Observation and Simulation of the Mixing in the Antarctic Circumpolar Vortex

South Pole - Movie of Polar Vortex

Image Description

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