission. I discovered the beauties of Ganymede and the men and women trying to get sense of what was going on there. His inspiration to me was so great that I decided to do my master thesis research under his supervisie.

My master thesis research was quite engineering oriented: Improving the current orbit determination of Cryosat-2 (the first one blew up), by looking at new laser ranging formats, trying different terrestrial reference frames and introducing a micromodel of the satellite for the solar radiation pressure modeling. All very engineering, however I discovered something very cool during my literature study.

My supervisor gave me a book (Satellite Altimetry and Earth Sciences, by Lee-Lueng Fu and Anny Cazenave) and said to read it, learn from it and try to summarize it. He gave me an extra task, which was the following. "Try to reproduce this figure."

This is not the true figure from the book, but my result, which looks the same (if you don't believe me, read the book and learn the wonders about satellite altimetry). What does this figure illustrates? I will try to explain.

First always look at the axis, what do they represent: x-axis, latitude in degree, ranging from -50 to 50 degrees (look at your globe, or use this), y-axis, propagation speed in cm/s (slow moving stuff). Then title: hmmm, that is not giving me any more information than when I not looked at it (google: hmmm, first find tells me something about an Interannual baroclinic Rossby wave, that is interesting!!!). Legend: There is something theoretical, something to do with basins (ah, ocean things, so the stuff must be propagation speed of waves (Rossby wave in particular). Just by not looking at the middle of the figure, I am getting there) and I see different names of altimeter satellites. In the end, looking at the figure: the dots, squares and asterix sort of follow the theoretical line, but not quite.

What we see is the dispersion relation of slow moving waves (Rossby waves) with their latitude dependence. Rossby waves move slower in areas with greater latitude and they move faster towards the equator. What we also see is that the theory doesn't quite fit the data, why this is I did not know, but I had some ideas (I think today researchers found the reason for the misfit with the linear theory, but I have to find the article (I will come back to you with that, maybe...)). 

But the most interesting thing is what are these so-called Rossby waves and how can you (yes, you, just like me) observe them. Rossby waves (oceanic ones, there are also atmospheric ones, the more famous ones, but I am with the underdog ;)) are waves that propage from east to west (against Earth's rotation) in the oceans. They are one of the only means to give climate information of eastern part of oceans to the western part. Their amplitude is usually a few centimeters, but their wavelength can go up to 100 kilometers. Due to these sizes you will not notice them when they pass you if your on a boat, but they are visible from outer space.

Altimeter satellites are measuring the oceanic surface for decades now. Satellites like ERS-1 and -2, TOPEX/Poseidon Envisat and Jason-1 and -2 have high accurate altimeters onboard, measuring the distance between the satellite and the ocean surface up to mm accuracy. After precise determination of the position of the satellite (which was my master thesis research goal, for Cryosat-2) the geometry of the ocean surface is measured. This data is available for everybody, so if you want to look at waves, go for example to the rads database. A database (set-up by our group) containing all space altimetry observations.

To view the Rossby waves, extract a long time series (few months, this is the time a Rossby wave needs to cross the North Atlantic) of one particular latitude of the tidal-free ocean height measurements. An area like depicted in the figure:

Collect all the measurements in bins of a few days of this area and plot the pixel rows below each other like a time serie (Just like the waterfall plot in my previous blog posts (hmm, science looks a lot a like)). The results would look like the following:

Here time runs from up to down and the x-axis depicts the longitude in degrees. I've normalized the height measurements. In the lower figure the bathymetry (ocean floor) is illustrated (I hoped to see some ocean floor dependency). In the color plot, Rossby waves are seen as stripes from up-right to low-left (reddish colors). To estimate the velocity of these waves, you should calculate the slope angle of these lines. If you do this correctly the velocity will be around 2-3 cm/yr (at 34 degree latitude). A clear distinction between the west and east part of the plot is seen (angle is smaller in west part than in east part), which tells us that Rossby waves flow faster in the west part at latitude 34 degree. 

At this latitude the Gulf stream is flowing, which is a bit counter intu├»tieve, because the Gulf stream flows the other way (west-to-east), against the Rossby wave. However due to some energy interaction (I don't have a clue), this results in an increase of propagation speed of the Rossby wave. They flow faster against the stream.

These kind of waves will never be seen on television or other media, but I think they are cool. We try to model these waves, which can only be observed by satellites, but still they show us strange interactions and can give new insights in ocean modeling. They are a good example of science that is purely curiosity based. But could have a great impact in modeling (for example in climate models, which is news and media important, ah well...).

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