2002 Ozone-Hole Splitting - Background
Since Farman et al. (1985) published their discovery of the ozone hole over Antarctica, polar ozone-hole events have been regularly recorded, mapped, and analyzed. During the 1990s, the records show that the patterns of the Antarctic ozone hole are similar from year to year. Typically, the ozone hole appears in August, continuously increases in size to a maximum in late September or early October, and then disappears in December.
A more complete description of the ozone-hole event involves the
seasonal movement of the global atmosphere (e.g., Schoeberl et al.,
1992; Solomon, 1999). Usually, during late March (i.e., Antarctic
autumn equinox), the Antarctic atmospheric temperature goes down,
and the density of the polar air increases. The pressure along the
boundary of the polar region decreases and drives the warmer air in
low- and mid-latitude regions to move upwards and towards the
poles. Because of the earth's rotation and shape (i.e., the radius
of a latitude circle decreases as the latitude increases away from
the equator), the winds, also called jet streams, are from west to
east and with increasing speed and height in higher
latitudes. Finally, the jet streams form a huge westerly circulation
in the stratosphere called the polar night jet. Inside the polar
night jet is the so-called polar vortex. The warmer air can only
move along the edge of the polar vortex, but not enter it. Within
the vortex, the cold polar air becomes cooler and cooler with
neither warmer air from lower latitudes nor energy from the sun
during the polar night. The conditions are made even worse due to
the thermal radiation in Antarctica. In the Antarctic winter, the
stratospheric temperature is usually below -80oC, which causes the
formation of the polar stratospheric clouds (PSCs), a prerequisite
for substantial ozone chemical depletion. PSCs accelerate the
conversion of some chemical material, such as Chlorofluorocarbons
(CFCs), into free chlorine and bromine that can directly destroy ozone
molecules. Therefore, the total column ozone continuously decreases until
the end of the polar night. During September and October (the
Antarctic polar "morning"), the edge of the polar vortex gradually
becomes unstable because the energy from the sun changes the
atmospheric-temperature distribution and the associated wind
pattern. The polar vortex becomes weaker and weaker, and finally
breaks down in November. Some researchers explain this process from
instability to breakup with the planetary wave or the Rossby wave theory (McIntyre and Palmer, 1983; Polvani and Saravanan, 2000). After the destruction of the vortex, warmer air with much higher ozone concentration enters the polar area and breaks down the ozone hole. In other words, the total column ozone in the region increases to values greater than 220 DU.
While the ozone hole typically has a standard cycle, a series of unusual events occurred in 2002. In
September 2002, the polar vortex became so unstable, that a strong jet
stream from the mid latitudes broke through the polar vortex and split
it into two vortices with each holding a smaller ozone hole. One
vortex, with its ozone hole, disappeared in several days. The other
one moved polewards and stayed above the polar area until early
November. This vortex and its ozone hole were destroyed one month
earlier than the normal polar vortex.
The 2002 shrinking and splitting of the ozone hole excited the mass media. For researchers, two important questions needed to be answered:
(1) How can the whole ozone-hole splitting event be explained?
(2) Is the 2002 ozone hole a significant signal of the improvement (i.e., decrease) of ozone loss?
By using three-dimensional ozone data from the TM3DAM model and the
wind data from ECMWF model, Siegmund et al. (2004) simulated the 2002
ozone-hole event and explained that the ozone-hole splitting is likely
the result of a strong zonal-mean meridional poleward ozone mass flux
between 20hPa and 40hPa.
Newman and Nash (2003) analyzed the southern hemisphere stratosphere
in the 2001/2002 winter based on the planetary wave theory. They
concluded that during the entire 2001/2002 winter the air was much
warmer (by up to 15oK) than normal in the mid-latitude region, but only
slightly warmer in the polar stratosphere. The planetary waves from
the troposphere heated the edge of the polar vortex, and one of the
waves was so strong that the zonal wind was diverted and broke through
the polar vortex. Therefore, the unusual 2002 wave events is thought to be the main reason for the unusual splitting of the ozone hole.
Figure 1. Total Column Ozone observations on September 27, 2002. Missing data are colored in gray. The data come from the Total Ozone Mapping Spectrometer (TOMS).
Although researchers have developed many theories and models to
explore the mystery of the ozone layer, ozone datasets have their
limitations. Ground-based stations or balloons can observe ozone level at
extremely limited locations, and observations from satellites also
have some problems. First, the orbits of Polar Orbiting Satellites
(POS) result in incomplete coverage of the low-latitude
regions. Second, there is no sunlight in the polar region during the
polar winter, which is necessary for a POS to obtain TCO
data. Third, the POS hardware is always a natural source of noise
that masks the signal.
The Multi-Resolution Spatial Model (MRSM) of Johannesson and Cressie
(2004) is an effective statistical method for prediction (or
smoothing) of spatial processes based on the change-of-resolution
Kalman filter (Chou et al. 1994; Huang et al. 2002; Johannesson et
al. 2007) and
variance-covariance likelihood inference. This TCO webpage shows
spatial statistical prediction using the MRSM, for the 2002
ozone-hole-splitting event along with a measure of the uncertainty
in each predicted value; see Figures 1, 2, and 3. By using similar procedures, the MRSM can be easily applied in other massive-data-processing scenarios. The purpose of this website is to encourage more researchers to recognize, understand, and apply the MRSM in their own fields.
Figure 2. Estimated Total Column Ozone values on September 27,
2002. The map is the output of the Multi-resolution Spatial Model (MRSM). Notice that the ozone hole has split into two.
Figure 3. Total Column Ozone uncertainty (standard deviation or SD) of the estimated Total Column Ozone on September 27, 2002.
Chou, K. C., Willsky, A. S., and Nikoukhah, R. (1994). Multiscale systems, Kalman filters, and Riccati equations. IEEE Transactions on Automatic Control, 39, 479-492.
Farman, J. C., Gardiner, B. G., and Shanklin J. D. (1985). Large losses of total ozone in Antartica reveal seasonal ClOx/NOx interaction. Nature, 315, 207-210.
Huang, H.-C., Cressie, N., and Gabrosek, J. (2002). Fast, resolution-consistent spatial prediction of global processes from satellite data. Journal of Computational and Graphical Statistics, 11, 63-88.
Johannesson, G. and Cressie, N. (2004). Variance-covariance modeling and estimation for multiresolution spatial models. In geoENV IV - Geostatistics for Environmental Applications (eds X. Sanchez-Vila, J. Carrera, and J. J. Gomez- Hernandez). Kluwer: Dordrecht, 319-330.
Johannesson, G., Cressie, N., and Huang, H.-C. (2007). Dynamic
multi-resolution spatial models. Environmental and Ecological
McIntyre, M. E. and Palmer, T. N. (1983). Breaking planetary waves in the stratosphere. Nature, 305, 593-600.
Newman P. A. and Nash, E. R. (2003). The unusual southern hemisphere
stratosphere winter of 2002. Journal of Atmospheric Sciences:
Special Issue on the Southern Hemisphere Sudden Stratospheric
Warming of 2002, in press.
Polvani, L.M. and Saravanan, R. (2000). The three-dimensional
structure of breaking Rossby waves in the polar wintertime
stratosphere. Journal of Atmospheric Sciences, 57, 3663-3685.
Schoeberl, M. R., Lait, L. R., Newman, P. A., and Rosenfield, J. E. (1992). The structure of the polar vortex. Journal of Geophysical Research, 97, 7859-7882.
Siegmund, P., Eskes, H., and Velthoven, P. (2004). Antarctic ozone transport and depletion in Austral spring 2002. Journal of Atmospheric Sciences:
Special Issue on the Southern Hemisphere Sudden Stratospheric
Warming of 2002, in press.
Sinnhuber, B.-M., Weber, M., Amankwah, A., and Burrows,
J. P. (2003). Total ozone during the unusual Antarctic winter of 2002.
Submitted to Geophysical Research Letters.
Solomon, S. (1999). Stratospheric ozone depletion: A review of concepts and history. Reviews of Geophysics, 37, 275-316.
Dobson Unit (DU): the equivalent ozone thickness in mm under standard ground atmospheric conditions (1024Pa, 300K). See: http://www.atm.ch.cam.ac.uk/tour/dobson.html.
Total Column Ozone (TCO): a measurement of the total amount of
atmospheric ozone in a given column. Usually, TCO is measured in
Dobson Units (DU).
Ozone Hole: the region where TCO values are less than 220 DU.
Antarctic Polar Vortex: see http://www.cfm.brown.edu/people/sean/Vortex/.
Rossby Wave (also called the planetary wave): see http://www.soc.soton.ac.uk/JRD/SAT/Rossby/Rossbyintro.html.