The long-term evolution of the Antarctic ozone hole is studied based on the TOMS data and the JMA data-set of stratospheric temperature in relation with the possible role of polar stratospheric clouds (PSC's). The effective mass of depleted ozone in the ozone hole at its annual mature stage reached a historical maximum of 55 Mt in 1991, 4.3 times larger than in 1981. The ozone depletion rate during 30 days before the mature ozone hole does not show any appreciable long-term trend but the interannual fluctuations do, ranging from 0.169 to 0.689 Mt/day with the average of 0.419 Mt/day for the period of 1979 - 1991. The depleted ozone mass has the highest correlation with the region below 195 K on the 30 mb surface in June, whereas the ozone depletion rate correlates most strongly with that in August. The present result strongly suggests that the long-term evolution of the mature ozone hole is caused both by the interannual change of the latitudinal coverage of the early PSC's, which may control the latitude and date of initiation of ozone decrease, and by that of the spatial coverage of the mature PSC's which may control the ozone depletion rate in the Antarctic spring

Akagi, K.

Ito, T.

Matsubara, K.

Naganuma, H.

Sakoda, Y.

Shibata, S.

Takao, T.

Watanabe, Y.

English

NASA Technical Reports Server

N95- 11041
QUANTITATIVE CHARACTERIZATION OF THE ANTARCTIC OZONE HOLE
T. ITO, Y. SAKODA, K.MATSUBARA, T. TAKAO, K. AKAGI
Y. WATANABE, S. SHIBATA and H. NAGANUMA
Observations Department, Japan Meteorological Agency
1-3-4, Otemachl, Chiyodaku, Tokyo 100.
ABSTRACT
The long-term evolution of the Antarctic ozone
hole is studied based on the TOMS data (ver. 6) and
the J_lh data-set of stratospheric temperature in
relation with the possible role of PSCs. The effec-
tive mass of depleted ozone in the ozone hole at
its annual mature stage reached an historical maxi-
mum of SS Mt in 1991, 4.3 times larger than in
1981. The ozone depletion rate during 30 days be-
fore the mature ozone hole does not show any appre-
ciable long-term trend but the inter-annual fluctu-
ations do, ranging from 0.169 to 0.689 Mt/day with
the average of 0.419 Mr/day for the period of 1979
- 1991. The depleted ozone mass has the highest
correlation with the region below 195 g on the 30
mb surface in June, whereas the ozone depletion
rate correlates most strongly with that in August.
The present result strongly suggests that the long-
term evolution of the mature ozone hoIe is caused
both by the inter-annual change of the latitudinal
coverage of the early PSCs, which may control the
latitude and date of initiation of ozone decrease,
and by that of the spatial coverage of the mature
PSCs which may control the ozone depletion rate in
the Antarctic spring.
1. INTRODUCTION
The remarkable depletion of the ozone layer
(so-called the ozone hole) has been appearing over
Antarctica in the spring since the late 1970's. In
order to study the complete mechanism for the long-
term evolution of the ozone hole, which remains
unexplained (Turco et al., 1989), it is required to
establish quantitative measures which enable us to
express the Intensity of the ozone hole appropri-
ately. In this paper, we tentatively define the
quantitative measures to characterize the ozone
hole evolution (hereafter called the ozone hole
indexes) and examine the cause of the long-term
evolution of the ozone hole.
2. DEFINITION OF OZONE HOLE INDEXES
The area of the ozone hole Is defined as the
area enclosed by the contour of 220 m atm-cm, south
of 50"S. The threshold of 220 m atm-cm Is taken as
the value which had never appeared before 1980 as
mentioned by Newman et al. (1991) and confirmed
with the total ozone data at Amundsen-Scott station
archlved In the Word Ozone Data Centre. The depth
of the ozone hole is defined as the daily minimum
total ozone south of 50"S. Another ozone hole index
expressing the volume of the ozone hole is needed
to complete the expression of the ozone hole inten-
sity. The depleted mass of ozone, which is used in
place of the volume of the ozone hole, is defined
as the deficit of ozone mass in the area south of
50"S from the long-term global average of total
ozone of 300 m atm-cm.
The daily values of these three ozone hole
indexes are calculated using the daily grid data
(vet. 6) of Total Ozone Mapping Spectrometer (TOMS)
for the period from 1 August to 31 December for the
years from 1979 to 1991, except for the period from
1 November to 31 December 1991 (unavailable at
present). In the calculation, unavailable grid
points due to the polar night are assumed to have
the same total ozone value as the most southern
available value that day.
Based on the daily values thus calculated, the
annual peak values, that is, the annual maximum
area of the ozone hole, the annual minimum of total
ozone in the ozone hole and the annual maximum of
depleted ozone mass in the ozone hole are taken as
the annual representative indexes of the ozone hole
in its annual mature stage (hereafter called the
mature ozone hole).
3. QUANTITATIVE CHARACTERIZATION OF THE OZONE HOLE
Figure 1 shows the ozone hole indexes in the
mature ozone hole from 1979 to 1991. In Fig. 1, the
ozone depletion rate in the developing stage of the
ozone hole is also shown, which is calculated by
the linear least square fitting to the daily values
of depleted ozone mass during the 30 days before
its peak date.
In Fig.. 1, it is seen that the long-term evo-
lution of the mature ozone hole became apparent in
1982. In 1991, the depth, area and depleted ozone
mass in the mature ozone hole reached _ist_rical
extremes which w?re 108 m atm-cm, 17.4x10" km_ and
55 Mt, respectively. When these values are compared
with those in 1981, it can be seen that, over the
last I0 years, the minimum total ozone in the ma-
ture ozone hole has decreased by about 47%, the
582
https://ntrs.nasa.gov/search.jsp?R=19950004628 2020-06-16T11:32:12+00:00Z
area has increased by about 13 times, and the dep-
leted ozone mass has increased by about 4.3 times.
In contrast to these quantities, the ozone
depletion rate does not show any appreciable long-
term trend but the Inter-annual fluctuations do
range from 0.169 to 0.689 Ht/day with the average
of 0.419 Mr/day for the years from 1979 to 1991. It
is interesting to note that in 1990, the depleted
ozone mass In the mature ozone hole was 48 Mt which
was only 54 lower than that in 1989 whereas the
depletlon rate in 1989 was 3 times larger than that
in 1990 which was the largest in the years from
1979 to 1991. Daily plots of ozone hole indexes
{Fig. 2) clearly suggest that the initiation of
ozone decrease in 1990 was earlier than In 1989.
Thus, the long ozone-decrease duration due to an
early start of ozone decrease causes the large
depleted ozone mass In the mature ozone hole, even
with the small depletion rate in 1990. Similarly an
early start in ozone depletion was also evident in
1991 and the depletion rate was the second histori-
cal maximum, about 104 smaller than in 1989. This
resulted in the historical maximum of the depleted
ozone mass in the mature ozone hole In 1991.
4. RELATION BETWEEN THE PRIOR STRATOSPHERIC
TEMPERATURE AND THE INTENSITY OF THE MATURE
OZONE HOLE
The process leading to the ozone hole forma-
tion is strongly related with the occurrence of
polar stratospheric clouds (PSCs). On PSCs parti-
cles, chlorine reservoirs (HC1, CIONO2 etc.) under-
go heterogeneous conversion, emitting chlorine gas
(C12) and fixing HNO3 in the PSCs particles (Solo-
mon et al., 1986; McElroy et al., 1986}. During
winter, C12 is accumulated in the Antarctic strato-
sphere. In spring when the sunlight returns to the
Antarctic stratosphere, C12 is photolyzed to produ-
ce a large quantity of reactive chlorines forming
the ozone hole. The fixing of HNO3 in PSCs parti-
cles which are eventually removed from the strato-
sphere by their gravitational settling Is also
important to form the ozone hole, inhibiting the
fixing of chlorine in its more stable reservoirs.
Satellite observations indicate an onset of PSCs
formation at approximately 195 K. The temperatures
of 195 K and 187 K are thought to be the threshold
of the onset of Type-I PSCs {mainly of nitric acid
trlhydrate solid particles} and Type-II PSCs (main-
ly of water solid particles) formation, respective-
ly (Turco et al., 1989}. The minimum stratospheric
TOTALOZONE
t
79 88 81 82 83 84 85 86 87 88 89 98 91
Mt/day DEPLETIONRATE
'1
79 88 81 82 83 84 85 86 87 88 89 98 91
Fig. 1. The long-term change of the minimum total
ozone, area enclosed by the 220 m atm-cm
contour and depleted ozone mass in the
mature ozone hole and the ozone depletion
rate.
"_- C C
r
u!Jlj 'ii r')l T ,NOJ DEC GLJG c;[ F' OCT NOU DEC
r: 1989 1998
m at_ cm
3_0 "'
<
]88
0
hUG SEP OCT NOU DEC
1991
F g. 2. The daily plot of the minimum total ozone (A), area enclosed
by the 220 m atm-cm contour (H) and depleted ozone mass (C).
583
temperatureor the area enclosed by a given tempe-
rature contour on a stratospheric isobaric surface
may be considered as a parameter expressing the
intensity or areal coverage of PSCs. In this sec-
tion, the correlation between the ozone hole index-
es in the mature ozone hole and the stratospheric
temperature before the mature ozone hole is examin-
ed to consider the possible cause of the long-term
evolution of the ozone hole In relation to the role
of PSCs.
The temperature data used in this study is the
analyzed stratospheric grid data (5" mesh) of
monthly mean temperature edlted by the Japan Mete-
orologtcal Agency, available for the isobaric sur-
faces of 10, 30 and 50 mb. From this grid data, the
following three types of parameters are calculated;
the minimum temperature in the Southern Hemisphere,
the areas enclosed by the 195 K contour, and that
by the 187 K contour on the 10, 30 and 50 =b sur-
faces, for the months of June, July, August and
September.
Then. the correlation coefficients are calcu-
lated between the ozone hole Indexes and the tempe-
rature parameters, and between the ozone depletion
rate and the temperature parameters. Since there
are 13 data points for each quantities for the
period 1979 to 1991, the correlation is considered
to be statistically significant with 99_ confidence
when the correlation coefficient is larger than
0.7. Table 1 shows the resultant correlation coef-
ficients for the 30 mb surface.
The 50 mb surface, which is located in the
main part of the ozone layer and also the main part
of the ozone depleting layer in the ozone hole,
exhibits no significant correlation between the
temperature parameters and the ozone hole indexes
including the ozone depletion rate. The 10 mb sur-
face, which is located at about 10 km above the
main part of ozone layer, does show a few signifi-
cant correlations as follows; between the minimum
total ozone and the area below 195 K for the months
from June to August; between the depleted ozone
mass and the area below 195 K in July; between the
depleted ozone mass and the minimum temperature in
August and between the depletion rate and the mini-
mum temperature in September.
However a numbers of significant correlations
are seen between the temperature parameters on the
30 mb surface and the ozone hole indexes including
the ozone depletion rate. The 30 mb surface Is
located near the top of the ozone depleting layer
in the ozone hole. The highest correlation coeffi-
cients (0.79-0.85) are obtained between the ozone
hole indexes and the area below 195 K on the 30 mb
surface in June, although correlation coefficients
higher than 0.7 are also seen in July. The ozone
depletion rate has the highest correlation coeffi-
cient (0.86) with the area below 195 K on the 30 mb
surface In August. In September, it is the second
highest (0.74).
Figure 3 shows the scatter diagram between the
depleted ozone mass in the mature ozone hole and
the area below 195 K on 30 =b surface in June. The
numbers attached to the plotted points indicate the
years. The distribution of years in the diagram
Tab. 1. Correlation coefficients between tempera-
ture parameters and ozone hole indexes.
OZONE HOLE INDEIES
DEPLETION
TENPEHATUHE PARAMETERS MINIMUM AHEA OF DEPLETED
TOTAL OZONE OZONE HOLE ]¢_SS HATE
JUN 0,73 0 61 0.58 0,05
MINIMUM OF JiJL 0 52 - O. 51 O. 5 | 0 . 35
TEMPERATURE AUG O. 3 9 - O. 4 3 O. 4 5 O. 4 2
SEP 0.16 O_t 7 0.25 065
JUN 0.85 0.82 -0.79 029
AREA JUL -068 0.74 -0.78 -0.42
!O=b BELOW 195 g htiO O 49 O. 59 - O. 64 O. 86
SEP 043 0.46 -0.63 -0_74
JUN -0 57 049 -0.44 0_[3
AREA JUL 0.73 0.74 -0.74 -0.43
BELOW 187 [ AUG -063 066 -0.70 -0.64
SEP
-10
v
-20
c)
-3_
_ -40
-50
8_
"81
"83
"?9
"84
"82
•86 "88
"85
' 9_,
"89
°87o91
16 I2 _4 16 18 2@ 22
aREABELOW4I95K{10Skr"2) ON38rob[N JUNE
Fig. 3. The scatter diagram of the depleted ozone
mass in the mature ozone hole and the area
below 195 K on 30 mb surface for June.
-.1
x::
t.u
-.3
_, -.4
-.5
o
-.6
-.7
"79.88
"86
"81
"9@
"82
"84
"SB
*85
"83
"91
"87 89
_5 18 2_ tt t_ t6
AREABELOW195K{106km2) ON30mbIN _UGUST
Fig. 4. The scatter diagram of the ozone depletion
rate and the area below 195 K on 30 mb
surface for August.
584
suggests that the high correlation seen in the
diagram results mainly from the simultaneous long-
term trend of the depleted mass in the mature ozone
hole and the area below 195 K on the 30 mb surface
in June, but also partly from the simultaneous
oscillation of the two variables. This is not the
case for the scatter diagram between the depletion
rate and the area below 195 K on the 30 mb surface
in August, shown in Fig. 4. This suggests that the
high correlation results mostly from simultaneous
Inter-annual oscillations of the two variables.
5. DISCUSSION
The important findings of the present analysis
are that the long-term evolution of the mature
ozone hole is highly correlated with the area below
195 Z on the 30 mb surface near the top of ozone
depleting layer in June, which is about four months
before the mature ozone hole. Since the temperature
of 195 K means the threshold of the onset of Type 1
PSCs formation, the area below 195 K is considered
to express the possible area covered wlth Type 1
PSCs.
The large spatial coverage of Type 1PSCs in
the winter would cause the large spatial distribu-
tion of photolyzable chlorines such as C12 spread-
ing to the relatively lower latitudes in the polar
vortex. Since the lower latitudes in the polar
vortex receive the solar light earlier than the
higher latitudes In the late. winter and early
spring, the destruction of ozone molecules by reac-
tive chlorines produced by the photolysls of C12
etc. would Initiate earlier there. Thus, large
spatial coverage of PSCs in the winter would lead
to the earlier onset of the ozone hole formation.
The early onset of the ozone hole formation tends
to lead to the larger depleted ozone mass in the
mature ozone hole, as was the case in 1990 and
1991.
In late August 1991, the total ozone observed
at Amundsen-Scott (archived In WODC) were mostly
larger than the provisional values of total ozone
observed at Halley and Faraday (Dr. Shanklln, pri-
vate communication}, Arrival Heights (Dr. Clarkson,
Private communication) and Syowa, suggesting the
ozone decrease started from the lower latitude in
the polar vortex, ieadlng to an earlier onset in
ozone hole formation In 1991. This fact may support
the present speculation.
Since the main decrease of the ozone amount
occurs below the 30 mb surface from August to Sep-
tember, the subsidence current in the polar vortex
Is required to exist from June to August in order
to explain the highest correlation between the area
below 195 K on 30 mb surface In June and the ob-
served ozone depletion. The required subsidence alr
current is approximately same In magnitude as the
descending velocity of PSCs detected by McCormick
et al. (1985).
On the other hand, the ozone depletion rate in
the ozone hole is considered to be approximately
proportional to the total amount of reactive chlo-
rine which would depend on the total surface area
of the PSCs particles available for the heterogene-
ous production of the photolyzable chlorines. The
large area below 195 K is also considered to be the
region where a large amount of the total surface
area of PSCs particles exists, if the particles
have grown well In size. According to the satellite
observations, the maximum extinction by PSCs Is
observed from July to August {McCormick et al.,
1989). Thus, the existing high correlation between
the ozone depletion rate and the area below 195 K
on 30 mb surface in August may be reasonable when
the slze of PSCs partlcles is taken into account.
6. CONCLUDING REMARKS
The present analysis strongly suggests that
the existing long-term evolution of the mature
ozone hole Is caused by the possible year-to-year
change of the spatial coverage of PSCs In both
their early- and mature-stages, which is controlled
by the stratospheric temperature field, while the
effect of the global trend in chlorofluorocarbons
concentration also plays an important role In the
ozone hole evolution. The year-to-year change in
the temperature field in the Antarctic stratosphere
may be caused by the change In the planetary wave
activity in the Southern Hemisphere as demonstrated
by Newman and Randel (1989), even in June and Au-
gust. Also, the long-term trend in the stratosphe-
rlc concentration of the green-house gases may be
responsible for the change in the temperature field
in the Antarctic stratosphere, as demonstrateO Dy
Shine (1988).
ACKNOWLEDGEMENTS: The authors would like to thank
Richard D. McPeters, Arlin J. Krueger of NASA
GSFC, members of the TOMS Nimbus Experiment and
Ozone Processing Teams, and the National Space
Science Data Center/ World Data Center-A for Roc-
kets and Satellites for supplying the TOMS data
used in this study.
REFERENCES
McCormick, M. P., P. Hamlll, and U. O. Farrukh,
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J. Meteor. Soc. Jpn., 63, 267-276.
mccormick, M. P., C. R. Trepte, and M. C. Pittes
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Newman, P., R. Stolarskl, M. Schoeberl, R.
McPeters, and A. Krueger, 1991: The 1990
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Geophys. Res. Lett., 18, 661-664.
Newman, P. A., and W. J. Randel, 1988 : Coherent
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Quantitative characterization of the Antarctic ozone hole

Quantitative characterization of the Antarctic ozone hole

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