Observation and Simulation of the Mixing in the Antarctic Circumpolar
Vortex

Image Description

Each spring in the stratosphere over Antarctica (Spring in the
southern hemisphere is from September through November.), atmospheric
ozone is rapidly destroyed by chemical processes.  Over the course of
two to three months, approximately 50% of the total column amount of
ozone in the atmosphere disappears.  At some levels, the losses
approach 90%.  This has come to be called the Antarctic ozone hole.
While the details are not fully understood, the ozone loss is known to
be the result of chemical reactions with compounds containing
chlorine.  The source of the chlorine is manmade chemicals, such as
chlorofluorocarbons (CFCs), which are widely used as refrigerants and
in many industrial processes.

The rapid ozone destruction is confined to the Antarctic because of
the unique meteorological conditions in the springtime stratosphere.
The extremely cold temperatures of the Antarctic stratosphere (less
than 190 K) allow the formation of clouds composed of ice and nitric
acid, called Polar Stratospheric Clouds (PSCs).  Elsewhere, the
stratosphere is too warm and too dry for clouds to form, so these
clouds are largely confined to the polar regions.  The clouds remove
reactive nitrogen compounds from the stratosphere that would otherwise
react with chlorine, preventing it from causing ozone destruction.
The ice crystals also provide a reaction surface for the ozone
destruction reactions, much like the catalytic converter in a car.
The net result is the rapid destruction of ozone in the lower
stratosphere.


Clouds do form during the winter, but many of the chemical reactions
require sunlight, so the ozone destruction does not begin until the
polar night ends in the late winter or early spring.  The northern
hemisphere is warmer than the southern hemisphere, and it warms up
earlier in the spring as a result of the differing weather patterns in
the two hemispheres.  Thus, by the time there is sunlight available,
the clouds have already disappeared.  This appears to explain why
there is no Arctic ozone hole (yet).  Recent observations show that
the Arctic stratosphere is significantly chemically perturbed,
however.

Later in the spring, as the stratosphere warms, the clouds evaporate
and the ozone destruction ceases.  At this time the circulation also
undergoes major changes, and region of low ozone, which is confined
near the pole, is mixed with air from lower latitudes.  This is
largely a transport process, not a chemical one.  Finally, ozone
levels gradually recover during the summer, setting the stage for the
process to repeat itself the following spring.

The purpose of my research is to understand how large-scale mixing
affects the evolution of the ozone hole.  The animation sequences
compare observations of total column ozone from the Total Ozone
Mapping Spectrometer on the NASA Nimbus 7 satellite with simulations
with a simple one-layer numerical model of the stratospheric
circulation.  Radiative heating and cooling processes tend to force
the circulation into a large, symmetric vortex centered on the pole.
Atmospheric disturbances, called planetary- scale waves, tend to make
the vortex asymmetric and produce mixing.  These experiments endeavor
to understand the factors controlling the mixing.

The two sequences of grayscale images show daily snapshots of ozone in
the polar vortex in October of 1983 and November of 1981 .  In the
October sequence, the vortex remains intact, while planetary-scale
waves strip material off the exterior of the vortex by folding and
stretching of the stratospheric air.  This can be seen as tongues of
primarily high-ozone air (light shades), being pulled from the
high-ozone area in a doughnut around the pole.  The low-ozone air
inside the vortex (dark shades) is not mixed with air from lower
latitudes.  Occasional large wave events do pull air from the interior
of the vortex (an example is most easily visible on 24 October).  This
behavior is typical of the time before the vortex breaks down.

(Note: the TOMS images are too widely separated in time to make smooth
animation sequences.  It is best to step through the images manually
in order to be able to see how the ozone is transported around the
vortex.) The situation changes in the November 1981 sequence.  During
this month the vortex mixes thoroughly with air from lower latitudes.
The ozone hole, which is prominent early in the month, is completely
mixed away by the end of the month.  This major mixing event occurs
every year at the end of the spring season.  The numerical model is
used to help understand the reasons for the differences in mixing
behavior.  The other two sequences of images show the motion of
particles placed in simulated stratospheric circulations with winds
characteristic of early October and late November.  For the October
winds the mixing occurs only on the exterior of the vortex, and the
folding and stretching behavior can be seen in the rings of particles
initially placed a constant latitude.  The behavior for the late
November winds is quite different.  In this case the mixing occurs in
the interior of the vortex.  The behavior of the model compares well
with theories of the interaction between the vortex and the waves.
Since chlorine levels in the stratosphere are expected to rise for
several decades, despite the planned phase-out of many chlorine
containing chemicals, it is important to understand the

differences between the northern and southern hemispheres and the
processes that control the size and duration of the ozone hole.

The numerical model used here was developed by Murry L.  Salby and
colleagues at the University of Colorado and the National Center for
Atmospheric Research.  A description of the model can be found in:

Salby, Murry L., R.  R.  Garcia, D.  O'Sullivan, and J.  Tribbia,
1990.  Global Transport Calculation with an Equivalent Barotropic
System, J.  Atmos.  Sci., 47, 188-214.

Details of the research described here can be found in: 
    Bowman, K. P., 1992.  
      Observations of Deformation and Mixing of the Total Ozone
      Field in the Antarctic Polar Vortex, J.  Atmos.  Sci., submitted.
    Bowman, K.  P., 1992.  
      Barotropic Simulation of Large-Scale Mixing in
      the Antarctic Polar Vortex, J.  Atmos.  Sci., submitted.

Total Ozone Mapping Spectrometer data is from the TOMS Gridded Ozone
Data CD-ROM, available from the National Space Science Data Center at
NASA Goddard Space Flight Center, P.  Guimares and R.  McPeters, eds.
(USA_NASA-UARP-OPT-001) TOMS data are processed by the Goddard Ozone
Processing Team.

This research is funded by the National Aeronautics and Space
Administration, Office of Space Sciences and Applications through
grant NAGW-992 to the University of Illinois.

Reseacher



Dr.  Kenneth P.  Bowman 
Climate System Research Program 
Department of Meteorology 
Texas A&M University 
College Station, TX 77843-3150 

bowman@uiatma.atmos.uiuc.edu 










