CAN VARIATIONS OF CONVECTION AND CIRCULATION IN THE TROPICS AND
SUBTROPICS PLAY A ROLE IN THE NATURAL VARIABILITY OF THE ANTARCTIC
OZONE?
1
Leila M. V. Carvalho and 2Charles Jones
Abstract
This article investigates some relevant aspects in the relationships between variations in the tropical
dynamics and the modes of variability of the extratropics. The importance of coupling tropicsextratropics low-frequency variations to the natural variability of the Antarctic ozone and the ozone
hole is also examined. We show that intraseasonal activity in the circulation and convection in the
tropics during austral spring may play an important role in determining intraseasonal variations in
the stratospheric polar vortex. These disturbances have interannual variations and are key
dynamical mechanisms for the spatial characteristics and reconstruction of the total Antarctic ozone
during spring. Moreover, it is suggested that the observed changes in the Antarctic ozone trend after
1990 may be linked to natural variations in the tropical troposphere with a stronger association with
the lower stratosphere modes of variability when compared to the period pre 1990.
Resumo
O presente artigo investiga alguns aspectos relevantes na relação entre variações na dinâmica dos
trópicos aos modos de variabilidade dos extratrópicos. A importância do acoplamento tropicosextratrópicos em baixa-frequência para a variabilidade natural do ozônio Antártico e do buraco de
ozônio também é examinada. Nós mostramos que a atividade intrasazonal na circulação e na
convecção nos trópicos
durante a primavera austral podem ter um papel importante na
determinação das variações intrasazonais no vórtice polar stratosférico. Estes distúrbios possuem
variação interanual e são mecanismos dinâmicos fundamentais para as características espaciais e
reconstrução do ozônio Antártico total durante a primavera. Além disso, sugere-se que a mudança
observada na tendência do ozônio Antártico após a década de 90 pode estar ligada à variações
naturais na troposfera tropical com uma associação mais forte com os modos de variabilidade da
baixa estratosfera quando comparado à década anterior.
1
Dept. Ciências Atmosféricas, IAG, USP
R. do Matão 1226, CEP 05508-900, SP
Fone: (11) 3091-4713 FAX: (11) 3091-4714
e-mail: leila@model.iag.usp.br
2
Institute for Computational Earth System Sciences
University of California Santa Barbara, USA
1
Key words: Antarctic Ozone, ozone hole, Antarctic Oscillation, ENSO, Intraseasonal variations
1- Introduction
The Antarctic ozone hole (AOH) was first discovered by the British Antarctic Survey with a
Dobson spectrophotometer at Haley Bay station in the 1981-1983 period. Satellites detected later
large springtime (Sept-Oct) continent-wide losses between 1975 – 1984 (300-200 DU from the
1960’s). A few theories appeared then to explain the ozone losses. They were basically divided in
two groups: the dynamical and heterogeneous chemistry theory (e.g., Thrush 1988). The dynamical
theory, which is less accepted, explains the AOH by considering that the atmosphere circulation
over the Antartica changed such that air from the troposphere (with less ozone) was carried into the
polar lower stratosphere. The heterogeneous Chemistry theory (Molina 1991,1996; Midya et al.
1994; Solomon 1990) considers that the AOH is the result of reactions occurring at the surface of
tiny cloud particles formed in extremely cold conditions (polar stratospheric clouds – PSC) (Leu et
al. 1988). Compounds formed by the reaction on these PSCs allow non-reactive compounds
containing chlorine to become reactive compounds. These reactive chlorine compounds
catalytically destroy ozone at an extremely rapid rate. The chlorine compounds are mostly from
CFCs human production (Baumgartner et al. 1994; Lal et al. 1991). This theory has been more
accepted so far.
Nevertheless, it is recognized that the stratospheric circulation plays a key role in modulating
polar temperature and the ozone layer (Salby and Callaghan, 2004 and references therein).
Moreover, variations in the stratospheric circulation are modulated by the troposphere through
planetary waves and by the quasi-biennial oscillation (QBO) (e.g, Gray and Pyle 1989; Fusco and
Salby 1999; Tung and Yang, 1994 Hu and Tung 2002; Salby and Callaghan, 2004)
The Antarctic Oscillation (AAO) is known as the leading mode of variability of extratropical
circulation in the Southern Hemisphere. The AAO, also called Southern Hemisphere highlatitude
or annular mode (Thompson and Wallace 2000), is one of the most important modes of variability
in the mid and high latitudes of the Southern Hemisphere. The AAO was originally identified by
Walker (1928) as changes in the surface pressure belt across Chile and Argentina with opposite
signals over the Weddell and Bellingshausen seas. Many subsequent studies have determined that
the AAO is a zonal pressure fluctuation between mid and high latitudes of the Southern Hemisphere
also observed in geopotential heights (Kidson 1988; Yoden et al. 1990; Shiotani 1990; Hartmann
and Lo 1998; Gong and Wang 1999; Thompson and Wallace 2000; Carvalho et al. 2004). The
annular and zonally symmetric structure involves exchanges of mass between mid and high
2
latitudes. Carvalho et al. (2004) indicated a link between AAO phases and convection in the tropics
during austral summer (December-February – DJF). Thompson and Solomon (2002) have
suggested a trend toward the high index polarity (positive phase) of the AAO linked to the negative
trend of the total Southern Hemisphere ozone (or increase in the ozone hole).
During the polar night, the stratosphere cools off by emitting IR radiation to space. Weather
systems in the stratosphere warm the polar region and during the winter they are very weak. The
Polar Jet (or vortex) results from the very cold polar region, weak weather systems in the
stratosphere and infrared cooling. Moreover, reactions required for ozone loss also involve sunlight.
Cold temperatures with PSCs and sunlight are conditions observed in September and October, when
a minimum in ozone and, therefore, a maximum in the ozone hole is observed (e.g., Leu et al.
1988). Salby and Callaghan (2004) show that the summertime structure over the Southern
Hemisphere stratosphere changes coherently with anomalous forcing of the residual circulation.
Therefore, it is reasonable to think that troposphere - lower stratosphere coupling mechanisms may
interact with the dynamics of the polar vortex and disturb its zonal structure and intensity and, thus,
influence the stratospheric ozone annual cycle and the ozone trend. Those mechanisms may vary on
many timescales and, therefore, need to be recognized in order to provide realistic predictions of the
Antarctic ozone trend and climate impacts. In the present work we investigate whether tropical and
subtropical modes of variability in the circulation play a role in modulating the lower stratospheric
Antarctic polar jet and total ozone during the spring (September to October).
2. Data:
In this study we used 25 years of reanalysis (1979-2003) of the following variables (time
resolution in pentads): zonal wind U 50hPa (U50); 700hPa geopotential height (H700), temperature
50hPa (T50), Outgoing Longwave Radiation (OLR) from 90S to 20S (2.5x2.5 resolution). A lowpass FFT filter was applied to the reanalysis fields (periods retained > 380 days). Twenty three
years of Total Ozone Mapping Spectrometer (TOMS) data from Nimbus-7 and Earth Probe
satellites (1979-1993 from Nimbus-7 and 1996-2003 from Earth Probe) from September to October
(in pentads) is used in this study to evaluate the total ozone 60S-poleward.
3. Methods and Results.
3.1 Tropics and Extratropics interaction pattern
In this section we discuss the relationships between tropics and extratropics by considering the
variability of the Southern Hemisphere annular mode (Thompson and Wallace 2000), the
correspondence with the lower stratosphere 20oS – poleward and convective activity equatoward of
3
60o latitude in both hemispheres. For this purpose, combined EOF analysis was applied to the
following low-pass filtered variables (September – October): U50, T50, H700 and OLR. The first
combined EOF (EOF-1) explains ~ 29% of the total variance and is shown in Fig 1. It indicates that
positive EOF-1 is related to negative phases of the AAO (H700 – first panel), warm temperatures in
the stratosphere (T50 – second panel), and a weakening of the polar vortex (U50 – third panel).
These features appear associated with an enhancement of convective activity in the central Pacific
toward the subtropics, in the tropical Atlantic and Indian oceans, and weakening over the Maritime
Continent and eastern subtropical Australia toward the date line. Conversely, negative EOF-1 is
related to positive AAO phases, the enhancement of the polar jet and cooling of the polar
stratosphere, which is consistent with Thompson and Solomon (2002). The association between
convective activity in the tropics and subtropics and the most relevant extratropical mode of
variability, the AAO, has been shown in Carvalho et al. (2004). The combination of the annular
pattern observed in Fig. 1 (H700 – Fig. 1 first panel) along with the lower stratosphere features
(T50 and U50, Fig. 1 second and third panels, respectively) and convective activity in the tropics
and subtropics (OLR – Fig. 1 fourth panel) identified from EOF-1 will hereafter be referred to as
Tropics-Extratropics Interaction Pattern (TEIP). The TEIP therefore relates the Southern
Hemisphere annular mode (Thompson and Wallace, 2000) to a tropical zonal wavenumber 3
structure (Fig. 1).
Fig. 1. EOF-1 correlation with variables (clockwise): H700, T50, U50, OLR. Only correlations
above the 5% significance level are shown. The number of degrees of freedom (Dof) is considered
equal to the number of seasons (Dof=25)
4
The TEIP time-coefficients distribution shows bi-modal characteristics (Fig. 2 left). Figure 2
(right) indicates the annual distribution of positive and negative TEIP phases. One important
characteristic of Fig. 2 (right) is the interannual persistence of the modes. For instance, the early
eighties (1980-1982) were characterized by positive TEIP phase, whereas from 1983 to 1998
(except 1988) negative TEIP phase was dominant. From 1991 to 1997 positive phases dominate,
except in 1993 when negative TEIP features prevailed. From 1998 to 2003 the same number of
years in positive and negative TEIP phases was observed.
To further understand relationships between TEIP and other known tropical and extratropical
indices, we correlated the EOF-1 time coefficients monthly averages (September and October) with
monthly mean AAO, sea surface temperature (SST Niño 3.4), Quasi-Biennual Oscillation (QBO),
Pacific North-America Pattern (PNA), North Atlantic Oscillation (NAO) and Pacific Decadal
Oscillation (PDO). Results are shown in Table. 1. Monthly TEIP has strong negative correlation
with monthly AAO, as expected based on the H700 pattern associated with the EOF-1 (Fig. 1 first
panel). Nevertheless, the low positive correlation between TEIP and Niño 3.4 indicates that the
OLR pattern showed in Fig. 1 cannot be totally explained by ENSO phases. Another interesting
result is the negative (and statistically significant) correlation between TEIP and NAO.
Table-1 Correlation between monthly TEIP and AAO, SST (Niño 3.4), QBO, NAO and PDO
(September-October).
Indexes
AAO
Niño 3.4
QBO
PNA
NAO
PDO
Correlation
-0.74
0.29
0.076
0.12
-0.35
0.17
3.2 TEIP and the springtime Antarctic ozone.
In the period investigated in this study, the springtime Antarctic ozone shows a considerable
interannual variation (Fig. 2). Nonetheless, there is a clear negative trend from the early eighties to
early nineties. From mid nineties to early 21th century an apparent decrease in the negative trend is
observed, albeit with large oscillations. However, some remarkable peaks in the mean springtime
ozone concentration occurred in the whole period, the most relevant being the 1988 and 2002
seasons. It is interesting to note that the existence of significant ozone depletion in the mid eights
comparatively to the decades before (e.g. Thrush, 1988) was concomitant with the longest period
(from 1979-2003) in negative TEIP phase (1983-1990, exception for 1988 - Fig. 2). This sequence
of years in negative TEIP occurred after three consecutive years (1980-1982) in positive TEIP
5
phases (Fig. 2). This observation is consistent with the pronounced trend observed in the mean
seasonal stratospheric ozone from 1979 to 1990 (Fig.3).
Number of Pentads/Year (Sep-Oct)
EOF-1 Time Coefficients Distribution
14
45
40
35
30
25
20
15
10
5
0
Frequency (Pentads)
12
10
8
6
4
2
Positive-EOF1
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1988
1987
1986
1985
1984
1983
1982
1981
1980
1979
0.1
0.08
0.06
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
-0.1
0
Negative- EOF1
Fig. 2. EOF-1 time coefficients distribution (left). Annual variation of positive and negative TEIP
phases (right).
60.5S TO 89.5 S : September to October average
350
Dobson Unity (Du)
330
2
79
Mean Ozone (Du)= 0.3142t - 11.465t + 333.78
2
310
R = 0.6449
80
81
290
02
88
82
83
84
86
270
250
91
85
89
230
92
90
87
210
96
00
97
93
99
98
03
01
190
170
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1988
1987
1986
1985
1984
1983
1982
1981
1980
1979
150
Fig 3. Annual September-October average ozone obtained from TOMS instruments poleward of
60.5oS. The non-linear trend line and respective determination coefficient are shown in the figure.
To investigate relationships between extreme TEIP phases and the Antarctic ozone, the 75th and
25th percentiles of the EOF-1 time-coefficient distribution (Fig. 2 left) were considered. With this
definition, pentads with EOF-1 below (above) the 25th (75th) percentiles during October were
separated and the years when they appeared were identified. Composites of the total ozone obtained
from satellite in positive and negative TEIP phases are shown in Fig. 4. Years observed in each
sample are indicated in the figure. They clearly show that extreme positive TEIP phases (Fig. 4,
first panel) were related to larger total ozone than negative TEIP phases (Fig. 4, second panel). The
percent differences (Fig. 4, third panel) indicate an excess of more than 30% in the total ozone over
the Antartica for pentads in positive TEIP phases compared to negative phases. The reasons for the
6
difference are straightforward. Cold and undisturbed stratosphere with a strong polar vortex during
the spring, when the sun light returns to the Southern Hemisphere high latitudes, is related to the
ozone loss and the augmentation of the ozone hole. However, an enhancement of the total ozone is
observed along with opposite situations, that is, the weakening of the polar jet and warming of the
lower stratosphere. These features are associated with positive TEIP and, therefore, with the pattern
of convection in the tropics and subtropics as indicated in Fig.1 (fourth panel).
Fig. 4. Ozone composites for positive TEIP (first panel), negative TEIP (second panel) and ratio
positive/negative TEIP (third panel)
3.3. Interannual variations of tropical and subtropical intraseasonal activity.
Since ENSO alone does not explain a large fraction of the natural variability of the tropical and
subtropical convective pattern in TEIP (Fig. 1 fourth panel), a question arises: what mechanisms in
the tropics and subtropics and in what time-scales could affect circulation and convective activity
such that TEIP represents the interannual variations shown in Fig. 2(right)? It has been reported in
many studies the importance of tropical-extratropical interactions associated with intraseasonal
anomalies (Ferranti et al. 1990; Hendon and Salby 1994; Jones 2000, Carvalho et al. 2004).
Carvalho et al. (2004) showed that negative (positive) AAO phases are related to an enhancement
(weakening) of intraseasonal activity in the Southern Hemisphere. Moreover, in the same paper, the
authors show evidence of interannual variations in the intraseasonal activity and suggest that they
may be independent of ENSO events, which is consistent with Jones et al. (2004). These previous
results indicate that some important aspects need to be investigated in details: can interannual
variations of intraseasonal disturbances in the tropics and subtropics be linked to TEIP? If so, can
the lower stratosphere be subjected to variations on intraseasonal time-scales during the springtime
that can affect the strength and zonal structure of the polar jet?
To examine variations in circulation of the upper troposphere and lower stratosphere on
intraseasonal time-scales we computed an intraseasonal index (hereafter referred to as ISI) (Slingo
et al 1999; Jones 2000). The upper tropospheric ISI (T-ISI) was obtained by filtering 200hPa zonal
wind (U200) in frequency domain with Fast Fourier Transform (FFT) and cut-off periods between
7
10 and 90 days. The filtered U200 were then squared and smoothed with a 101 days moving
average window (Jones 2000). Twenty five years of October-September T-ISI climatology (19792003) is shown in Fig. 5 (top). The subtropics of the Southern Hemisphere (~ 30 o S) and midlatitudes of the Northern Hemisphere (~ 45o N) show the largest T-ISI values. Nonetheless, there is
a local maximum T-ISI in the tropical Pacific, located approximately at 135oW - 90oW and 0o 10oS. The stratospheric ISI (S-ISI) was analogously computed by considering U50. The OctoberSeptember S-ISI climatology shows a maximum coinciding with the climatological location of the
Southern Hemisphere Polar Jet (Fig. 5 bottom).
.
Fig. 5. September-October tropospheric ISI (top) and stratospheric ISI climatology (bottom).
The stratospheric and tropospheric ISI show both seasonal and interannual variability in all
latitudes. The existence of seasonal variations in tropical intraseasonal anomalies is well
documented. One example is the increase in frequency and intensity of tropical intraseasonal
convective anomalies during the austral summer (e. g. Jones et al. 2004). In midlatitudes, on the
other hand, the maximum in the tropospheric intraseasonal activity is observed during the
wintertime in both hemispheres (not shown). The seasonality of the S-ISI is very intense, with the
wintertime being the months with most intense activity.
8
The lower stratospheric and upper tropospheric ISI also presents interannual variations in all
latitudes. This is consistent with Jones et al. (2004) results indicating large interannual variability in
frequency and in properties of tropical intraseasonal anomalies in 25 years of observations. To
eliminate high frequency variations and to understand possible links between tropospheric and
stratospheric ISI and TEIP, a low-pass FFT filter was applied to T-ISI and S-ISI time series and
periods above 380 days were retained. The low-frequency (tropospheric and stratospheric) ISI
anomalies will be hereafter referred to as ISILF.
a) TEIP and upper troposphere ISILF (T-ISI LF)
The hypothesis we will examine is whether TEIP phases during the austral spring are related to
an enhancement/weakening of intraseasonal activity on interannual time-scales from the tropics to
the extratropics and how this pattern of variability depends on latitude and hemisphere. To examine
these issues, T-ISILF composites were performed in distinct TEIP phases considered according to
the quartiles described in section 3.1. The statistical significance was then assessed by considering
as degree of freedom the number of seasons in each TEIP phase (that is, 6 independent events in
each phase). To verify the latitudinal variation of intraseasonal activity, zonal averages of T-ISILF
were then performed by considering only grid points that passed the t-test. The resulting zonal
averages were then scaled by the square root of the cosine of the latitude. Evidently, negative and
positive T-ISILF zonal averages are possible as well as zonal means approximately equal to zero
when anomalies cancel each other in a given circle of latitude. Figure 6 shows the zonal average
(top) and difference (bottom) observed for distinct TEIP phases. It clearly shows that, despites
latitudinal fluctuations, positive (negative) TEIP phases prevail during periods of increased
(decreased) intraseasonal activity in the troposphere of the SH from the tropics to the extratropics.
Moreover, Fig. 6 indicates that negative (positive) TEIP phases are favored (disfavored) by
increased (decreased) intraseasonal activity in the troposphere of the NH, particularly near 45oN.
Carvalho et al. (2004) showed consistent relationships with respect to the AAO.
b) TEIP and lower stratosphere ISILF (S-ISILF)
Similar procedure as described above was used to verify the relationships between TEIP and SISILF. Composites for positive and negative TEIP are shown in Fig. 7. Positive TEIP phases, which
imply negative AAO phases, are observed in association with an enhancement of the lower
stratosphere intraseasonal activity ~ 70oS (Fig. 7 left). On the other hand, negative TEIP phases
(related to positive AAO) show a pronounced decrease in S-ISILF near 70oS and 50oS (Fig. 7 left).
As a result, the pattern of differences positive – negative TEIP shows two maxima, one near 70oS
and an other approximately at 50oS.
9
Fig. 6 Composites of tropospheric ISILF latitudinal variation for positive and negative TEIP (left)
and difference between the two phases (right)
Fig. 7. The same as Fig. 6 but for the lower stratosphere
c) T-ISILF and S-ISILF relationships
The results above suggest that an enhancement of intraseasonal activity in the troposphere, from
the tropics to the extratropics of the Southern Hemisphere, is likely related to an enhancement of
intraseasonal activity in high latitudes of the lower stratosphere. The stratospheric activity is more
intense where the polar jet has its climatological position (between 50 and 70S). To verify the
hypothesis of link between T-ISILF and S-ISILF during the austral spring, the total annual T-ISILF (
S-ISILF) was computed as the summation of contributions from 40oS – 2.5oS (70oS – 50oS) each
season. The range in latitudes was based on the relationships with TEIP phases (Figs 5-7). The
annual variation of T-ISILF along with S-ISILF (Fig. 8) shows for the period of 25 years a linear
correlation ~0.62. Interesting enough, there is a considerable interdecadal change in the correlation
coefficient: ~0.20 in the 1979-1990 period to ~0.80 in the 1991-2003 period. Moreover, as indicated
in the figure, there is a clearer relationship between high (low) ISILF and warm (cold) ENSO
episodes in the 1991-2003 years. It is worth noticing that an apparent change in the rate of ozone
loss (less abrupt) is observed from the nineties on, which coincides with the period when a closer
correspondence between T-ISILF, S-ISILF and ENSO is observed.
10
40000
8000
6000
EN
EN
20000
10000
0
2000
LN
0
-20000
Correlation: 0.62
LN LN
-30000
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1988
1987
1986
1985
1984
1983
1982
1981
1
1980
-10000
LN
EN
LN
4000
EN
EN
EN
EN
-2000
-4000
Total T-ISI anomalies (m2/s2)
Correlation: 0.80
30000
1979
Total S- ISI anomalies (m2/s2)
Correlation: 0.21
-6000
U50: 70S- 50S
U200:40S-2.5S
Fig. 8. Annual variation ISILF during October for the lower stratosphere (full line) and upper
troposphere (dashed line). The lower stratosphere ISILF was obtained from 70oS – 50oS and the
upper troposphere ISILF from 40oS to 2.5oS. Moderate to strong El Niño (EN) and La Niña (LN)
episodes are indicated in each year
3.4 Intraseasonal circulation in the stratosphere and the ozone hole.
To show the importance of TEIP and S-ISILF for the variability of the total ozone, we
reconstructed the evolution of the October ozone cycle along with the intraseasonal circulation in
the stratosphere (20-90 days) observed in two distinct years representative of opposite TEIP phases:
1993 (negative TEIP) and 2002 (Positive TEIP) (Figs. 9 and 10).
Fig. 3 shows that in 1993 the
seasonal ozone average was among the lowest during the period with available satellite data,
whereas 2002 showed ozone averages comparable to the early eighties. Panels at the top of Figs. 9
and 10 show U50 intraseasonal anomalies in distinct pentads of October 2002 as indicated in the
figure. Panels at the bottom of Figs. 9 and 10 show the total ozone observed in the respective
pentads indicated at the top panels.
The 2002 ozone cycle
On the fist pentad of October 2002 (October 3-7) easterly intraseasonal U50 anomaly are
observed around Antarctica (Fig. 9 first panel, first row). Two pentads later (October 13-17) the
easterly anomalies were replaced by strong westerly anomalies extending toward the Southern Pole
(Fig. 9 second panel, top row) with an annular pattern around Antarctica. This sharp change in
direction of the wind anomalies (20-90 days) indicates an intense intraseasonal activity in the
period. Two pentads later (October 23-27) the westerly anomaly annular pattern begins to break
near the Antarctic Peninsula. One pentad later (October 28 – November 01) westerly anomalies are
11
Composites of U 50hPa anomalies (20-90 days)
10/03/2002
10/13/2002
10/23/2002
10/28/2002
Composites of Total Ozone (TOMS)
Fig. 9. U50 intraseasonal anomalies (top panels) and total ozone (bottom panels) during an active
stratosphere in October 2002
Composites of U 50hPa anomalies (20-90 days)
10/03/1993
10/13/1993
10/23/1993
10/28/1993
Composites of Total Ozone (TOMS)
Fig. 10. The same as Fig. 9 but for a quiescent stratosphere during October 1993.
restricted to high latitudes around South Australia. The average total ozone is consistent with the
variability of the intraseasonal winds and, therefore, the variation in the intensity of the polar vortex
on intraseasonal time-scales. The minimum total ozone and the largest ozone hole are observed
along with the intensification of the intraseasonal westerlies by October 13-17 and formation of the
12
annular structure (compare Fig. 9 second panel top and bottom). The ozone increases and the ozone
pattern modifies as the westerlies anomalies weaken and the vortex loses its annular aspect (third
and fourth panels at the bottom of Fig. 8). It is likely the break in the polar vortex by intraseasonal
anomalies in the stratosphere and the mixing with ozone from mid-latitudes (with larger
concentration) were what disturbed the ozone hole and increased the ozone amount by the end of
the October season.
The 1993 ozone cycle
The 1993 October ozone cycle was characterized by a quiescent stratosphere (Fig. 8), with
very weak intraseasonal activity (Fig. 10 top panels). The undisturbed polar vortex, particularly in
the beginning of October, inhibited polar ozone from mixing with midlatitudes higher concentration
ozone. The result was one of the lowest ozone concentrations from 1979 to 2002 by the end of
October (Fig. 10 bottom panels). A comparison between Fig. 9 (bottom panels) and Fig. 10 (bottom
panels) clearly shows that the ozone hole was deeper in all 1993 October pentads. This evidences
the importance of intraseasonal activity in the stratosphere for the reconstruction of the total ozone
and the possible role of the tropical troposphere for this mechanism.
CONCLUSIONS
The low-frequency coupling tropics-extratropics mode of variability (TEIP) was found in this
study with EOF-analysis using low frequency anomalies. It explains about 29% of the total variance
of the data and clearly relates the most important mode of extratropical-high latitudes variability,
the Antarctic Oscillation, with the stratospheric polar jet, the Antarctic lower-stratosphere
temperature and variations in convection in the tropics. Disturbances in the polar jet, therefore, are
related to phases of the AAO and variations in circulation and convection in the tropics on
intraseasonal to interannual time-scales. During the austral spring, the interannual variations in
convective activity and circulation in the tropics and subtropics of the Southern Hemisphere seem
to be related to variations in high latitudes of the lower stratosphere, where the polar jet has its
climatological position. The annual variability of intraseasonal disturbances in circulation between
troposphere and lower stratosphere are in phase particularly from 1991 to the present. Moreover, a
clear relationship between these activities and ENSO is observed in this period.
The total Antarctic ozone changes with time and shows a negative trend in the last decades.
Nonetheless, ozone presents large temporal variability which influences the magnitude and
behavior of the negative trend. We show evidence that variations in the dynamics of the polar jet
are crucial for the annual cycle of ozone. Therefore, low-frequency natural variability in the tropics
13
and its coupling with the extratropics should be considered in addition to the heterogeneous
chemistry theory to assess actual trends of the Antarctic ozone. Moreover, planning future human
activities depends on the possible realistic scenarios of global change. In this regard, this study
presents a further step for the understanding of the natural variability of climate.
Acknowledgements
Leila M. V. Carvalho acknowledges the financial support of CNPq (proc: 302203/02-8)
CNPq/PROANTAR (550363/02-5)
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