Letters to the Editor
Ozone Hole Split
Letters to the Editor
The Southern Hemisphere Ozone Hole Split in 2002
Costas Varotsos
University of Athens, Department of Applied Physics, Panepistimiopolis, GR-157 84 Athens, Greece
([email protected]; [email protected])
Among the most important aspects of the atmospheric pollution problem are the anthropogenic impacts on the stratospheric ozone
layer, the related trends of the total ozone content drop and the solar ultraviolet radiation enhancement at the Earth's surface level.
During September 2002, the ozone hole over the Antarctic was much smaller than in the previous six years. It has split into two
separate holes, due to the appearance of sudden stratospheric warming that has never been observed before in the southern
hemisphere. The analysis of this unprecedented event is attempted, regarding both the meteorological and photochemical aspects, in
terms of the unusual thermal field patterns and the induced polar vortex disturbances.
Introduction. The total ozone content (TOC) observations
currently performed by the Total Ozone Mapping
Spectrometer (TOMS) flown on the NASA Earth Probe
satellite and the NOAA-16 Solar Backscatter Ultraviolet
instrument (SBUV/2) showed that the size of the Antarctic
ozone hole was around 15 million km2 during the last two
weeks of September 2002, which is well below the morethan 24 million km2 seen during the last six years in this
season. Apart from the smaller size (as it was in 1988), the
Antarctic ozone hole split into two holes during late
September, an event seen for the first time since global
maps for the stratosphere of the southern hemisphere
became available (Kondratyev and Varotsos 2000).
According to the September 30 Press Release from NASA
and NOAA (NASA/NOAA 2002), this year's warmer-thannormal temperatures around the edge of the polar vortex
over Antarctica are responsible for the smaller ozone loss
(Varotsos 2002) [Fig. 1].
This note attempts a better insight into this exceptional event
through an analysis of the thermal field patterns over the
Antarctic in late September; it proposes a plausible
mechanism of this appearance.
Results and Discussion. According to the analysis of
the climatological data, the coldest temperatures over
Antarctica typically occur in August–September (depletion
of the ozone layer), while temperatures usually begin to
warm by early October (the ozone layer starts to recover).
However, the corresponding data analysis shows that in
late September 2002 (especially during 21–26/09/2002)
some unusual synoptic features have occurred regarding
the stratospheric weather patterns over Antarctica. In
particular, an inspection of the polar stereographic charts
(10 hPa geopotential height) shows that the basic polar
vortex in the stratosphere of the southern hemisphere
was step-by-step elongated up to the occurrence of a
dramatic change the polar vortex split into two centres. To
be more concrete, on September 21 the basic polar
vortex elongated (as shown by the roughly elliptical height
contours) and centered just off the pole. On September
22, this structure rotated eastward and elongated further
with a weakening of the polar vortex (accompanied by a
quasi-stationary anticyclone which appeared much earlier
than normal) at the middle. On September 23 and 24 this
rotation and elongation continued, and the weakening
became gradually stronger. On September 25, an
opposite to the first, secondary anticyclone started to
develop, and
Fig. 1: What a difference a year makes. Southern hemisphere ozone image September 26, 2001 and 2002 (Source:
http://toms.gsfc.nasa.gov/news/press_release_2002.html)
ESPR - environ Sci & Pollut Res 9 (6) 375 - 376 (2002)
© ecomed publishers, D-86899 Landsberg, Germany and Ft. Worth/TX, USA • Tokyo, Japan • Mumbai, India • Seoul, Korea
Ozone Hole Split
event occurred with the vortex splitting into two
cyclonic centers. From the temperature field data it
is evident that relatively warm air extended to the
South Pole, which indicates a polar stratospheric
major warming. In this respect, the amplitude of the
zonal wave-number 2 (derived from the Fourier
analysis of the isobaric height and temperature
fields) reached the peak when the polar vortex split
into two circulations (reversal of the zonal mean
wind). The latter has also been interpreted by using
dynamical diagnostics suggested by the wave,
mean-flow interaction theory (e.g. Eliassen-Palm
flux). It is worth recalling that such a sudden
stratospheric warming is usually accompanied with
'blocking events'; these are characterised by
quasistationary distortions of the tropospheric flow.
It should be emphasised that the above-described
meteorological situation, which prevailed during
21–26/09/2002 and led to the split of the southern
ozone hole, is not identical with that during the
period of 5–13/09/2002, when a first trial of the
ozone hole to split was observed. Thus, according
to the aforementioned analysis, the polar vortex in
the stratosphere of the southern hemisphere split in
two, due to a major warming, leading to a
corresponding split of the ozone hole. It should be
mentioned that such an event has never been
reported in the literature before. Conventionally,
major warming events only occur in one of two
northern winters, and they do not occur in the
southern hemisphere. So far, the total absence of
major warming events in the southern winter
seasons was partly attributed to: 1) the fact that the
winter vortex is stronger in the southern than in the
northern hemisphere, and thus more deceleration
(more wave forcing) would be required to reverse it,
and 2) the comparative lack of the large-amplitude
quasi-stationary waves in the southern hemisphere
(Andrews et al. 1987). However, it is concluded
from the analysis that in September 2002 the latter
was not the case. As far as the photochemical
aspects of the ozone hole in the Antarctic winter of
2002 are concerned, the existing understanding of
photochemical ozone-depletion processes is
adequate to assess the timing and magnitude of
the chemical ozone loss which is proportional to the
exposure to sunlight (von der Gathen et al. 1995,
Rex et al. 1998, Reid et al. 1998, Schulz et al.
2000).
Conclusions. The data analysis performed for this
note led to the following conclusions which provide
a plausible observational explanation of the
unusual behaviour of the ozone hole over
Antarctica this year: 1) The basic polar vortex in the
stratosphere of the southern hemisphere in late
September 2002 was gradually elongated and up
to the splitting into two centres, which resulted in a
corresponding split of the ozone hole. 2) The
meteorological situation during 21–
376
Letters to the Editor
26/09/2002 which led to the split of the southern ozone hole, is not
exactly identical with that during the period of 5–13/09/2002, when
a first trial of the ozone hole to split was observed. 3) Relatively
warm air extending to the pole indicates that a polar stratospheric
major warming was taking place. The amplitude of the zonal wavenumber 2 reached the peak when the polar vortex split into two
circulations
(reversal of the zonal-mean wind is observed).
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ESPR - environ Sci & Pollut Res 9 (6) 375 - 376 (2002)