Mixing in the oceans. Some extraordinary phenomena.

We often tend to think of the ocean as a big homogeneous soup, but it is nevertheless a complex stratified system. The ocean is structured in layers of different density with a very dynamic nature, allowing for mixing in all sorts of spatial and temporal scales. From the tiniest slow molecular diffusion to the large turbulent mixing in storms.

Double diffusion generating salt fingers. The different rates of diffusion from heat (faster) than composition/salinity (slower) allow for a previously buoyancy-stable layer to become unstable.

Mixing in the oceans is important for it has a principal role transferring heat and energy, as well as balancing oceanic properties. Therefore there is an important amount of research going on. Furthermore it is responsible for some of the most beautiful phenomena that we find in the oceans. Taking advantage of some lab experiments that I got to do recently, I would like to introduce some aspects of mixing and to show some of those wonderful phenomena.

The smallest sort of mixing is molecular diffusion; the intrinsic dynamics of the fluid’s molecules results in a very slow mixing. Imagine a trickle of milk on a steady cup of tea. Without stirring, the milk would eventually mix with the rest but would take a very long time. Thus, we usually stirr our tea to create turbulent motion and blend it faster.

Tea diffusing into water

Turbulence occurs in many ways, some of it as waves, tides and eddies. But there are many other interesting ways. An example of those are the Kelvin-Helmholtz instabilities, as the ones pictured below. You may have seen them as clouds, shaped as waves; also in the bands of Saturn or the Red Spot of Jupiter. But they can also occur in the water. When two overlaying layers move at different speed, they generate a vertical shear. Depending on the conditions, that shear can become unstable and then break the interface in wave-like structures that mix the proximities of those layers. You can see it in the picture below or in the video.

Kelvin-Helmholtz instabilities generated in the lab. The red water layer is denser than the overlying transparent water. When tilting the tank by lifting the right-side, red water starts moving to the left and the upper layer to the right. As the shear increases, the instabilities occur and mix the water.

Video explaining how to generate these instabilities in the lab.

Another cool kind of instabilities are the Rayleigh-Taylor instabilities. When we suddenly have a layer of water much lighter than the layer above, buoyancy is unstable and the lighter layer will push upwards towards the denser layer, generating mixing again. This phenomenon is harder to observe in the oceans, but for instance it can also be seen in salt domes, weather inversions and even after Super Novas, such as the Crab Nebula. Isn’t it great that the same physics and phenomenon occur in such different places? You can see the beautiful structures generated by this instability in the picture below. Also another video of the experiment (note the internal waves generated in the second part of the experiment!)

Evolution of Rayleigh-Taylor instabilities as two layers of different density and buoyancy-unstable meet together. In a tank, a lighter green layer is first separated by a screen from the overlaying denser transparent water. As the screen is removed, the instabilities and mixing occur.

Video explaining the Rayleigh-Taylor instabilities. First with just two layers, then with 3 layers (the coloured one being middle range density). In the latter case, note the internal wave generated after the mixing.

So here we have seen just some examples of phenomena which generate mixing. Although there are many other ways and scales at which mixing occurs. You can see that the topic is a broad world, and at the same time a very important protagonist in the study of the oceans and the transfer of energy and properties.