Those foam stripes parallel to the coast — again!

I think I might be getting obsessed with those stripes parallel to the coast. We saw them as foam stripes, eel grass stripes and now today: leaf stripes!

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Or should it be leaves stripes instead of leaf stripes?

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Interestingly enough, that day there wasn’t just one stripe, but in some places there were even two. It’s a little difficult to see in the pictures, but it was very clear in person.

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See?

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A little further downwind foam also appeared, but only inshore of the innermost leaf stripe.

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And then a little further downwind, several parallel foam stripes appeared. Now this I could imagine being Langmuir circulation. And all the other stripes must be on individual convergence zones, too?

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Someone should hire a PhD student to figure this out, it is really bugging me that there is a phenomenon that we can observe pretty much every time we look at Kiel fjord, yet I can’t find anything on what is going on there.

Luckily, the day I took those pictures, my famous oceanographer friend J was with me, and it was bugging her almost as much as it was bugging me :-) We decided the most likely explanation was that someone had pulled all those leaves on thin strings and put them out in the water just to see whether someone would notice…

The importance of playing in outreach activities.

Some time ago, I wrote two blog posts on the importance of playing in outreach activities for the EGU’s blog’s “educational corner” GeoEd. Both have now been published, check them out! Here is the link on EGU’s website (here) and in case that ever stops working, it is also available on my own website (here – including a lot of bonus materials that didn’t make the cut over at EGU)

What do you think? What makes for the best outreach activities?

Continuity

What do you see when you look at an aquarium?

When I was in Gothenburg last year for EMSEA14, one night we got to hang out at the Sjöfartsmuseet Akvariet there, and, even cooler, had the whole place to ourselves. A lot of the staff was around and happy to chat, including people who actually designed the exhibitions, so that was really exciting. But those are the times when I realize that I am really a physical oceanographer at heart. I like looking at colorful anemones or fishies or sea horses, sure!

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But what I am most excited about is stuff like this: When there is enough suspended stuff in the water to visualize a flow field to recognize hydrodynamic principles, like in this case continuity.

Eddies in a jar

Rotating experiments in your kitchen.

Do you know those Saturday mornings when you wake up and just know that you have to do oceanography experiments? I had one of those last weekend. Unfortunately, I didn’t have a rotating table at hand, but luckily most of my experiments work better than the exploding water balloon time-lapse I showed you on Monday, so this is what I did:

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Dye spiral

I took a large cylindrical jar, filled it with water, stirred, let it settle down a little bit and then injected dye at the surface, radially outward from the center. Because the rotating body of water is slowed down by friction with the jar, the center spins faster than the outer water, and the dye streak gets deformed into a spiral. The sheet stays visible for a very long time, even as it gets wound up tighter and tighter. And you can see the whole eddy wobble a bit (or pulsate might be the more technical term) because I introduced turbulence when I stopped stirring. So pretty, the whole experiment. And so satisfying if you need a really quick fix of oceanography on a Saturday morning!

Watch the movie below if you want to see more. Or even better: Go play yourself! It’s easier than making one of those microwave mug cakes and sooo goooooood :-)

Hadley cell experiment

Cooling and rotation combined. (deutscher Text unten)

I can’t believe I haven’t blogged about this experiment before now! Pierre and I have conducted it a number of times, but somehow the documentation never happened. So here we go today! Martin and I ran the experiment for our own entertainment (oh the peace and quiet in the lab!) while the kids were watching a movie. But now that we’ve worked out some of the things to avoid (for example too much dye!), we’ll show it to them soon.

This is a classical experiment on general atmospheric circulation, well documented for example in the Weather in a Tank lab guide. The movie below shows the whole experiments, though some parts are shown as time lapse.

Für unsere eigene Unterhaltung haben Martin und ich dieses Experiment gemacht, während die Kinder mit allen Gruppen gemeinsam einen Film gesehen haben. Himmlische Ruhe im Labor! Aber wir werden es bald auch der Gruppe vorführen.

Dieses klassische Experiment zeigt, wie die großskalige atmosphärische Zirkulation in der Hadley-Zelle angetrieben wird und ich weiß auch schon, wie wir es beim nächsten Mal noch eindrucksvoller hinbekommen als bei diesem Mal!

Langmuir circulation, take 2

Attempt at mechanistic understanding of Langmuir circulation.

After  complaining about how I didn’t have mechanistic understanding of Langmuir circulation recently, and how I was too lazy to do a real literature search on it, my friend Kristin sent me a paper that might shed light on the issue. And it did! So here is what I think I understand (and please feel free to jump in and comment if you have a better explanation).

First, let’s recap what we are talking about. My friend Leela (and it was so nice to have her visit!!!) and I observed this:

Long rows of foam on the surface of the fjord, more or less aligned with the direction of the wind (we couldn’t tell for sure since we were on a moving boat, and since it was a tourist cruise we couldn’t ask them to stand still for a minute to satisfy our oceanographic curiosity). Foam is – and so much makes sense – accumulated in regions of surface convergence.

But let’s see. The explanation that Kristin forwarded me is from the paper “Upper ocean mixing” by J.N. Moum and W.D. Smyth for Academic Press Encyclopedia of Ocean Sciences, 2000According to my understanding of their paper and others, Langmuir circulation is related to Stokes drift.

Stokes drift is the small current in the direction of wave propagation that is caused by orbital wave motions not being completely closed (even though they are as a first order explanation, and that’s what you always learn when you think about rubber ducks not being laterally moved by waves).

As the wave orbital motions decrease with depth, there is a shear in the Stokes drift, with strongest velocities being found at the surface. At the same time, if there are small disturbances in the wind field, there are small inhomogeneities in the resulting surface current, hence shear that generates vertical vorticity.

The combination of horizontal and vertical vorticity causes counterrotating vortices at the ocean surface. The convergences between two adjacent rows concentrate the wind-driven surface current into a jet at the convergence, hence providing a positive feedback.

Voila: Stokes drift!

Water seeks its level.

A solution for the siphon problem of the fjord circulation experiment.

After having run the fjord circulation experiments for several years in a row with several groups of students each year, Pierre and I finally figured out a good way to keep the water level in the tank constant. As you might remember from the sketch in the previous post or can see in the figure below, initially we used to have the tank separated in a main compartment and a reservoir.

 But there were a couple of problems associated with this setup. Once, the lock separating the two parts of the tank fell over during the experiment. Then there are bound to be leaks. Sometimes we forget to empty the reservoir and the water level rises to critical levels. In short, it’s a hassle.

So the next year, we decided to run the experiment in a big sink and tip the tank slightly, so that water would just flow out at the lower end at the same rate that it was being added on the other side. Which kinda worked, but it was messy.

So this year, we came up with the perfect solution. The experiment is still being run in a sink, but now a hose, completely filled with water, connects the main tank with a beaker. The hight of the rim of the beaker is set to the desired water level of the big tank. Now when we add water to the big tank, there is an (almost – if the hose isn’t wide enough) instant outflow, so the water level in the tank stays the same.

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New setup: A bubble-free hose connecting the tank and a reservoir to regulate the water level in the tank.

This way, we also get to regulate the depth from where the outflowing water is being removed. Neat, isn’t it?

Fjord circulation

Tank experiment on a typical circulation in a fjord.

Traditionally, a fjord circulation experiment has been done in GEOF130’s student practicals. Pierre and I recently met up to test-run the experiment before it will be run in this year’s course.

This is the setup of the experiment: A long and narrow tank, filled with salt water, a freshwater source at one end and an outlet at the other end. This sets up a circulation from the head towards the mouth of the fjord close to the surface, and a deep return flow.

Watch the movie below to see how different circulations are set up depending on the depth of the freshwater source. As in the picture, velocity profile 1 is for the case where freshwater is being added close to the surface, and in case 2 the freshwater is being added deeper down.

Langmuir circulation

We think we observed Langmuir circulation, but we don’t understand the mechanism causing it.

Recently, my friend Leela came to visit Bergen and we went on a fjord cruise to make the most of a sunny October day. We observed foam streaks on the fjord. The structures were long and persistent, and being the oceanographers we are, of course we knew that they had to have been caused by Langmuir circulation.

Langmuir circulation on Østerfjorden, Norway.

But then we started wondering about the mechanism driving the Langmuir circulation. Textbook knowledge tells us that Langmuir cells are spiraling rows with convergences (the foamy stripes) and divergences (in between the foamy stripes) at the surface. They are, according to common knowledge, caused by wind that has persistently blown over the surface for more than some 10 hours, and by Ekman processes. Plus there might be some interaction with waves.

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More Langmuir circulation

But that’s about where my knowledge ends, and I have absolutely no mechanistic understanding of Langmuir circulation. Literature research was unsuccessful (at least in the period of time I was willing to spend on this), a quick poll of my colleagues didn’t help, so now I am turning to you, dear readers: Do you have a simple mechanism for me that explains Langmuir circulation? Please help!