When I (Mirjam) was visiting Elin at GFI last year, we set up Nansen’s “dead water” experiment in the 6m long tank in GFI’s basement to be used in GEOF213 to make things a little less theory-heavy and a little more easy to grasp. And since it’s about now that the experiment will be run again in GEOF213, I wanted to take the opportunity to remind you of how cool an experiment this is!
Out considerations for using this specific experiment in teaching are described here, including the learning outcomes we hope to achieve with the experiment. Students read original literature, determine the exact setup of the experiment, compare their theory-based predictions to actual observations. How much more fun can it get? Last year’s students even wrote a blog post about the experiment, which you can find here.
In 1893, Nansen described a phenomenon he observed in the Arctic: “When caught in dead water Fram appeared to be held back, as if by some mysterious force, and she did not always answer the helm. In calm weather, with a light cargo, Fram was capable of 6 to 7 knots. When in dead water she was unable to make 1.5 knots. We made loops in our course, turned sometimes right around, tried all sorts of antics to get clear of it, but to very little purpose.” (cited in Walker, J.M.; “Farthest North, Dead Water and the Ekman Spiral,” Weather, 46:158, 1991)
The experiment we set up shows the mechanism that explains Nansen’s observation. Energy from the propulsion of the ship is used to generate internal waves at the interface between a shallow, fresh surface layer and the denser, more salty deep layer below. If the ship is moving slowly enough that the internal wave it generates has the chance to catch up with the ship, an interaction between the internal wave and ship will take place. This will slow down the ship much the same way that Nansen described.
Instructions for how to set up that experiment can be found here.
Calculating the phase velocities of shallow water and deep water waves from the dispersion relation sometimes seems a bit pointless to students (at least it sure did to me (Mirjam) when I had to do it during my studies years ago). So Elin and I played around with it a bit (thanks to a suggestion by Tor Gammelsrød, who always comes to visit us in the lab!), and now there is a new experiment included in GEOF213 to complement the theoretical exercises that were already in place.
Look at Elin exciting shallow water waves in the picture below. It’s quite easy to imagine how one could measure the waves’ phase speed in the lab, just by taking the time it takes for them to run over a known distance, right? (Btw, this is the shallow water experiment that is part of the 2nd-year instruction, so students should already be familiar with shallow water waves)
Things get a little more complicated if there is more water in the tank, as you see in the picture below. Not only do waves have a smaller amplitude (because we didn’t want to risk flooding the lab), but also there is the thing about phase velocity and group velocity in deep water, that makes both of them a lot harder to observe! We don’t want any spoilers here, but you know what I am talking about…
This is such a simple experiment to run, but having the 6m long tank really helps because it gives us at least some time to observe waves before the reflections from the far end come back to haunt us.
And it is quite difficult to excite waves with more or less constant wave lengths. “Allegro!” is what Elin gave me as instructions for what kind of waves she wanted. Playing with a tank with Elin is always the best!
This is probably the first – and last – time I give a lecture in a long dress and high heels! Every year, on Fritjof Nansen’s birthday, the Norwegian Science Academy invites its members (and a few others) to “Nansen’s memorial lecture”. The title of this year’s lecture was “From cold to warm – Norwegian Oceanographic Research in the Weddell Sea” – and the presenter was me!
When preparing for the talk I learnt a lot about the first Antarctic research expeditions and the history of oceanography in Bergen, and I had the pleasure to have Arne Foldvik tell me his stories from the “old days” down south – I’ll try to share some of those with you here later, but first some photos from the festive evening in Oslo!
The presentation was followed by a very fancy dinner!
In the image above, we see planetary Rossby waves. They are propagating along the slope with shallow water to the right. But why? This is the kind of thing one might learn in GEOF213: “Dynamics of Ocean and Atmosphere”. This is theoretical subject, with equations filling the blackboard in most of the classes. To make it more fun, to help understanding of mechanisms and to motivate why a little theory really can’t be avoided, Elin and I (Mirjam) set up a couple of experiments over the last couple of weeks. Some working better than others, but that was to be expected…
But one that worked super well are planetary Rossby waves. We use a square tank with a sloping bottom which is spun up to solid body rotation. Then, a colored ice cube is placed in the shallow eastern corner of the tank. As it starts melting, a column of melt water forms below it. Because the melt water column is being stretched as it is sinking, it starts spinning. Once it reaches the sloping bottom, it is stretched even further. In order to conserve potential vorticity, it moves back up the slope again, starting to form a Rossby wave which then propagates westward.
Below you see an experiment both from the top (upper left corner) and the side.
What I find super cool is that the ice cube, sitting on top of its rotating Taylor column, spins in the same direction as the tank, but even faster than the tank itself! Physics says it has to, of course, but this is the kind of counterintuitive stuff that is just really nice to directly observe.