Higher Atmosphere

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Dynamics of the Stratosphere

Compared to the troposphere processes in the stratosphere take place very slowly. The layering is very stable and there is only little exchange with the troposphere. But little exchange is more than no exchange ...

Stratosphere-Troposphere Exchange (STE)

The global transport of air is driven by the sun. The sun's radiation warms up land, sea surface and air in the tropics more than in higher or mid latitudes. Therefore convection is stronger in the tropics and goes to higher altitudes. Above the tropopause the sunlight leads to a warming of the stratosphere by ozone absorption, which is lower in the polar regions and goes to zero in polar winter. The consequence is, that slow motions transport air, which rises in the tropics, towards the poles (1).

Stratosphere troposphere exchange may occur, if layers of constant (potential) temperature cross the tropopause (2) or  if there are disturbances and convective transport occurs in the mid-latitudes (3). In any case the vertical exchange of the troposphere takes hours to days, the mixing of the stratosphere months to years.
This is why after strong volcano eruptions (e.g. Mt Pinatubo 1991) a disturbance of the stratospheric equilibrium can last for 1-2 years. Have a look on the illustrations below, in order to see the impact of the eruption.

This little exchange between the layers, also called stratosphere-troposphere-exchange (STE), is important for the tropospheric ozone pool, which is in major parts supplied from the stratosphere. Stratospheric ozone initiates OH formation and the cycles of photochemical formation and destruction of ozone in the troposphere.

Table:
The tropospheric ozone budget:

Tropospheric ozone formation and destruction is a circulation, which has its major driving source in stratospheric ozone.

Production/loss process Tg / year Transport from stratosphere + 600 a) Photochemical formation + 3500 b) Photochemical destruction - 3400 Sum a+b: Net in situ formation + 100 Surface deposition - 700

Brewer Dobson Circulation

The cross section of air streams, indicated in figure 1 for one hemisphere, is shown on the left for both hemispheres as annual average. The medium ozone distribution shows that ozone is accumulating near the poles. The transport of air masses is called Brewer Dobson circulation. The causes for this air flow can only be understood taking into account complicated processes in the Earth's radiation equilibrium, planetary waves and subsidence processes in the polar vortex. Each hemisphere has its own circulation. The exchange between the hemispheres is poor.

However, there are differences between the Northern and the Southern hemisphere. In the North the distribution of water and land is more inhomogeneous and the polar vortex is weaker. Also the seasons have to be taken into account. Fig. 3 shows the average distribution through the year. But with the seasons and the zenith of the sun also the centre of air buoyancy in the tropics is shifted either to the North or to the South. Figure 4 illustrates that the distribution of temperature and wind are not homogeneous in January. In parallel also the Brewer Dobson circulation is shifted.

Polar Vorticity

The polar vortex is a circumpolar wind which forms basically over both of the poles but preferably over the Antarctic continent. The Arctic vortex is less stable, since the structure of the landscape and the alternating surface of oceans and continents disturbs the formation of such a vortex. Within the Antarctic vortex however, very low temperatures can be reached and like in a swirl air from higher regions (e.g. containing compounds relevant for the ozone hole destruction) are sucked to lower regions.

About this page:

author: Dr. Elmar Uherek - Max Planck Institute, Mainz
educational proofreading: Michael Seesing - Uni Duisburg - 2003-08-07
last published: 2004-04-20