the antarctic ozone hole during 2011 - bureau of meteorology · ozone value within the ozone hole...

Australian Meteorological and Oceanographic Journal 64 (2014) 293–311

293

The Antarctic ozone hole during 2011

A.R. Klekociuk1, M.B. Tully2, P.B. Krummel3, H.P. Gies4, S.V. Petelina5, S.P. Alexander1, L.L. Deschamps6, P.J. Fraser3, S.I. Henderson4, J. Javorniczky4, J.D. Shanklin7, J.M. Siddaway5 and K.A. Stone8

1Climate Processes and Change, Australian Antarctic Division, Australia2Bureau of Meteorology, Australia

3Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Australia

4Australian Radiation Protection and Nuclear Safety Agency, Australia5Department of Physics, La Trobe University, Australia

6Centre for Australian Weather and Climate Research, Bureau of Meteorology, Australia7British Antarctic Survey, United Kingdom

8School of Earth Sciences, Melbourne University, Australia

(Manuscript received November 2013; revised March 2015)

The Antarctic ozone hole of 2011 is reviewed from a variety of perspectives, mak-ing use of various data and analyses. The ozone hole of 2011 was relatively large in terms of maximum area, minimum ozone level and total ozone deficit, being ranked amongst the top ten in terms of severity of the 32 ozone holes adequately characterised since 1979. In particular, the estimated integrated ozone mass effec-tively removed within the ozone hole of 2011 was 2119 Mt, which is the 7th largest deficit on record and 82 per cent of the peak value observed in 2006. The key fac-tors in promoting the extent of Antarctic ozone loss in 2011 were the relatively low temperatures that occurred in the lower stratosphere of the polar cap region over most of the year, and the fact that the stratospheric vortex was relatively strong and stable, at least up to mid-spring. Dynamical disturbance of the polar vortex from mid-spring increased Antarctic ozone levels in the latter part of the ozone hole’s evolution and helped to limit the overall severity of depletion. Through ex-amination of regression of various ozone metrics against expected levels of equiv-alent effective stratospheric chlorine, we suggest that recent changes in averaged ozone levels over Antarctica show some evidence of the recovery expected due to international controls on the manufacture of ozone depleting chemicals, albeit at a statistically low level of confidence due to the influence of meteorological factors that largely dictate year-to-year variability of Antarctic ozone loss.

Introduction

Ozone loss associated with the Antarctic ozone hole is a key climate driver that is gaining stronger recognition as not only having had a significant influence on southern hemisphere climate since at least the late 1970s, but which may potentially extend over much of the 21st century (Arblaster et al., 2011; Canziani et al., 2013; Hi et al., 2011; Kang et al., 2011; Karpechko et al., 2010; Perlwitz et al., 2008;

Polvani et al., 2011a,b; Sigmond et al., 2011). An important factor in stronger appreciation of the influence played by ozone depletion on climate are advances in incorporating interactions between ozone, atmospheric chemical species and atmospheric dynamics in chemistry–climate and earth system models. An additional factor is that a great deal of high quality measurements, particularly from networks of ground-based stations and a fleet of earth observations satellites have been accumulated, and in many cases the heritage of these measurement now extends back over one or more decades.Corresponding author address: Andrew Klekociuk, Australian Antarctic

Division, Hobart, email: [email protected]

294 Australian Meteorological and Oceanographic Journal 64:4 December 2014

greatest ozone loss etc.; rank 2 = second largest, etc. We use 220 DU as the threshold in total column ozone to define the location and occurrence of the ozone hole, and restrict our analysis to poleward of 35°S. The definitions of the metrics presented in Table 1 are:1. Maximum 15-day averaged area: The largest value (in

each year) of the daily ozone hole area averaged using a 15-day sliding time interval.

2. Daily maximum area: The maximum daily value of the ozone hole area.

3. Minimum 15-day averaged total column ozone: The minimum of the 15-day averaged column ozone amount observed south of 35°S.

4. Daily minimum total column ozone: The minimum of the daily column ozone amount observed south of 35°S. This metric effectively measures the ‘depth’ of the ozone hole.

5. Daily minimum average total column ozone: The minimum of the daily column ozone amount averaged within the ozone hole. This metric effectively measures the ‘average depth’ of the ozone hole.

6. Maximum daily ozone deficit: The maximum value of the daily total ozone deficit within the ozone hole. This metric effectively measures the combined area and depth of the ozone hole.

7. Integrated ozone deficit: The integrated (total) daily ozone deficit for the entire ozone hole season. This metric effectively measures the overall severity of ozone depletion.

The ozone hole in 2011 was ranked between 6th and 11th over the seven metrics. Figure 1(a) shows the daily ozone hole area throughout the 2011 season. The annual maximum value of the daily area (metric two) is commonly used to assess the overall size of ozone hole; in 2011 this metric was the 11th largest on record, attaining 25.7 million km2 in early October. As shown in Fig. 1(b), the progression of the daily ozone hole depth during 2011 was generally below the 1979–2010 mean, and at a minimum (metric four) in early October, ranked 8th. Figure 1(c) shows the daily ozone deficit, which measures the total mass of ozone effectively depleted within the area of the ozone hole. The maximum daily ozone deficit (metric six) for 2011 of 37.5 million tonnes (Mt), which was attained in early October, was ranked 9th.

An abrupt decrease can be seen in the daily deficit during early October (Fig. 1(c))—this also resulted in a small overall decrease in the area metric (Fig. 1(a)) at this time. These decreases coincided with the off-pole displacement of the vortex associated with an episode of strong mid-latitude planetary wave activity which is further discussed in the section on vertically resolved ozone measurements below.

Trends in ozone hole metricsFigure 2(a) shows the minimum daily average total column ozone value within the ozone hole from 1979 to 2011 (metric four). This metric shows a consistent downward trend in

In advancing our understanding of the influence of ozone change, it is important to assess how Antarctic ozone depletion progresses as each additional year unfolds. This has two benefits: firstly, it enables us to test our understanding of the processes responsible for ozone depletion, and secondly, it enables assessment of how strongly ozone depleting substances (ODS) are influencing ozone levels. Here we provide the next installment in a series of analyses on the Antarctic ozone hole that provides an overview of conditions in 2011 from various perspectives and presents a range of Australian data and analyses including Bureau of Meteorology (the Bureau) meteorological analyses, ODS measurements and analyses by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Marine and Atmospheric Research, ozone measurements obtained by the Australian Antarctic Division (AAD) and the Bureau, and Antarctic ultraviolet measurements from the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) biometer network. A variety of data from satellite missions and ground-based instruments are also presented. We focus on a description of meteorological conditions and their relation to the overall level of Antarctic ozone depletion in 2011. This work complements analyses of earlier Antarctic ozone holes reported by Tully et al. (2008, 2011) and Klekociuk et al. (2011).

Further information on Antarctic atmospheric conditions during 2011 can be found in World Meteorological Organisation (WMO) Antarctic Ozone Bulletins (www.wmo.int/pages/prog/arep/gaw/ozone/index.html), the National Aeronautics and Space Administration (NASA) Ozone Watch web site (http://ozonewatch.gsfc.nasa.gov), the upper-air summaries of the National Climate Data Center (NCDC; www.ncdc.noaa.gov/sotc/upper-air) and the National Oceanic and Atmospheric Administration (NOAA) – NCDC State of the Climate Report (www.ncdc.noaa.gov/bams-state-of-the-climate; Blunden et al., 2012).

Total column ozone measurements

Ozone hole metric summary and rankingsWe use total column ozone data processed with the NASA TOMS Version 8.5 algorithm from the Ozone Monitoring Instrument (OMI) on board the Aura spacecraft and the Total Ozone Mapping Spectrometer (TOMS) instruments on the earth probe and earlier spacecraft to evaluate specific ozone hole metrics. The OMI data have been interpolated to the same spatial resolution as the TOMS data (1.5° in longitude, 1.0° in latitude). These metrics are evaluated on a semi-regular basis and are available at www.environment.gov.au/protection/ozone/publications/antarctic-ozone-hole-summary-reports. They also complement similar analyses provided by NASA at http://ozonewatch.gsfc.nasa.gov/statistics/annual_data.html.

Table 1 contains the rankings for 32 ozone holes since 1979 using seven metrics that measure the ‘size’ of the Antarctic ozone hole: rank 1 = lowest ozone minimum, greatest area,

Klekociuk et al.: The Antarctic ozone hole during 2011 295 Ta

ble

1.

Ran

ked

An

tarc

tic

ozo

ne

ho

le m

etri

cs o

bta

ined

fro

m T

OM

S/O

MI s

atel

lite

dat

a. T

he

2005

met

rics

are

the

aver

age

of T

OM

S a

nd

OM

I dat

a; ‘N

aN’ i

nd

icat

es in

suffi

cien

t mea

sure

-m

ents

ava

ilab

le.

Met

ric

1. M

axim

um

15-d

ay a

vera

ged

2. D

aily

max

imum

area

3. M

inim

um

15-d

ay a

vera

ged

tota

l col

umn

ozon

e

4. D

aily

min

imum

tota

l co

lum

n oz

one

5. D

aily

min

imum

6. D

aily

max

imum

oz

one

defic

it7.

Inte

grat

ed o

zone

defic

it

Ran

kYe

ar10

6 km

2Ye

ar10

6 km

2Ye

arD

UYe

arD

UYe

arD

UYe

arM

tYe

arM

t

120

0028

.720

0029

.820

0693

.420

0684

2000

138.

320

0645

.220

0625

79.1

220

0627

.620

0629

.620

0093

.519

9886

2006

143.

320

0044

.919

9824

19.9

320

0326

.920

0328

.419

9896

.820

0089

1998

146.

720

0343

.420

0122

98.2

419

9826

.819

9827

.920

0198

.920

0391

2003

147.

519

9442

.719

9922

49.6

520

0826

.120

0527

.119

9999

.920

0191

2005

148.

619

9841

.119

9621

75.7

620

0125

.720

0826

.920

1110

0.9

2005

9320

0114

8.8

2008

39.6

2000

2164

.0

720

0525

.619

9626

.820

0310

1.9

1991

9419

9914

9.3

2001

38.5

2011

2119

.3

819

9625

.020

0126

.420

0910

2.6

2011

9520

0915

0.0

2005

37.7

2008

2000

.2

920

1125

.019

9325

.820

0510

2.7

2009

9620

0815

0.5

2011

37.5

1994

1981

.8

1019

9324

.819

9925

.719

9310

4.0

1999

9719

9615

0.6

2009

35.7

2005

1901

.0

1119

9424

.320

1125

.719

9610

6.0

1997

9920

1115

1.2

1999

35.3

2003

1893

.6

1219

9224

.019

9425

.219

9710

7.2

2008

101

1997

151.

319

9734

.519

9318

32.9

1319

9924

.020

0725

.120

0810

8.5

2004

102

2007

154.

619

9633

.920

0918

09.0

1420

0723

.919

9725

.119

9211

1.5

1996

103

1993

155.

219

9233

.520

0717

85.1

1520

0923

.919

9224

.920

0711

2.2

1993

104

1992

156.

320

0733

.019

9717

58.6

1619

9723

.320

0924

.419

9111

3.4

1992

105

1991

162.

519

9332

.619

9215

28.6

1720

1021

.620

0422

.719

8711

5.7

2007

108

1987

162.

619

9126

.619

8713

65.6

1819

8721

.419

8722

.420

0411

6.0

1989

108

1990

164.

419

8726

.220

1013

39.9

1920

0421

.119

9122

.319

9011

7.8

1987

109

2010

164.

720

1026

.119

9011

80.7

2019

9121

.020

1022

.219

8912

0.4

1990

111

1989

166.

219

9024

.319

9199

8.3

2119

8920

.720

0221

.820

1012

3.9

2010

118

2004

166.

719

8923

.620

0497

4.7

2219

9019

.519

8921

.619

8513

1.8

1985

124

2002

169.

820

0223

.219

8991

6.7

2320

0217

.719

9021

.020

0213

6.0

2002

131

1985

177.

120

0422

.819

8562

9.6

2419

8516

.619

8518

.619

8615

0.3

1986

140

1986

184.

719

8514

.520

0257

4.8

2519

8613

.419

8414

.419

8415

6.1

1984

144

1984

190.

219

8610

.519

8634

6.2

2619

8413

.019

8614

.219

8316

0.3

1983

154

1983

192.

319

849.

219

8425

5.5

2719

8811

.319

8813

.519

8816

9.4

1988

162

1988

195.

019

837.

019

8819

7.6

2819

8310

.119

8312

.119

8218

3.3

1982

170

1982

199.

719

886.

019

8318

4.0

2919

827.

519

8210

.619

8020

0.0

1980

192

1980

210.

019

823.

719

8273

.1

3019

802.

019

803.

219

8120

4.0

1979

194

1979

210.

219

800.

619

8012

.8

3119

811.

319

812.

919

7921

4.7

1981

195

1981

210.

219

810.

619

813.

5

3219

790.

219

791.

219

94n

/a

1994

n/a

1994

n/a

1979

0.3

1979

1.0


Fig 1. Estimated daily ozone hole metrics from OMI: (a) area, (b) depth and (c) ozone mass deficit. The values were obtained using measurements from the OMI satellite instrument (from 2005) and from the TOMS satellite instruments before 2006.

Klekociuk et al.: The Antarctic ozone hole during 2011 297

Fig 2. Annual values of (a) minimum southern hemisphere daily total column ozone values, (b) daily maximum Antarctic ozone hole area, and (c) estimated total ozone deficit with the ozone hole (interpolation for missing daily data). The green line shows regression to Equivalent Effective Stratospheric Chlorine (EESC) using a mean age of air of 5.5 years.


ozone levels at Halley for 2005 to 2011 (155 ± 16 DU) are higher than those observed from 1996 to 2001 (141 ± 4 DU), although the 1σ uncertainties just overlap. If we remove 2002 and 2004 data from Fig. 3, the remaining data for 1996–2011 show significant ozone growth (recovery) of 1.4 ± 0.7 (1σ) DU/yr. For the period 1993–2011 the ozone growth is 1.8 ± 0.5 (1σ) DU/yr, although the early 1990s data may be low due to the impact of the Mt. Pinatubo eruption.

Vertically resolved ozone measurements

MLS partial columnsIn Fig. 4(a), ozone hole area metrics are provided for three partial columns in the lower stratosphere, mid-stratosphere and full stratosphere (denoted as ‘low’, ‘mid’ and ‘full’ partial columns respectively) based on measurements by the Aura MLS instrument during 2010 and 2011. As apparent from Table 1, ozone depletion in 2010 was relatively benign, and as suggested by Klekociuk et al. (2011), this was the result of the polar vortex being relatively warm and weak. MLS provides height-resolved ozone information during day and night, and is thus able to measure the portion of the vortex that is within continuous darkness during winter and spring and inaccessible to the OMI instrument.

The ‘full’ partial column time series for the two years show similar values and variations to the OMI data presented in Fig. 1(a). The ‘low’ partial column timeseries rose and fell at similar times of the year, with the areal extent being slightly larger in 2011 than in 2010; the larger area is indicative of a stronger vortex at this height.

The ‘mid’ partial column time series were distinctly different in the two years shown. In 2011, the onset occurred approximately two weeks earlier than in 2010, and the area was consistently and appreciably larger than in 2010. The depletion in the ‘mid’ column was evidently responsible for the earlier growth and larger overall size of the ozone hole in 2011 compared with 2010. An interesting feature is the

average ozone from the late 1970s until the mid 1990s, with some sign of ozone recovery by 2011. The 1996–2001 mean was 148 ± 5 DU while the 2006–2011 mean was 152 ± 7 DU. In this figure, we show the regression against Equivalent Effective Stratospheric Chlorine (EESC), obtained using

Y(t) = a1 × EESC(t) + a0.

where Y(t) is the regression value at time t, and a1 and a0 are coefficients obtained from the linear regression y(t) = a1 × EESC(t) + a0, with y(t) in this case being the annual minimum daily average total column ozone value. EESC is an important measure used to relate abundances of ODSs at the surface to stratospheric chlorine and bromine levels, and hence the impact on stratospheric ozone levels (see the section ‘Ozone depleting substances’ below). The values used for the regression were obtained from Fraser et al. (2013) assuming a mean age of air of 5.5 years. The upward trend in the regression against EESC since around 2000 suggests the commencement of ozone recovery, but the uncertainty intervals overlap. If we remove data for 2002 and 2004 (years in which the polar vortex was unusually disturbed by planetary waves), the remaining data for 1996–2011 show signs of a linear ozone growth (recovery) of 0.5 ± 0.3 (1σ) DU/yr.

Figure 2(b) shows the annual time series of maximum ozone hole area (metric two). Disregarding 1988 and 2002 when the polar vortex broke up unusually early, this metric suggests that the ozone hole has stopped growing, but has not yet started to decline in area. The 1996–2001 mean was (25.6 ± 2.0) × 106 km2, while the 2006–2011 mean was (24.7 ± 2.1) × 106 km2; the change in these values suggests ozone recovery, although the increase is not statistically significant at the 1σ confidence limit. If we remove 2002 and 2004 data from Fig. 2(b), the remaining data for 1997–2011 do not show a significant decrease in ozone hole area. The data suggest that the maximum in annual ozone hole area occurred around 2000.

Figure 2(c) shows the integrated ozone deficit from 1979 to 2011 (metric seven). The ozone deficit rose steadily from the late 1970s until the late 1990s/early 2000s, where it peaked at approximately 2300 Mt, and then started to decline. Excluding 2002 and 2004, this metric does not show evidence of a statistically significant ozone recovery. The 1996–2001 mean was 2180 ± 230 Mt while the 2006–2011 mean was 1940 ± 410 Mt, suggesting an ozone increase, but these means overlap at the 1σ level. If we remove 2002 and 2004 data from Fig. 2(c), the remaining data for 1996–2011 indicate a decline in ozone deficit of 25 ± 16 (1σ) Mt/yr.

An often quoted metric to defining the severity of the ozone hole is the average minimum ozone level observed at Halley, Antarctica (75.5°S, 26.5°W) in October (Fig. 3). This was the metric that was first reported in 1985 by scientists from the British Antarctic Survey to identify the significant ozone loss over Antarctica. The minimum ozone level was observed in 1993, and this has been attributed to residual volcanic effects from the 1991 eruption of Mt. Pinatubo (Farman et al., 1985). Ignoring the warm years of 2002 and 2004, the mean October

Fig 3. October monthly mean total column ozone values for Halley station for 1957–2011 (magenta points and line) and regression to EESC (green line) using a mean age of air of 5.5 years.


OSIRIS stratospheric ozone profilesVertical profiles of stratospheric ozone number density obtained from the Optical Spectrograph and Infra-Red Imager System (OSIRIS) instrument onboard the Odin satellite are presented in this section. Odin is an ongoing polar-orbiting satellite, launched in 2001, with the typical latitude coverage (in the orbit plane) from ~83°N to ~83°S. OSIRIS ozone profiles, provided since November 2001, are retrieved from the limb-scattered spectral solar irradiances with a 1 km vertical step. As such measurements can be obtained only in the sunlit hemisphere, data for the southern hemisphere are available from late August until early March each year. A detailed instrument description and validation of OSIRIS stratospheric ozone retrievals are given in Llewellyn et al. (2004), Petelina et al. (2004) and Roth et al. (2007).

Figure 5 shows time-height sections of daily zonal mean OSIRIS ozone number density in the range from 12 km to 34 km poleward of 60°S. White gaps indicate time periods when no measurements were performed. The 2002–2010 plot (Fig. 5(a)) is an average of daily zonal mean ozone values over the preceding nine years. The onset of stratospheric ozone depletion during the first half of September is not observed in the 2011 data (Fig. 5(b)) due to technical issues that prevented the collection of data. In the second half of September, ozone depletion is observed with the ozone hole increasing its vertical extent by the beginning of October. Over the month of October until mid-November, there is substantial ozone depletion extending up to 20–22 km altitude, suggesting a stronger-than-average polar vortex.

Daily zonal mean ozone vertical profiles averaged over the 60°–70°S latitude range for 2011 are shown in Fig. 5(c). This figure shows a clear ozone density maximum at 17–

pronounced decrease in the ‘mid’ partial column ozone hole area in early October; this was associated with a decrease of the ozone hole area for the ‘full’ partial column, but not for the ‘low’ partial column.

Figure 4(b) shows a polar stereographic map of the partial ozone column from the upper troposphere through to the mesosphere on 7 October. The distribution of ozone is typical of the following one week period, and shows the depleted region of the ozone hole being somewhat asymmetric and displaced off the pole generally towards the region of the southeast Pacific Ocean. A ridge of relatively high partial column ozone values (in excess of 350 DU) can be seen extending from generally east of South America to east of New Zealand, and coming close to the edge of the Antarctic Continent near longitudes of 90˚E (bottom meridian). The ozone ridge is a feature typically seen in the spring at these latitudes. Most of the ozone in the partial column is in the lower stratosphere; the concentration in this region is enhanced by the poleward-flowing branch of the Brewer-Dobson circulation and the dynamical barrier to poleward flow created by the stratospheric polar vortex, and disturbed by the circulation eddies associated with quasi-stationary and zonally-propagating planetary waves. Overall, the asymmetries apparent in the shape of the ozone hole and its off-pole location shown in Fig. 4(b) are consistent with the planetary waves having an important effect on the edge of the vortex during this period.

Fig. 4. Aura MLS version 3.3 ozone data (Froidevaux et al., 2008). (a) Ozone hole area metrics for 2010 and 2011 derived for three partial columns: 146–68 hPa (red, ‘low’ column), 46–10 hPa (blue, ‘mid’ column) and 464–0.1 hPa (black, ‘full’ column), for which the ozone hole thresholds of 25 DU, 70 DU and 220 DU have been used, respectively. The thresholds have been chosen to enclose regions with a similar meridional gradient of ozone relative to the general level inside and outside the vortex. (b) Map of the ‘full’ column data for 7 October. Swath values have been interpolated to a uniform horizontal grid with latitude and longitude cells of 1.25° and 1° respectively.


Fig. 5. OSIRIS daily zonal mean stratospheric ozone profiles for (a) latitudes 60°–83°S for 2002–2010, (b) 60°–83°S for 2011–12 and (c) 60°–70°S for 2011–12.


squares), particularly around mid- year. Also apparent, again by comparing Fig. 6(a) with Fig 15(a) in Klekociuk et al. (2011), is that low ozone associated with the ozone hole from spring to early summer (September to December), most obvious over the height range of approximately 12–20 km, was less pronounced in 2011 than in 2010. Figure 6(b) shows large positive excursions during the spring of 2011 on days 280 (7 October), 314 (10 November) and 334 (30 November). The 12–20 km partial column value on 7 October was the largest value observed for this time of year since measurements began in 2003.

Antarctic ultraviolet radiation

A series of broadband ultraviolet radiation (UVR) detectors (UVBiometer Model 501, Solar Light Co., Philadelphia USA) have been used by ARPANSA to monitor the solar UVR in Antarctica since the mid-1990s. With assistance from the AAD these detectors are currently operating at Casey (66.3°S, 110.5°E), Davis (68.6°S, 78.0°E) and Mawson (67.6°S, 62.9°E) stations and the data are transmitted back to ARPANSA for analysis. UVR measurements made by ARPANSA detectors take into account cloud cover, ozone levels and other environmental factors that computer models can only approximate. A detailed description of the detectors and calibration methods used by ARPANSA in Antarctica has been published previously (Tully et al., 2008).

The UV Index is a measure of the intensity of solar UVR at the earth’s surface taking into account its biological effects on human skin (WHO, 2002). UV Index values are grouped into exposure categories such that readings of two or less are denoted a low exposure, three to five are moderate, six to

22 km persisting from mid-September to early-October, before a sharp decrease in ozone across all altitudes. By mid-October, there was an increase in ozone density above 19 km at 60°–70°S (Fig. 5(c)) that is not significant when averaged over the full measurement extent poleward of 60°S (Fig. 5(b)). This abrupt increase in ozone density coincides with the episode of distortion in the polar vortex during the second and third weeks of October noted above.

Comparison of vertical structures in the Figs 5(a) and 5(b) suggests that the vertical extent of ozone depletion was ~4–6 km larger from mid-September to mid-November 2011 than for the corresponding month intervals in the 2002–2010 mean.

Ozonesonde measurements at DavisAustralian measurements of Antarctic ozone have been made at Davis since 2003 under a collaborative program between AAD and the Bureau. During 2011, ozone sondes were launched at weekly intervals throughout the year; during the preceding years weekly launches were made only between mid-May and mid-November, and monthly at other times of the year. A summary of the Davis ozonesonde data is presented in Fig. 6. In Fig. 6(a), the time-height evolution of ozone partial pressure is shown, with lower values corresponding to lower ozone concentrations. The ozone partial pressures in the height range 15–25 km up to the beginning of September were generally less in 2011 than in 2010 (comparing Fig. 6(a) with Fig 15(a) in Klekociuk et al., 2011). The reduced ozone concentration in 2011 can also be seen by examining Fig. 6(b), where the partial column in the height range 12–20 km for 2011 (red squares) are mostly less than those for the corresponding period in 2010 (blue

Fig. 6. Summary of ozone sonde measurements at Davis, Antarctica. (a) Time-height cross-section of ozone partial pressure. The data have been interpolated to a uniform grid with resolution 500 m (vertical) by eight days in time. (b) Time series of partial column ozone for the height interval 12–20 km. Shown are data for all years of measurement, with data for 2011 highlighted in red. The grey line is a climatological mean from Fortuin and Kelder (1998) interpolated to the location of Davis. The loca-tion of the thermal lapse rate tropopause is shown by the black line and dots. A measurement gap occurred between 20 July (day 201) and 26 August (day 238) due to issues with the hydrogen balloon-filling system at Davis.


MonthCasey Davis Mawson

2007 2008 2009 2010 2011 2007 2008 2009 2010 2011 2007 2008 2009 2010 2011

Jul 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Aug 2 5 8 1 5 3 2 6 3 4 7 6 8 0 3

Sep 3 11 9 10 12 15 21 20 20 23 17 21 21 18 27

Oct 0 7 9 7 3 20 17 26 22 9 23 20 29 23 8

Nov 4 4 4 4 0 10 9 2 13 1 8 11 1 14 4

Dec 0 0 0 0 0 5 0 0 4 3 4 1 0 3 5

All 9 27 30 22 20 53 49 54 62 40 59 59 59 58 47

MonthCasey Davis Mawson

2007 2008 2009 2010 2011 2007 2008 2009 2010 2011 2007 2008 2009 2010 2011

Jul 0.1 0.1 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0

Aug 0.4 0.4 0.5 0.2 0.3 0.3 0.3 0.3 0.2 0.3 0.3 0.4 0.4 0.3 0.3

Sep 1.0 2.3 1.6 1.9 1.6 1.6 2.0 1.5 1.5 1.6 1.5 2.3 1.8 1.9 2.1

Oct 3.2 4.1 4.4 4.6 4.1 4.5 4.4 6.0 5.3 4.0 5.2 5.8 7.1 6.0 4.3

Nov 6.3 5.9 5.9 7.2 5.0 7.2 7.0 4.9 7.7 5.2 6.3 7.6 5.2 8.5 6.3

Dec 6.7 6.9 6.0 7.3 6.9 7.4 6.9 6.2 6.8 8.3 n/a 8.1 7.2 8.0 9.0

All 2.7 3.3 3.1 3.5 3.0 3.4 3.4 3.2 3.6 3.2 2.6 4.1 3.6 4.1 3.6

CategoryCasey Davis Mawson

2007 2008 2009 2010 2011 2007 2008 2009 2010 2011 2007 2008 2009 2010 2011

Low 91 87 101 87 93 90 84 93 90 96 88 76 87 82 85

Moderate 21 46 36 43 44 38 41 41 34 39 33 33 34 27 36

High 9 28 29 22 27 24 35 34 28 19 21 31 30 31 27

Very High 13 21 17 26 13 23 23 16 27 23 9 38 30 30 27

Extreme 0 1 1 6 0 4 1 0 5 5 0 3 1 14 3

All 144 183 184 184 177 179 184 184 184 182 151 181 182 184 178

Table 2. Number of ozone hole events (defined as less than 220 DU) by month for the past five years over each of the Australian Antarctic stations.

Table 3. Monthly mean UV Index for the past five years measured at each of the Australian Antarctic stations. No data are available for December 2007 at Mawson.

Table 4. Number of days where the UV Index falls in each exposure category for the past five years (July to December) measured at each of the Australian Antarctic stations.

Table 5. Individual ODS contributions to the decline in EESC-A and EESC-ML observed in the atmosphere in 2011 since their peak values (4.14 ppb in 2000 and 1.93 ppb in 1998 respectively).

Species EESC-A decline ppb CL EESC-ML decline ppbCl

methyl chloroform 0.26 0.19

methyl bromide 0.10 0.07

CFCs 0.04 0.03

carbon tetrachloride 0.04 0.03

halons -0.09 -0.04

HCFCs -0.05 -0.02

Total decline 0.31 0.25


seven are high, eight to ten are very high, while 11 or more is extreme. ARPANSA records the solar UVR at ten minute intervals, reporting the maximum daily ultraviolet radiation as a UV Index value.

Figures 7(a), 7(b) and 7(c) show daily peak values of the measured UV Index for Casey, Davis and Mawson, respectively, over the 2011 season. Also shown in these figures are the daily total column ozone values derived from OMI satellite observations. The UV Index exhibits a seasonal dependence due to variations in the minimum solar zenith angle (apparent height of the sun in the sky at solar noon), together with variability due to cloud effects and varying stratospheric ozone. For clear-sky days there is a strong anti-correlation between the measured UV Index and total column ozone. UV Index levels recorded at the Australian stations on the edge of the Antarctic continent occasionally reach extreme levels when clear skies coincide with the presence of the ozone hole during the Antarctic spring and, on occasions, during summer as seen at Davis and Mawson.

Defining total column ozone values reported by the OMI satellite of less than 220 DU as an ozone hole event, Table 1 shows the number of days for which this occurred for each of the Australian stations in Antarctica over the past five years. This year there were fewer ozone hole events at all stations during October and November compared to previous years. Total number of ozone hole events over Davis and Mawson were also down this season. Casey recorded a similar number to previous years but they all occurred early in the season.

The monthly mean of the (daily peak) UV Index recorded at each of the Australian stations in Antarctica over the past five years is listed in Table 2. The December monthly mean UV Index for Davis and Mawson was higher this season than in previous years, whilst October and November show a lower monthly mean UV Index for all sites. Note that the Mawson UV Index data record is missing three days in mid-December, during an ozone hole event; therefore it is likely that the monthly mean UV Index was actually even higher.

Table 3 shows for each of the past five years the number of days between July and December on which the UV Index measured at the Australian Antarctic stations fell within each of the WHO exposure categories. Despite missing three days data in mid-December, during an ozone hole event, the number of extreme UV Index days for Mawson is substantially lower than last season’s unusually high count. The situation in Casey and Davis was unremarkable.

Most of the ozone hole events this season occurred early enough that the midday sun was still low in the sky (large minimum solar zenith angles) and the consequent long atmospheric path meant that UV levels generally did not reach the highest exposure categories prior to December. The most notable feature of this season’s ozone hole in relation to the Australian stations was the dramatic drop in ozone levels in mid-December over Davis and Mawson. The presence of ozone depleted air above Davis and Mawson at this time coincided with clear skies and gave rise to particularly large UV Index values.

Fig. 7. Total column ozone in Dobson units (left axis) and daily UV Index (right axis) during 2010 for (a) Casey (66.3°S, 110.5°E), (b) Davis (68.6°S, 78.0°E), and (c) Mawson (67.6°S, 62.9°E). Also shown is the line for 220 Dobson units, where ozone values lower than this are defined as being part of the ozone hole (left axis).


Ozone depleting substances

Ozone recovery over Antarctica is complex to model (for a recent review of progress, see Dameris and Reimann, 2014). Apart from the future level of ozone depleting chlorine and bromine in the stratosphere, temperature trends and variability in the stratosphere, the impact of major volcanic events and the future chemical composition (for example H2O, CH4 and N2O) of the stratosphere are likely to be important factors in determining the rate of ozone recovery. Model results and observations show that the solar cycle changes have maximum impact on tropical ozone and do not significantly impact on stratospheric ozone levels over Antarctica.

Fig. 8. (a) Equivalent Effective Stratospheric Chlorine (EESC – see text) derived from global measurements of all the major ODSs at Cape Grim and other AGAGE stations and in Antarctic firn air from Law Dome following Newman et al. (2007). EESC-A is calculated with fractional release factors using a 5.5 year mean age of air (plotted with a 5.5 year lag) as a driver of Antarctic ozone depletion. EESC-ML is calculated with fractional release factors using a three-year mean age of air (plotted with a three-year lag) as a driver of mid-latitude ozone depletion. Arrows indicate dates when the mid-latitude and Antarctic stratospheres return to pre-1980s and pre-ozone hole levels respectively. (b) ODGI-A and ODGI-ML indices (Hofmann and Montzka, 2009) derived from AGAGE ODS data using ODS fractional release factors from Newman et al. (2007).

Chlorine and bromine levels are likely to decline steadily over the next few decades leading to reduced ozone destruction (Figs 1–15 of Carpenter and Godin-Beekmann, 2014). Figure 8(a) shows Equivalent Effective Stratospheric Chlorine (EESC) for the Antarctic (EESC-A, 5.5 year mean age of air) and mid-latitude (EESC-ML, three-year mean age of air) stratospheres, calculated using the fractional release factors from Newman et al. (2007), but derived from CSIRO and the Advanced Global Atmospheric gases Experiment (AGAGE) global measurements of ODSs, including Cape Grim (Tasmania) measurements and measurements in air trapped in Antarctic firn from Law Dome, Antarctica as described in Fraser et al. (2013).


EESC-A and EESC-ML peaked at 4.14 ppb in 2000 and 1.93 ppb in 1998 respectively, falling to 3.83 and 1.68 ppb by 2011, which represent declines of seven per cent and 13 per cent respectively. Table 5 shows the species contributing to the declines in EESC-A (0.31 ppb) and the decline in EESC-ML (0.25 ppb) since their peak values in 2000 and 1998 respectively. The declines are dominated by methyl chloroform (CH3CCl3), followed by methyl bromide (CH3Br), the chlorofluorocarbons (CFCs) and carbon tetrachloride (CCl4). The halons and hydrochlorofluorocarbons (HCFCs) have made an overall growth contribution to EESC since the respective peaks in Antarctic and mid-latitude values.

The initial (one to two decades) decline in EESC-A and EESC-ML have been and will be dominated by the shorter-lived ODSs, such as methyl chloroform and methyl bromide, whereas the long-term decline will be dominated by CFCs and carbon tetrachloride. Based on EESC values from scenarios of ODS decline (Velders and Daniel, 2014), ozone recovery at mid-latitudes will occur at about 2050 and ozone recovery in the Antarctic stratosphere will occur about the early 2080s.

Hoffman and Montzka (2009; and recent updates see www.esrl.noaa.gov/gmd/odgi/) have defined an index that neatly describes the state of the atmosphere, in relation to stratospheric halogen (chlorine plus bromine) levels

and ozone recovery at mid-latitudes (ODGI-ML) and over Antarctic (ODGI-A). Figure 8(b) shows the CSIRO version of the ODGI-ML and ODGI-A indices derived from global AGAGE data including data from Cape Grim. Based on data up to 2011, the ODGI-A and ODGI-ML indices have declined by 15 per cent and 33 per cent respectively since peak EESC-A and EESC-ML values in 2000 and 1998 respectively, indicating that the atmosphere is approximately 15 per cent and 33 per cent along the way toward a halogen level that should allow an ozone-hole free Antarctic stratosphere and a ‘normal’ (pre-1980s) ozone layer at mid-latitudes (assuming a one-to-one correspondence between ozone and EESC levels at 1980). The CSIRO version of the ODGI uses ODS fractional release factors from Newman et al. (2007).

Discussion

Polar temperatures and atmospheric indicesFrom January to June of 2011, monthly mean temperatures in the lower stratosphere above Antarctica were generally below normal. Figure 9a shows monthly temperature anomalies for the latitude range 90°S to 65°S from National Center for Environmental Prediction (NCEP) Reanalysis-2 data (Kanamitsu et al., 2002) with respect to the base period 1979–2010 for three pressure levels. At all levels and months

Fig. 9. (a) Monthly temperature anomalies (K) from zonal means for the latitude range 90°S to 65°S from NCEP Reanalysis-2 data relative to the monthly climatology for 1979–2010 at pressure levels of 10 hPa (top), 50 hPa (middle) and 100 hPa (bottom). Coloured bars show monthly anomalies for 2011, and diamonds connected by solid lines show maximum and minimum anomalies for 1979–2011. Numbers at the top of each panel are the rank of 2011 relative to years 2002–2011 (1 [10] = most positive [most negative] anomaly), and numbers at the bottom of each panel are values (K) of the monthly anomalies for 2011. (b) (top) NCEP standardised 30 hPa Quasi-Biennial Oscillation (QBO) index, (middle) standardised surface Southern Annular Mode (SAM) index (Marshall, 2003), and (bottom) standardised SAM index evaluated at 50 hPa (see text for details). The indices are expressed in standard deviations relative to base period of 1981–2010 (for QBO) and 1971–2000 (for SAM). Diamonds connected by solid lines show maximum and minimum anomalies for each index over the period 1979–2012.


shown in Fig. 9(a) (with the exception of December at 10 hPa), temperatures were below the climatological mean, but no record extremes were set relative to the 1979–2010 period. Relative to the decade 2002–2011, a few extreme low temperatures were set in 2011 (indicated by a rank of ten at the top of each panel in Fig. 9(a)), most notably at all of the three levels shown in October. From comparison with Klekociuk et al. (2011), temperatures in the Antarctic lower stratosphere were generally cooler in 2011 compared with 2010, particularly at and above the 50 hPa level in the period July to October.

During 2011, the NCEP standardised 30 hPa Quasi-Biennial Oscillation (QBO) index (www.cpc.ncep.noaa.gov/data/indices/qbo.u30.index) shifted from a positive (eastward) anomaly in the months up to October to a strongly negative (westward) anomaly by the end of the year (Fig. 9(b), top panel). The QBO modulates the ability of upward propagating planetary waves to influence extratropical latitudes. A weaker and more disturbed polar vortex is preferentially observed during the negative (westward) QBO index (Baldwin and Dunkerton, 1998). As shown in the top panel of Fig. 9(b), the QBO transitioned from a positive to a negative index during mid- to late austral spring (October–November).

The surface standardised Southern Annular Mode (SAM) index (Marshall, 2003 and www.antarctica.ac.uk/met/gjma/sam.html) vacillated during the year (Fig. 9(b), middle panel), and was mildly negative in late winter and early spring. The surface SAM expresses the sense of meridional pressure gradient anomalies, with positive (negative) values indicating anomalous positive (negative) pressure gradients between mid- and high latitudes. As discussed in Baldwin and Dunkerton (2001), positive (negative) anomalies in the stratospheric SAM tend to occur preferentially under conditions of a strong (weak) polar vortex (corresponding

to positive (negative) QBO phase)—these conditions are generally confined to the winter and spring seasons in the southern hemisphere when the polar vortex occurs. Stratospheric and tropospheric SAM variations are sometimes independent, but as noted in Baldwin and Dunkerton (2001), SAM anomalies just above the tropopause tend to favour tropospheric SAM anomalies of the same sign. The bottom panel of Fig. 9(b) shows the SAM index for 50 hPa evaluated using empirical orthogonal function analysis of NCEP Reanalysis-2 data, following the approach used by the NOAA Climate Prediction Center for their 700 hPa Antarctic Oscillation index (www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/aao_index.html). The surface and 50 hPa SAM indices shown in Fig. 9(b) were of the same sign in all months with the exception of April and November, while the index at 50 hPa trended to more positive values following its most negative value for the year in July.

As shown in Fig. 9(b), SAM and QBO were of the same positive sign (strong vortex condition) in January–February (summer), May–June (winter), and October (mid-spring). This was in contrast to the situation in 2010 when the QBO index favoured a weak polar vortex (negative index) up until late winter (August). Overall, the combination of the QBO and SAM indices in 2011 suggests that conditions favoured generally strong vortex conditions up to mid-winter, which weakened during spring.

Wave activity

Figure 10 shows the time-height temperature cross section above the South Pole during 2011 from the Australian Community Climate and Earth System Simulator (ACCESS) numerical weather prediction (NWP) analysis on the global domain (ACCESS-G). The overall evolution of the South Pole

Fig. 10. Daily time-pressure section of air temperature at 90°S from the global domain of the Australian Community Climate and Earth System Simulator (ACCESS) numerical weather prediction model. The black contour delineates the 195 K contour, which is the approximate warmest formation threshold for polar stratospheric clouds.


temperature profile in 2011 was fairly typical of recent years. Comparing with Fig. 2 of Klekociuk et al. (2011) it can be seen that temperatures in the lower stratosphere (pressures lower than 100 hPa in the figure) were colder in winter and early spring than in 2010; this is particularly evident by examining the extent of the 195 K contour. A short warming occurred in early October (shown by the period of temperatures above 205 K at 100 hPa), followed by a few weeks of relatively stable temperatures, and then a progressive warming from early November.

Daily measurements by the Microwave Limb Sounder (MLS) on the Aura spacecraft averaged over the Antarctic region are shown in Fig. 11. The temperature changes in October and November noted for Fig. 10 are seen more clearly in the large-scale average provided by Fig 11(a). Despite the warmings noted above, MLS measured anomalously cold stratospheric temperatures relative to 2004–2010 from August through to November, as shown in Fig. 11(b), which are of similar magnitude to the monthly average anomalies over this period shown in Fig. 9(a).

The poleward transport of heat provides a useful indicator of dynamical disturbances to the polar atmosphere produced by planetary waves at low- and mid-latitudes. Figure 12 shows the evolution of heat flux (measured by the product of the zonal anomalies in temperature and meridional wind speed) during 2011 using assimilated meteorological data from the United Kingdom Meteorological Office (UKMO). Outbreaks of poleward heat transport in the mid- and upper stratosphere (pressure levels 10–0.5 hPa) and above occurred from July through to November at mid-latitudes (Fig. 12(a)). In particular, the deepest penetration (lowest altitude) of

enhanced heat flux into polar latitudes (Fig. 12(b)) coincided with the reduction in the ‘mid’ and ‘full’ partial ozone column area metrics in early October shown in Fig. 4(a). This was near the time when the polar vortex was displaced off-pole (Fig. 4(b)) and when unusually high 12–20 km partial column ozone was observed at Davis (Fig. 6(b)).

From comparison of Fig. 12 with similar time-height sections for 2010 shown in Fig 6. of Klekociuk et al. (2011), it can be seen that while dynamical activity in this region was generally more consistent in 2011 compared with 2010, strong events of the size seen in July 2010 were absent in 2011. Indeed, the dynamical events in 2011 evidently did not start to significantly penetrate into the polar cap region until mid-September, as evident by comparing Figs 12(a) and 12(b).

The stratospheric polar vortex

The structure of the southern hemisphere stratospheric polar vortex during 2011 is shown in Fig. 13 for the 500 K potential temperature (isentropic) surface, which is near the top of the region of maximum springtime ozone loss. The central region of the vortex edge at this level was situated at approximately -70 degrees equivalent latitude from May (when the vortex formed) until late July. During most of August, the centre of the vortex edge moved equatorward by approximately ten degrees before retreating back to -70 degrees equivalent latitude during September. From late November, the vortex at this level began to break down, resulting in the steady poleward retreat of the edge region through to the end of December. Davis station in Antarctica

Fig. 11. (a) Daily time-height section of zonal average air temperature for latitudes 85°S to 65°S from Aura Microwave Limb Sound-er (MLS) quality controlled version 3.3 data for 2011. The solid black line marks the height of the warm-point stratopause in 2011, while the white dashed line marks the average warm-point stratopause height based on MLS data from 8 August 2004 (the start of measurements) to 31 December 2010. The white bar marks missing data. (b) Daily temperature anomaly obtained by subtracting the average daily temperature for all data prior to 2011 from the 2011 daily data. The warm-point stratopause height is marked as in the top panel. Single diagonal hatches marks anomalies that are outside the interdecile range based on measurements prior to 2011. Crossed diagonal hatching marks anomalies that are at the daily maximum or minimum value for all measurements up to and including 2011.


was located inside the vortex edge for the majority of 2011 (as shown by the location of the white contour in Fig. 13 in relation to the region of maximum PV gradient at the edge of the vortex). Excursions of Davis outside the core of the vortex can be seen in early October (around the time of observed high 12–20 km partial column ozone in Fig. 6(b)), and on several occasions in November (white contour situated close to or equatorward of the central edge of the vortex). Comparing with the situation in 2010 (Fig. 8 of Klekociuk et al., 2011), the overall areal extent of the strongest potential vorticity gradients was less in 2011, particularly for gradients at and above 3 × 10-6 K m2 kg-1 s-1/degree from August through October. This suggests that the

vortex in 2011 was potentially more permeable to meridional air exchange during the peak ozone loss period than in 2010.

Time series of proxies for the areal extent of the stratospheric polar vortex are shown in Fig. 14 for the 450 K and 850 K isentropic surfaces. On both isentropes, the vortex area was mainly above the climatological mean, particularly from August through November when the area exceeded or was close to the 95th percentile for the period since 1992. Figure 14(b) suggests that a rapid reduction in the vortex size on the 850 K isentrope (~31 km height) occurred in late September, which subsequently slowed until early October, after which the vortex at this height rapidly shrank to disappear by the end of the month.

Fig. 12. Daily eddy heat flux averaged between latitudes of (top) 55°S to 35°S and (bottom) 85°S and 65°S as a function of pressure evaluated from UKMO Stratospheric Assimilated Data. Negative values indicate poleward transport of heat. The zero contour is outlined in white.

Fig. 13. Potential vorticity gradient (expressed in Potential Vorticity Units (PVU; 1 PVU = 10-6 K m2 kg-1 s-1) per degree of equivalent latitude) as a function of time and equivalent latitude for the 500 K potential temperature (ϴ) isentrope (~20 km height), derived from the UKMO stratospheric assimilation. Equivalent latitude is derived using the method of Nash et al. (1996). The white contour denotes the equivalent latitude of Davis station (68.6°S, 78.0°E). Additional red contours show the location of the ‘inner’, ‘central’, and ‘outer’ limits of the vortex edge as defined by Nash et al. (1996).


The size of the stratospheric vortex for the two afore mentioned isentropes is compared for separate months over the years since 1992 in Fig. 15. For 2011, the vortex in the lower stratosphere was relatively long-lived (as shown by the relatively large area in December for the 450 K isentrope in Fig. 15(a)). Additionally, the size of the vortex in the mid-stratosphere during late winter was larger than for earlier years (as shown in Fig. 15(b)), particularly in comparison with the situation for 2010.

Overall, the meteorological data reviewed in this section indicate that polar stratospheric temperatures in 2011 were generally below average, and relatively cold compared with the preceding decade, while dynamical activity resulting in disturbance to the polar vortex became most apparent in early October. Additionally, the polar vortex was relatively large, and in the lower stratosphere, was relatively persistent with final breakdown occurring in December. All of these factors helped to promote generally large levels of ozone loss in the Antarctic spring.

Fig. 14. Southern hemisphere vortex area evaluated on potential temperature (ϴ) surfaces of (a) 450 K (~18 km height) and (b) 850 K (~31 km height). The time series for 2011 is shown in black; the blue time series is the mean for 1992–2011, while the lower and upper red time-series in each graph show the 5th and 95th percentiles, respectively, for 1992–2011. The vortex area is evaluated using data from the United Kingdom Meteorological Office (UKMO) stratospheric assimilation, and represents the surface area enclosed by potential vorticity contours of (a) -30 Potential Vorticity Units (PVU; 1 PVU = 10-6 Km2 kg-1 s-1) and (b) -600 PVU.

Fig. 15. Monthly averages of daily southern hemisphere vortex area on potential temperature (q) surfaces of (a) 450 K for Decem-ber and (b) 850 K for August, obtained using data and methods described in Fig. 14.


Conclusions

We have examined meteorological conditions and ozone concentrations in the Antarctic atmosphere during 2011 using a variety of data sources, including meteorological assimilations, satellite remote sensing measurements, and ground-based instruments and ozone sondes, and summarise our findings are follows.

The ozone hole of 2011 was relatively significant in terms of maximum area, minimum ozone level and total ozone deficit, being ranked amongst the ten most severe of the 32 ozone holes adequately characterised since 1979. The integrated ozone mass effectively removed within the ozone hole in 2011 was estimated as 2119 Mt, which is the 7th largest deficit on record and 82 per cent of the all-time peak observed in 2006.

Polar stratospheric temperatures in 2011 were mostly below average, and amongst the coldest of the preceding decade. The cold temperatures were conducive to significant ozone loss over Antarctica, and were favourable for the size and depth of the ozone hole to be relatively large.

The dynamical conditions promoted by the positive phases of the QBO and SAM up to the middle of the year were such to favour a strong polar vortex during this period, which aided in promoting chemical changes during winter that led to significant spring ozone depletion. The relative strength of the polar vortex and coldness of the polar cap were likely key factors in causing the ozone hole to be larger in 2011 than in 2010, rather than changes to background levels of ozone depleting substances.

The QBO index noticeably weakened from June, moved into a negative phase in late spring. The SAM index was negative in late winter and early spring, and, taken together with the weakened QBO index, appeared more conducive to the dynamically disturbed conditions that occurred in the polar cap in spring. These dynamical disturbances improved polar ozone and UVR levels from early October by reducing the strength of the polar vortex and allowing air to mix between the polar cap and lower latitudes.

In examining 2011 in the context of the ozone holes observed since 1979, it is evident that year-to-year variability in the meteorology of the Antarctic stratosphere plays a significant role in dictating the severity of ozone loss in spring. However, there are indications that the largest of the ozone holes may now be in the past and that signs of stabilisation and possible recovery in Antarctic springtime stratospheric ozone levels consistent with expected trends in the atmospheric reservoir of man-made ozone depleting substances are emerging.

Acknowledgments

We acknowledge the Department of Sustainability, Environ- ment, Water, Population and Communities for support of this work, and the assistance of the following people: Jeff Ayton and the Australian Antarctic Division’s Antarctic

Medical Practitioners in collecting the solar UV data, Nada Derek of CSIRO for preparation of figures, Bureau personnel for collecting upper air measurements in Antarctica and ODS data at Cape Grim, Tasmania, and for expeditioners of the British Antarctic Survey for collecting the Halley measurements. Odin is currently a third-party mission for the European Space Agency. OSIRIS operations and data retrievals are primarily supported by the Canadian Space Agency. The OMI ozone data are courtesy of the Ozone Processing Team at NASA Goddard Space Flight Center. Aura/MLS data used in this study were acquired as part of the NASA’s Earth-Sun System Division and archived and distributed by the Goddard Earth Sciences (GES) Data and Information Services Center (DISC) Distributed Active Archive Center (DAAC). UKMO data were obtained from the British Atmospheric Data Centre (http://badc.nerc.ac.uk). NCEP Reanalysis-2 data were obtained from the National Oceanic and Atmospheric Administration Earth System Research Laboratory, Physical Sciences Division. ODS data were collected as part of the NASA-funded AGAGE Program. Part of this work was performed under Projects 737 and 4012 of the Australian Antarctic Science programme.

ReferencesArblaster, J.M., Meehl, G.A. and Karoly D.J. 2011. Future climate change in

the Southern Hemisphere: Competing effects of ozone and greenhouse gases, Geophys. Res. Lett., 38, L02701, doi:10.1029/2010GL045384.

Baldwin, M. P. and Dunkerton, T.J. 1998. Quasi-biennial modulations of the Southern Hemisphere stratospheric polar vortex, Geophys. Res. Lett., 25, 3343-3346.

Baldwin, M.P. and Dunkerton, T.J. 2001. Stratospheric harbingers of anomalous weather regimes. Science, 294, 581-4.

Blunden, J., and D. S. Arndt, Eds., 2012: State of the Climate in 2011. Bull. Am. Meteorol. Soc., 93 (7), S1–S264.

Canziani, P.O, O´Neill, A., Schofield, R., Raphael, M., Marshall, G., Redaelli, G., and Mitchell, D. 2013. WCRP Special Workshop on Climatic Effects of Ozone Depletion in the Southern Hemisphere: Assessing the evidence and identifying gaps in the current knowledge—25 February–1 March, 2013, Buenos Aires, Argentina. SPARC Newsletter, 41, 29-34.

Carpenter, L. J. and Reimann, S. (Lead Authors), Burkholder, J. B., Clerbaux, C., Hall, B.D., Hossaini, R., Laube, J.C., and Yvon-Lewis, S.A. 2014. Ozone-Depleting Substances (ODSs) and Other Gases of Interest to the Montreal Protocol, Chapter 1 in Scientific Assessment of Ozone Depletion: 2014, Global Ozone Research and Monitoring Project – Report No. 55, World Meteorological Organization, Geneva, Switzerland.

Dameris, M., and Godin-Beekmann, S. (Lead Authors), Alexander, S., Braesicke, P., Chipperfield, M., de Laat, A. T. J., Orsolini, Y., Rex, M., and Santee, M. L. 2014. Update on Polar ozone: Past, present, and future, Chapter 3 in Scientific Assessment of Ozone Depletion: 2014, Global Ozone Research and Monitoring Project – Report No. 55, World Meteorological Organization, Geneva, Switzerland.

Farman, J. C., Gardiner, B. G., and Shanklin, J. D. 1985. Large losses of total ozone in Antarctica reveal seasonal CLOx/NOx interaction, Nature, 315, 207-210..

Fortuin, J.P.F and Kelder, H. 1998. An ozone climatology based on ozonesonde and satellite measurements, J. Geophys. Res. 103, 31709-31734.

Fraser P., Dunse, B., Krummel, P., Steele P., and Derek, N. 2013, Australian Atmospheric Measurements and Emissions of Ozone Depleting Sub-stances and Synthetic Greenhouse Gases, Report prepared for Depart-ment of the Environment, CSIRO Marine and Atmospheric Research, Centre for Australian Weather and Climate Research, Aspendale, Australia, iv, 42 pp.


10.1175/JCLI-D-11-00015.1Roth, C.Z., Degenstein, D.A., Bourassa, A.E. and Llewellyn, E.J. 2007.

The retrieval of vertical profiles of the ozone number density using Chappuis band absorption information and a multiplicative algebraic reconstruction technique, Can. J. Phys., 85(11), 1225–43. .

Schwartz, M. J., Lambert, A., Manney, G.L., Read, W.G., Livesey, N.J., Froidevaux, L., Ao, C.O., Bernath, P.F., Boone, C.D., Cofield, R.E., Daffer, W.H., Drouin, B.J., Fetzer, E.J., Fuller, R.A., Jarnot, R.F., Jiang, J.H., Jiang, Y.B., Knosp, B.W., Krüger, K.R.,. F. Li, J.-L Mlynczak, M.G., Pawson, S., Russell III, J.M., Santee, M.L., Snyder, W.V., Stek, P.C., Thurstans, R.P., Tompkins, A.M., Wagner, P.A., Walker, K.A., Waters, J.W., and Wu, D.L. 2008. Validation of the Aura Microwave Limb Sounder temperature and geopotential height measurements, J. Geophys. Res., 113, D15S11, doi:10.1029/2007JD008783.

Sigmond, M., Reader, M.C., Fyfe, J.C., and Gillett, N.P. 2011. Drivers of past and future Southern Ocean change: Stratospheric ozone versus greenhouse gas impacts, Geophys. Res. Lett., 38, L12601, doi:10.1029/2011GL047120.

Son, S.-W., et al. 2010. Impact of stratospheric ozone on Southern Hemisphere circulation change: A multimodel assessment, J. Geophys. Res., 115, D00M07, doi:10.1029/2010JD014271.

Stolarski, R.S., Krueger, A.J., Schoeberl, M.R., McPeters, R.D., Newman, P.A. and Alpert, J.C. 1986. Nimbus 7 satellite measurements of the springtime Antarctic ozone decrease, Nature, 322, 808–11.

Swinbank, R., and O’Neill, A.A.1994. Stratosphere-troposphere data assimilation system. Mon. Weather Rev., 122, 686-702.

Tully, M.B., Klekociuk, A.R., Deschamps, L.L., Henderson, S.I., Krummel, P.B., Fraser, P.J., Shankin, J.D., Downey, A.H., Gies, H.P. and Javorniczky, J. 2008. The 2007 Antarctic Ozone Hole. Aust. Met. Mag., 57, 279–98.

Tully, M.B., Klekociuk, A.R., Alexander, S.P., Dargaville, R.J., Deschamps, L.L., Fraser, P.J., Gies, H.P., Henderson, S.I., Javorniczky, J., Krummel, P.B., Petelina, S.V., Shanklin, J.D., Siddaway, J.M. and Stone, K.A. 2011. The Antarctic Ozone Hole during 2008 and 2009. Aust. Meteorol. Oceanogr. J., 61, 77-90.

Velders, G. J. M. and Daniel, J. S. 2014. Uncertainty analysis of projections of ozone-depleting substances: mixing ratios, EESC, ODPs, and GWPs, Atmos. Chem. Phys., 14, 2757-2776, doi:10.5194/acp-14-2757-2014.

WHO (World Health Organization) 2002. Global Solar UV Index: A Practical Guide, ISBN 92 4 159007 6, Geneva.

Froidevaux, L., Jiang, Y. B., Lambert, A., Livesey, N. J., Read, W. G., Waters, J. W., Browell, E. V., Hair, J. W., Avery, M. A., McGee, T. J., Twigg, L. W., Sumnicht, G. K., Jucks, K. W., Margitan, J. J., Sen, B., Stachnik, R. A., Toon,G. C., Bernath, P. F., Boone, C. D., Walker, K. A., Filipiak, M. J., Harwood, R. S., Fuller, R. A., Manney, G. L., Schwartz, M. J., Daffer, W. H., Drouin, B. J., Cofield, R. E., Cuddy, D. T., Jarnot, R. F., Knosp, B. W., Perun, V. S., Snyder, W. V., Stek, P. C., Thurstans, R. P., and Wagner, P. A. 2008. Validation of Aura Microwave Limb Sounder stratospheric ozone measurements, J. Geophys. Res., 113, D15S20, doi:10.1029/2007JD008771.

Hofmann, D., and Montzka, S. Recovery of the ozone layer: the Ozone Depleting Gas Index, Eos, 90: 1, 1-2, 2009.

Hu, Y., Xia, Y., and Fu, Q. 2011. Tropospheric temperature response to stratospheric ozone 491 recovery in the 21st century, Atmos. Chem. Phys., 11, 7687-7699, doi:10.5194/acp-11-7687-492 2011.

Kanamitsu, M., Ebisuzaki, W., Woollen, J., Yang, S.-K., Hnilo, J.J., Fiorino, M. and Potter, G.L. 2002. NCEP-DEO AMIP-II Reanalysis (R-2). Bull. Am. Meteorol. Soc., 83(11), 1631–1643.

Kang, S., Polvani, L.M., Fyfe, J.C. and Sigmond, M. 2011. Impact of polar ozone depletion on subtropical precipitation, Science, 332, 951-954.

Karpechko, A.Y., Gillett, N.P., Gray, L.J. and Dall’Amico, M. 2010, In-fluence of ozone recovery and greenhouse gas increases on Southern Hemisphere circulation, J. Geophys. Res., 115, D22117, doi:10.1029/2010JD014423.

Klekociuk, A.R., Tully, M.B., Alexander, S.P., Dargaville, R.J., Deschamps, L.L., Fraser, P.J., Gies, H.P., Henderson, S.I., Javorniczky, J., Krum-mel, P.B., Petelina, S.V., Shanklin, J.D., Siddaway, J.M. and Stone, K.A. 2011. The Antarctic Ozone Hole during 2010. Aust. Meteorol. Ocean-ogr. J., 61, 253-267.

Llewellyn, E.J., Lloyd, N.D., Degenstein, D.A., Gattinger, R.L., Petelina, S.V., Bourassa, A.E., Wiensz, J.T., Ivanov, E.V., McDade, I.C., Solheim, B.H., McConnell, J.C., Haley, C.S., von Savigny, C., Sioris, C.E., McLinden, C.A., Griffioen, E., Kaminski, J., Evans, W.F.J., Puckrin, E., Strong, K., Wehrle, V., Hum, R.H., Kendall, D.J.W., Matsushita, J., Murtagh, D.P., Brohede, S., Stegman, J., Witt, G., Barnes, G., Payne, W.F., Picha, L., Smith, K., Warshaw, G., Deslauniers, D –L., Marchand, P., Richardson, E.H., King, R.A., Wevers, I., McCreath, W., Kyrola, E., Oikarinen, L., Leppelmeier, G.W., Auvinen, H., Mégie, G., Hauchecorne, A., Lefèvre, F., de La Noë, J., Ricaud, P., Frisk, U., Sjoberg, F., von Schëele, F. and Nordh, L. 2004. The OSIRIS Instrument on the Odin spacecraft. Can. J. Phys., 82, 411–422.

Marshall, G. J. 2003. Trends in the Southern Annular Mode from observations and reanalyses. J. Clim., 16, 4134-4143.

Nash, E.R., Newman, P.A., Rosenfield, J.E. and Schoeberl, M.R. 1996. An objective determination of the polar vortex using Ertel’s potential vor-ticity, J. Geophys. Res., 101 (D5), 9471–9478.

Newman, P.A., Daniel, J.S., Waugh, D.W. and Nash, E.R. 2007. A new formulation of equivalent effective stratospheric chlorine (EESC), Atmos. Chem. Phys., 7, 4537-4552.

Petelina, S.V., Llewellyn, E.J., Degenstein, D.A., Lloyd, N.D., Gattinger, R.L., Haley, C.S., von Savigny, C., Griffioen, E., McDade, I.C., Evans, W.F.J., Murtagh, D.P. and de La Noeet, J. 2004. Comparison of the Odin/OSIRIS stratospheric ozone profiles with coincident POAM III and ozonesonde measurements, Geophys. Res. Lett., 31, L07105, doi:1029/2003GL019299.

Perlwitz, J., Pawson, S., Fogt, R.L., Nielsen, J.E. and. Neff W.D. 2008. Impact of stratospheric ozone hole recovery on Antarctic climate, Geophys. Res. Lett., 35, L08714, doi:10.1029/2008GL033317.

Polvani, L.M., Previdi, M. and Deser, C. 2011a. Large cancellation, due to ozone recovery, of future Southern Hemisphere atmospheric circulation trends, Geophys. Res. Lett., 38, LO4707, doi:10.1029/2011GL046712.

Polvani, L.M., Waugh, D.W., Correa G.J.P. and Son S.-W. 2011b: Stratospheric ozone depletion: the main driver of 20th Century atmospheric circulation changes in the Southern Hemisphere,

J. Clim., 24, 795–812.Rienecker, M.M., Suarez, M.J., Gelaro, R., Todling, R., Bacmeister, J., Liu,

E., Bosilovich, M.G., Schubert, S.D., Takacs, L., Kim, G.-K., Bloom, S., Chen, J., Collins, D., Conaty, A., da Silva, A.,Gu, W., Joiner, J., Koster, R.D., Lucchesi, R., Molod, A., Owens, T., Pawson, S., Region, P., Redder, C.R., Reichle, R., Robertson, F.R., Ruddick, A.G., Sienkiewicz, M., and Wollen, J. 2011. MERRA - NASA’s Modern-Era Retrospective Analysis for Research and Applications, J. Clim., 24, 3624-3648, doi:

the antarctic ozone hole during 2011 - bureau of meteorology · ozone value within the ozone hole...

Documents