Salt fingering

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.

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This is a zoom in on one of the bottles shown in this experiment: In the warm bottle, the red food dye acts as salt to form salt fingers!

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.

Double-diffusive mixing

On the coolest process in oceanography.

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:

[vimeo 83429427]

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”.

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Salt fingering happening with the red food dye acting as “salt”.

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)

Ship-generated internal waves

A tank experiment showing ship-generated internal waves.

When entering a fjord from the open ocean by ship, it can sometimes be noted that the speed of the ship changes even though apparently nothing else changed – the wind didn’t change, the position of the sails didn’t change, the settings on the engine didn’t change – whatever was driving the ship didn’t change. And yet, the ship slowed down. How can that be?

According to the legend (that I like to propagate in my classes), when this phenomenon was first noticed, people attributed it to sea monsters latching onto the ship and slowing it down. Or if not monsters, than at least mollusks and other not-quite mostery monsters. But then Bjerknes came along and, together with Ekman, set up experiments that explain what is taking all the energy away from propulsion. I’ll give you a hint:

Yes – the ship excites internal waves at a density interface. Since the stratification in a fjord is much stronger than in the ocean, driving into a fjord means loosing a lot more energy towards the generation of internal waves.

See the movie here: