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! 🙂
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! 🙂
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!
Have a great weekend, everybody, and we are looking forward to seeing the tank filling with water and starting to rotate on Monday! We will keep you posted! 🙂
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.
Remember this little office on top of the tank? Exciting stuff going on there: Testing of the cameras that were just being installed in the last post! (Of course it will get even more exciting once we start rotating! :-))
Of course we need to know which part of the tank the camera captures exactly, and what any given length on the pictures relates to in real life. That’s why we measured the topography in the last blog post.
Below, you see that office again and additionally get a glimpse one floor down into the tank, and people moving a large grid there:
This grid, with exactly known dimensions and positioned in different ways above our topography, lets us calculate the focal length of the camera, which will ultimately give us the possibility to calculate back from pictures taken from far above to actual lengths in the tank. This is necessary to calculate the currents inside of the tank.
Easy as this sounds, I can assure you that it is not. So many considerations need to go into this now, for example where do we want to put the origin of the coordinate system when we interpret the data? And how should the axes be oriented? We are in the Southern Hemisphere, so should we use this to determine the direction of the x-axis? Or should the direction of the jet that we will be introducing into the tank give us our x-axis orientation? The source (where we will have the inflow of the jet later) seems like an obvious origin of a coordinate system, but we will move it around on the topography, so it really is not.
Decisions made now will make it a lot more convenient (or inconvenient) to work with the data for years to come, so lots of different considerations going on right now…
And there are two cameras that need to play well together, or at least the data coming from them needs to be connected seamlessly to each other…
We have arrived at the Coriolis platform in Grenoble and it is seriously impressive. When you have heard us talk about a 13-meter-diameter swimming pool that is being rotated, you could not have possibly imagined this GIANT 13 METER DIAMETER SWIMMING POOL THAT WILL BE PUT INTO ROTATION! At least I know that even though I theoretically knew the dimensions, I had absolutely no idea of how massive the structure would actually be when you stand in front of it or even climb in.
Let me show you around a little.
This is what the whole thing looks like when seen from the outside. It’s very difficult to imagine the scale of it all, but you can see the dark floor at the bottom of the tank, then on the left, there is a second level, and a little further up a third one. Those are all normal floors — on the second level there even is a little office!
When we climb down as low as possible, we see how the whole tank rests on all kinds of very heavy duty structures. And it must be, considering that it is supporting not only a lot of moving water, but also an office! I can’t wait to see everything rotating! Here you can maybe gauge the scale a little from the stair cases and handrails?
Below, the green metal wall (inside of that black-and-yellow striped zone, which is what will be turning, and the black safety guard) is the outside of the actual tank, that will contain the water later.
This is the office space on the second floor I talked about: you can kinda imagine the size of the tank underneath from the curvature of this room. And this will be rotating with the tank!
And here you see Elin down in the tank. The clear structure to her feet is the topography of Antarctica that we will use in the first experiment (more on that later).
When you look up from the tank, you see a lot of scaffolding and two very nice technicians (can you spot both?) installing fancy camera equipment for us (more on what we are going to do with those in later posts).
So this is where we’ll be for the next couple of weeks!
The first thing we did today was to measure the topography so we have a reference for what is actually in the water later (rather than what we thought there should be). You see Elin (on the left) sitting on the bottom of the tank, and Nadine (on the right) climbing on the topography.
And now we are busy sorting out all the things like access to the servers so we can see the data we’ll be measuring, VPN connections so Matlab finds its licence back home, and all the other fun stuff. But we will obviously keep you informed of every exciting new development, the super awesome science, and we are hoping to start calibrating the cameras later today! 🙂
I was told (by our outreach expert) to introduce myself to you… so here we go! You already know the basics – my name is Elin and I am an oceanographer. I live in Bergen, Norway where I moved from the flat southern Sweden as a student a long time ago. I quickly fell in love with the mountains, bought a pair of rubber boots and learnt to live with the (eternal) rain… and just before the summer, I (finally) got a permanent position as an associate professor at the University of Bergen, so I guess I’ll keep wearing those rubber boots for a while.
I went to Svalbard and UNIS at 78N as a student, and science wise I never really left the high latitudes. Snow, ice and cold water are so fascinating, so interesting and so beautiful! I now work mostly in Antarctica – which, for a sea-going oceanographer means that you every now and then disappear many weeks (or even months) at a time. That is not always easy when you’ve got two young daughters at home. Last time when I was about to go south, Sara wondered why I didn’t just put my instrument in the ocean next to our summerhouse – “There’s plenty of water there too!”
Antarctica is far away – but it is plays a key role in our climate system and we know so little about what happens there and about how it all fits together. There are so many exciting and important questions to answer! This time however, we will try to answer a few of them from Grenoble, a little bit closer to home.
The shelf break and the ice shelf front—Two topographic barriers
So far, we’ve explained you that outside the Antarctic continent, warm and saline water (Circumpolar Deep Water, CWD) is located beneath fresh and cold water (Surface Water). We’ve also explained you that if this warm and saline water gets onto the continental shelf and mixes with high salinity shelf water, it melts the ice shelf from below. You think that sound easy? Well, it’s not! The ice shelves are actually pretty well protected from this warm subsurface water…
In Figure 5 from our post on Tuesday you can see that especially the largest ice shelves—Filchner-Ronne and Ross Ice Shelves—are protected from warm water through the continental shelves that keep the warm water at a distance of several hundreds of kilometers. Oceanic currents tend to flow along the bathymetry (slopes), not across it. The continental slope—the steep slope connecting the deep Southern Ocean to the continental shelf— thus acts like a wall and limits the flow of warm water onto the shelf. In the Amundsen and Bellinghausen Sea, however, the warm water already reaches on the continental shelf, and it reaches all the way to the ice shelf front. The ice shelf front reaches many hundreds of meter down into the water, and it forms a second wall that the water has to cross in order to reach the cavities beneath the ice shelves. The pathway of the warm water across these two walls, or topographic barriers as we like to call them, is still poorly understood and therefore the main focus of our project. How does the warm water that is located outside the continental shelf and in a depth of hundreds of meters flow onto the continental shelf and beneath the ice shelf?
Figure 7. Sketch for the project TOBACO of the uncertainties in the warm water flow across the shelf break and the ice shelf front.
Topographic steering
The rotation of the Earth causes ocean currents flow parallel to topographic slopes, i.e. to roughly follow lines of constant depth. The currents around Antarctica therefore follow the continental slope, and water from the slope doesn’t easily make it onto the continental shelf. Similarly, at an ice shelf front currents mainly flow parallel to the front instead of entering the ice shelf cavity. The shelf break and the ice shelf front form a topographic barrier
Warm water has been measured on the continental shelf and beneath ice shelves. In fact, the water can cross topographic barriers after all!
Certain processes help the warm Circumpolar Deep Water to cross the barriers:
Troughs crosscutting the continental shelf and the shelf break reduce the barrier effect and enable on-shelf transport.
Eddies formed within the currents are able to move across the “barriers” bringing (warm) water with them.
During our time at the Coriolis platform, we will investigate these points by varying the trough geometries, current thickness and density. If you are interested, please follow our blog throughout the experiments and learn about the control of topography on ocean currents and ice shelf melt!
The Antarctic Ice Sheet is melting because it is losing more mass through increased air and ocean temperatures than it gains mass by snow fall. Melting of ice is most efficient through contact with water, because water has a higher heat conductivity compared to air; simply said this means that water removes heat easier from the ice than air. In case of the Antarctic Ice Sheet it is also of importance that air temperatures are mainly far below the freezing point, whereas the ocean is always warmer than or close to the freezing point. The strongest melting in Antarctica therefore occurs beneath ice shelves. You want to know more about the melting process and what happens beneath an ice shelf?
The ice pump beneath ice shelves
Ice shelves are melting from beneath as ocean water reaches into the cavity. To explain the processes, we come back to the sketch of an ice shelf that we already showed you before.
In front of ice shelves the ocean freezes to sea ice, which is a process that produces very saline and dense water (high salinity shelf water). Because of its high density, it sinks down and mixes with circumpolar deep water (CDW) that spills over the continental shelf edge. The water mixture is then relatively dense and warm. It flows along the bottom of the continental shelf that is—in many cases—deepening inland and therefore reaches far beneath the ice shelf. When the water reaches to the ice shelf base it is far below sea level and at a higher pressure. Water there can still be liquid at temperatures down to below -2⁰C! And the ice starts melting as soon as the temperatures reach above this pressure melting point! If the ocean water is warmer than this temperature, it melts the ice shelf from below. The melt water is fresh with a low density and brings water out of the ice shelf cavity in form of a buoyant melt plume. All together, this forms an ocean circulation beneath the ice shelf, called ice pump.
Figure 4. This sketch shows a typical Antarctic ice shelf that starts floating from the grounding line and enables the flow of warm circumpolar deep water (CDW) underneath the ice shelf. The ice therefore melts from below (basal melt) and this melt water rises as a buoyant melt plume. Image credit: Helen Amanda Fricker, Professor, Scripps Institution of Oceanography, UC San Diego.
Where does the warm water come from?
The rates at which all Antarctic ice shelves melt from beneath are estimated to be about 1325 Gt/yr (gigatons per year; or 3.7 mm sea level equivalent per year). Yes, this is a lot! We are wondering where all the energy comes from and how the warm water reaches onto the continental shelf…
The dynamics governing the flow of warm water towards and underneath the ice shelves are non-trivial and still poorly understood. However, we know that the heat reservoir threatening the ice shelves is located off the continental shelf, in the deep Southern Ocean, where relatively warm water resides below a shallow, cold and fresh surface layer. Well, you may wonder how cold water can float on top of warm water!? If you have ever been swimming in a lake or in the ocean, you may have realized that it usually gets colder the further down you get.
Water layers in the oceans are stratified due to their different densities, with the densest water masses lying at the bottom of the sea floor and the lightest water masses floating at the surface. Density is given by the salt content and the temperature: high salinity and low temperatures lead to the densest water masses. In the tropics, the temperature is more important and it usually gets colder with depth. Closer to the poles, however, salinity gets more important. In the Southern Ocean, the warmer water can stay beneath the cold and fresh surface layer, because of its high salt concentration. The warm subsurface water it transported with the Antarctic slope current, which flows westwards along the Antarctic slope almost around the whole Antarctic continent.
West Antarctic threatened by a warm ocean
In West Antarctic, especially in the Amundsen and the Bellinghausen Sea, the water found on the continental shelf is relatively warm compared to East Antarctic (Figure 5), resulting in high melt at the base of the ice shelves. Some of the ice shelves therefore thin tens of meters per decade! This thinning also expands upstream to the proximal grounded glaciers which then causes a sea level rise. The figure shows that already today one drainage area in West Antarctica alone lost almost 100 Gt/yr to the ocean, with a trend towards increasing numbers due to basal malt. These basal melt rates are challenging to estimate, which makes the contribution of West Antarctic Ice Sheet melt to sea level rise to the largest source of uncertainty in the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC).
Figure 5. Water temperatures of the Southern Ocean around the Antarctic Ice Shelf and the resulting ice shelf thickness change per year, as well as the ice loss from the grounded ice sheet that actually contributed to sea level rise (circles) (Pritchard et al., 2012).
West Antarctic – A marine ice sheet
Besides the ocean temperatures, there is another substantial difference between West and East Antarctica that leads to the high mass loss in the former one: The bedrock beneath the West Antarctic Ice Sheet lies far below sea level (up to more than 1 km) in most parts (the blue areas in Figure 6). Thus, most of the ice sheet’s base is located far below sea level, what we call a marine ice sheet. Marine ice sheets are particularly instable for several reasons:
Their bed slopes inland, which generally is an instable configuration. As submarine melt moves the grounding line—the transition zone between grounded and floating ice—further inland, it reaches into deeper areas, that make the glacier floating again and also causes a larger flux through the grounding line. Once this process is triggered, the glacier retreats continuously until the bed geometry changes again.
The pressure melting point—the temperature at which ice freezes or melts—is reduced when the pressure increases. When the glacier base reaches into deeper waters where the pressure is higher, the melting point therefore decreases and the surrounding water is warm relative to the melting point. It may be warm enough to melt the ice from below.
The uncertainty regarding the West Antarctic Ice Sheet melt to sea level rise therefore results from the exposure of the ice sheet to relatively warm water. The ice sheet is also close to a tipping point at which the ice sheet may irreversibly lose large amounts of ice. To better estimate future sea level rise, we need a good estimate on the ocean heat flux to the ice shelves and good knowledge on the response of the ice sheet.
Figure 6. The bed topography beneath the Antarctic Ice Sheets reveals that most parts of West Antarctica lie beneath sea level, which makes the West Antarctic Ice Sheet sensitive to ocean warming (Fretwell et al., 2013)
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.
