Foam stripes mystery — closer to figuring out what’s going on?

I think I might be getting closer to understanding the foam stripe mystery. Remember how we’ve always observed them going in parallel to the coast?

Yesterday I saw this again, looking up the coast in one direction…

…and down the other direction. I’ve had the hypothesis that they might be somehow related to Langmuir circulation, but in any case there must be some kind of convergence zone there.

But let’s move closer to that pier we see in the background of the picture above. Here we see a foam stripe parallel to the pier, but at a 90 degree angle to the see wall that I am standing on and that has a foam stripe running in parallel, too! And even more curious: at the edge of the pier, the foam strip detaches and runs toward the coast! See?

Looking down the coast again, we see that foam stripe coming in at an angle, and running in parallel to the coastal stripe in the far back.

Looking up the coast from the pier right where it meets the sea wall, we see both foam stripes running in parallel (as we saw in the picture above):

I think what is happening here is that the foam of the foam stripes doesn’t form locally (which was an implicit assumption I had whenever I was staring at the water, trying to observe more wave breaking there than in other places). Instead, foam forms somewhere else (probably pretty much all over the place) and just accumulates in those stripes. That’s actually pretty likely if we think back to the eel grass or leaf stripes: the eel grass and leaves were clearly advected from somewhere else, too. And actually that’s the same with Langmuir circulation, too: stuff just accumulates in convergence zones but isn’t formed there.

So for some reason there is a convergence parallel to the sea wall as well as the pier, and foam just accumulates there. And as for the part of the stripe that detaches from the pier and runs to the coast? It is going more or less downwind. So it’s probably just part of the stripe parallel to the pier that gets advected around the corner and blown toward the coast.

Why does that stripe end up in parallel to the one at the coast rather than joining it? I don’t know yet. But at least now I only need to figure out why there are convergences in some places and I can let go of the obsession with foam formation in the stripe itself :-)

Do you have any idea that might explain those foam stripes? I’d love to hear from you!

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…

Foam stripes parallel to the coast.

On my way to Heligoland the other day I noticed a phenomenon that I found really intriguing and that I should probably be able to explain. I first saw it on the screen of the boat’s web cams when we were about to leave the port of Hamburg. Unfortunately I could, at that point, only take a photo of the screen (but see how I excited I was to actually take a photo of the screen? ;-)).

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Can you spot what I mean? Yes, that foam stripe running pretty much parallel to the pier! The place where it bends is right in front of our boat, which you see the railing off as that white stuff at the bottom of the screen).

But then, on Heligoland, I saw it again and became even more intrigued. Right in front of the place I stayed at, you could see it looking down the coast to your left…

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…and to your right!

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Here we can also see the stripe bending at some point, but here again the bend coincides with a change in the coast line. Similarly to what we saw in the port of Hamburg above, the stripe stays at more or less the same distance from the coast, so it is parallel while the coast is straight, and bends out when there are obstacles (like the catamaran above or the rocks below).

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So how do we interpret the whole thing? I am not quite sure. I seem to have a very vague recollection that it should have something to do with half a wave length of the dominant wave, and foam collecting in a node point. And that makes sense intuitively. Except that I have several (ha! understatement of the month) minutes of video footage of the above, and I cannot for the life of me spot anything that would explain the stripe. If it is a node point, it is a very well-disguised one and I am surprised the foam can find it!

But there must be something different about that location than about all the other places closer to or further away from the coast. Any ideas, anyone?

More wave phenomena on a lake, and a bit about wind

Waves on Aasee in Münster. By Mirjam S. Glessmer

Last week I showed you the results of my “wave hunt expedition” on Aasee in Münster. Today, I am following up with the same lake on the day after and the day after that. Even more wave phenomena to observe!

First, on my second day in Münster on my way to the conference:

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Clearly it had been windy for a while with more or less constant winds: You see Langmuir circulation cells.

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So imagine my surprise when, on day 3, I wake up to this view:

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Absolutely no waves at all, and no wind! Reason enough for a pre-breakfast stroll.

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As I was walking the wind picked up, as you can see in the increased surface roughness in the middle of the lake.

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But many parts of the lake were still completely calm. For example that weird building, which I sat at for the next half hour or so.

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Sitting there, I watched the “sea state” turn to slightly more wavy (see above — aren’t those pretty reflection patterns? :-))

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And I love how you have those tiny wave trains. So pretty!

At some point it got too windy for my liking, and I wandered on. And noticed a spot that I had missed on my last walk: A drain going into the lake, making more pretty patterns!

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Eventually I had walked all the way around the lake again into the lee of the land, which would have been really boring if it had not been for some duckies:

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Oh, and of course more pretty reflections.

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Hope you have a great day, too! :-)

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!

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!