Annual mean
In the atmosphere, GHG and ODS abundances have increased simultaneously and
nonlinear interactions can occur. The difference between the sum of the
single forcings (grey) and the change of simultaneously increased GHG and ODS
mixing ratios (black) is shown by the red line in Fig. .
Throughout the stratosphere the nonlinear
contribution to the annual mean global mean ozone change is positive
(Fig. a). The largest nonlinear effect is found in
the upper stratosphere, where it is as large as 1.2 %. Here, the ozone
change due to nonlinearity is about half as large as the ozone change induced
by GHG changes. Statistically significant nonlinear contributions are found
above 100 hPa.
Latitude-height section of the nonlinear contribution to the annual
mean ozone change (a) between 1960 and 2000 in percent and the separation
into the contributions from ozone loss (b), ozone
production (c) and ozone transport (d). Red/blue shading
indicates positive/negative changes. The contour lines indicate the regions
where the changes are larger than ±2σ and ±3σ. The bold
dashed line shows the mean tropopause location of the R1960 simulation for
the annual mean. Note that the contributions from chemistry (b+c) and
transport (d) do not exactly add up to the total (a)
because of the residual term.
Top panels: vertical profile of the relative ozone change due to
chemical ozone destruction (black) and its attribution to GHGs (blue), ODSs
(green) and nonlinear interactions (red) for the annual mean at
30∘ N (a) and at 60∘ S (b). The results
based on the calculation with the tool StratO3Bud are shown as solid lines.
For comparison, the result of the total change calculated accordingly to
Fig. b is shown as black line with circles. Bottom:
vertical profile of the nonlinear contribution to the loss-induced ozone
change (red; see top panel) and the separation into the contributions from
the different ozone loss cycles, i.e. the Ox (purple), HOx (blue),
NOx (green), ClOx (orange) and BrOx (magenta) loss cycles for
the annual mean at 30∘ N (c) and at
60∘ S (d). The bars denote the 95 % confidence level of
the changes. The contributions from the single loss cycles add up to the
total loss change. Note the different scales of the
subfigures.
The vertically integrated nonlinear contribution for the different latitudes
is shown in red in Fig. b. Significant positive
changes are found in the extra-polar regions. At SH midlatitudes the
nonlinear term causes up to 1.1 % increase. Nonlinearity has a slightly
negative (not significant) contribution in the SH polar region in the annual
mean but a slightly positive contribution (not significant) in the NH polar
region. All in all, due to nonlinear interactions between changing GHG and
ODS concentrations, the resulting ozone loss in the recent past is slightly
smaller than expected from the single forcings.
To analyse the processes that underlie the nonlinear ozone changes, the
regions with significant nonlinear changes have to be identified. In
Fig. a, showing the vertically and latitudinally resolved
annual mean nonlinear ozone change, two stratospheric regions are found: the
extra-polar upper stratosphere and the SH midlatitude lower stratosphere.
Both regions exhibit positive nonlinear contributions to the overall ozone
change of 1–2 %. These regions of statistically significant nonlinear
changes are in relatively good agreement with the regions identified by for the future.
In the following we investigate which processes exhibit nonlinear
interactions in the different regions. For this purpose the annual mean
nonlinear ozone change is separated into the contributions from chemical
ozone loss, chemical ozone production and ozone transport, shown in
Fig. b–d respectively. For the interpretation, it should
be noted that not the changes in the processes are shown, but the changes in
ozone that are attributed to the changed processes. Since the ozone tendency
is inversely proportional to the (positive definite) chemical ozone loss, a
positive ozone change attributed to chemical loss implies slowed ozone loss.
It is found that in the upper stratosphere, the nonlinear ozone changes are
caused by nonlinearities in the ozone chemistry, with a positive effect from
ozone loss and a smaller negative from ozone production (Fig. b
and c). In the tropical lower stratosphere and at
NH midlatitudes the significant nonlinear effects from ozone loss and
production nearly compensate each other, leading to insignificant changes in
ozone. The positive nonlinear signal in the lower stratosphere at SH
midlatitudes results from the contribution from both ozone chemistry and
ozone transport. Nonlinear processes affecting the ozone transport cause an
ozone increase in the tropical and SH midlatitudinal lower stratosphere and a
decrease in the SH polar region (Fig. d). This
indicates a reduced ozone transport into the SH polar stratosphere. However,
to identify the involved processes it is necessary to analyse the seasonal
changes in detail, since the BDC exhibits strong seasonal variability (see Sect. ).
The next step is to understand how the nonlinear interactions are caused and
which processes are responsible. First we analyse the reasons for the
nonlinearity of the chemical ozone loss by separating the contributions from
the different ozone loss cycles, applying the tool StratO3bud. For
illustration, we show the attribution of the ozone changes due to ozone loss
at 30∘ N and at 60∘S (Fig. ). Note that the use
of StratO3bud can lead to quantitatively different results compared
to Fig. b, which is indicated by the additional
contour line (black with circles) in the top panel. In the upper stratosphere
at NH midlatitudes (Fig. a), the nonlinear processes are
acting in the same direction as the increasing GHG concentrations and are
reducing the efficiency of the ozone loss, whereas the increase of the
halogen loading causes an ozone decrease due to enhanced ozone loss. In the
lower stratosphere both the GHG and ODS increase enhance the ozone loss. The
nonlinear contribution, however, remains positive. At 60∘ S
(Fig. b) the sign of the ozone changes attributed to increasing
GHG and ODS concentrations is the same as at NH midlatitudes, but the ozone
loss due to ODSs is clearly larger in the lower stratosphere, which is linked
to the evolution of the ozone hole. The nonlinear contribution to the ozone
change is very small and not significant between 50 and 10 hPa and even
slightly negative at 5 hPa, but in the lower stratosphere 8 % of the overall
annual mean ozone change is explained by nonlinear interactions.
By analysing the nonlinear contributions from different loss cycles
(Fig. c and d), we find that at NH midlatitudes the nonlinear ozone
increase is determined by a reduced ozone loss in the catalytic chlorine loss
cycle (orange) above 70 hPa. In the upper stratosphere this increase is
slightly counteracted by an enhanced ozone loss in the Chapman cycle
(purple). In the middle stratosphere nonlinear interactions modify the
NOx-catalysed O3 loss, while in the lower stratosphere the HOx- and
BrOx-catalysed O3 loss are affected. In contrast to the nonlinear
effect on the ClOx and Chapman cycles in the upper stratosphere, which
varies only quantitatively but not qualitatively with latitude, the sign of
the nonlinear ozone change due to the NOx cycle depends on the
geographical region. In the NH the nonlinear ozone change related to the
NOx cycle is relatively small and not statistically significant. In the
SH, however, ozone is significantly decreased by up to 2 % in the upper
stratosphere at midlatitudes (Fig. d) and increased in the
middle stratosphere in the polar region due to a nonlinearly modified
NOx-catalysed ozone loss (not shown). This causes the hemispheric
asymmetries in the nonlinear ozone change attributed to chemical loss in
Fig. b. In the lower stratosphere the nonlinear ozone
change due to HOx is positive at all latitudes, but statistically
significant increases occur only at high latitudes. In the annual mean the
total nonlinear decrease of the chemical O3 loss in the Antarctic lower
stratosphere is caused by a reduced HOx-, ClOx- and BrOx-catalysed
O3 loss (Fig. d).
Latitude-height section of the nonlinear change of the annual mean
ClOx mixing ratio (a) and the September to November mean
NOx mixing ratio (b) between 1960 and 2000. The contour lines
indicate the regions where the changes are larger than ±2σ and
±3σ. The bold dashed line shows the mean tropopause location of
the R1960 simulation for the annual mean and the SON mean
respectively.
Which nonlinear processes are affecting the ozone loss cycles? Since the loss
rate of a specific reaction is determined by the (temperature dependant) rate
coefficient and the concentration of the involved species, nonlinear effects
can occur either because of nonlinear temperature changes or/and nonlinear
changes of the radical and ozone abundances. We find that the nonlinearity in
the ClOx-induced ozone loss is primarily caused by a reduced concentration
of ClOx radicals if ODSs and GHGs are changed simultaneously, as compared
to the sum of the single forcings (Fig. a). In the
upper stratosphere the ClOx increase between 1960 and 2000 is about
300 %, while the changes due to ODSs (≈+350 %) and GHGs
(≈ -10 %) add up to ≈ +340 % (not shown). This is explained
by a nonlinear effect on the partitioning of inorganic chlorine, consistent
with the study by . From 1960 to 2000 the ratio between
reactive (ClOx) and inorganic chlorine is reduced more than expected from
the single forcings. This is caused by the interaction between the chlorine
species and the GHGs CH4 and N2O. While CO2 is chemically quasi-inert in the atmosphere and primarily influences the radiative budget of the
system, CH4 and NO2 (a product species from N2O) can react with
chlorine compounds and form HCl and ClONO2 respectively, which are the
most abundant chlorine reservoir species in the stratosphere. Thus, the
formation of chlorine reservoir species is enhanced if the GHG concentrations
are increased simultaneously with the chlorine loading. This is also valid
for the BrOx-catalysed O3 loss in the lower stratosphere through the
formation of BrONO2. In addition, nonlinear processes lead to a reduced
abundance not only of chlorine radicals but also of the total amount of
inorganic chlorine in the stratosphere (not shown). This is related to a
reduced conversion of the chlorine source gases to inorganic compounds in the
tropical stratosphere. Here, the reduced short-wave radiation reaching the
lower stratosphere due to the O3 increase above lowers the photolysis
rate of organic chlorine. Furthermore, circulation changes can play a role
for the chlorine release as discussed in .
The positive nonlinear effect on ozone shown here is contrary to the findings
in , who found a larger ozone decrease for the combined
change of ODSs and CO2. The main difference to the study by
is that not only CO2 concentrations
but also the CH4 and N2O abundances are increased. This means that the nonlinear
effect due to a reduced temperature sensitivity of ozone is smaller than the
nonlinearity that originates from changing atmospheric abundances of CH4
and N2O and their interactions with chlorine species.
The rate limiting reaction of the Chapman loss cycle (O3 + O) exhibits a
strong temperature dependency resulting in reduced ozone loss if temperatures
decrease and enhanced loss if temperatures increase. The annual mean
nonlinear temperature change between 1960 and 2000 (Fig. )
is positive and statistically significant in the
tropical upper stratosphere and lower stratosphere at SH midlatitudes. Thus,
the stratospheric cooling in the tropical upper stratosphere is weaker by up
to 0.4 K if ODSs and GHGs are changed simultaneously, with the consequence
that the ozone loss via the Chapman cycle is slightly increased. The
temperature change pattern is linked to the nonlinear ozone increase due to
the ClOx cycle and the concomitant increase in ozone heating rates, but it
is modulated by dynamical processes, especially in the polar regions. The
warming in the SH polar upper stratosphere is related to a dynamically
induced adiabatic descent that is probably caused by the cooling in the lower
stratosphere. The cooling can partly be explained by reduced downwelling (see
Sect. and Fig. d).
The hemispheric asymmetry in the nonlinear ozone change in the lower and
middle stratosphere is attributed to a larger nonlinear effect on the NOx
loss cycle in the SH that leads to a compensation of the ClOx-induced
ozone increase at SH midlatitudes and to a larger nonlinear ozone increase in
the polar region. This is mainly caused by processes in the SH spring season
and will be discussed in Sect. .
Same as Fig. a but for the nonlinear annual
mean temperature change (K) between 1960 and 2000. The contour lines indicate
the regions where the changes are larger than ±2σ and
±3σ.
Same as Fig. but for the SON (September–October–November) mean. See text for details.
Latitude-height section of the SON mean nonlinear O3 changes
due to the ClOx (a) and the NOx cycle (b) derived
from StratO3Bud.
The significant nonlinear annual mean ozone increase due to chemical loss in
the lowermost stratosphere at SH high latitudes (Fig. b)
is mainly caused by a reduced efficiency of
HOx-catalysed O3 loss (see Fig. d for 60∘ S).
At this altitude, the HOx cycle is primarily determined by the reaction of
OH with O3. Although the absolute abundance of HOx is increased due
to nonlinear processes, the partitioning between OH and HO2 is shifted in
favour of HO2 in this region (not shown). Thus, the loss efficiency is reduced.
In addition to chemical ozone loss, chemical ozone production contributes to
the nonlinear ozone signal. Figure c shows that ozone
production is reduced if interactions between increasing GHGs and ODSs occur.
It is mainly caused by a decrease of the photolysis rate due to the ozone
increase in the levels above (i.e. a reversed self-healing effect). The
nonlinear ozone increase attributed to production changes in the NH upper
troposphere, however, is found to be due to increased production via the
reaction path HO2 + NO (not shown).
The processes that are responsible for the nonlinear change in the ozone
transport are analysed in more detail from the seasonal point of view in the
next section. To investigate the seasonality of the nonlinear ozone changes,
the attribution method is applied to seasonal means as discussed in
Sect. . The largest nonlinear contributions are found in the
September to November (SON) season. Therefore we focus on the SON mean in the
following analyses.
Southern Hemisphere spring (SON)
Figure shows the nonlinear ozone change between
1960 and 2000 for the SH spring season (SON) and the attributions to chemical
ozone loss, production and transport analogous to Fig. .
Figure a shows that the
nonlinear ozone increase in the extra-polar upper stratosphere that was found
for the annual mean is a robust signal in austral spring (and in fact all
seasons; not shown). In the lower stratosphere, however, the nonlinear ozone
change in the SON mean exhibits a clear dipole pattern in the SH, with a
positive signal at midlatitudes and a negative signal in the polar region.
Furthermore, a statistically significant ozone increase due to nonlinear
interactions is found in the NH polar lower stratosphere.
The nonlinear ozone changes due to loss in the SON mean (Fig. b)
are qualitatively similar to the annual mean, but
in the SH polar region the changes are more pronounced. The nonlinear
contribution is positive in the upper and lower extra-polar stratosphere, as
in the annual mean, but an ozone decrease is attributed to nonlinear
processes at SH midlatitudes in the middle stratosphere and in the polar
region in the upper stratosphere and lower mesosphere. This decrease is
caused by significantly enhanced ozone loss through the NOx cycle – by
more than 2 % (Fig b) – which slightly exceeds the
ozone increase due to reduced ClOx-catalysed O3 loss
(Fig. a; see Sect. for more details to the
ClOx-catalysed O3 loss change). In the SH polar region, however, the
nonlinear NOx-catalysed O3 loss is decreased and thus ozone is
increased in the middle stratosphere between 50 and 5 hPa
(Fig. b). In the NH, no comparable nonlinear change pattern is
found in the spring season (March to May; not shown).
The nonlinearity in NOx-catalysed O3 loss originates from a nonlinear
change of the NOx mixing ratios in the atmosphere: it is positive at SH
midlatitudes and negative in the polar region (Fig. b). To understand this nonlinear behaviour, we first
explain the effect of the single forcings, since the NOx mixing ratios are
affected by both increasing GHGs and ODSs. In the stratosphere N2O is
destroyed either by photolysis or by the reaction with an excited oxygen atom
O1D. However, only the latter reaction path produces NOx. Increasing
halogen loading leads to a reduction of stratospheric NOx above the 50 hPa
level by diminishing the overhead ozone column and thus increasing the
photolysis rate of N2O, which mitigates the NOx production.
Furthermore, an enhanced formation of reservoir species (ClONO2,
BrONO2) may also contribute to the NOx reduction (not shown). In
contrast, increasing GHG concentrations cause a significantly larger
abundance of nitrogen radicals in the extra-polar stratosphere (not shown)
which is linked to increased N2O input into the stratosphere. In the upper
stratosphere and mesosphere, GHG-induced stratospheric cooling increases the
NOy loss reaction rate and therefore causes
a NOx decrease. The combined NOx change is dominated by the positive
GHG effect in the tropical middle stratosphere and by the negative ODS effect
in the polar regions and lower stratosphere. In the upper stratosphere and
lower mesosphere the total NOx change between 1960 and 2000 is negative.
This means that in the SH, the combined change of ODSs and GHGs leads to a
larger NOx decrease in the polar region than expected from the sum of the
single forcings (shown in Fig. b). At midlatitudes,
the NOx decrease is mitigated by nonlinear processes. Since this pattern
dominates also the annual mean change (not shown), seasonally asymmetric
processes must be involved. In the lower stratosphere the distribution of
NOx is determined by the release from reservoir species which are produced
from N2O and transported via the residual circulation. Thus, nonlinear
NOx changes in the lower stratosphere can be caused by changes in the
NOy production, in the circulation and/or in the NOx/NOy ratio. In
the upper stratosphere the dominant form of odd nitrogen is NOx. Due to
the chemical loss through the reaction NO + N in the upper stratosphere and
mesosphere, a maximum mixing ratio of NOx occurs at 3 hPa. Thus, air
masses that are transported downward from the mesosphere are characterized by
lower NOx values.
In the lower stratosphere we find qualitatively the same nonlinear change
pattern for NOy as for NOx, with only slightly masked absolute values
due to a modified partitioning of radicals and reservoir species. Since the
release from N2O shows no significant nonlinear change in the tropics (not
shown), a possible explanation for the nonlinear NOy change is an effect
of transport. In the upper stratosphere the larger ozone abundance due to
nonlinear processes can reduce the photolysis of NO which reduces the
efficiency of the NOx loss reaction .
Furthermore, the reduced cooling in the tropical upper stratosphere
(Fig. ) tends to decrease the loss. This leads to an
increase of NOx. However, the dipole pattern cannot be explained by these
processes. Therefore, transport changes must be involved. The circulation
changes due to nonlinear processes are discussed later in more detail.
The significant ozone decrease attributed to chemical loss in the SH polar
upper stratosphere in the SON mean (Fig. b) is
caused by increased O3 loss in the Chapman and the HOx cycle, which
together exceed the effect of the ClOx decrease (not shown). The enhanced
O3 loss in the Chapman cycle is explained by nonlinear warming (see
Fig. , because the SON nonlinear temperature change is
comparable to the annual mean), while the increased O3 loss due to
HOx is related to a nonlinear increase of the HOx mixing ratio in the
upper stratosphere (not shown).
While the ClOx-catalysed O3 loss is significantly reduced at all
latitudes and all seasons in the upper stratosphere due to nonlinear
processes, a significant nonlinear ozone decrease occurs in the SH polar
region between 20 and 5 hPa in the SON mean (Fig. a).
This is not explained by a nonlinear change of the ClOx mixing ratio but
is probably related to the reduced ozone loss in the NOx cycle that leads
to more Ox available for the catalytic ClOx cycle. However, the overall
nonlinear ozone change attributed to loss in this region is dominated by the
ozone increase due to NOx.
The nonlinear ozone change attributed to chemical production
(Fig. c) depends on the seasonality of the incoming solar
radiation and is therefore slightly different from the annual mean. The
contribution to the nonlinear ozone change, however, remains negative.
All in all, we find that ozone chemistry is affected by nonlinear changes,
but it cannot fully explain the nonlinear ozone changes, in particular the
ozone decrease in the Antarctic lower stratosphere in spring.
Figure d shows the nonlinear ozone change due to ozone
transport in the Antarctic spring season. The pattern is qualitatively
similar to that for the annual mean (Fig. d), which
indicates that the effect of nonlinear interactions on ozone transport is
largest in the SH spring season. We find a strong dipole signal in each
hemisphere: in the SH a significant decrease in ozone due to transport in the
polar stratosphere and an increase in the tropics and midlatitudes, and vice
versa in the NH. Hence, the nonlinear ozone change pattern in the SH is
primarily determined by the nonlinear changes in the ozone transport.
To understand why this dynamically driven nonlinearity is generated, we
analyse the changes in the residual mean mass stream function (Ψ).
Figure a shows the change in the mass
stream function between 1960 and 2000 for the SON mean. The contributions from
GHGs, ODSs and the nonlinear term are illustrated in
Fig. b–d respectively. The absolute field of the
stream function is positive for clockwise transport from the equator to the
north pole. The zero Ψ line of the 1960 reference simulation is shown in green.
The residual mean circulation is strengthened throughout the stratosphere in
the NH between 1960 and 2000 in the SON mean. In the SH the circulation is
enhanced in the upper stratosphere and weakened in the lower stratosphere.
This is consistent with the results by , who analysed
simulations with a CCM and reported a weakening of the downward motion in the
Antarctic lower stratosphere in SON for the 1960 to 2004 period and an
enhancement of the downwelling in the upper stratosphere.
Latitude-height section of the changes in the residual mean mass
stream function (Ψ) in 109 kg s-1 between 1960 and 2000 for
the SON mean (a) and the changes due to GHGs (b) and
ODSs (c) as well as the nonlinear contribution (d). The
light/dark grey shading indicates statistically significant changes on the
95/99 % confidence level respectively. The green contour line shows the
zero line of the absolute residual mean mass stream function of the 1960
reference simulation (R1960).
The change in the SH and NH upper stratosphere in the EMAC simulations can be
explained by the GHG and ODS forcings respectively (Fig. b
and c) but the weakening in the SH lower
stratosphere occurs only if ODSs and GHGs are changed simultaneously. This
result shows that in contrast to the findings by , we
detect a small, but significant nonlinear response in our time slice
simulations. This is potentially related to the different approach (time slice
vs. transient simulations) used in our study compared to
and also to the fact that the chemical effect of
increasing CH4 and N2O is solely included in our “GHG only” and not in
our “ODS only” simulation as it is in the study by .
Thus in our study, nonlinear effects on the dynamics arising from nonlinear
ozone changes are more likely to be detected.
Due to increasing GHG concentrations, the residual circulation is enhanced in
the NH upper stratosphere and in the lower stratosphere at low latitudes as
well as in the SH lower stratosphere (Fig. b). A reduced wave dissipation in the upper
troposphere (seen in the reduced Eliassen–Palm flux (EPF) convergence;
Fig. S1b in the Supplement) leads to enhanced wave propagation into the lower
stratosphere at midlatitudes in both hemispheres. In the SH the wave
dissipation is enhanced between 100 and 10 hPa, leading to a strengthening of
the circulation, particularly in the lower stratosphere; however, for the NH
midlatitudes, the atmospheric structure favours wave propagation (indicated
by the change in the refractive properties ; see
Fig. S2b) into the upper stratosphere, where the waves dissipate and drive
the change of the mean mass stream function in the upper part (Figs. S1b
and b).
In contrast, ODS increase leads to an enhancement of the mass transport in
the SH and a reduction in the NH (Fig. c),
which is also reported by . In the SH the source region of
wave energy (EPF divergence) in the upper troposphere/lower stratosphere (UTLS) between 30 and 60∘ S
is shifted poleward and intensified (see Fig. S1c). This is
probably related to a slight poleward shift of the SH subtropical jet, which
is caused by the cooling trend in the Antarctic lower stratosphere and an
increase of the latitudinal temperature gradient. The shift of the SH
subtropical jet is a known feature in summer months
e.g., but it already starts to develop in SON in the
time slice simulations. In addition, wave dissipation is reduced in the lower
stratosphere at midlatitudes, i.e. the atmosphere is more permeable, which
leads to increased EPF convergence in the middle and upper SH stratosphere
(see Figs. S2c and S1c respectively) and to a strengthening of
the SH residual circulation (Fig. c). The
improved conditions for wave propagation are linked to the positive change of
the zonal mean zonal wind (see Fig. S3c), which accompanies a later breakdown of the polar vortex (not shown). The NH weakening is
explained by , with an extension of the SH circulation change
into the NH leading to reduced downwelling at high NH latitudes.
Finally, nonlinear changes occur, for example, if changes in the atmospheric
conditions due to ODSs favour or mitigate the propagation of waves, which in
turn are caused by increasing GHGs. In our simulations we find that the
strengthening of the residual circulation in the SH lower stratosphere, which
arises from both GHG and ODS changes, is weaker for the combined forcing
(Fig. d). Here, different processes play a
role. On the one hand, the wave activity from below is decreased due to less
reduced (= increased) wave dissipation in the troposphere. This is linked to
a weaker increase of the zonal wind around 60∘ S (see
Figs. S1 and S3), which is associated with a weaker meridional temperature
gradient in the UTLS and a reduced poleward shift of the SH subtropical jet
(compared to the sum of the single forcings). This shift also induces a
weakening of the EPF divergence in the lowermost stratosphere (see
Figs. S1d and S3d). On the other hand, the middle stratosphere is
more permeable for waves (see Figs. S1d and S2d), which is
related to the greater persistence of the polar vortex in SH spring for the
combined forcings compared to the sum of the single forcings (not shown),
meaning a longer period of westerly winds in spring (see Fig. S3d). Thus,
while wave dissipation is reduced in the middle stratosphere, it
is enhanced in the upper stratosphere, driving the positive circulation
change there (Fig. d).
In the NH the weakening of the residual circulation caused by ODSs
and, in the polar lower stratosphere, by GHGs is compensated by nonlinear
interactions. The wave dissipation in the troposphere is decreased at
midlatitudes, allowing more waves to propagate into the stratosphere. As a
consequence the wave dissipation in the middle and upper stratosphere is
increased, driving the positive change of the residual circulation
(Figs. S1d and ).
This nonlinear behaviour of the mass stream function is consistent with the
changes of the ozone transport, since reduced transport from the tropics to
the polar regions causes ozone increase at midlatitudes and decrease at high
latitudes. At the same time, a strengthening of the mass stream function in
the NH lower stratosphere occurs, which causes an increased transport of
ozone to the higher latitudes. Moreover, the changes of the residual
circulation provide a possible explanation for the nonlinear NOx change
pattern in the lower stratosphere (Fig. b). A slower
mass transport from the tropics to the mid- and high latitudes goes along
with a longer transport time, which means that more time is available for the
chemical conversion of N2O. The reduced NOx values south of 70∘ S
are probably linked to the transport barrier at the edge of the polar vortex,
which is more persistent when ODSs and GHGs are increased simultaneously (not
shown). In the upper stratosphere, the increased downward motion transports
air with low NOx to the polar region and explains the NOx decrease.
Schematic figure of the annual mean nonlinear ozone change between
1960 and 2000 and the main processes we have identified.
(O3+)/(O3-) means positive/negative change of ozone due to the
indicated process.