Voices of Science

Well this is another favourite lecture demonstration. It shows another process that’s fundamental to understanding for instance how the ozone hole works, and I like to call it gyroscopic pumping. And this is a case of it that’s also called Ekman pumping, but never mind the technical jargon. What you do is you take a spinning mass of fluid, and with this paddle I can make this mass of fluid spin almost in solid rotation. That’s a bit surprising to a fluid dynamicist but it works remarkably well. So now the mass of fluid is spinning and it rapidly settles towards something close to solid rotation. However, on the bottom is a friction force that’s trying to drag it back; it’s a retrograde force against the sense of rotation. And whenever you have a retrograde force in a rotating mass of fluid like this, there’s a systematic pumping action that pushes fluid toward the rotation axis and the tea leaves sitting on the bottom are showing that by gathering in the middle of the container. I think you can see it fairly clearly now, and if you wait another few seconds it’ll become still clearer. So that’s gyroscopic pumping due to a retrograde frictional force. But the real point about this is that it doesn’t matter what causes the retrograde force; in the stratosphere the force has a completely different cause, namely breaking Rossby waves, and I think I talked about that at greater length in the interview. And that similarly pushes fluid towards the poles and that’s the main driver. It’s a mechanical pumping action, it drives the mean circulation that’s so important for understanding all the chemical processes, including the ozone destruction in the Antarctic in spring time, the ozone hole as it’s popularly known. This demonstration is sometimes called Einstein’s tea leaves because Albert Einstein, yes the Einstein, published a little paper in which he described this happening in a teacup with the tea leaves gathering on the bottom because he was interested in this kind of flow for a different reason, namely understanding how meanders form in rivers, and it’s equally fundamental to understanding that.