Global change in the Stratosphere and its impact on Tropospheric Climate

The atmosphere is commonly divided into a number of layers or 'spheres'. Some of these layers are characterised by an increase in temperature with height whilst other show an overall decrease in temperature with altitude. The Troposphere is the layer closest to the earth and is around 8km in height at the poles, rising to 16km at the equator. Within this layer average temperature decreases with height. The layer above the Troposphere is the Stratosphere which extends to around 48km above the earth's surface. In direct contrast to the layer below, average temperatures within the Stratosphere increase with altitude. This variation in temperature structures means that weather patterns are also very different. In the Troposphere, weather patterns are dominated by systems with typical horizontal scales of 1000km, which move from west to east. These are the low and high-pressure systems typically seen on TV weather forecasts. In the Stratosphere, weather patterns are dominated by a single, much larger low-pressure system. This system is called the Stratospheric polar vortex, and has a typical horizontal scale of 6000km. While most of the time the Stratospheric polar vortex is very stable, not moving or changing in size from day to day, occasionally during winter it is temporarily disturbed or destroyed. These disturbances causes temperatures near to the pole in the Stratosphere to increase very rapidly, often by as much as 50 degrees centigrade in only five days and hence are known as 'Stratospheric sudden warnings'. In the 1970s, scientists showed that Stratospheric sudden warnings were caused by long wavelength Rossby waves. The longest wavelength Rossby waves are generated in the Troposphere and propagate into the Stratosphere. When they reach the Stratosphere, they can break and disrupt the Stratospheric polar vortex. In effect this means that Stratospheric sudden warnings are an example of a process which occurs in the Stratosphere but which is caused by variations in the Troposphere. For most of the 20th century, scientists thought variations in the Stratosphere had little effect on weather systems in the Troposphere, because the Troposphere has about twice as much mass as the Stratosphere. However in the late 1990's and the early 2000's scientists started to show that in fact the Stratosphere does have a small impact on the Troposphere. It is already established that dynamics in the Stratosphere will change due to global warming, therefore if we want to understand what the earth's climate will look like in the future, we have to understand how the Stratosphere and Troposphere are linked. This project will focus on understanding how the Stratosphere will change in response to global warming and how this will affect Tropospheric climate. A range of computer models of the atmosphere will be used to better understand the relationship. One part of the research will use a model (HadGAM1) of the whole atmosphere to estimate how the Stratosphere will change due to increases in greenhouse gasses like carbon dioxide and water vapour and changes to ozone, and make the quantitative estimates of future Stratospheric climate. Other, more simple models of the atmosphere will be used to try to understand how the Troposphere and Stratosphere interact in relation to specific changes, for example, how changes to Greenhouse Gasses, Ozone, Water Vapour or Tropospheric Rossby Waves interact to affect Stratospheric climate, and how this might change future Tropospheric climate. By combining information from all of the models it will be possible to find out exactly what influence future changes to the Stratosphere will have on our climate and how important they will be in relation to other documented changes.