## Ekman spiral in a tank.

Tank experiment showing (the effects of) the Ekman spiral.

One experiments that has been run in GEOF130 forever is the “Ekman spiral” experiment. A tank filled with water is spun up to solid body rotation on a rotating table.

Then the tank is slowed down. The resulting friction between the water body and the tank creates a bottom Ekman spiral.

In the lab, you can observe the Ekman depth by looking into the tank from the side:

I’m showing you here what they see, except that you have a big advantage: The camera was mounted on the rotating table, so instead of watching the tank rotate in front of you (which makes it really difficult to focus on features without getting seasick) you are actually rotating with the tank.

It is very difficult to capture on video or photo, but Pierre and I are doing our best.

Stay tuned for a nice Ekman movie coming up soon!

## Why do we get an Ekman spiral?

Visualizing an Ekman spiral using a deck of cards.

To state this right upfront: this post will not explain why the surface layer is moving at a 45 degree angle to the wind direction, and if anyone has a great idea for a simple demo for that please let me know! It will also not explain why the layers are turning further and further the deeper down you go. But what I am trying to do today is give an intuitive understanding for why all the theoretical layers in the water column turn in response to the surface layer and hence why an Ekman spiral develops if we accept that the surface layer is turning relative to the wind direction.

You will need a deck of cards. Bonus points if they are “salmon fly” cards like mine (seriously – who could walk past a deck of cards with salmon flies on them? Plus I needed a deck of cards because I was already in Iceland when I realized I wanted to show this demo).

All you do now is put the stack in front of you. Put your hand on the top card, twist gently while applying a little bit of pressure. Voila – your Ekman spiral develops! It is turning the wrong way round, but the main point is that the twist is being transferred downwards from layer to layer and not only the top layer twisting while the other layers stay motionless.

And because people seem to always like movies: