How our experiments relate to the real Antarctica

After seeing so many nice pictures of our topography and the glowing bright green current field around it in the tank, let’s go back to the basics today and talk about how this relates to reality outside of our rotating tank.

Below, you see the area that we are trying to reproduce in the tank: A small part of the Wedell Sea, where we have idealized topography representing the Luipold coast and the Ronne Ice Shelf and represent the Filchner Depression by our canyon. Can you recognize it from yesterday’s post?

Figure 1 or Darelius, Fer & Nicholls (2016): Map. Location map shows the moorings (coloured dots), Halley station (black, 75°350 S, 26°340 W), bathymetry and the circulation in the area: the blue arrow indicates the flow of cold ISW towards the Filchner sill and the red arrows the path of the coastal/slope front current. The indicated place names are: Filchner Depression (FD), Filchner Ice Shelf (FIS), Luipold coast (LC) and Ronne Ice Shelf (RIS).

Above you see the red arrows indicating the coastal/slope front currents. Where the current begins in the top right, we have placed our “source” in our experiments. And the three arms the current splits into are the three arms we also see in our experiments: One turning after reaching the first corner and crossing the shelf, one turning at the second corner and entering the canyon, and a third continuing straight ahead. And we are trying to investigate which pathway is taken depending on a couple of different parameters.

The reason why we are interested in this specific setup is that the warm water, if it turns around the corner and flows into the canyon, is reaching the Filchner Ice Shelf. The more warm water reaches the ice shelf, the faster it will melt, contributing to sea level rise, which will in turn increase melt rates.

In her recent article (Darelius, Fer & Nicholls, 2016), Elin discusses observations from that area that show that pulses of warm water have indeed reached far as far south as the ice front into the Filchner Depression (our canyon). In the observations, the strength of that current is directly linked to the strength of the wind-driven coastal current (the strength of our source). So future changes in wind forcing (for example because a decreased sea ice cover means that there are larger areas where momentum can be transferred into the surface ocean) can have a large effect on melt rates of the Filchner Ice Shelf, which might introduce a lot of fresh water in an area where Antarctic Bottom Waters are formed, influencing the properties of the water masses formed in the area and hence potentially large-scale ocean circulation and climate.

The challenge is that there are only very few actual observations of the area. Especially during winter, it’s hard to go there with research ships. Satellite observations of the sea surface require the sea surface to be visible — so ice and cloud free, which is also not happening a lot in the area. Moorings give great time series, but only of a single point in the ocean. So there is still a lot of uncertainty connected to what is actually going on in the ocean. And since there are so few observations, even though numerical models can produce a very detailed image of the area, it is very difficult how well their estimates actually are. So this is where our tank experiments come in: Even though they are idealised (the shape of the topography looks nothing like “real” Antarctica etc.), we can measure precisely how currents behave under those circumstances, and that we can use to discuss observations and model results against.

Darelius, E., Fer, I., & Nicholls, K. W. (2016). Observed vulnerability of Filchner-Ronne Ice Shelf to wind-driven inflow of warm deep water. Nature communications, 7, 12300.

Water jet pumps, and why we don’t like them in our experiments

Just a quick update from the lab tonight: We are fixing more bugs by the hour 🙂

First: the bubble-free source.

I have previously written about how we thought we were going to get our source bubble-free (link here). Turns out, it’s not quite as easy as we thought — there were still plenty of bubbles everywhere! But luckily, Thomas came to our rescue and put some foam inside the source so the water has to pass through there before leaving the source through the honeycomb.  That effectively gets rid of all the bubbles since they just don’t fit through and surface inside the source box instead of outside of it in our experiment. We were really concerned about all those bubbles for two reasons: A) They might show up in the pictures we want to analyse and destroy any correlations we are hoping to find since they are there one second and then burst and disappear the next. And B) since the bubbles left the source below water level, they popped up to the surface and introduced vertical flow where we really didn’t want it.

But anyway, the bubbles are gone now! It’s amazing how well that works and all our (ok, my) prophecies of doom (the water is never to go through the foam! It is going over it and then enter the tank as a water fall! And even if it does go through, the particles we need to visualize the flow with, won’t!) were completely unnecessary.

Thomas kneeling on Antarctica, fixing our source

Second: The unwanted water jet pump.

We don’t actually know where it happened, but somewhere there was a leak and air got pulled into our inflow. Thomas and Samuel fixed this problem, too, but this is what happened: We have quite a fast flow from a reservoir sitting high above the tank down to the source. The faster the flow, the lower the pressure in it, which means that it sucks stuff (in our case air) from the surroundings, and entrains it. And that’s exactly the effect that is used in water jet pumps, except there people want it to happen…

Below you see an example of the Isère here in Grenoble, where a rather fast flow is causing a return flow as soon as the river bed widens a little.

Anyway, now it’s almost dinner time in our shared flat. But we’ll be back tomorrow with first results from our experiments! 🙂

What are we actually trying to measure in our rotating swimming pool?

You’ve heard us talk a lot about rotating swimming pools. Nadine has written about why we care about Antarctic ice shelf melting (link), why the ice shelf is melting (link) and how we are going to investigate it (link). Today, I am going to bring those explanations together with all that you’ve seen so far about our experiments in Grenoble. I hope! 🙂

Let’s start with a technical drawing of our “Antarctica”, the topography we have in the middle of the tank. You have seen it in many of our previous posts: It’s the thing that used to be in clear plastic, but that one early morning got painted black.

Drawing modified after Samuel Viboud with permission

What we are investigating is how a current, introduced at the “source”, will behave. We expect that it will flow along the shelf break and that some of it will turn left and flow around the corner into the canyon, while some of it will continue on straight ahead. How large a portion of the current takes which part depends on several parameters, which we will systematically change over the next couple of weeks: How large the source’s flow rate is (we are starting with 50 liter per minute), whether there is a density difference between the source water and the ambient water (we are starting with no density difference but will later have salt water in the tank and a fresher source) and what happens if we add a sharp corner to the nice and smooth corner of Antarctica and the shelf.

So far, so good. When we look down from the rotating first-floor office, things look a little different (an annotated version further down this page):

You see (parts of) the topography in the lower right corner of the image. And then you see a lot of green light: Our laser sheets! The laser sheets will illuminate thin layers of water, which we can take pictures of with cameras mounted perpendicularly to the sheets. That in itself isn’t so exciting, but we will have neutrally buoyant particles in the water, which light up when lit with the laser. If we take enough pictures of the particles, we can track individual particles as they get advected by the currents, and thus get a good idea of the flow field that is illuminated by the lasers.

And the cool thing is that we have not only one, but six vertical laser sheets, that are used sequentially. Below you see our first experiment and each picture in the animated gif is showing you a different layer, going from 5 cm below the surface down, so you get an idea of the vertical shape of the flow field. And you see that there is a lot more going on in the layers close to the surface!

Isn’t this amazing? How lucky are we that we got the opportunity to travel to Grenoble to be involved in all of this? 🙂

Introducing: Lucie Vignes

Written by Lucie Vignes:

I am Lucie, I will spend 3 weeks in Grenoble to work on the
topographic aspect of the experiment. I am starting a PhD in October
in LOCEAN (Paris) to work about water masses circulation and
transformation in the Weddell Sea, which is surrounding Antarctica.
Regarding my thesis subject, the experiments that are going to take
place in Grenoble will (I hope) help to have a better understanding of
the dynamical processes in the Weddell Sea.
I have a master in physical oceanography,  in which I studied oceanic
circulation, geophysical fluid dynamics and coastal dynamics.
I am super happy to work on the Coriolis platform and hope that we
will see nice phenomenon.

Getting rid of bubbles in our jet

Sometimes the devil is in the details…

On our first day at the Coriolis platform in Grenoble, I took a picture of the “source” in our experiments (see above): The plastic box that is fed by a hose from above and that has one open side with a “honeycomb” (or: a make-the-outflowing-water-nice-and-laminar thingy, technical term) that introduces the water into the tank that we want to follow around Antarctica.

This source is sitting against our topography, and will be partly submerged so that we introduce the jet at water level and below (instead of having a waterfall going in). The idea is to get a nice and bubble-free flow because — as we talked about yesterday — bubbles reflect the laser very strongly and disguise the signal that we are actually interested in.

So when we were doing our first tests last night, the first step was to flush out all the bubbles from all the pipes and hoses that supply the water to our source. Except that bubbles kept coming. And coming. Until, at some point, we realised that this was the problem:

The inflowing water was free-falling through air before hitting the water inside the source box, thus entraining a lot of air bubbles directly inside of the source. Good luck flushing them out… The solution was to add an extra piece of hose to just below the water surface so no air can be entrained.

So when we arrived at the lab this morning to an empty tank* we were delighted to see that the amazing Samuel and Thomas had already fixed the source!

*Yes, the tank really was empty again. Turns out that the reflection of the laser off the topography is so strong that it’s both a problem for data quality and that too much of the light gets scattered out of the water to be safe when we use the laser at it’s real setting for the experiments rather than at the super low setting we used for the tests… Disappointing, yes, but we were so surprised and pleased when we arrived this morning and the topography had already been painted AND the source  been fixed! We are super impressed with and grateful to the awesome team here in Grenoble! 🙂 And we are happy to report that there is water in the tank again and we can start measuring after lunch!

We have water in our rotating tank! Now testing the lasers

Above you see the very first water coming into our tank. Only a couple of hours, and the tank was full! And in solid body rotation (since it has been spinning all the time while being filled) which means that we can start doing the real experiments very soon! 🙂

Most of the afternoon has been spent testing the lasers that will be used later to measure flow velocities inside the water around our topography. Laser testing isn’t something where we can help with, but that doesn’t keep us from having fun with safety goggles! Although it took us a little while to figure out that while the goggles made the laser invisible (or, hopefully, blocked it from coming anywhere near our eyes) we could see on the displays of our cameras whether the laser was on or off!

Below you see the laser going through the water and illuminating the topography in the lower right corner of the image.

What needed to be done then was to make sure that the laser sheet is actually at exactly the position we want it to be.

When you look in from the side through the water, you see the shape of our topography illuminated and the vertical laser sheet coming in from the right.

Same if you look in from the top: Do you recognise our little Antarctica? Below we see a vertical laser sheet.

What the “official” camera sees can be observed on a screen in our second-floor office:

And what we saw is that there are way too many bubbles on the topography still, that show up as bright spots (which distract from the particles that we specifically seed to visualize currents). So: Someone needed to go in and clean…

We could observe on the screen how the bubbles were swept away!

Next, it was time to set the exact positions we want the laser sheets at.

For the horizontal sheets, this is done by having someone stand in the tank and actually measure the height at which the laser hits a ruler for a given setting.

But now I am going to pick up Lucie, or new team member, at the tram stop and hope that we are ready to start the real experiments first thing tomorrow morning! 🙂

Why do we go to all the hassle of rotating our swimming pool?

We have started rotating and  filling water into our 13-meter-diameter rotating tank! So exciting! Pictures of that to come very soon.

But first things first: Why do we go to the trouble of rotating the swimming pool?

The Earth’s rotation is the reason why movement that should just go straight forward (as we learned in physics class) sometimes seems to be deflected to the side. For example, trade winds should be going directly towards the equator from both north and south, since they are driven by hot air rising at the equator, which they are replacing. Yet we see that they blow towards the west in addition to equatorward. And that is because the Earth is rotating: So even though the air itself is only moving towards the equator, when observed from the Earth, the winds seem to be deflected by what is called the Coriolis force.

The influence of the Coriolis force becomes visible when you look at weather systems, which also swirl, rather than air flowing straight to the center where it then raises. Or when you look at tidal waves that propagate along a coastline rather than just spreading out in all directions. Or when you look at ocean currents. But all of these effects are fairly large-scale and not so easy to observe directly by just looking up in the sky or out on the ocean for a short while.

There are however easy ways to experience the Coriolis force when you play on a merry-go-round or with a record player or with anything rotating, really. Those are obviously spinning much faster than the Earth, and that’s exactly the point: The faster rotation makes it easy for us to see that something is going on. And obviously, Nadine and I had to test just that on the best merry-go-round that I have ever seen:

And that is what we’ll use in our experiments, too: Since our topography is a lot smaller than the real world it is representing, we also have to turn the tank faster than the real world is turning in order to get comparable flow fields. How to exactly calculate how fast we need to turn we’ll talk about soon. Stay tuned! 🙂

Introducing: Mirjam Glessmer

And another post introducing another team member working on the TOBACO project! Introducing today: Mirjam Glessmer.

My name is Mirjam. I am a physical oceanographer myself and have done a postdoc in Bergen (which – small world! – Elin sent me the advertisement to after – even smaller world! – a proposal for a postdoc position with Anna unfortunately didn’t get funded!). But while I love physical oceanography, love going to sea, and love doing tank experiments, I realized that I am not so keen on doing the sitting-in-the-office-and-struggling-with-software part of the research myself (but if you are interested in my oceanographic street cred, check out my publications on Nordic Seas fresh water, double-diffusive mixing, and lots of other cool stuff here). However, I am passionate about learning about other people’s research, and about communicating ocean and climate topics to the public! So I am here to support the outreach side of things.

When I am not in Grenoble, I work at the Leibniz Institute for Science and Mathematics Education in Kiel, Germany, and investigate outreach in ocean and climate sciences. How can we do it best? What design criteria can we use? In fact, I might be doing some research on who is reading this blog and for what reasons! 😉

For more information about me, you can have a look at my website, or – if you are interested in “kitchen oceanography”, random observations of all things water or science teaching – check out my blog!

Introducing: Nadine Steiger

I am Nadine and will spend the whole 2 months in Grenoble to make sure that I don’t miss anything exciting in the lab. Just one month ago I started my PhD with Elin in Bergen and I am still a bit new to the topic. So, I will be learning together with you and help keeping you up-to-date on what is happening at the Coriolis platform. My background is in meteorology and oceanography with a main focus on polar regions. Because I have been studying the retreat of marine-terminating glaciers during my master thesis, it really interests me why the beautiful ice has to melt! How does the warm ocean water can make it all the way into the ice shelf cavities and how will this change in a changing environment? I hope we will get closer to the answer during the experiments here in Grenoble.