Awesome outreach collaborations to continue: We won a Bjerknes Visiting Fellowship 2018!

We are excited and grateful for a great opportunity for continued collaboration that has recently presented itself: Elin won a Bjerknes Visiting Fellowship 2018 for me (Mirjam) to visit Elin and the rest of her team in Bergen for a month in 2018!

We have several goals for that visit, but the main one is to develop more hands-on experiments (which we lovingly call “kitchen oceanography”), which parents, teachers, and other educators can use to get children excited about oceanography (and obviously for the grown-ups to play with, too :-)). Between Elin and me, we do already have a lot of experiments which we use regularly and recommend (for Elin’s, check out this site, and mine are here). But we would love to bring them in a different format so that they are easy to find and use, and are well integrated with the ekte data project. And then, obviously, we want to let everybody in Bergen (and all of our faithful readers) know where to find the experiments, and how to use them in science communication.

So plenty of stuff to stay tuned for! We’ll absolutely keep you posted on our progress on here!

Scrolling back memory lane: Reminiscing of rotation

Hei, remember we were in Grenoble and did exciting experiments on the 13-m-dimeter pool on a merry-go-round only last autumn? Feels like a long time ago already. But here are two ways for you to scroll back memory lane:

  1. Check out the new way we’ve created to access to Elin & her team’s previous adventures on our “previous adventures” page. Did you know we have posts in English, Norwegian and Swedish, addressing audiences from primary school kids, over high school pupils, to teachers, our fellow oceanographers and friends and family? Quite impressive how much Elin has written over the years, and fascinating to read, too! And also check out her ongoing adventures like the student cruise to Bjørnafjorden!

2. Read the award-winning* poem below which I just found somewhere in my files. Yes, we did feel not quiiiite well when we were working on the platform at high rotation rates, but we still loved the whole experience, every minute of it! 🙂

*Yes, this poem really won an award in a competition run by @IAmSciComm on Twitter back in October. And here is the awesome bag and sea horse card we won 🙂

Mesmerizing flow patterns and sad goodbyes

Written by Anna Wåhlin

It is the final day of experiments here at the Coriolis platform. The apartment is emptied of personal belongings, bicycles are being returned, goodbyes are stretched out. The lasers will soon be dark, the platform will grind to a halt and the tank will be emptied. It has been a fantastic time! I am amazed at what we have accomplished together during these weeks – answers to some of the most basic questions that are currently asked about the future for the Antarctic ice sheet.

Figure 2a-d: More mesmerizing flow patterns

The last days have been spent re-running some of the experiments that needed an extra quality-check, and we finished the very last one only an hour ago. Next week I will stay behind alone to try to get some nice photographs of the flow for our future publications. In order to prepare for that we were testing some different dyes. Red dye absorbs the light of the laser efficiently and gives a dark shadow on the images. Our all time favorite is Rhodamin – it is a fluorescent dye that produces its own light if you shine on it with green laser. We spent a good hour simply staring at the eddies and flow, mesmerized by the motion and flowing patterns. A very fine ending to the week! And a suitable finale to the time we have spent here on the rotating platform.

Video: Visualization of a beautiful barotropic eddy created outside the channel. It stayed like this for a good hour. You can see the barotropic structure since it moves in unison on the surface and below the surface, in a nearly perfect two-dimensional motion.

We discovered a new galaxy! Or at least a very pretty vortex

When we move our wall back and forth, we create very strong wing tip vortices that persist for quite a long time.

Above, you see the vortex, lit by a laser sheet close to the surface. You can see the whole column rotating as one, that bright smudge below the swirl is the lower part of the column. There are so many of our neutrally buoyant particles in there that the column looks bright even though it isn’t directly lit by the laser.

And in the picture above, you see those bright smudges on the left of the picture? That’s particles that the vortex hoovered up and then dumped in its path, pretty much like a hurricane would.

And that’s what it looks like as a gif:

Introducing: JB Sallée

Written by JB Sallée
JB Sallée is an oceanographer interested in the dynamics of the ocean and climate with active research efforts on the study of the Southern Ocean and Antarctica shelf circulation. His research mostly focuses on the observational connection between the ocean surface and the deep ocean interior, with particular emphasises ocean ice-sheet interaction as well as on heat, salt and anthropogenic carbon sequestration in the Southern Ocean. His research tackles questions from oceanic turbulence to large-scale ocean circulation, as well as on the impact of ocean physics on biology.

“No one believes a theory, except the theorist. Everyone believes an experiment — except the experimenter.”

Different types of experiments, and why we use such a weirdly-shaped “Antarctica” and are happy with it.

When we want to show people images of our model experiments in a tank, people often imagine that they will be shown cute little miniature landscapes, looking much like the ones you see for really fancy model train setups. And then they are hugely disappointed when they see pictures like the one below and we tell them that yes! that’s our Antarctica that Nadine is climbing on, while Elin is sitting in the Southern Ocean.

The kind of experiment everybody hopes to see could, according to Faller (1981), be classified as a simulation: representing the natural world in miniature, including every detail. Data from those experiments — since they would in theory be realistic representations of the real world — could be used to fill in missing data from the real world in regions that are hard to get real data from, like for example the Southern Ocean. However, since those experiments are designed to represent the complexity of the real world, interpretation of the experiments is as complex as it is to interpret data from the real world: There are so many processes involved that it is hard to isolate effects of individual processes.

The kind of experiments we are doing would be classified as abstractions. Faller describes this kind of experiment as similar to abstract art: Only the main features, or better: the artist’s interpretation of the main features, are reproduced and everything else is omitted. That makes the art difficult to understand for anyone who isn’t well versed in abstract art, but for the experts it is obvious what the point is.

In case of our experiments that means that we have all the relevant features, or better: our interpretation of what we believe to be relevant features, of Antarctica present in the tank: the parts of topography that we think have an influence on how the current should behave, i.e. a V-shaped canyon, a source that supplies water of the correct properties into the ambient “ocean” water, an ice shelf. And when that ice shelf is tilted, we feel like our experiments are already becoming pretty realistic!

These abstractions are the kinds of experiments in which you can, because they are relatively simple, develop new theories when new features of the circulation emerge that you then have to rationalize and include in your theories after the fact.

We have actually also done another type of experiment, a verification. I wrote about it in this post: we tilted the ice shelf because this is a case for which we actually knew from theory how our current should behave, in contrast to all the previous experiments where we didn’t actually know what to expect, and we were happy when we observed exactly what we expected based on theoretical considerations. So in this case the experiment wasn’t about discovering something new, but rather making sure that our understanding of theory and what goes on in the tank actually match.

Faller describes a last type of experiment: the extension. That is the kind of experiment that you could perform after a successful verification experiment: Pushing the boundaries of the theory. Does it still hold if the current introduced in the tank is very fast or very slow? If the water is very deep? If the slope of the ice shelf is very large or small? Basically, every parameter could now be changed until we know for which cases the theory holds, and for which it does not.

So why am I writing all of this today? Faller’s (1981) article, before he goes on to describe the framework to think about geophysical fluid dynamics experiments that I mentioned above and which I find quite helpful to consider, starts with the sentence “No one believes a theory, except the theorist. Everyone believes an experiment — except the experimenter.” On this blog, our goal is to bring the two together and not make anyone believe either of them, but to show how both can work together to mutual benefit.

Faller, A. J. (1981). The origin and development of laboratory models and analogues of the ocean circulation. Evolution of Physical Oceanography, 462-479.

How to make sure the properties of water in a tank experiment are *just right*

For all our experiments here on the rotating platform in Grenoble, we have had a source, introducing an artificial current into our water-filled tank. With flow rates between 15 l/min and 60 l/min, and experiments running for about half an hour, that is a lot of water that has to come out of the source!

Below, you see a picture of the source during an experiment, and you see there is a pipe going into it, through which water is being supplied.

That water is coming from the very top of the rotating platform. There is a smaller tank up there which you see on the picture below. This is the tank where the particles which we use to visualize the flow field get added, and water in this tank needs to have the exact density we want our inflow to have. Not easy since it is sitting some 10 meters above the tank, where the air temperature is higher…

In fact, it’s an extremely complex system of tanks everywhere on and around the rotating platform. Below you see a picture of the screen through which most of them are operated:

There are three huuuuge water tanks in which water is prepared. You might have seen them rotating past in some of our videos, or you see them below (on the left you see the rotating tank). This picture doesn’t do them any justice: They are enormous. They are higher than the tank, and the mini tank on top of it, and the whole tent around all of it, and they start from the very bottom of the room (so not the level that seems to be the floor in the picture below).

We got to climb on one of them, which gave us a really great view of the tank (or at least that’s what Nadine says, and what the picture looks like — I was too busy getting over my fear of heights combined with the dizziness of a long working day on the rotating platform to enjoy it much ;-)).

Nadine has described earlier about how for some experiments, we add salt to spice things up. In the first set of experiments, for some, the whole tank was filled with salt water. And for this set of experiments, we sometimes added a small amount of salt to adjust the density of the inflow. But this is how producing the salt water actually works: Salt arrives in big bags, stacked on pallets. The salt pellets are put into the bin you see in the picture below, and get hoovered up into one of the huge tanks, where they are dissolved in water to make a saturated salt solution. That solution is then diluted to whatever salt concentration is desired for a certain experiment.

To fill a whole tank with salt water with approximately oceanic salinity, we need all the salt shown in the picture above!

We are pretty lucky that Thomas and Samuel take care of all the saltwater-making for us. That would be a huge task if we had to do it ourselves, and we are already now not getting bored 😉

And, btw, if you are wondering about how we are getting rid of the dense, salty current that we inject into the fresh ambient water in between experiments: The dense water eventually sinks to the bottom of the tank, slowly filling it up underneath the fresher water. You might have noticed those UFO-shaped flat plates on the bottom of the tank that you see in the picture below. They cover the outlets through which the tank can be drained, such that now water from the very bottom of the tank can be pumped out without introducing a vertical component (which would suck water from higher levels, too).

Quite a lot of effort going on not only to prepare the water, but also to get rid of it again! 🙂

Closing off our channel at the ice-shelf end to avoid unrealistic outflow

We are very deep into discussing all the different ice shelf experiments that have happened so far. As you see above, the white board in our office is filled with drawings of our interpretations of the experiments.

And as you know, things don’t always go exactly as planned. Or, in fact, most of the time they don’t.

One thing that has been happening in our experiments is that water flowed out of the channel underneath the ice shelf. Not a lot of water, but after long discussions, we decided that — since in reality there is no way for water to come out that end of the channel, because there is land closing off the channel at the end — even a little water was too much and that we needed a way to block off that end of the channel.

However, constructing anything inside a rotating tank full of water and with a lot of scaffolding just above the water level isn’t easy (as I found out when I was sweeping the tank, trying to duck under the scaffolding and flooded the waders I was wearing. Yep, true story…), so even after deciding that we needed a solution, it still took a lot more discussing until we actually had a solution that everybody was happy with.

So now we will build a wall! And it is going to be huge! And it will block all the water so nothing is coming out of the channel any more. And it is going to be the best wall in the world (and maybe Mexico will pay for it? ;-))

Actually, the wall is in the tank already, so now we’ll start investigating whether it actually has an effect or not!

One thing we found out already: Moving a large wall back and forth in a tank creates nice eddies (duh!). But look at how pretty they are! 🙂

And now on to even more realistic ice shelves!

We have already described experiments where our ice shelf was tilted, making the setup a little more realistic* than before (link here). But then later that day, we did two more experiments! And this time, the ice shelf wasn’t just tilted, it was also not going up all the way to the surface (or, well, it’s flat bottom did not, and then there was a sharp edge and the side of the ice shelf went out of the water). So we are expecting to see a mixture between the experiments shown is yesterday’s blog post: Some of the water being blocked by the ice shelf, but some possibly conserving its potential vorticity and going down the v of the canyon and then turning around and coming back up.

And that’s what we saw!

Can you spot the return flow that has come out from below the ice shelf in the lower layers before it gets obscured by all the stuff that got blocked by the ice shelf in the upper layers?

Nice when experiments really work out the way you expect them to do! 🙂

*I have a blogpost in the making on what “realistic” actually means in the context of geophysical fluid dynamics experiments, and if that is even something one should aim for (spoiler alert: not necessarily!), but I keep getting too distracted by all the cool stuff going on here in Grenoble, that it hasn’t progressed out of the draft stage. But I will finish it up and post it, I promise!

What happens when a current meets an obstacle? Topographic steering

As long as water depth and latitude stay the same, a current usually happily goes straight forward. However, a large part of what we are doing at the Coriolis tank in Grenoble has to do with what happens to ocean currents when they meet topography, so sea mounts, ridges or troughs under the water, and what happens then is called topographic steering.

Topographic steering basically means that a current will follow lines of constant potential vorticity (ω+f)/H. In this, ω is the rotation of the fluid (more on this here), f is the Coriolis parameter, and H is the water depth. So if a current is flowing  straight ahead (ω=0) in a sea of constant depth, it will stay at the one latitude where it started. If, however, there is a ridge or a canyon in its way, it will try to move such that it either changes its rotation or that it reaches a different latitude so that it stays on a path of constant (ω+f)/H.

What does that mean for our experiments?

In our experiments, we actually change the water depth not only by sloping the floor down into the canyon, we also change it by taking away height from the top by introducing ice shelves.

f in the tank is constant (explanation here), so only ω/H need to be conserved, meaning that the current needs to either follow lines of constant depth, or compensate for any depth change by changing its rotation. I have described in this post what that means for the flow in our tank: We expect — and observe visually (see picture on top of this post) — that an ice shelf that is tilted such that it is slowly decreasing the water depth will force the current down the slope of the canyon, until it reaches the deepest point, turns, and moves up again.

But now Nadine has plotted the actual measured data, and we see the same thing! Below you see a plot of the flow field on a level just below the upper edge of the canyon. I have drawn in where the ice shelf is situated and where the contours of the channel are, and, most importantly, that the flow field shows exactly the behaviour we were hoping for!

 

The messy flow field where the contours of the ice shelf are drawn in is probably because the data that is being plotted has been calculated from pictures that were taken from above the tank, through the ice shelf, so we don’t have good data in those spots. But all in all, we are very happy! And almost ready to call it a day. Almost ready, except it is still too exciting to think about our experiments… 😉