GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L04707, doi:10.1029/2011GL046712, 2011

Large cancellation, due to ozone recovery, of future Southern
Hemisphere atmospheric circulation trends
Lorenzo M. Polvani,1,2 Michael Previdi,2 and Clara Deser3
Received 7 January 2011; accepted 26 January 2011; published 25 February 2011.

[1] The role of stratospheric ozone recovery in the Southern
Hemisphere climate system, in the coming decades, isexamined by contrasting two 10‐member ensembles ofCommunity Atmospheric Model (CAM3) integrations,over the period 2000–2060. Model integrations in the first
ensemble are conducted with a complete set of forcings:
greenhouse gas concentrations from the A1B scenario,
SSTs from corresponding ocean‐atmosphere coupledmodel integrations, and ozone starting with severe depletionover the South Pole and recovering by mid‐century. The
integrations in the second ensemble are very similar to thefirst, except that only the transient ozone forcing is specified,and all other forcings are kept at year 2000 levels. Specifying
ozone recovery in isolation allows us to determine unambiguously how it impacts the atmospheric circulation. We findthat, in DJF, most key indices of atmospheric circulation show
significant trends in the second ensemble, due to the closingof the ozone hole. In the first ensemble, however, trends arefound to be statistically insignificant for nearly all key circulation indices. This suggests that ozone recovery will resultin a nearly complete cancellation (and possible reversal) of
the atmospheric circulation effects associated with increasing
greenhouse gases, in Southern Hemisphere summer, over the
coming half century. Citation: Polvani, L. M., M. Previdi, and
C. Deser (2011), Large cancellation, due to ozone recovery, of future
Southern Hemisphere atmospheric circulation trends, Geophys. Res.
Lett., 38, L04707, doi:10.1029/2011GL046712.

1. Introduction[2] It is now widely documented that stratospheric ozone
depletion has played a major role in causing the atmosphericcirculation changes that have been observed in the Southern
Hemisphere (SH) during the second half of the 20th century[see, e.g., Polvani et al., 2011, and references therein]. It isthus likely that the projected ozone recovery will have aconsiderable impact in the coming decades: understandingthat impact is the goal of this paper. It may be worth recalling,as originally pointed out by Shindell and Schmidt [2004], thatin the late 20th century the depletion of stratospheric ozone has
added to the circulation changes associated with increasing
greenhouse gases (GHGs), whereas in the 21st century ozone

1
Department of Applied Physics and Applied Mathematics andDepartment of Earth and Environmental Sciences, Columbia
University, New York, New York, USA.
2
Lamont-Doherty Earth Observatory, Earth Institute at Columbia
University, Palisades, New York, USA.
3
CGD, National Center for Atmospheric Research, Boulder,
Colorado, USA.

Copyright 2011 by the American Geophysical Union.
0094‐8276/11/2011GL046712

recovery will subtract from them. The opposite effects ofozone recovery and increasing GHGs have been highlighted ina recent study of Arblaster et al. [2011], who have suggested
that climate sensitivity may play an important role in the
future cancellation.
[3] In fact, this cancellation is well documented in the model
integrations performed for the World Climate Research Programme’s (WCRP’s) Coupled Model Intercomparison Project
phase 3 (CMIP3) used for Intergovernmental Panel on Climate
Change (IPCC) Fourth Assessment Report, as reported by Son
et al. [2009], and also in those performed for the Stratospheric
Processes and their Role in Climate (SPARC)/Chemistry‐
Climate Model Validation project phase 2 (CCMVal‐2), as
reported by Son et al. [2010]. Neither of these intercomparison projects, however, yields a clean picture of the effect ofozone recovery on the climate system in the next half century,
since the ozone fields were not varied independently of theother forcings.
[4] The goal of this paper, therefore, is to perform a cleansensitivity study on the role of ozone recovery on the atmospheric circulation of the SH over the period 2000–2060. We
use an IPCC‐class atmospheric model and compare twoensembles of integrations, one with all forcings specified and
one with ozone forcing alone: using a relatively large numberof ensemble members (10) we show that, in the coming half
century, stratospheric ozone recovery will cause a nearlycomplete cancellation of all GHG‐induced summer circulation trends in the SH.

2. Methods[5] The numerical model used in this study is the Community Atmospheric Model, version 3 (CAM3). It is integrated at T42 horizontal resolution (approximately a 2.8° ×
2.8° grid in latitude and longitude) and with 26 hybrid
vertical levels, 8 of which are located above 100 hPa. All
integrations are carried out over the period 2000–2060, andan ensemble of 10 distinct integrations is constructed byinitializing the model with different days, between December
1999 and January 2000, of a 20th century integration of theCommunity Climate System Model Version 3 (CCSM3); thelatter is based on CAM3, but also includes an ocean and acryosphere model (see, for all details, Deser et al. [2010,
hereafter DEA10]).
[6] All 10 integrations in the first ensemble are forced
with GHGs following the A1B scenario, stratospheric ozone
fields starting with a severe depletion in 2000 and recovering by 2060, and smaller forcings (in the SH) related tosulfate aerosols and black carbon: these forcings are identical to the ones used for the A1B integration contributed by
CCSM3 to the CMIP3 project, and all details are given by
Meehl et al. [2006]. (The ozone field used in our study is

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documented on page 2599 of Meehl et al. [2006], and its SH
polar cap trend, over the period 2000–2060, is nearly
identical to the one in the AC&C/SPARC Ozone Dataset(I. Cionni et al., Multi‐model mean ozone time series insupport of CMIP5 simulations, manuscript in preparation,2010).) In addition to these radiative forcings, the sea surface temperature and sea ice concentrations (SSTs for short)
are specified, for all 10 integrations, using the mean of alarge ensemble (30 members) of CCSM3 integrations, overthe same period, and with identical forcings, as described by
DEA10. This first ensemble of 10 integrations is labeled“ALL”, to make it clear that all forcings are operative.
[7] To clearly bring out the impact of stratospheric ozone
recovery on the atmospheric circulation, a second ensembleof 10 integrations is performed with the same model(CAM3). In this second ensemble, labeled “OZONE”, theintegrations are identical in most respects to the ones of the
ALL ensemble, except that only the ozone field is changed
over the period 2000–2060: all other forcings have no timedependence other than a seasonal cycle. The SSTs are held
fixed at the 2000–2009 mean, and the values of GHG concentrations and all other forcings taken from the year 2000.
[8] Owing to limited computational resources, we havenot performed the complementary ensemble of integrations,
i.e., with all forcings changing in time except for ozone
recovery. Nonetheless, assuming additivity, the difference
between the means of the ALL and OZONE ensembles isshown in all plots below: this difference is labeled “GHG/
SST”, as it gives an indication of the effects associated withincreasing GHGs and the accompanying warming of the
SSTs. As a note of reassurance, we note that Deser andPhillips [2009], in a very similar exercise to the one presented in this paper, found close additivity for the SAM trends
in DJF during 1950–2000 [see Deser and Phillips, 2009,
Figure 6].

3. Results[9] The DJF ensemble mean SH climate responses associated with the ALL and OZONE ensembles are shown in
Figure 1 (left and middle, respectively); also in Figure 1(right), we show their difference (GHG/SST). In all panels,
we present the response in terms of trends, computed using
linear, least-square fits, over the period 2001–2060.
[10] Consider first the temperature response. In Figure 1c,
one can see the familiar patterns associated with GHG
induced climate change: an overall warming of the troposphere, with maximum warming in the tropical upper troposphere, together with a cooling of the stratosphere. This,
however, differs greatly from the result when all forcings areincluded (Figure 1a): most of the differences are in the lower
stratosphere, which warms by more than 8° K near the pole,
as a direct radiative consequence of ozone recovery in thepolar lower stratosphere. Note that, in the absence of GHG
increases the warming would be even larger, as can be seenin Figure 1b, where ozone recovery alone produces a
warming of 10–12° K in the polar lower stratosphere.
[11] It is perhaps not widely appreciated that the direct
radiative effect of ozone recovery, while confined to the polar
lower stratosphere, is able to influence the atmospheric circulation over the entire SH. Consider the response of the zonal
mean zonal wind, shown in Figure 1 (middle). The effect ofozone recovery (Figure 1e) is to produce a strong equatorward

L04707shift of the midlatitude jet, which is opposite in sign to the
response associated with increasing GHG (Figure 1f): thisresults in a near cancellation of the trends, so that the combined response (Figure 1d) basically vanishes below 500 hPa.
[12] Another way to visualize the surprisingly large surface signature associated with stratospheric ozone recovery
is to take a look at the sea‐level pressure (SLP), which is
closely related to the Southern Annular Mode (SAM), a
widely used metric of the extratropical atmospheric circulation. The SLP response for the ALL and OZONE integration ensembles, and their difference, in DJF, is shown in
the Figure 2 (left). The negative SAM response accompanying ozone recovery (Figure 2b) overwhelms the positive
one associated with GHG increases (Figure 2c), and results
in a weak, negative, and not statistically significant (seebelow) response for the ALL integrations (Figure 2a).
[13] Next, recall that the position of the midlatitude jet ishighly correlated with the edge of the Hadley circulation in
SH summer, both in terms of the internal climate variability
on interannual time scales [Polvani et al., 2011; Kang andPolvani, 2011], and in terms of the climate response toexternal forcings on centennial timescales [Lu et al., 2008].
Hence it is not surprising that, together with an equatorward
shift of the midlatitude jet, ozone recovery will induce a
considerable contraction of the tropical circulation, as can beseen in Figure 1h, where the response of the mean‐meridional streamfunction y is shown. Again, this effect is
opposite to the one induced by GHGs (Figure 1i), and the
combined response again yields extremely weak, overall
Hadley cell expansion (Figure 1g) which is not, in fact,
statistically significant (see below).
[14] Finally, closely related with the mean meridional
circulation, we illustrate how the hydrological cycle will
be directly affected by stratospheric ozone recovery. In
Figure 2 (right), the P − E (precipitation minus evaporation)
fields are shown; the black contour indicates the latitudes
where P − E = 0 during 2001–2010. Note how the large
poleward expansion of the subtropical dry zones (i.e., thebrown regions where P − E < 0) associated with increasing
GHGs (Figure 2f) is largely canceled by the dry zone contraction due to ozone recovery (Figure 2e), so that the overall
response consists of a relatively weak poleward expansion.
[15] In order to quantify the statistical significance of ourresults, we report in Table 1, the ensemble mean and theassociated 95% confidence interval for several key metricsof the atmospheric circulation, for the ALL and OZONE
ensembles (the rows labeled GHG/SST simply show thedifference between these two ensemble means). Statisticallysignificant responses are those in which the 95% confidence
interval does not bracket zero (the null hypothesis being thatthe response is zero).
[16] Consider the DJF rows first. Note that for the
OZONE ensemble, the response in nearly all metrics is
statistically significant, whereas for the ALL ensemble only
the polar cap temperature and the P − E responses are. The
lack of statistical significance in most atmospheric circulation trends in the ALL ensemble is directly attributable toozone recovery, which severely reduces the amplitude of the
response. As shown by DEA10 with the coupled version of
the same model, over the same period, with the same forcings as the ALL ensemble, detection of statistically significant SLP trends in DJF requires approximately 15–20
ensemble members. This may appear surprising, but simply

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Figure 1. Ensemble mean DJF (top) zonal mean temperature, (middle) zonal wind and (bottom) mass streamfunction
responses, during 2001–2060, for the integrations in (left) the ALL ensemble, (middle) the OZONE ensemble and (right)
their difference. Contour intervals are, top to bottom: 0.2° K/decade, 0.22 m/s/decade, and 0.15 × 109 kg/s/decade. Black
contours: the climatological mean (2001–2010), with contour intervals of 10° K, 5 m s−1 and 1.5 × 1010 kg s−1, respectively,
and negative contours dashed (and thicker zero contour).

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Figure 2. (left) DJF ensemble mean SLP response, 2001–2060, for (top) the ALL ensemble, (middle) the OZONE
ensemble, and (bottom) their difference. (right) As in Figure 2 (left), but for P − E (thick black lines: climatological mean
P − E = 0 during 2001–2010). Statistically significant responses, at the 95% level, are hatched.

Table 1. DJF and JJA Responses During 2001–2060 of 100 hPa Polar Cap (60° S–90° S) Temperature, Latitude of 850 hPa Zonal Wind
Maximum, SAM Index, Latitude of y = 0 at 500 hPa, and Latitude of P − E = 0a
Ensemble Name

Polar Cap T (° K)

Jet Maximum (° Latitude)

SAM Index (hPa)y(500 hPa) = 0 (° Latitude)

P − E = 0 (° Latitude)

ALL
OZONE
GHG/SST

5.24 ± 0.57
7.62 ± 0.70
−2.38 ± .93

0.24 ± 0.58
1.33 ± 0.52
−1.09 ± 0.49

DJF
−1.16 ± 1.79
−4.93 ± 2.11
3.77 ± 2.00

−0.02 ± 0.30
0.68 ± 0.31
−0.70 ± 0.31

−0.23 ± 0.16
0.18 ± 0.19
−0.41 ± 0.22

ALL
OZONE
GHG/SST−1.90 ± 0.58
0.68 ± 0.46
−2.58 ± 0.75

−0.73 ± 0.69
0.11 ± 1.01
−0.84 ± 1.38

JJA
3.17 ± 1.52
−0.11 ± 1.37
3.28 ± 2.37

−0.61 ± 0.20
0.06 ± 0.09
−0.67 ± 0.24

−1.00 ± 0.25
0.01 ± 0.13
−1.01 ± 0.32

a
The ensemble mean and associated 95% confidence interval are given for the ALL and OZONE ensembles, with the rows labeled GHG/SST showing
the difference between these two ensemble means. Boldface type indicates statistically significant responses.

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Figure 3. SAM index (anomalies from 2001–2010) in DJF
for the two, 10‐member ensembles. Thin red and blacklines: individual members in the ALL and OZONE ensembles, respectively. Thick red and black lines: the corresponding ensemble means. Thick blue line: the difference of the
thick red and black lines, i.e., the SAM response due toall forcings other than ozone recovery.

reflects the presence of very large internal variability whichmakes even broad, integral measures of the circulation (suchas SLP) difficult to predict without large ensembles.
[17] To make this more immediately clear, we show in
Figure 3, the yearly time series of SAM in DJF, for all
20 integrations in the two ensembles we have discussed in
the paper. The SAM index is here computed as the anomalous (relative to 2001–2010) zonal mean SLP difference
between 40° S and 65° S [Marshall, 2003]. The thin lines
are the individual ensemble members, and the thick lines theensemble mean. Note how small the trend one is trying tocapture is, in comparison with the year‐to‐year oscillations.
A discernible (and statistically significant) negative trend
can be seen in the thick black line (the mean of the OZONE
ensemble), but the trend in the thick red line (mean of the
ALL ensemble) is very small, and not statistically significant. Figure 3 illustrates the challenge of determining trends
in the atmospheric circulation, and raises issues about many
earlier modeling studies that often used a very small numberof ensemble members to determine trends.
[18] Finally, from Table 1, we note that nearly all JJA
trends for the OZONE ensemble are not significant, as one
would expect from the high seasonal nature of ozone
recovery, which reaches its maximum in SON and has its
largest effect in DJF. More importantly, note that all JJA
metrics are statistically significant in the ALL ensemble, inagreement with DEA10. Contrasting the trends for the ALL
ensemble in JJA and DJF, again highlights the key roleof stratospheric ozone recovery in canceling the effects ofincreasing greenhouse gases during SH summer in the
coming decades.

4. Discussion[19] This paper confirms and extends an earlier study of
Shindell and Schmidt [2004], and demonstrates that stratospheric ozone recovery will likely lead to large cancellations

L04707(and possibly reversals) in SH atmospheric circulation
trends in the first half of the 21st century. Our sensitivity
study corroborates the results of both the CCMVal‐2 and theCMIP3 multimodel ensemble projections (provided ozone
recovery is taken in to account).
[20] Our results are obtained with an IPCC‐class atmospheric model, and this might suggest that interactive
stratospheric chemistry and a well resolved stratosphericcirculation may not be needed to capture the effect of ozone
recovery on the tropospheric circulation. This, however,remains unclear. First, the amount of cancellation accompanying ozone recovery depends on the magnitude of theozone trends, as clearly illustrated by Karpechko et al. [2010].
Second, the rate of recovery of ozone depends on GHG
concentrations, as documented by Waugh et al. [2009a]
and Eyring et al. [2010], so that stratospheric chemistry
may be needed for a chemically consistent calculation.
Third, unlike the ozone fields used in this study (and most
IPCC class models), substantial zonal asymmetries exist inpolar stratospheric ozone, and these have important effects[Waugh et al., 2009b; Gillett et al., 2009].
[21] Finally, a comment on the role of the ocean, which
we have neglected up to this point. One might argue that theocean temperatures would also respond to ozone recovery,
and thus integrations of an atmospheric model with specified SSTs are not fully consistent. However, the radiative
effects at the surface associated with ozone recovery areprobably small, and unlikely to fundamentally alter the results presented here. As evidence, the 40‐member ensembleof CCSM3 integrations discussed by DEA10 – which employ
the same atmospheric model used in this study yet fully
coupled to ocean and cryosphere models, and with radiative
forcings identical to those prescribed in our ALL ensemble
over the same 2000–2060 period – show nearly identical
SLP trends in the SH to the ones presented here (compare
our Figure 2a with Figure 1a of DEA10). Moreover, therecent study of Sigmond et al. [2010], has carefully addressed the issue of whether the ocean impacts the atmospheric response to stratospheric ozone depletion, andconcluded that the addition of an ocean model has little effect.
(After our manuscript had been completed, Dr. Ted Shepherd
made us aware of the existence of a related paper[McLandress et al., 2011], which will soon appear. In that
paper the main conclusion of our study is confirmed, from
integrations of a coupled ocean‐atmosphere model withactive stratospheric chemistry).
[22] Acknowledgments. This work was funded, in part, by a grantfrom the US National Science Foundation to Columbia University. All
model integrations were performed at the National Center of AtmosphericResearch, which is sponsored by the US National Science Foundation.
LMP is grateful to Judith Perlwitz for thoughtful comments on an earlyversion of this paper.

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C. Deser, CGD, National Center for Atmospheric Research, PO Box
3000, Boulder, CO 80307, USA. (cdeser@ucar.edu)
L. M. Polvani, Department of Applied Physics and Applied Mathematics,
Columbia University, MC 4701, 500 West 120th St., New York, NY 10027,
USA. (lmp@columbia.edu)
M. Previdi, Lamont‐Doherty Earth Observatory, Earth Institute atColumbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964,
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