I’m back at my happy place — the teaching lab at GFI in Bergen! :-)
Here a quick look at what we’ve been doing today: Filling the large wave tank! With clear fresh water and then salty pink water that forms a layer below. As the pink water flows underneath the clear water, there is shear between the two layers, waves form and then they break. Beautiful Kelvin-Helmholtz shear instabilities!
Why have we filled the large tank? Just you wait and see… ;-)
The experiment presented on this page is called the “slightly more complicated version” because it builds on the experiment “oceanic overturning circulation (the easiest version)” here.
One of the first concepts people hear about in the context of ocean and climate is the “great conveyor belt”. The great conveyor belt is a very simplified concept of the global ocean circulation, which is depicted as a single current that spans the world oceans (see Figure 1 below). In this simplified view of the global circulation, water flows as a warm, global surface current towards the North Atlantic, where it cools, sinks and finally returns southward and through all the world oceans near the bottom of the ocean. Water is transported back to the surface through mixing processes and starts over its journey again as a warm surface current. While in reality some part of the conveyor belt is wind-driven and many processes come to play together, a large part of the circulation can be explained by the water sinking due to cooling at high latitudes.
Figure 1: The great conveyor belt. My sketch on top of a map from http://www.free-world-maps.com
This can be very easily represented in a demonstration or experiment.
What we need for this experiment:
The container is filled with lukewarm water. On the “poleward” end, we add the cold pad, the warm one at the “equatorward” end of the tank.
Blue dye is tripped on the cold pad to mark the cold water, red dye on the warm pad as a tracer for warm water.
Thermally-driven overturning circulation: Warm water flowing near the surface from the warm pad on the left towards the right, cold flow from the cool pad at the bottom right to left.
A circulation develops. If you drop dye crystals in the tank, the ribbon that formed gets deformed by the currents for yet another visualization of the flow field.
Thermally-driven overturning circulation. In the middle of the tank you see a ribbon of dye, caused by falling dye crystals, being transformed by the currents in the tank.
Here is the video:
Besides the obvious observation, watching, there are a couple of things you can ask your audience to do.
For example, if they carefully slide their fingers up and down the side of the tank, they will feel the warm water near the surface and the cold water at the bottom.
If you have a clear straw, you can use it as plunging syphon to extract a “column” of water from the middle of the tank, showing again the stratification of red, clear, blue.
If you put little paper bits on the surface, you will see them moving with the surface current.
Can you come up with more?
Someone recently asked me whether I had ideas for experiments for her course in ocean sciences for non-majors. Since most of the experiments I’ve been showing on this blog were run in the context of Bachelor or Master oceanography-major courses, she didn’t think that the experiments were as easily transferable to other settings as I had claimed.
So here is proof: You can do pretty complex experiments with non-university level students. To prove my point, let’s go to a primary school.
Me running the overturning experiment with a primary school class in 2012.
The overturning experiment as seen by the teacher (2012).
Of course, you can adapt this experiment to different levels of prior knowledge. For example, in the primary school, I introduced this experiment by showing pictures of lions and penguins and other animals that the pupils knew live in warm or cold climates, and we talked about where those animals live. In the end this aimed at how temperatures are a lot colder at the poles than at the equator. This is the differential heating we need for this experiment to work. While this is something that I felt the need to talk about with the primary school kids, this can be assumed as a given with older students (or at least that is the assumption that I made).
With the university-level courses, one of the points that I made sure came up during the discussion are the limitations of this model. For example that we apply both heating and cooling over the full depth of the water column. How realistic is that? Or the fact that we heat at one end and cool at the other, rather than cooling on either end and heating in the middle?
Let me zoom in on something in the picture above.
Curious features in the thermal conveyor experiment. Do you know what this is about?
Do you see these weird red filaments? Do you think they are a realistic part of the thermal circulation if it was scaled up to a global scale?
Of course not. What we see here is salt fingering. This is a process that is caused by the different diffusivities of heat and of the red dye. And while it is pretty large scale in our small tank, you cannot scale it up just like that when talking about the real ocean. And it is also really difficult to get rid of salt fingers for this experiment, in fact I haven’t yet managed. But I am open to suggestions! :-)
Another point that I would talk about with university-level students that I would probably not bring up with primary school kids (- although, why not if I had more time than just those 45 minutes per class?) is that ocean circulation is driven by more than just differential heating. Even when just considering the density-driven circulation, that is additionally influenced by changes in salinity. Put that together with wind-driven circulation and we are starting to talk about a whole new level of complicated…
But anyway. My point is that even primary school kids can benefit from doing this kind of experiments, even if what they take away from the experiments is not exactly the same as what older students would take away.
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P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on January 13th, 2016.
“The easiest” in the title of this page is to show the contrast to a “slightly more complicated” version here.
One of the first concepts people hear about in the context of ocean and climate is the “great conveyor belt”. The great conveyor belt is a very simplified concept of the global ocean circulation, which is depicted as a single current that spans the world oceans (see Figure 1 below). In this simplified view of the global circulation, water flows as a warm, global surface current towards the North Atlantic, where it cools, sinks and finally returns southward and through all the world oceans near the bottom of the ocean. Water is transported back to the surface through mixing processes and starts over its journey again as a warm surface current. While in reality some part of the conveyor belt is wind-driven and many processes come to play together, a large part of the circulation can be explained by the water sinking due to cooling at high latitudes.
Figure 1: The great conveyor belt. My sketch on top of a map from http://www.free-world-maps.com (used with permission)
Since the global conveyor belt is such a basic concept that we come across in many different contexts, it is very useful to have a good demonstration of what is happening in the world ocean. Plus demonstrations and experiments are always fun!
I here present a very simple experiment that can be used for many different purposes. In science outreach, for example on a fair or in a talk, to catch people’s attention and raise an interest in oceanography. In schools to do the same, or to connect the fascination of the ocean to school physics and talk about density, convection, heat. At university to do all of the above, as well as to practice writing lab reports, talk about the scientific method or the validity of simplifications in theoretical or physical models.
All we need to run this experiment is
Ideally we’d also have a thermos or some other kind of insulation to keep the ice cubes frozen until we start running the experiment. To prepare the experiment, all we need to do a half a day ahead is mix some blue food dye into the water that we put in the ice cube tray, and freeze the ice cubes.
To run the experiment, we start out by filling our “tank” with lukewarm water. Let it settle for a bit. Now we decide for one end of your tank to be the “equator” end. There, we add some red food dye (see Figure 1).
Figure 2: Tank with luke warm water. Red food dye added to the “warm” end of the tank.
Then we add the blue ice cubes to the “poleward” end of our tank (see Figure 3).
Figure 3: Blue ice cubes melting at the poleward end of the tank. The cold melt water sinks to the bottom of the tank and then spreads “equatorward”.
The cold melt water from the ice cubes is denser than the lukewarm water in the tank and therefore sinks to the bottom of the tank where it spreads “equatorward”, pushing below the warmer water. This can be seen where the red water is pushed upwards and “poleward”.
Of course, the processes at play here are not exactly the same as in the real ocean.
For one, deep water formation is NOT due to ice cubes melting in lukewarm water. In fact, melting of sea ice will in most cases not lead to any kind of sinking of water, since the melt water is fresh and the surrounding ocean water is salty and hence denser than the melt water. Cooling in the ocean happens through many processes at the surface of the ocean, like radiation into space and evaporation.
Heating is also represented in an extremely simplified way in this experiment. Heating in the ocean occurs mainly (with the negligible exception of thermal springs in the ocean) by radiative heating from the sun, and at the surface only. We “heat” throughout the whole depth of the ocean by filling the whole tank with lukewarm water.
Also, the mixing processes that, in the real ocean, bring deepwater back to the surface are not represented here at all. Our tank will eventually fill with a layer of cold water at the bottom (See Figure 4), and the circulation will stop once all the ice has melted.
Figure 4: Blue ice cubes melting at the poleward end of the tank. The cold melt water sinks to the bottom of the tank and then spreads “equatorward”. Slowly, the tank fills with cold water.
Even with all the simplifications described above, this experiment is a great first step in becoming intrigued by the ocean, and towards understanding ocean circulation. Seeing the melt water sink from the ice cubes is fascinating no matter how little interest one might have in the physics that cause it. Sliding a finger up and down the side of the tank lets you experience – feel! – how the temperature changes from warm near the surface to very cold near the bottom. Actually physically feeling this is a lot more impressive than just watching the experiment or even just being shown temperature sections of the ocean. And the experiment invites you to play: What if you added little pieces of paper on the water surface, would you see them move with the flow towards the cold end of the tank? Or if you dropped a dye crystal in the middle of the tank, would the dye ribbon that forms be deformed by the currents in the tank? And what if you added twice as many ice cubes, would the currents be twice as fast?
This is pretty much the easiest experiment you can imagine. If you are afraid of what food dye might do in the hands of your participants, you don’t even need to let them handle it themselves, even when they are working in small groups with individual tanks: just go around dripping the dye in and then add the dyed ice cubes yourself. While someone might still tip over a tank and spill the water, this has yet to happen to me. Especially since, before running the experiment, you will have pointed out that they need to make sure not to bang against the tables as to not disturb the experiment. And now apart from making sure that the ice cubes are frozen when you want to run this experiment, there is nothing that can go wrong. So why not try this experiment next time you want to talk about global ocean circulation?
Watch a video of the experiment here:
Next time, I would pay attention to which end of my tank will represent the equatorward and poleward side of the ocean. Not that it matters much, but in most graphics of sections through the North Atlantic, the northern end will be on the right side and the southern end on the left. If the experiment is set up the other way round (as on all pictures and movies above) you will need to remember to point it out (or even mark it on the tank with a sharpie or such).
And for all of you who hesitate doing awesome experiments because it looks like you need so much equipment: No you don’t. Here is a “making of” shot from how I did this experiment on my coffee table while sitting on my couch. The background is the back of an old calendar sheet, clipped to the back of a chair. And that’s it.
The setup for my experiment.
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P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.
Rotating experiments in your kitchen.
Eddies, those large, rotating structures in the ocean, are pretty hard to imagine. Of course, you can see them on many different scales, so you can observe them for example in creeks, as shown below:
Eddies in the Pinnau river, and their dark “shadows”.
If you can’t really spot them in the image above, check out this post for clues and a movie.
So how can you create eddies to observe their structure?
Dye spiral caused by an eddy in a jar
I took a large cylindrical jar, filled it with water, stirred, let it settle down a little bit and then injected dye at the surface, radially outward from the center. Because the rotating body of water is slowed down by friction with the jar, the center spins faster than the outer water, and the dye streak gets deformed into a spiral. The sheet stays visible for a very long time, even as it gets wound up tighter and tighter. And you can see the whole eddy wobble a bit (or pulsate might be the more technical term) because I introduced turbulence when I stopped stirring.
Watch the movie below if you want to see more. Or even better: Go play yourself!
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P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 27th, 2015.
A wind stress is applied to the surface of a stratified and a non-stratified tank to cause mixing.
This is a pretty impressive experiment to run if you have a lot of time, or to watch the time-lapse of if you don’t. The idea is that a density stratification will make mixing harder than it would be in the unstratified case, because more energy has to be used to break up the stratification.
To look at this, we ran two experiments, one after the other.
In the first one, we took a tank full of freshwater, added dye droplets and switched on a hair dryer, set to blow pretty much along the surface of the tank, to force mixing through the wind stress. After about a minute, the tank was fully mixed.
In the second experiment, we created a density stratification: salt water with approximately 35 psu, and freshwater. We then added the dye droplets. The droplets never penetrated into the salty layer but instead layered in at the interface between the two layers. (See how there are internal waves on the interface, which is why the dye seems to penetrate much deeper on the right? If you watch the movie at the bottom of this page, you see the internal wave very clearly) We then added the hair-dryer wind stress.
After a minute, the surface layer was well mixed, but there was no mixing penetrating into the bottom layer. (We added blue dye at some point, which makes the picture below a little confusing.) To fully mix the whole depth, the wind forcing ran for 86 minutes (and I am proud to report that my hair dryer survived this ordeal! Don’t leave this experiment on its own, not every hair dryer might make this without catching fire!).
Mixing in a non-stratified tank (left) and in a stratified tank (right). See the stop watch at the bottom of the panels for an impression of the time scales involved!
This is a great demonstration of how mixing is inhibited by stratification. We had expected to see a difference, but we were really surprised that the difference was so large. Of course, the stratification in our tank was pretty harsh, but still.
Watch a short movie below and a movie containing the full time lapse even further down!
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P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 27th, 2015.
Explore how melting of ice cubes floating in water is influenced by the salinity of the water. Important oceanographic concepts like density and density driven currents are visualized and can be discussed on the basis of this experiment.
This hands-on experiment is suited for many different audiences and can be used to achieve a wealth of different learning goals. Audience ranges from first-graders over undergraduates in physical oceanography to outreach activities with the general public. Depending on the audience, this activity can be embedded in very different contexts: For children, either in their physics teaching to motivate learning about concepts like density, or in the context of learning about the climate system and ocean circulation. For college/university students the activity can either be used in physics teaching to get a different view on density; in oceanography/Earth science to talk about ocean circulation and processes that are important there; to motivate the scientific process; or to practice writing lab reports (you can be sure that students will at some point be tasting the water to make sure they didn’t accidentally swap the salt water and fresh water cup – a great teachable moment for a) Never putting anything in your mouth in a laboratory setting, and b) Always documenting exactly what you are doing because stuff that you think you will definitely remember obviously isn’t remembered that clearly after all). For the general public, this is typically a stand-alone activity.
It helps if the concept of density is known, but the experiment can also be used to introduce or deepen the understanding of the concept.
I use this activity in different ways: a) as a simple in-class experiment that we use to discuss the scientific method, as well as what needs to be noted in lab journals and what makes a good lab report, or density-driven circulation; b) to engage non-majors or the general public in thinking about ocean circulation, what drives ocean currents, … in one-off presentations.
Students learn about concepts that are important not only in physical oceanography, but in any physical or Earth science: density in general; density of water in particular, depending on the water’s temperature and salinity; how differences in density can drive currents both in the model and in the world ocean; how different processes acting at the same time can lead to unexpected results; how to model large scale processes in a simple experiment. After finishing the activity, they can formulate testable hypotheses, are able to reason based on density how a flow field will develop and they can compare the situations in the cups to the “real” ocean.
Students learn about and practice the use of the scientific method: formulation of hypotheses, testing, evaluating and reformulating.
Students practice writing lab reports, making observations, working in groups.
(per group of 2-4 students):
Before the experiment is started, students are asked to make a prediction which ice cube will melt faster, the one in salt water or the one in fresh water. Students discuss within their groups and commit to one hypothesis.
Students then place the ice cubes into the cups and start a stop watch/note the time. Students observe one of the ice cube melting faster than the other one. When it becomes obvious that one is indeed melting faster, a drop of food dye can be added on each of the ice cubes to color the melt water. Students take the time until each of the ice cubes has melted completely.
The ice cube in the cup containing the fresh water will melt faster, because the (fresh) melt water is colder than the room-temperature fresh water in the cup. Hence its density is higher and it sinks to the bottom of the cup, being replaced by warmer waters at the ice cube. In contrast, the cold and fresh melt water in the salt water cup is less dense than the salt water, hence it forms a layer on top of the salt water and doesn’t induce a circulation like the one in the fresh water cup. The circulation is clearly visible as soon as the food dye is added: While in the freshwater case the whole water column changes color, only a thin meltwater layer on top of the salt water is colored (for clarification, see images in the presentation below)
Students usually assume that the ice cube in salt water will melt faster than the one in fresh water, “because salt is used to de-ice streets in winter”. Have students explicitly state their hypothesis (“the one in salt water will melt faster!”), so when they measure the time it takes the ice cubes to melt, they realize that their experiment does not support their hypothesis and start discussing why that is the case. (Elicit the misconception, so it can be confronted and resolved!)
My experience with this experiment is that all groups behave very consistently:
It is always a good idea to have plenty of spare ice cubes and salt/fresh water at room temperature ready so people can run the experiment again if they decide to either focus on something they didn’t observe well enough the first time round, or try a modified experiment like the ones described above.
A reviewer of this activity asked how easily students overcome the idea that water in the cup has to have just one temperature. In my experience this is not an issue at all – students keep “pointing” and thereby touching the cups, and in the thin-walled plastic cups I typically use the temperature gradient between “cold” melt water and “warm” salt water is easily felt. The (careful!) touching of the cups can also be explicitly encouraged.
This experiment can be used in many different ways depending on the audience you are working with.
Pro tip: If you are not quite sure how well your students will be able to cope with this experiment, prepare ice cubes dyed with food coloring and use them in a demonstration if students need more help seeing what is going on, or even let students work with colored ice cubes right from the start. If ice cubes and hence melt water are dyed right away, it becomes a lot easier to observe and deduct what is happening. Feel free to bring the photos or time lapse movie below as a backup, too!
Dyed ice cubes about to be put into fresh water (left) and salt water (right)
When the ice cubes start melting, it becomes very clear that they do so in different manners. In the left cup, the cold meltwater from the ice cube is denser than the lukewarm water in the cup. Hence it sinks to the bottom of the beaker and the water surrounding the ice cube is replaced by warmer water. On the right side, the lukewarm salt water is denser than the cold melt water, hence the cold meltwater floats on top, surrounding the ice cube which therefore melts more slowly than the one in the other cup.
The ice cube in the fresh water cup (left) is almost completely gone and the water column is fairly mixed with melt water having sunk to the bottom of the beaker. The ice cube in the salt water cup (right) is still a lot bigger and a clear stratification is visible with the dyed meltwater on top of the salt water.
And here a time-lapse movie of the experiment.
Another way to look at the experiment: With a thermal imaging camera!
Cold (dark purple) ice cubes held by warm (white-ish) fingers over room-temperature (orange) cups with water
After a while, both cups show very different temperature distributions. The left one is still room temperature(-ish) on top and very cold at the bottom. The other one is very cold on top and warmer below.
When you look in from the top, you see that in the left cup the ice has completely melted (and the melt water sunk to the bottom), whereas in the right cup there is still ice floating on top.
Depending on the audience I use this experiment with, the learning goals are very different. Therefore, no one assessment strategy can be used for all different applications. Below, I am giving examples of what are possible ways to assess specific learning goals:
– Students apply the scientific process correctly: Look at how hypotheses are stated (“salt melts ice” is not a testable hypothesis, “similar-sized ice cubes will melt faster in salt water than in fresh water of the same temperature” is).
– Students are able to determine what kind of density-driven circulation will develop: Suggest modifications to the experiment (e.g. ice cubes are made from salt water, or ice cubes are held at the bottom of the cups while melting) and ask students to predict what the developing circulation will look like.
– Students can make the transfer from the flow field in the cup to the general ocean circulation: Let students compare the situation in the cup with different oceanic regions (the high Arctic, the Nordic Seas, …) and argue for which of those regions displays a similar circulation or what the differences are (in terms of salinity, temperature, and their influence on density).
In general, while students run the experiment, I walk around and listen to discussions or ask questions if students aren’t already discussing. Talking to students it becomes clear very quickly whether they understand the concept or not. Asking them to draw “what is happening in the cup” is a very useful indicator of how much they understand what is going on. If they draw something close to what is shown on slide 28 of the attached slide show, they have grasped the main points.
Don’t worry, it is totally feasible to bring all the equipment you need with you to run the experiment anywhere you want. This is what we brought to EMSEA14 to run the workshop three times with 40 participants each:
What we brought to EMSEA14 to run workshops on the ice cubes melting in fresh and salt water experiment
In one big grocery bag:
And if you are my friend, you might also get the “ice cube special” — a pink bucket with all you will ever need to run the experiment! Below is what the ice cube experiment kit looks like that I made for Marisa, with labels and everything…
An “ice cube experiment” kit that I made for a friend. Want one, too?
This activity has been discussed before, for example here:
I have also written about it a lot on my blog, see posts tagged “melting ice cubes experiment“.
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P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.
I’ve been wanting to develop a good fjord circulation experiment for a long time now — I wasn’t happy with the one we used back when I was teaching in Bergen, and then 1.5 years ago I talked with Steffi and Ailin who took over the tank experiments in Bergen a while back and we wanted to do something about it, but it just never happened. Life, you know, and jobs with other foci…
But then when I couldn’t sleep, I decided that 4:15 in the morning is a very good time to sketch out how I would develop a learning sequence on fjord circulation. Let’s see how I feel about it at a more normal hour after some more sleep, but right now I am happy with it, and excited to flesh out the sketch a little more with actual instructions for experiments. Would you be interested in reading that? And where would you look for instructions like that (except for on my blog)? I am hoping to maybe publish it somewhere “official”…
Rotating the tank
Currents flowing around “Antarctica”
You know I like tank experiments, but what I am lucky enough to witness right now is NOTHING compared to even my wildest dreams. Remember all the rotating experiments we did with this rotating table back in Bergen?
Those were awesome, no question about that. But the rotating tank I am at now? 13 meters diameter.
Yes, you read that correctly. 13 METERS DIAMETER!
I’m lucky enough to be involved in Elin Darelius & team’s research project on topographically steered currents in Antarctica, and I will be blogging on her blog about it:
Follow the blog, or like us on Facebook!
In any case, don’t miss the opportunity to see what is going on in a tank this size:
Yes, they are both INSIDE the tank. Elin (on the left) is sitting on the tank’s floor, Nadine (on the right) is climbing on the topography representing Antarctica. For more details, head over to the blog!