Processes in the stratosphere take place very slowly compared to those in the troposphere.
The stratosphere is extremely stable and there is only little air
exchange with the troposphere. This small air exchange is, however,
extremely important to our climate.
1. The direction of global circulation and stratosphere troposphere exchange (STE). by Elmar Uherek. Please click to enlarge!
Stratosphere-Troposphere Exchange (STE)
Transport of air around the globe is driven by the Sun. Radiation
from the Sun warms up the land, sea surface and air. Heating is
greatest in the tropics, less so in the mid and high latitudes. This
means that convection is strongest in the tropics and air rises to
higher altitudes here compared to elsewhere on Earth. Above the
tropopause, absorption of solar radiation by ozone leads to a warming of
the stratosphere. This warming is highest over the tropics, lower in
the polar regions and goes to zero in the polar winter. Figure 1. shows
the consequences of this, warm air rises in the tropics, cools and
moves towards the poles (shown as 1 on the figure).
Exchange of air between the stratosphere and the troposphere
can occur if layers of constant (potential) temperature cross the
tropopause (point 2 on Figure 1) or if there are disturbances and
convective transport occurs in the mid-latitudes (point 3).
Vertical air exchange in the troposphere takes hours to days whereas
mixing in the stratosphere takes months to years, This is why, after
large volcanic eruptions such as Mount Pinatubo in 1991, it can
take between one and two years for the stratosphere to return to its
stable state. Have a look at the illustrations below to see the impact
of this eruption.
The small stratosphere-troposphere-exchange (STE) is an important
source of ozone from the stratosphere to the troposphere. Stratospheric
ozone initiates hydroxyl (OH) radical formation and the cycles of
photochemical formation and destruction of ozone in the troposphere.
The tropospheric ozone budget
There is a cycle of ozone formation and destruction in the
troposphere. The major driver of this cycle is inputs of ozone from the
stratosphere.
Production/loss process
Tg / year
Transport from the stratosphere
+ 600
a) Photochemical formation
+ 3500
b) Photochemical destruction
- 3400
Sum a+b: Net in situ ozone formation
+ 100
Deposition to the ground
- 700
2. a) Eruption of Mt. Pinatubo in the Philippines in June 1991.
2. b) Aerosol absorption: Absorption by particles
in the atmosphere after the eruption of Mt. Pinatubo in June
1991 immediately increased with the eruption and decreased only slowly
over the next 2-3 years. The figure shows that particles reached the
stratosphere. Data from SAGE I + II, please click to enlarge! (50 K)
3. The average annual flow of the Brewer Dobson
Circulation shows that air moves from the Tropics (in the middle) to the
poles. An average ozone distribution for the year is laid underneath
and shows that ozone accumulates in the polar regions. The north pole is
on the right. Data source: Nimbus 7 website.
Brewer Dobson Circulation
The pattern of air movement around the atmosphere is known as the
Brewer Dobson circulation. Since the processes which control this
circulation pattern (the radiation budget of the Earth, planetary waves,
subsistence processes in the polar vortex) are very complicated, we
won't go into them in detail here. Very simply, air rises in the
tropics and sinks at the poles, each hemisphere has its own circulation
and exchange of air between the hemispheres is poor.
There are slight differences in how the air circulates in
the Northern and Southern hemispheres. In the north, the distribution
of oceans and land masses is more inhomogeneous than in the south and
the polar vortex is weaker. In addition, seasons have to be taken into
account. Figure 3. shows the average circulation through the year, but
as the seasons and angle of the Sun change over the year, the position
where air rises in the tropics shifts either to the north or to the
south. Figure 4. shows how the temperature and wind patterns vary
between the two hemispheres in January, this in turn causes the Brewer
Dobson circulation to vary.
The polar vortex (very simply, a whirlwind) is a circumpolar
wind which forms over both poles, more often over the Antarctic
continent than over the Arctic. The Arctic vortex is less stable since
the alternating surface of oceans and continents on the Earth disturbs
the formation of the vortex. Very low temperatures can be reached within
the Antarctic vortex and air from higher regions (containing
compounds responsible for ozone hole destruction) can be sucked down to
lower altitudes by the wind.
5. a) Three dimensional illustration of the wind speed in the polar vortex and the temperature. Data from: NASA / Goddard Space Flight Centre, simulation by IBM. Please click to enlarge!
5. b) Three dimensional illustration of the wind
speed in the polar vortex and the ozone destruction in October 1987.
Data from: NASA / Goddard Space Flight Centre, simulation by IBM.
Please click to enlarge!
About this page:
author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz, Germany. educational proofreading: Michael Seesing - Uni Duisburg, Germany - 2003-08-07 last published: 2004-04-20
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