Sometimes it seems like articles in newspapers are published as if they were planned to coincide with my syllabus. This was one such week. We set the drifters adrift on Tuesday and only two days earlier, on Sunday, the New York Times published a piece on surface currents in Monterey Bay, Finding Order in the Apparent Chaos of Currents.
The story has a big flashy graphic that decorated most of the cover of the Sunday Science section of the Times, and comes complete with an accompanying flash video and podcast. Click image for a larger, more readable version.
The article reveals how theoretical scientists are doing some pretty wild stuff with Lagrangian flow models and “Lagrangian coherent structures”. Now, I can barely pronounce “Langrangian”, much less explain it to my students, but there are fascinating revelations in all kinds of fields such as blood flow from ventricles, turbulence in atmospheric flow patterns, and, of course, dispersal of pollutants in the ocean currents. On this image, the bright colors show the “amount of stretching and chaotic stirring in surface water”. I’m OK with the chaotic stirring, but stretching? Every semester I explain to my oceanography class about the incompressibility of water is the basis of hydraulic brakes working properly. Now I have to explain that water gets stretched somehow? Maybe now that you have taken the time to read this can explain this one to me.
In any case, the article is very cool, even though I still don’t get a real, physical feel for how water circulates through the bay. It’s a different way of looking at flow with which I have no experience.
I haven’t decided yet whether to ask my students to read this article. What do you think?
Drifters come out of the bay today. More on that later.
While this analysis technique is even above my head, the key point to remember here is that while the ocean current measurements are of the surface, that is 2D, there is another dimension that is not directly taken into account here. So when they’re talking about “stretching” or “squeezing” they’re really talking about the 2D flow.
In reality, what is really happening is that there is a divergence or convergence in the flow field, which if the fluid was compressible would cause it to increase or decrease in density. Of course, water is not very compressible, so in this case, the divergence (convergence) causes a much smaller upward (downward) vertical velocity. This has a huge effect on oceanic mixing and greatly affects the chemical and biological makeup of the ocean. As they say, we’re really only scratching the surface here.
Ahhh… That makes a lot of sense.