Citation: Gerber, Edwin P., 2012: Stratospheric versus Tropospheric Control of the Strength and Structure of the Brewer–Dobson Circulation. J. Atmos. Sci., 69, 2857–2877.
The stratosphere lies above the troposphere (which is where most ‘weather’ happens) between about 10-50 km altitude. Its circulation is in many ways counter-intuitive, thanks mainly to the influence of waves.
Wave motions are ever-present in the atmosphere on a huge variety of scales – from turbulent eddies to thousand-kilometre planetary waves. They can basically be understood as motions which dump momentum (which changes the wind speed) when they break. Think of waves surging onshore at the beach. Out at sea, the wave doesn’t actually move water horizontally. It just moves water up and down. The wave moves but the water doesn’t. As the shore gets closer and the sea shallower, the wave breaks and water rushes forward. Edwin Gerber’s paper describes the behaviour of the wave-driven Brewer-Dobson circulation in the stratosphere.
The Brewer-Dobson circulation (BDC) transports air poleward from the tropics. This happens because waves from the troposphere are produced by mountains (think about putting a rock in a stream and imagine the swirls this produces), move up into the stratosphere and break. This breaking decreases the wind speed and air drifts towards the pole and then sinks. This is because of the principle of conservation of angular momentum. Basically, if you are spinning in a computer chair really fast, you will find it difficult to pull your arms in. If you slow down (imagine this is the effect of some waves) you will be able to pull your arms towards your body (your ‘pole’). The BDC is driven by the breaking of very large planetary waves (thousands of kilometres long)
Gerber uses a fairly simple atmospheric model to look at the behaviour of the BDC. The model isn’t meant to be like the real world. Instead it’s meant to simulate the fluid dynamics of the atmosphere in a way which looks something like the real world. It follows the same physical laws as the real world. This means the physics we learn about using the model can be applied to the real world.
This paper picks out two ways in which the BDC can change.
- Tropospheric control. Increase the wave activity and the BDC will become stronger.
- Stratospheric control. Increase the strength of the polar vortex (westerly winds in the winter hemisphere of the stratosphere) and the upward movement of waves will change.
The effects of the control mechanisms on the BDC can be easily seen by looking at the ‘age‘ of the air. This is the time since the air was at a given level in the atmosphere (in this paper, 100 hPa, or around 20 km). The picture below shows the age for different model setups. Red colours show older air, indicating a slower circulation.
Tropospheric control is the easier one to understand. If the BDC is driven by waves, increasing the wave activity will make it stronger. What does ‘increasing the wave activity’ mean? Well, the amplitude (‘size’) of the waves can be increased by changing the height of the mountains producing them. This is the what the study does. It sounds silly, because for the foreseeable future the mountains on this planet are going to stay much the same. But this is just the simple approach Gerber took in the paper. In fact, the amplitude of the planetary waves can be changed by large-scale changes in the air pressure in the lower atmosphere (such as the ‘Arctic Oscillation’, which is an expression of the latitude of the jet stream).
The stratospheric control mechanism is a little more subtle than the tropospheric one. Changing the winds in the stratospheric polar vortex changes the way waves behave. If the vortex is very weak, waves are trapped at the base of the stratosphere (under 20 km). This means the bottom part of the BDC speeds up, but the top part slows down. If the vortex is strong, waves can move all the way up into the stratosphere, so the top part speeds up.
Gerber’s experiments agree with some previous work indicating the BDC has two branches: an upper and a lower (you can see this in the first picture: the lower part is around 20 km while the upper part goes up to around 50 km). They seem to affected in different ways. The lower branch is most susceptible to changes in the waves coming from below; the upper branch is modified by changes in the way waves behave once they are in the stratosphere.
Why is this important? The BDC transports ozone to the poles, and ozone is important for protecting living things from harmful ultra-violet sunlight. Changes in the upward motion at the bottom of the stratosphere also change the level of the tropopause (thick blue line in the first picture). This is important because the level of the tropopause governs how much water vapour gets into the stratosphere. In the troposphere, extra water just rains out, but in the stratosphere extra water vapour exerts a warming influence on the Earth’s surface. If the tropopause gets lower it moves to a warmer region of the atmosphere, which allows more water vapour to get into the stratosphere. So it’s important we understand how and why the BDC does what it does.
It is unusual to find a single-author paper of such detail nowadays. It is also unusual to find such a readable paper on a highly technical subject. As I have said I find stratospheric dynamics quite tricky, but this paper really nicely illustrated the important physical principles without giving me a headache. There is a lot more in the paper than in this summary, but I hope it has at least given you a flavour of the clear, focused experimental design and the excellent presentation of its arguments.