Using the continuous salinity stratification created yesterday, Rolf and Daniel conducted a really cool experiment: They forced internal waves and watched them develop. I’ve converted their movie into a time-lapse; watch it below.
Mit der kontinuierlichen Salzschichtung, die Daniel und Rolf gestern gebastelt haben, haben sie danach noch weiter experimentiert. Sie haben einen durch einen kleinen Motor angetriebenen Stempel in die Schichtung eingeführt und auf und ab bewegt. Das Wellenfeld, das sich dadurch entwickelt hat, sieht man im Film oben im Zeitraffer (einige kurze Abschnitte zwischendurch zeigen auch Echtzeit). Farbkristalle, die nachträglich hinzugefügt wurden, helfen, die Strömungen zu visualisieren.
And watching internal waves – a data-model comparison. (deutscher Text unten)
In an experiment similar to the one done by the group looking at the effects of temperature and salinity on density, the wave group, supported by Rolf, started looking at how to create a continuous stratification through internal wave action. Two water masses, one saline and one fresh, were separated in a tank. When the separation was removed, an internal wave developed.
Salinity and tank dimensions were recreated similarly in the tank and in a model, and you can watch the comparison below. Impressive, isn’t it?
Mit der Unterstützung von Rolf hat die Wellengruppe angefangen zu untersuchen, wie eine kontinuierliche Salzschichtung durch Vermischung durch interne Wellen erstellt werden kann. Genau die gleichen Bedingungen wie im Tank (Dimensionen und Salzgehalt) hat Rolf auch in seinem Modell losgelassen und hier ist die Simulation zum Vergleich. Eindrucksvoll wie ähnlich sich die Natur und die Modelllösung sind, oder?
Removing a barrier between waters of different densities and watching what happens. (deutscher Text unten)
Today, one of the groups performed a classical experiment (shown for example here) – but the awesome thing is that they came up with the planning pretty much by themselves in order to determine the effects of temperature and salinity on density. They compared water of the same temperature, but one fresh and one salty; warm salty vs cold fresh water; and cold salty vs warm fresh water. They predicted the outcome correctly, and we are showing two movies below: One normal movie and one in slow motion. Enjoy!
Heute hat eine Gruppe ein klassisches Experiment reproduziert. Allerdings haben sie es quasi selbstständig entwickelt.
Um den Effekt von Temperatur und Salzgehalt auf die Dichte zu bestimmen, werden zwei Wassermassen in einen Tank gefüllt, durch ein Wehr getrennt. Das Wehr wird herausgezogen und die dichtere Wassermasse schichtet sich unter die weniger dichte. Die Gruppe hat drei Fälle verglichen: Wasser gleicher Temperatur mit und ohne Salz; warmes salziges Wasser mit kaltem süßen; und warmes süßes Wasser mit kaltem salzigen. Der Film unten zeigt eine Zeitlupe der Bewegung.
Today I’m excited to bring to you a guest post from Innsbruck, Austria, written by my friend Kristin Richter. Kristin ran the oceanography lab in Bergen before I took over, and she is a total enabler when it comes to deciding between playing with water, ice and food dye, or doing “real” work. Plus she always has awesome ideas of what else one could try for fun experiences. We just submitted an abstract for a conference together, so keep your fingers crossed for us – you might be able to come see us give a workshop on experiments in oceanography teaching pretty soon! But now, over to Kristin.
A little while ago, I made an interesting experience while presenting some science to students and the general public on the “Day of Alpine Science” in Innsbruck using hands-on experiments. Actually, my task was to talk about glaciers but being a physical oceanographer I felt like I was on thin ice. Well, glaciers, I thought, hmmm … ice, melting ice, going into the sea, … sea, … sea ice! And I remembered how Mirjam once showed a nice experiment to me and some friends about melting ice in fresh and salt water. And suddenly I was all excited about the idea.
To at least mention the glaciers, I planned to fill two big food boxes with water, have ice float (and melt) in one of the tanks and put ice on top of a big stone (Greenland) in another tank filled with water to show the different impact of melting land ice and sea ice on sea level. Since melting the ice would take a while (especially on a chilly morning outside in early April) I would have enough time to present the “actual” experiment – coloured ice cubes melting in two cups of water – one with freshwater, and the other one with salt water.
As we expected many groups with many students, I needed a lot of ice. I told the organizers so (“I need a lot of ice, you know, frozen water”) and they said no problem, they will turn on their cooling chamber. The day before, I went there and put tons of water into little cups and ice cube bags into the chamber to freeze over night.The next morning – some hundreds of students had already arrived and were welcomed in the courtyard – I went to get some ice for the first group. I opened the cooling chamber,… and froze instantly. Not so very much because of the cold temperature but because I was met by lots of ice cube bags and little cups with… water. Like in LIQUID WATER! Cold liquid water, yeah, but still LIQUID! Arrrghhhh, my class was about to begin in a few minutes and I had NO ICE. “Ah, yes”, volunteered the friendly caretaker, “come to think of it, it is just a cooling chamber!”I started panicking, until a colleague pointed out the Sacher Cafe (this is Austria after all) and their ice machine across the road. I never really appreciated ice machines, but that one along with the friendly staff saved the day. Luckily, I brought some colored ice cubes from at home – so I was all set to start.
And the station was a big success, the students were all interested, asked many questions and were excited about the colored melt water sinking and not sinking. :-) I even managed to “steal” some students from the neighboring station of my dear meteorology colleagues. That was something I was particularly proud of as they could offer a weather station, lots of fun instruments to play with and a projector to show all of their fancy data on a big screen. (Actually, I also abandoned my station for a while to check out their weather balloon.)
Anyway, I had a lot of fun that day and could definitely relate to Mirjams enthusiasm for this kind of teaching. I can’t wait for the next opportunity to share some of those simple yet cool experiments with interested students. I will bring my own ice though!
Is there an equation of state for hypersaline water at very cold temperatures?
A friend of mine is looking to calculate changes in density of a hypersaline Antarctic lake from summer to winter. Apparently, this lake is about 10 times saltier than the ocean and often cools down to -17C at the bottom.
My own spontaneous answer was that I am not aware of such an equation of state, and that I doubt that there is a lot of empirical data in that property range. Plus from talking to Dead Sea researchers while working on double diffusion, I know that measuring salinities that are that high is not at all easy – the constancy of composition of sea water breaks down (at least in the Dead Sea) which has consequences for the measurement methods that can be used, and in any case CTDs aren’t calibrated for those salinities. But I am hoping that the collective wisdom of my readers will come up with a better answer.
So, dear readers. Do you know of an equation of state that applies to that range of properties, or do you have any other comments on the issue? Please leave a comment below or get in touch with me! That would a) really help my friend, and b) help satisfy my curiosity :-)
Why do heat and salt diffuse at different rates? This seems to always be puzzling students when talking about double diffusion.
Well, why should they diffuse at the same rate? The processes of molecular diffusion of heat and salt are very different.
In the case of heat, a transfer of heat only means that particles hit and transfer energy. The warmer the substance, the faster the particles’ movements. So faster particles hitting slower particles will transfer momentum, and by slowing down one of the particles and speeding up the other one, this is de facto a heat exchange.
A transfer of salt on the other hand means that ions have to be transported from a region of higher concentration to a region of lower concentration. In order for this to happen, they have to travel over physical distances larger than just the wiggling connected to molecular movement, and they have to exchange place with water molecules and clusters.
Clearly, this second process is a lot more time-consuming than the first one?
How to easily set up the stratification for the salt fingering process.
Setting up stratifications in tanks is a pain. Of course there are sophisticated methods, but when you want to just quickly set something up in class (or in your own kitchen) you don’t necessarily want to go through the whole hassle of a proper lab setup.
For double diffusive mixing, there are several methods out there that people routinely use.
For example the hose-and-funnel technique, where the less dense fluid is filled in the tank first and then the denser fluid is slid underneath with the help of a hose and a funnel. And a diffuser at the end of the hose. And careful pouring. And usually a lot more mixing than desired.
Or the plastic-wrap-to-prevent-mixing technique, where the dense fluid is put into the tank, covered by plastic wrap, and then the lighter fluid is poured on top. Then the plastic wrap is removed and by doing so the stratification is being destroyed. (No video because I was frustrated and deleted it right away)
Or some other techniques that I tried and didn’t find too impressive. (No videos either for the same reason as above)
But then accidentally I came across this one:
Granted, this is not a realistic model of an oceanic stratification. But as you can see towards the end of that movie, that turns out to be a blessing in disguise if you want to talk about the process in detail. As you see in the movie, the salt fingers inside the bottle are much smaller than the salt fingers outside the bottle. Because, clearly, inside the bottle the warm water is cooled both at the interface with the cold water inside the bottle, and by heat conduction through the walls of the bottle, since the water is surrounded by cold water. The warm water that flowed out of the bottle and up towards the water’s surface is only cooled at the interface with the water below (the air above is warmer than the cold water). So this gives you the perfect opportunity to discuss the scaling of salt fingers depending on the stratification without having to go through the pains of actually preparing stratifications with different gradients in temperature or salinity.
So in two courses (at different universities) I asked students in the exam to describe what would happen in a stably stratified body of water, where cold and fresh water overlies warm and salty water. And in both courses I have been surprised (read: shocked) by the responses I got.
The by far most common response goes along these lines: “Cold water is denser than warm water, so it will sink to the bottom and the warm water will rise”.
What I find frustrating about this (besides the fact that they didn’t notice that I clearly stated in the question that the stratification was stably stratified) is that whenever I talked about density, I mention how density depends on both temperature and salinity.
The next most common response is then this: “Heat diffuses a factor 100 faster than salt. Hence, salt fingers will form at the interface”. This answer then continues on describing salt fingering and never even mentions that the stratification I described in the question was actually the opposite one to the one they are assuming. So here, students clearly jumped to the conclusion that if I bothered describing a stratification, it clearly had to be the one for my favorite process (even though during those discussions I made sure to mention diffusive layering, too, but without talking it through in as much detail as salt fingering).
But then there are always students (usually the ones who don’t have a lot of confidence in their oceanography skills) who take the questions I ask at face value. Those are the students who go on to write something like this (numbering referring to the sketch below):
1) The initial stratification is stable in density, with cold and fresh water over warm and salty water. This means that the salinity stratification outweighs the temperature stratification in terms of density.
2) Since temperature diffuses a factor 100 faster than salinity, a thin layer with an intermediate temperature will form around the interface in salinity, that will persist for a while.
3) Focussing above the interface now, we have a stratification where cold and fresh water overlies lukewarm and fresh water. This stratification is hence unstable in temperature and convective overturning will occur. Below the interface, a similarly unstable layer has formed: lukewarm and salty water over warm and salty water. Again, convective overturning will occur.
The thickness of those layers depends on the initial temperature stratification and on how quickly temperature exchange happens during the overturning. In the end, two new temperature interfaces will have formed.
And yes – that is exactly the response I wanted to hear!
So why do only so few students answer this question correctly? Don’t they understand that when I talk about salt fingering it is only an example of a double-diffusive process and not the only double-diffusive process there is? That was my initial thought after I saw the exams in the first class. So for the second class, I made sure to mention diffusive layering even more, and to explicitly say that I was talking through only one of the processes and that it might be helpful if they went through the other one on their own. Yet in the exams, the results did not change. And I have no idea. Do you? Then please let me know!
How to show my favorite oceanographic process in class, and why.
As I mentioned in this post, I have used double-diffusive mixing extensively in my teaching. For several reasons: Firstly, I think that the process is just really cool (watch the movie in this post and tell me that it isn’t!!!) and that the experiments are neat and that everybody will surely be as excited about them as I am. Secondly, because it shows that understanding of small processes can be really important in order to understand the whole eco- and even climate system. And thirdly, because it helps to demonstrate a way of thinking about oceanography.
When I introduce salt fingering, I talk students through the process in very small steps. It goes something like this (Numbering is referring to the sketch below):
1) Initially, you have a stratification where warm and salty overlies cold and fresh water. This stratification is stable in density (meaning the influence of the temperature stratification on density outweighs that of the salinity stratification).
2) Since molecular diffusion of temperature is about a factor 100 faster than that of salinity (we will talk about why that is in a later blog post), the interface in salinity is initially basically unchanged, whereas a temperature exchange is happening across that interface, and a layer of medium temperature is forming.
3) At the salinity interface, we now have a stratification that is no longer stable in density: while the water now has the same temperature in a thin layer above and below the interface, it is still more salty on top and less salty below the interface. This means that the saltier water in this thin layer is denser than the less salty water below. This leads to finger-shaped instabilities at the interface: The salty water will sink and the fresh water will rise.
The individual salt fingers now have a much larger surface than the original interface, hence molecular diffusion of salt will happen much more efficiently and eventually the salinity inside and outside of the salt fingers will be the same, hence the growth of the fingers will stop.
At the depth where the salt fingers stopped, a new interface has formed. This new interface can also develop salt fingering, leading to a staircase-like structure in temperature and salinity.
After salt fingering has been introduced, there are usually several other occasions where it, or its effects, can be pointed out, like for example when showing this experiment (see picture below), when talking about the hydrographic properties in the area of the Mediterranean outflow or the Arctic, or when talking about nutrients in subtropical gyres.
While talking about salt fingering, since I focus so much on the process, I have always been under the illusion that students actually understand the reasoning behind it and that they can reproduce and transfer it. Reproduce they can – transfer not so much. Stay tuned for the next post discussing reasons and possible ways around it.
My favorite oceanographic process, as all of my students and many of my acquaintances know, is double-diffusive mixing. Look at how awesome it is:
Double-diffusive mixing happens because heat and salt’s molecular diffusion are very different: Heat diffuses about a factor 100 faster than salt. This can lead to curious phenomena: Bodies of water with a stable stratification in density will start to mix much more efficiently than one would have thought.
In the specific case of a stable density stratification with warm, salty water over cold, fresh water, finger-like structures form. Those structures are called “salt fingers”, the process is “salt fingering”.
Even though salt fingers are tiny compared to the dimensions of the ocean, they still have a measurable effect on the oceanic stratification in the form of large-scale layers and stair cases, and not only the stratification in temperature and salinity, but also on nutrient availability in the subtropical gyres, for example, or on CO2 drawdown.
Over the next couple of posts, I will focus on double diffusive mixing, but less on the science and more on how it can be used in teaching. (If you want to know more about the science, there are tons of interesting papers around, for example my very first paper)