impact of antarctic ozone depletion and recovery on ...swson/papers/purich-son...ozone...

Impact of Antarctic Ozone Depletion and Recovery on Southern HemispherePrecipitation, Evaporation, and Extreme Changes

ARIAAN PURICH AND SEOK-WOO SON

Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

(Manuscript received 3 July 2011, in final form 19 September 2011)

ABSTRACT

The possible impact of Antarctic ozone depletion and recovery on Southern Hemisphere (SH) mean and

extreme precipitation and evaporation is examined using multimodel output from the Climate Model In-

tercomparison Project 3 (CMIP3). By grouping models into four sets, those with and without ozone depletion

in twentieth-century climate simulations and those with and without ozone recovery in twenty-first-century

climate simulations, and comparing their multimodel-mean trends, it is shown that Antarctic ozone forcings

significantly modulate extratropical precipitation changes in austral summer. The impact on evaporation

trends is however minimal, especially in twentieth-century climate simulations. In general, ozone depletion

has increased (decreased) precipitation in high latitudes (midlatitudes), in agreement with the poleward

displacement of the westerly jet and associated storm tracks by Antarctic ozone depletion. Although weaker,

the opposite is also true for ozone recovery. These precipitation changes are primarily associated with changes

in light precipitation (1–10 mm day21). Contributions by very light precipitation (0.1–1 mm day21) and

moderate-to-heavy precipitation (.10 mm day21) are minor. Likewise, no systematic changes are found in

extreme precipitation events, although extreme surface wind events are highly sensitive to ozone forcings.

This result indicates that, while extratropical mean precipitation trends are significantly modulated by ozone-

induced large-scale circulation changes, extreme precipitation changes are likely more sensitive to thermo-

dynamic processes near the surface than to dynamical processes in the free atmosphere.

1. Introduction

Southern Hemisphere (SH) climate changes over the

last few decades have been extensively documented in

recent studies. They include an expansion of the Hadley

cell (Seidel et al. 2008; Johanson and Fu 2009), a shift in

atmospheric mass from high to midlatitudes (Thompson

and Solomon 2002; Marshall 2003), a poleward displace-

ment of the westerly jet and storm tracks (Thompson

and Solomon 2002; Marshall 2003; Fyfe 2003), an in-

crease in surface wind speeds over the Southern Ocean

(Boning et al. 2008), anomalously dry conditions over

southern South America, New Zealand, and southern

Australia, and anomalously wet conditions over much

of Australia and South Africa (Gillett et al. 2006). A

freshening and warming of the Southern Ocean (Wong

et al. 1999; Gille 2002; Boning et al. 2008) and a significant

warming of the Antarctic Peninsula (Thompson and

Solomon 2002) have also been observed. While some of

these changes have been attributed to circulation changes

induced by the increase in anthropogenic greenhouse

gases (Fyfe et al. 1999; Kushner et al. 2001; Cai et al.

2003), such changes in austral summer have also been

influenced by Antarctic ozone depletion (Thompson and

Solomon 2002; Shindell and Schmidt 2004; Arblaster and

Meehl 2006; Perlwitz et al. 2008; Son et al. 2009, 2010;

McLandress et al. 2011; Polvani et al. 2011; Kang et al.

2012). It is known that both increasing greenhouse gases,

occurring year-round, and ozone depletion, which occurs

most significantly in late spring and summer, have driven

SH extratropical circulation changes in a similar way,

the cumulative effect resulting in more significant tro-

pospheric climate change in austral summer than in other

seasons. In the future, the effects of these two forcings

are, however, predicted to oppose each other (Shindell

and Schmidt 2004; Perlwitz et al. 2008; McLandress et al.

2011) as Antarctic ozone concentrations are anticipated

to increase owing to implementation of the Montreal

Protocol (Austin et al. 2010).

Corresponding author address: Seok-Woo Son, Department of

Atmospheric and Oceanic Sciences, McGill University, 805 Sher-

brooke West, Montreal QC H3A 2K6, Canada.

E-mail: [email protected]

1 MAY 2012 P U R I C H A N D S O N 3145

DOI: 10.1175/JCLI-D-11-00383.1

� 2012 American Meteorological Society

While the surface climate impact of increasing green-

house gases is relatively well understood, our under-

standing of stratospheric ozone-related climate change

at the surface, especially its mechanisms, is somewhat

limited. In particular the impact of stratospheric ozone

changes on the hydrological cycle in the SH is not well

understood. A series of recent studies have shown that

stratospheric ozone depletion has likely enhanced aus-

tral summer precipitation changes in the subtropics and

high latitudes but reduced them in midlatitudes, consis-

tent with the poleward displacement of the westerly jet,

or equivalently the positive trend in the Southern An-

nular Mode (SAM) index (Son et al. 2009; McLandress

et al. 2011; Polvani et al. 2011; Kang et al. 2012). Although

they are crucial for understanding salinity changes in

the Southern Ocean, net hydrological changes, including

evaporation, are not yet well understood. In addition,

and arguably more importantly, potential changes in

extreme precipitation events are yet to be investigated.

It is known that individual precipitation events are

likely to get more intense as the climate warms (Emori

and Brown 2005; Sun et al. 2007; O’Gorman and

Schneider 2009). Previous studies suggest that it is pre-

dominantly thermodynamics that control changes in

extratropical extreme precipitation (Emori and Brown

2005; O’Gorman and Schneider 2009). Thus, it is ques-

tionable whether extreme precipitation events will re-

spond to dynamical changes driven by the Antarctic

ozone hole.

The purpose of this study is to bridge the existing gap

in understanding the relative contributions of anthro-

pogenic greenhouse gas emissions and stratospheric

ozone changes in forcing changes in the hydrological

cycle. Multimodel output from the Climate Model In-

tercomparison Project 3 (CMIP3) (Meehl et al. 2007) is

analyzed. By grouping models into those with prescribed

ozone depletion and recovery and those without it, we

show that Antarctic ozone forcings significantly affect

seasonal-mean precipitation trends in the extratropics

during austral summer but play a minimal role in evap-

oration and extreme precipitation trends.

2. Data and methods

CMIP3 data from the twentieth-century climate sim-

ulations (20C3m) and twenty-first-century climate sim-

ulations with the special report on emissions scenarios

A1B forcing (A1B) are analyzed. From all available

models, the models that archived daily precipitation

are first selected. For those models, evaporation is cal-

culated from surface latent heat flux as outlined in Yu

et al. (2008). Each model’s precipitation and evapo-

ration climatologies are then compared with Global

Precipitation Climatology Project version-2 (GPCP)

precipitation (Adler et al. 2003) and Objectively Ana-

lyzed Air–Sea Heat Fluxes version-3 (OAFlux) global

ocean evaporation data (Yu et al. 2008). Those models

with significant biases are discarded, and 19 models are

selected for the analyses as described in Table 1.1

All CMIP3 models have prescribed stratospheric

ozone concentrations with a seasonal cycle. However,

not all models have incorporated stratospheric ozone

depletion in the latter part of the twentieth century and

recovery in the twenty-first century, as anthropogenic

ozone forcings were not mandated in the CMIP3 (Meehl

et al. 2007). Ten models prescribed ozone depletion and

ozone recovery, while nine models simply used clima-

tological ozone fields.2 As such, models are grouped into

four sets: those with and without ozone depletion in

the twentieth century and those with and without ozone

recovery in the twenty-first century. For each group, the

multimodel-mean climatologies and trends are calcu-

lated for the fields of interest (precipitation and evapo-

ration) over the twentieth and twenty-first centuries. As

in Son et al. (2009), climatologies and trends are first

calculated for each ensemble member and averaged

over all available ensemble members of a given model.

The ensemble average of each model is interpolated

onto a 48 latitude by 48 longitude grid and averaged over

all available models within a group. Hatching is used on

trend maps to denote where the multimodel mean trend

is greater than or equal to one standard deviation of

the trends of different models within that group. By

comparing the multimodel means of each group, the

impact of Antarctic ozone forcings on hydrological cli-

mate changes is systematically examined. Although this

approach does not necessarily reveal ozone-related sur-

face climate changes, as each group comprises different

models, it is known that trend differences resulting from

different ozone forcings are likely larger than those as-

sociated with model-dependent internal variabilities

(Son et al. 2009).

Since daily data are archived only for selected decades

in the A1B runs, long-term trends are estimated in this

1 Flexible Global Ocean–Atmosphere–Land System Model grid-

point version 1.0 (FGOALS1.0g) [Institute of Atmospheric Physics

(IAP), China] is discarded as twentieth-century precipitation in the

high-latitude region is found to be unreasonably higher than observa-

tions and all other models. Goddard Institute for Space Studies (GISS)

Model E-R (GISS-ER) (NASA, United States) is discarded as 1971–99

daily precipitation appears to be erroneous across the extent of the SH.2 Certain CMIP3 models prescribed ozone depletion in the

twentieth century but did not prescribe ozone recovery in the twenty-

first century; however, for other reasons, such models were not in-

cluded in this study.

3146 J O U R N A L O F C L I M A T E VOLUME 25

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study using decadal differences. The twentieth-century

change, reflecting the impact of ozone depletion, is de-

fined by the difference between 1990–99 and 1961–70

means. Likewise the twenty-first-century change, reflect-

ing the impact of ozone recovery, is defined by the differ-

ence between 2056–65 and 1990–99 means. Since decadal

differences are qualitatively similar to the linear trends

computed from monthly-mean data over 1960–99 and

2000–79 (not shown), they are simply referred to as

‘‘trends’’ in this study. The possible changes in ex-

treme precipitation events are examined by decomposing

seasonal-mean precipitation trends into three regimes

(Sun et al. 2007): very light (0.1–1 mm day21), light (1–

10 mm day21), and moderate-to-heavy (.10 mm day21)

precipitation changes. Five extreme precipitation indices

are also examined. They are the sum of precipitation on

all wet days divided by the number of wet days, the sum of

rainfall on days exceeding the 95th percentile threshold

as determined for the base period of 1961–90 (hereafter

95th percentile precipitation), the sum of rainfall on days

exceeding the 99th percentile threshold as determined for

the base period of 1961–90, seasonal maximum one-day

precipitation, and seasonal maximum five-day consecu-

tive precipitation (ETCCDI/CRD 2009).

3. Results

Multimodel-mean trends of austral summer [December–

February (DJF)] precipitation and evaporation are pre-

sented in Fig. 1. Only the extratropics, poleward of 308S,

are shown, as tropical and subtropical trends are noisy

and largely insignificant. In the twentieth century the

FIG. 1. Multimodel-mean trends of (first row) precipitation, (second row) evaporation, and (third row) precipitation minus evaporation

in DJF. Trends are calculated by differencing 1961–70 from 1990–99 averages and 1990–99 from 2056–65 averages for twentieth- and

twenty-first-century trends, respectively. (left to right) Multimodel-mean trends are shown for models with and without ozone depletion

in the twentieth century and for models with and without ozone recovery in the twenty-first century, respectively. Cool colors denote an

increasing freshwater flux (increasing precipitation or decreasing evaporation) while warm colors denote a decreasing freshwater flux

(decreasing precipitation or increasing evaporation). Hatched areas denote where the multimodel-mean trend is greater than or equal to

one standard deviation. Contours show the climatology, with contour intervals of 100 mm season21 for all panels.


poleward displacement of storm tracks by increasing

greenhouse gases causes a dipolar trend in pre-

cipitation (Yin 2005). This is evident in the models

without ozone depletion; however, trends are only

weak. A similar pattern but with much stronger

magnitude is found in the models with prescribed

ozone depletion, indicating the combined effects of

increasing greenhouse gases and ozone depletion on

extratropical precipitation changes. The opposite is

generally true in the twenty-first century: models with

prescribed ozone recovery show relatively weaker

precipitation trends than models with fixed ozone forc-

ing. As shown in previous studies (Son et al. 2009;

McLandress et al. 2011; Polvani et al. 2011), this sen-

sitivity is observed only in austral summer.

In contrast to the annular-like trends of precipitation,

evaporation shows relatively weak trends in the extra-

tropics, which lack organization. The sensitivity of evap-

oration trends to ozone forcings is also weak, although

there is a hint that models with ozone depletion have

a weaker decreasing trend in high-latitude evaporation

than those without ozone depletion, presumably because

of the acceleration of surface westerlies by ozone de-

pletion. This result, combined with the findings related

to precipitation trends, indicates that Antarctic ozone

forcings modulate long-term trends of surface freshwater

flux (or equivalently, precipitation minus evaporation)

by primarily affecting precipitation trends. Figure 1 (third

row) in particular suggests that Antarctic ozone deple-

tion has likely contributed to the observed freshening

of the Southern Ocean (e.g., Boning et al. 2008); however,

its recovery would have little impact on Southern Ocean

freshening, as ozone-related precipitation changes in the

twenty-first century are partly cancelled by evaporation

changes.

Figure 2 presents the relative contributions of very

light, light, and moderate-to-heavy precipitation changes

to the DJF-mean precipitation changes shown in Fig. 1. It

is evident that mean precipitation trends are dominated

by light precipitation trends in the models (second row),

the precipitation regime that is somewhat overestimated

in the CMIP3 models (Dai 2006). Note that, although

very light precipitation events also show significant trends,

their cumulative impact is very weak (shading interval in

the first row is one-tenth of that in the second row). More

importantly, their sensitivity to ozone forcings is exactly

opposite to those of seasonal-mean precipitation trends

(cf. the first rows of Fig. 1 and Fig. 2). This is somewhat

surprising, but similar results—opposite trends between

very light and light precipitation events—are also found

in the precipitation response to anthropogenic warm-

ing (e.g., Sun et al. 2007). For instance, as shown in

the rightmost column of Fig. 2, very light and light

precipitation trends in the twenty-first-century with

fixed ozone but with increasing greenhouse gases show

opposite trends in the high latitudes. It appears that the

similar excitation of the SAM by both increasing

greenhouse gases and varying ozone affects the pre-

cipitation distribution. The mechanisms for this are,

however, unknown. The response may be accounted

for in terms of thermodynamic changes by increasing

greenhouse gases (i.e., a warm atmosphere can hold

more moisture, likely causing the probability distribu-

tion function of precipitation events to shift away from

very light to light precipitation events), but is more

perplexing in terms of ozone-forced dynamic changes.

Further studies are needed.

The trends in moderate-to-heavy precipitation events

(Fig. 2 third row), which are quantitatively similar to 95th

percentile precipitation trends (fourth row), are relatively

weak. Unlike trends in very light and light precipitation,

they show no dipole pattern: trends are predominantly

positive across the extent of the SH, particularly in the

twenty-first century, reflecting more intense and fre-

quent extreme precipitation events with anthropogenic

warming. More importantly, no notable sensitivity to

ozone forcings is observed. Although not shown, a lack

of sensitivity is also found in the other extreme pre-

cipitation indices described in the data and methods

section. This suggests that Antarctic ozone forcings only

play a minimal, if any, role in extreme precipitation

changes. Here, it should be noted that extreme precipitation

events are sensitive to model resolution. Comparisons of

the MIROC3.2(hires) (T106) with MIROC3.2(medres)

(T42), and of CGCM3.1 (T63) with CGCM3.1 (T47), in

fact, show stronger trends in the higher-resolution models

(not shown). However, that each model group contains

models with a variety of resolutions and that multimodel-

mean trends in extreme precipitation are quantitatively

similar between models with time-varying ozone and

models with fixed ozone, although the former have gen-

erally higher resolution than the latter (Table 1), suggests

that the conclusion that Antarctic ozone forcings play

a minimal role in extreme precipitation changes holds.

The above results suggest that Antarctic ozone forc-

ings affect hydrological climate changes in the SH extra-

tropics by modifying dynamics that in turn modify light

precipitation trends. It may be questionable whether re-

sults are affected by model groups having different cli-

mate sensitivities: models with varying ozone are known

to have higher climate sensitivity to increasing green-

house gases (Miller et al. 2006). However, that in the

twenty-first-century models with varying ozone exhibit

trends in mean precipitation consistent with ozone

recovery suggests that models with varying ozone are

not simply responding more strongly to increasing


greenhouse gases, as the response to ozone recovery is

in the opposite direction. Nevertheless, results should be

treated with caution, as they are based solely on simple

multimodel averaging. One limitation of multimodel av-

eraging, among others, is because individual models have

different climatologies (e.g., Barnes and Hartmann 2010).

For instance, the DJF-mean climatological jet, defined by

the maximum westerly wind at 925 hPa, varies from 438

to 568S among models (not shown). Models with fixed

ozone forcing generally have a jet in lower latitudes

(46.28S 6 2.98) compared to those with ozone de-

pletion (49.88S 6 2.78) [as expected, the latter is closer

to the real atmosphere (508S)]. Since storm tracks are

located on the poleward side of the westerly jet, the

simple multimodel averaging used in this study, which ig-

nores individual model bias, could over- or underestimate

precipitation trends by averaging stormy regions with dry

ones.

FIG. 2. Multimodel-mean trends in (first row) very light (0.1–1 mm day21), (second row) light (1–10 mm day21), (third row) moderate-

to-heavy (.10 mm day21), and (fourth row) 95th percentile precipitation in DJF: other details as in Fig. 1. Note that the color scale is an

order of magnitude less for very light precipitation so that details can be seen. Contour intervals of climatologies are 20 mm season21 for

very light precipitation panels and 50 mm season21 for all other panels.


Figure 3 shows seasonal-mean precipitation clima-

tology and long-term trends for individual models and

multimodel means, shifted relative to the climatological

jet. Only zonally averaged fields are shown as precipi-

tation trends are largely homogeneous in the zonal di-

rection (Figs. 1 and 2). The CMIP3 models reproduce

DJF-mean precipitation reasonably well in the high lati-

tudes (2308–08 relative to the jet). However, they gen-

erally underestimate it in midlatitudes (08–208 relative to

the jet) and overestimate it in the subtropics and tropics

(208–508 relative to the jet). Intermodel variation is

particularly large in the subtropics and tropics, making

any influence of ozone forcings difficult to distinguish. In

fact, although a recent study by Kang et al. (2012) has

shown that ozone depletion may have enhanced sub-

tropical precipitation in the SH, no sensitivity of sub-

tropical precipitation trends to ozone forcings is found

in multimodel-mean trends (second row). A noticeable

difference in subtropical precipitation trends is only

found in the twenty-first century (second row, right

column). This is, however, unlikely to be associated

with ozone recovery: it is, instead, thought to be a result

of the intensification of moderate-to-heavy (fifth row,

right column) tropical precipitation, caused by anthro-

pogenic warming, among models with different tropical

climatologies.

FIG. 3. (first row) Zonal-mean precipitation climatology and (second row) total, (third row) very light, (fourth row)

light, and (fifth row) moderate-to-heavy precipitation trends in DJF plotted as a function of jet-relative latitudes in

the SH. Zonal-mean values are shown for individual models (varying ozone models in blue and fixed ozone models in

red), multimodel averages (bold blue and red lines), and GPCP precipitation (bold black line). Both (left) twentieth-

century and (right) twenty-first-century simulations are shown.


Returning to extratropical precipitation trends, Fig. 3

confirms that ozone depletion tends to increase high-

latitude precipitation trends but decrease midlatitude

precipitation trends by modulating light precipitation

events. The role of ozone recovery is the reverse: de-

creasing (increasing) high- (mid-) latitude precipitation

trends, relative to greenhouse-gas-induced trends alone.

This sensitivity is statistically significant at the 99% con-

fidence level, as summarized in Table 2. Note that, while

the midlatitude mean precipitation trend difference in

the twentieth century is not significant, the decrease in

midlatitude light precipitation by ozone depletion is

statistically significant. Note also that, although the

sensitivity of high-latitude precipitation to ozone

forcings is not significant in the twenty-first century

in Table 2, it becomes significant if linear trends of

monthly-mean precipitation, which are generally stron-

ger than decadal differences, are used. The identical

analyses are further performed for other seasons and

no significant difference in multimodel-mean trends be-

tween the models with and without time-varying ozone

forcings is found.

4. Discussion

The multimodel analyses, based on CMIP3 models,

show that Antarctic ozone forcings significantly affect

austral summer precipitation trends in the SH extra-

tropics. Their effects are primarily realized by changes in

light precipitation events (1–10 mm day21), with negli-

gible changes in extreme precipitation events attribut-

able to ozone forcings. This lack of sensitivity of extreme

precipitation events to ozone forcings is somewhat

contradictory to the atmospheric circulation changes

by ozone forcings: ozone depletion is known to strengthen

extratropical westerlies or, equivalently, to favor the pos-

itive polarity of the SAM (Son et al. 2008, 2010;

McLandress et al. 2011; Polvani et al. 2011). As shown

in Fig. 4, trends of both seasonal-mean and frequency

of occurrence of 95th percentile surface westerlies are

strengthened by ozone depletion (second row). While

mean tropospheric circulation changes are consistent

with mean precipitation changes, no link is estab-

lished between extreme events (cf. first and second

rows). In other words, strengthening of the 95th per-

centile wind by ozone depletion does not lead to

strengthening of 95th percentile precipitation. This

result suggests that thermodynamic effects are likely

more important than dynamic effects in the extra-

tropical extreme precipitation changes as discussed

by Emori and Brown (2005) and O’Gorman and

Schneider (2009). In fact, surface air temperature

trends, which control water vapor content in the at-

mosphere, show a negligible sensitivity to ozone forc-

ings (third row).

As previously stated, the findings of this study are

based solely on multimodel averaging and thus should

be treated with care. Although multimodel averaging

can reduce model biases, it does not allow direct attri-

bution of SH surface climate changes to Antarctic ozone

forcings. This approach may also underestimate surface

climate responses to ozone forcings by averaging models

with realistic climatologies and trends with those with-

out them. More quantitative studies using climate model

sensitivity tests (e.g., McLandress et al. 2011; Polvani

et al. 2011) are needed for better quantifying strato-

spheric ozone-related hydrological climate changes in

the SH.

TABLE 2. Summary of differences in percentage change between the models with time-varying ozone forcings and those with fixed

ozone forcing: percentage change is defined as a decadal change normalized by long-term climatology, calculated for the high-latitude

(averaged over 48–248 south of individual model’s climatological jet) and midlatitude (averaged over 128–08 north of individual

model’s climatological jet) regions. Only the values statistically significant at the 99% confidence level are shown. Significance tests

are based on a Monte Carlo approach. This approach selects one group of 10 models (eight for evaporation) and one group of 9 models

at random and calculates the percentage change difference between the means of the two groups. This is repeated 50 000 times to get

a statistical distribution. The actual difference between the mean of the varying ozone group and the fixed ozone group is then

compared with this statistical distribution at the 99% confidence level. Although not shown, overall results are qualitatively similar to

a two-sided Student’s t test at the 99% confidence level. Only results from DJF are shown, as no significant values are found in other

seasons.

Precipitation

High latitudes Midlatitudes

Twentieth century Twenty-first century Twentieth century Twenty-first century

DJF DJF DJF DJF

Mean 3.1% 2.6%

Very light 3.5%

Light 4.2% 24.1%

Moderate to heavy


Acknowledgments. We thank Jacques Derome for

helpful discussion in the course of this research and the

anonymous reviewers for helpful comments that im-

proved this manuscript. We also thank the Program

for Climate Model Diagnosis and Intercomparison

(PCMDI) for collecting and archiving the IPCC/AR4

model data, the JSC/CLIVAR Working Groups on Cou-

pled Modeling (WGCM) and their Coupled Model In-

tercomparison Project (CMIP) and Climate Simulation

Panel for organizing the model data analysis activity,

and the IPCC WG1 TSU for technical support. The

IPCC Data Archive at Lawrence Livermore National

Laboratory is supported by the Office of Sciences, U.S.

Department of Energy. This study is funded by the Korea

Polar Research Institute (KOPRI) grant under Project

PE 11010. The work by A.P. is in part supported by

the Global Environmental and Climate Change Centre,

Montreal, Quebec, Canada. A.P. is also grateful of the

support of the Stephen and Anastasia Mysak Graduate

Fellowship in Atmospheric and Oceanic Sciences, McGill

University, during the course of this research.

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