When I was talking about foam stripes the other day, you might have noticed that the foam stripe wasn’t a foam stripe all the way, but became “just a stripe”. And since I have been thinking about surfactants a lot recently, I think that’s actually what we are seeing in places where the stripe is just a stripe: Some kind of film on the water.
Inside “the stripe” the water looks a lot smoother and waves are dampened out.
At first I thought that it was maybe due to being sheltered from the wind, but clearly that wasn’t the case. If anything, the stripe was in a location where it was more windy (see where it comes out underneath the pier? That should be funneling wind around that corner, not sheltering from the wind!). But surfactants would make sense if they collected in the convergence zone of the stripe.
See how the reflections from the sun are different where there is stuff on the water and the waves are dampened out?
As you might have noticed, I am getting a little obsessed with those foam stripes. Another day, a little more wind, looking up the coast:
And down the coast…
Do you notice the irregularities in the foam stripe in the pictures above? Those are the places where, in much calmer weather, you see the funny waves. I.e. there are steps that disturb the wave pattern and hence the foam stripe.
Its taking a very similar path like it did last time, but this time it joins the one parallel to the sea wall, rather than forming a second stripe parallel to the first one.
And if we continue further down the coast, we see a similar phenomenon (we are now walking towards that edge in the background of the picture below).
Looking back, we see another foam stripe coming from the other edge of the pier, joining the one parallel to the sea wall.
See, this is how they meet at the sea wall?
And, funnily enough, a similar stripe can be seen going through the sailing harbour: Entering it through an opening in the pier and then going across the harbour and out the other end, until it finally joins the stripe at the sea wall.
This stripe finally convinced me: There don’t need to be convergences for the stripes to exist, at least not for those that aren’t running in parallel to the coast. Because I cannot imagine a convergence zone running in such a way through the harbour that is partly sheltered from the wind, has pylons in it, and just has completely different conditions than the open fjord. Or at least the mechanisms forming that convergence zone would have to be very different from those forming the other stripes. So now I am thinking those stripes are just advecting foam from places where it accumulated (in front of the pier) to new places where it accumulates some more.
And finally it just gets stranded on land:
See how nicely the foam stripe is going around the obstacle? :-)
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!
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!
Or should it be leaves stripes instead of leaf stripes?
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.
A little further downwind foam also appeared, but only inshore of the innermost leaf stripe.
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?
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…
I recently got a new comment on my blog post on foam stripes parallel to the coast, and since you guys hardly ever comment on my blog (I like getting your emails! Really! But why not comment on here? ;-)) it spiked my interest enough to look out for more foam stripes. So about a month ago, I saw this in Kiel: Yes! A foam stripe parallel to the coast!
I’m actually pretty sure that they are there most of the time, at least when there is some wave action going on, but I just never noticed since they are so close to the sea wall and it’s easy to just look out over the fjord and never look down.
But again, as much as I tried, I could not see how the position of the stripe related to the wave field.
But now that I was intrigued, I went back the next morning to take a look. A lot less wind than the night before, and shorter waves. And what do we see?
At first glance, there is no foam stripe, but instead there is a stripe where floating sea weed accumulated (indication of a convergence zone? Can you see it? Sorry about the bad picture).
But then in other spots, there is a little bit of foam, too, where the sea weed accumulated. And this time I could actually see where it came from: That is the area where most of the wave breaking happens when reflected waves meet incoming waves. Mind, though, I could not observe that on previous occasions!
Plus, waves break when they meet the sea wall, and that creates more foam that sits between the foam stripe and the sea wall.
Since at that point I was really intrigued, I went back around lunch time the same day. And what do we see?
Foam stripes are gone, but there is a lot more sea weed now! And all confined to a narrow stripe along the coast.
Actually, if we look really carefully, we can see that there still is a foam stripe parallel to the coast, but very very close to the sea wall now.
And looking down the other direction, there are even two stripes with sea weed, and only the one closer to the coast also has foam on it. It gets weirder and weirder :-)
Anyway. I guess what we need for foam stripes is enough choppy wave action that waves break (waves alone are not enough as you can see in this post on standing waves which happens to talk about the exact same spot), because if waves weren’t breaking, where would the foam come from? Although sea weed could still accumulate, I guess?
I will investigate further. In the meanwhile, does anyone have any more ideas of what is going on? Do you now see those foam stripes everywhere, too? :-)
P.S.: Kiel peeps, btw, you probably know exactly when I took those pictures, since there is the Sweden ferry coming in on one and then the Norway ferry going out on the other… :-)