Tag Archives: salinity
24 Days of #KitchenOceanography — Mediterranean Outflow
24 Days of #KitchenOceanography — Melting ice cubes!
Brine rejection and overturning, but not the way you think! Guest post by Robert Dellinger
It’s #KitchenOceanography season! For example in Prof. Tessa M Hill‘s class at UC Davis. Last week, her student Robert Dellinger posted a video of an overturning circulation on Twitter that got me super excited (not only because as of now, April 15th, it has 70 retweets and 309 likes. That’s orders of magnitude more successful than any kitchen oceanography stuff I have ever posted! Congratulations!).
Robert is using red, warm water on one side and melt water of blue ice cubes on the other side to provide heating and cooling to his tank to create the overturning. Why did I get so excited? Because of this: the head of the meltwater plume was very clearly not blue (see above)! Rob kindly agreed to write a guest post about these observations:
“I first and foremost want to start off by thanking Dr. Mirjam Glessmer for doing a phenomenal job at SciCom through Kitchen Oceanography. I was able to replicate her physical oceanography experiment regarding oceanic overturning circulation for my oceanography class with Dr. Tessa Hill.
As mentioned in her previous post, oceanographic currents are often simplified to give an easier understanding of how oceanic overturning circulation operates. The top 10% of our oceans are controlled by wind-driven currents and tidal fluctuations while the bottom 90% of our ocean currents are controlled by density-dependent movements.Originally this process was defined as thermal circulation but was later expanded to thermohaline circulation. Thermohaline circulation is dependent upon both temperature (thermal) and salinity (haline.) These density-dependent reactions occur when either freshwater fluxes meet saltwater and from thermal differences in water masses. Due to differential heating in our planet, colder formations of dense water masses are formed at the poles, which in turn causes the convective mixing and sinking of water masses driving oceanic circulation.
(Video by Robert Dellinger; thanks for letting me use it!)
In this experiment, we primarily focus on the thermally dependent reactions between two water masses. As seen in the video, the warmer water mass is dyed red, while the cold water mass formed by ice melting, is blue. As expected the more dense water mass (cold) is pushed under the warmer mass once they meet. One feature I would like to point out is the clear plume head feature in my experiment (see picture on top of this post). My theory is that part of the ice cube that was not dyed melted first and was pushed under the warm water mass. This feature is most likely due to the ice cube experiencing unequal cooling, which in turn led to an uneven dye distribution as seen in the previous post “Sea ice formation, brine release, or: What ice cubes can tell you about your freezer.”
As seen in the experiment, thermohaline circulation is thermally driven therefore, the role of salinity causes the system to be non-linear. Salinity serves as a positive feedback mechanism by increasing the salinity of deeper water and strengthens the circulation. Furthermore, current studies are focusing on how atmospheric warming is altering thermohaline circulation attributed by increases in ocean heat absorption and freshwater fluxes (primarily from melting ice caps.)”
On melting ice cubes and molecular diffusion of heat
First of all, let me say how much I love having chats like the one Elin and I had over the weekend (which you only see the very beginning of above). I had gotten into a bit of a rut kitchen oceanography-wise, which, I am happy to report, is over now! Thanks, Elin! :-)
One of my favourite experiments of all times are the ice cubes melting in fresh water and salt water. I’ve written about this experiment many times before, so if you aren’t familiar with it, check out my introductory post or all the posts related to the original experiment, because now we are going to take it one step further.
Last year, Elin told me about a conversation she had had with Prof. Emeritus Arne Foldvik (our hero when it comes to tank experiments!) about someone considering towing icebergs from Antarctica to some tropical place to use as freshwater supply. Arne mentioned that when the water in which the iceberg swims gets warmer than 27°C, the situation changes, as in that the iceberg’s melt water now is denser than the warm saltwater it is swimming in. So the assumption from that would be that the melt water would sink, rather than form a layer floating around the iceberg.
And that’s the experiment I had been wanting to try and only got around to doing when Elin reminded me on Saturday. Results were … a bit disappointing. At least at first:
So what is happening is that even though the melt water is initially denser than the salt water, it doesn’t stay that way for very long, because diffusion of temperature is very fast and the fresh meltwater plumes don’t have to warm up by very much before they are less dense than the warm saltwater, so that happens very quickly.
In the movie below we see evidence of this: Around minute 1 (marked in the video) we can spot plumes of dense water sinking down, but at the same time we very clearly see a (green) freshwater layer forming on top of the salt water.
(The “Happy Birthday Arne” in this movie refers to Arne Foldvik’s 90th Birthday which was yesterday!)
For the experiment, I kinda eyeballed the salinity and also my thermometers might not be the most suitable choice for measuring water temperatures, at least in that temperature range. But as Elin and I discussed as the chat above went on: I think it won’t make a big difference to fiddle with temperatures and salinities, in the end the dominant process will be the molecular diffusion of heat that will always quickly warm the meltwater, making it buoyant. And I actually think that this makes this experiment even more interesting — to show how different processes are acting at the same time, and it’s not always obvious right away which one will be the most important one. Kinda similar to what I showed in yesterday’s post: molecular diffusion of heat will sneak up on you faster than you think :-)
Would the same thing also happen if we didn’t just have small ice cubes with low meltwater “production”, but icebergs, where the meltwater plumes would have a larger volume and so wouldn’t be warmed up as easily? Who knows… But my kitchen is too small to try and I’m too lazy to do the maths, so now it’s your turn! :-)
Temperature dependency of molecular diffusion, and convection taking over
I saw the idea for this experiment on Instagram (Max is presenting it for @glaeserneslabor) and had to try it, too!
The idea is to put drops of dye into hot and cold water and observe how in hot water the dye is mixed a lot faster than in cold water — after all, molecules in hot water should move a lot more due to more energy and thus more Brownian motion. And we see that nicely in the upper panel of the picture: In hot water, structures look blurred, whereas in the cold water, we nicely see the vortex rings of dye falling into the water.
But what I found super interesting: Molecular diffusion of dye is only the dominant process in the very beginning of the experiment! Very quickly, molecular diffusion of heat is taking over. By warming the dye, we now get a convective flow that moves dye upward in the warm water (see lower panel).
For someone who worked on double-diffusive mixing (i.e. me) this is very exciting: It’s so nice to observe the effects of both diffusion of dye and diffusion of heat in one experiment! And to be able to show how different processes are important at different times.
What’s next? I think next time I’ll use dye at the different temperatures of the two glasses, that should get rid of the convection. Very curious to see what will happen then! :-)
Ice cubes melting in fresh water and salt water
Today we are doing the melting ice cubes experiment in fancy glasses, because Elin is giving a fancy lecture tonight: The Nansen Memorial Lecture of the Norwegian Science Academy in Oslo! Cheers!
We each had green ice cubes in our glasses, but one of our glasses contained fresh water and the other one salt water, both at room temperature. Can you figure out who got which glass?
This time lapse might give you a clue…
To read more about this experiment, check out this blog post!
A “Siel” – the valve in a dyke that lets freshwater out but no salt water in
Ok now, after complaining about how I dislike mud and the “no water” (i.e. low water) times in the Wadden Sea yesterday, today I’ll tell you about some stuff I really love about the North Sea coast. For example, the pretty little villages full with shrimp trawlers everywhere!
Picture above is Neuharlingersiel, below is Dornumersiel.
And the pictures below are from my favourite “…siel” so far, Greetsiel. Even though I unfortunately didn’t take any pictures of the village itself! I was so fascinated with all the shrimp fishing vessels.
The boom trawlers with all the colorful nets and ropes and everything are just too pretty, even on an otherwise fairly miserable day!
But what I find fascinating in all the “…siel”s is how they bring together a nautical atmosphere (after all, most of these ships have been out on the open North Sea all night!) and a very inland-pretty-old-town vibe. And here is how they do that: with the “Siel”. The “Siel” is basically a valve that lets freshwater out into the sea, but which doesn’t let salt water come back in. The way that is done by doors that are opened if the pressure on the inside (i.e. the land side) is higher than on the outside, and that close if the pressure on the outside is higher than on the inside. Pretty smart, ey? That way the land gets drained during low water, but no salt water comes inland to mess up the agricultural land. But I find it fascinating how this one little door in a dyke can separate two completely different habitats that coexist peacefully just one step away from each other.
Above, you are looking from the inland on such a “Siel”, which is currently closed because the water is higher on the outside than on the inside.
Oh, and what I always love: Light houses!!!
Yep, so I definitely have to go back. Stay tuned! :-)
Demonstration: Nansen’s “dead water” in a tank!
A ship that is continuously pulled with a constant force suddenly slows down, stops, and then continues sailing as if nothing ever happened? What’s going on there? We will investigate this in a tank! And in order to see what is going on, we have dyed some of the water pink. Can you spot what is going on?
The phenomenon of “dead water” is probably well known to anyone sailing on strong stratifications, i.e. in regions where there is a shallow fresh or brackish layer on top of a much saltier layer, e.g. the Baltic Sea, the Arctic or some fjords. It has been described as early as 1893 by Fridtjof Nansen, who wrote, sailing 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)
When observing the experiment, whether in the movie above or in the lab, the obvious focus is on the ship and the interface between the clear fresh water layer (the upper 5cm in the tank) and the pink salt water layer below. And yes, that’s where a large-amplitude internal wave develops and eats up all the energy that was going into propulsion before! Only when looking at the time lapse of the experiments later did I notice how much more was going on throughout the tank! Check it out here:
The setup for this experiment is discussed here and is based on the super helpful website by Mercier, Vasseur and Dauxois (2009). In the end, we ended up without the belt to reduce friction, and with slightly different layer depths than we had planned, but all in all it works really well!
Accidental double-diffusive mixing
When setting up the stratification for the Nansen “dead water” demo (that we’ll show later today, and until then I am not allowed to share any videos, sorry!), I went into a meeting after filling in layer 4 (the then lowest). When I came back, I wanted to fill in layer 5 as the new bottom layer. For this experiment we want the bottom four layers to have the same density (so we would actually only have one shallow top layer and then a deep layer below [but we can’t make enough salt water at a time for that layer, so I had to split it into four portions]), and I had mixed it as well as I could. But two things happened: a) my salinity was clearly a little fresher than the previous layer, and b) the water in the tank had warmed up and the new water I was adding with layer 5 was cold tap water. So I accidentally set up the stratification for salt fingering: warm and salty over cold and fresh! Can you spot the darker pink fingers reaching down into the slightly lighter pink water? How cool is this??? I am completely flashed. Salt fingering in a 6 meter long tank! :-D