the antarctic ozone hole during 2012 - bureau of meteorology · ozone hole and the 2012 ozone hole...

Australian Meteorological and Oceanographic Journal 64 (2014) 313–330

313

The Antarctic ozone hole during 2012

A.R. Klekociuk1, M.B. Tully2, P.B. Krummel3, H.P. Gies4, S.P. Alexander1, P.J. Fraser3, S.I. Henderson4, J. Javorniczky4, S.V. Petelina5,

J.D. Shanklin6, R. Schofield7,8 and K.A. Stone7,8

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

6British Antarctic Survey, United Kingdom7School of Earth Sciences, University of Melbourne, Australia

8ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney, Australia

(Manuscript received March 2014; revised March 2015)

We review the 2012 Antarctic ozone hole, making use of various meteorological reanalyses, remotely sensed ozone measurements and ground-based measure-ments of ultra-violet radiation. Based on analysis of 33 years of satellite records, we find that the ozone hole of 2012 was one of the least severe since the late 1980s in terms of maximum area, minimum ozone level and total ozone deficit. In particular, the estimated integrated ozone mass effectively depleted within the ozone hole of 2012 was approximately 720 Mt, which is the 12th smallest deficit on record and 28 per cent of the peak deficit observed in 2006. The key factor in limiting the extent of Antarctic ozone loss in 2012 was the relatively warm tem-peratures that occurred in the Antarctic stratosphere from early July. These warm temperatures, which were driven by dynamical activity, limited the activation of ozone depletion chemistry within the polar vortex during the latter part of the polar winter. Additionally, dynamical disturbances to the polar cap region during spring were aided by the prevailing phase of the Quasi-Biennial Oscillation (QBO) which was strongly negative (westward) and favouring the poleward propagation of heat flux anomalies; these disturbances resulted in the steady erosion of the vortex and caused it to breakdown relatively early compared to recent years. The metrics for the Antarctic ozone hole of 2012 showed some similarity with those of 1988 and 2002 (which were years of anomalously small ozone holes) despite all three years having distinctly different QBO indices indicating variant strengths of the polar vortex (and severity of ozone loss).

Introduction

Over the past few decades, the most significant changes in southern hemisphere climate have included [Canziani et al., 2014]: (1) a southward shift and intensification of the tropospheric eastward jet over the Southern Ocean

region—resulting in a tendency for the extratropical storm tracks to move poleward; (2) anomalously dry conditions have occurred over southern Australia, New Zealand and southern South America, and anomalously wet conditions over northwestern Australia and South Africa; (3) a warming of the Southern Ocean and its waters have become less saline; (4) a warming of the Antarctic Peninsula and its oceanic margins; (5) an expansion the tropical zone and a shift of Corresponding author address: Andrew Klekociuk, Australian Antarctic

Division, Hobart. Email [email protected]

314 Australian Meteorological and Oceanographic Journal 64:4 December 2014

atmospheric mass from mid to higher latitudes (Hendon et al., 2007; Calvo et al., 2012; Gonzalez et al., 2013; Turner et al., 2013); and (6) an increase in Antarctic average sea-ice extent. These changes are largely consistent with industrial era effects expected from increasing levels of anthropogenic greenhouse gases and the more recent effects caused by ozone depletion, and are largely though not exclusively related to observed changes in the meridional location of the mid-latitude tropospheric jet during recent decades (Lin et al., 2009; Fu et al., 2010). The observed shift in the southern hemisphere mid-latitude tropospheric jet location during summer since the latter part of the 20th century has primarily been attributed to effects from the Antarctic ozone hole (Previdi and Polvani, 2013). Changes in the location of this jet are implicated in temperature trends at high southern latitudes (Turner et al., 2013) and cloud-radiative anomalies over the Southern Ocean (Grise et al., 2013). In modelling studies, hemispheric and regional changes in precipitation patterns also appear to result from changes in the jet location (Purich and Son, 2012; Kang et al., 2013; Gonzales et al., 2013). While ozone depletion appears to have contributed to aspects of recent trends in southern hemisphere climate during summer, it is becoming apparent that the trends are likely to be ameliorated over the coming few decades while dynamical changes brought about by increasing greenhouse gas levels and ozone recovery will largely oppose each other (Arblaster et al., 2011).

Of importance in evaluating overall trends and variability in ozone-induced climate change is evaluation of the drivers and effects of the intra- and inter-annual variability of ozone depletion. In this paper, we focus on the description of meteorological conditions and their relation to the overall level of Antarctic ozone depletion in 2012 using a range of Australian data and analyses including Bureau of Meteorology (the Bureau) meteorological analyses, 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 is also presented. This work complements analyses of previous Antarctic ozone holes reported by Tully et al. (2008, 2011) and Klekociuk et al. (2011, 2014), and other analyses of Antarctic atmospheric conditions and ozone depletion during 2012 provided by the World Meteorological Organisation (WMO) Antarctic Ozone Bulletins (www.wmo.int/pages/prog/arep/gaw/ozone/index.html), upper-air summaries of the National Climate Data Center (NCDC; www.ncdc.noaa.gov/sotc/upper-air) and by Blunden and Arndt (2013; www.ncdc.noaa.gov/bams-state-of-the-climate).

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 ranking for all 33 ozone holes recorded since 1979 using eight metrics that measure the ‘size’ of the Antarctic ozone hole. The first seven metrics in Table 1 measure various aspects of the area and depth of the ozone hole and the 2012 ozone hole was ranked between 22nd and 23rd in terms of severity across these metrics. Figure 1(a) shows the daily ozone hole area throughout the 2012 season. The annual maximum value of the daily area (metric 2) is often used to describe the overall size of ozone hole; in 2012 this metric was the 22nd largest on record, attaining 21.2 million (M) km2 in late September (compared for example with surface areas of Antarctica and Australia of 14 M km2 and 7.7 M km2, respectively), and was the smallest value observed over since 1990. As shown in Fig. 1(b), the progression of the daily minimum total column ozone amount during 2012 was generally above the 1979–2011 mean, particularly from early October. The overall minimum total column amount (metric four) of 124 DU attained in late September was ranked the 22nd lowest annual minimum observed. 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) of 22.5 million tonnes (Mt) was attained in early October, which was the 23rd largest annual maximum observed. A metric which gauges the overall severity of the ozone hole is the total annual deficit (metric seven); at 720 Mt, this was the 22nd largest deficit on record and only 28 per cent of the peak value observed in 2006.

In terms of the overall development of the ozone metrics shown in of Fig. 1, the behaviour for 2012 was clearly different to most of the other years shown, with the exception of 2002. In 2002, temperatures during winter and spring in the Antarctic stratosphere were well above average, and the circulation pattern in the region was unusually disturbed by anomalous planetary wave activity (Newman and Nash, 2005). The variability in the metrics for 2002 (light green curves of Fig. 1) from late September to mid October was due to the unprecedented sudden stratospheric warming (SSW) that occurred on September 22 of that year. At the time of the SSW, the extent of ozone loss was suddenly reduced as part

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before late September and after mid-October was similar to the situation in 2002 and 2004 (not shown).

Figure 2 shows the estimated ‘recovery’ date of the ozone hole as a function of year; the recovery date is defined as the day of year when the daily area metric (metric two) falls

of the polar vortex detached and dispersed equatorward. Subsequently, the vortex somewhat regained its strength and stability, allowing a region of depleted ozone to persist and dissipate in a more normal manner. From inspection of Fig. 1 it can be seen that the behaviour for 2012 in the period

Fig 1. Estimated daily (a) ozone hole area, (b) ozone hole depth and (c) ozone mass deficit based on OMI satel-lite data. The 2002 data in the above figures are from the TOMS satellite instrument.

Fig 2. Estimate date (day of year) of ozone hole recovery (metric 8) as a function of year.

Metric definitions:

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.

8. Recovery date: the final date at which the daily maximum area (metric 2) falls below 0.5 Mkm2.

Note that the metrics use 220 DU as the threshold in total column ozone to define the location and occurrence of the ozone hole.

Klekociuk et al: The Antarctic ozone hole during 2012 317

Fig 3. Annual values of (a) minimum 15-day average southern hemisphere daily total column ozone, (b) maximum 15-day average Antarctic ozone hole area, and (c) estimated total ozone deficit with the ozone hole (interpolation for missing daily data). Observations are shown as purple dots. The vertical bars in (a) and (b) show the range of values in the 15-day window. The green solid line shows regression to Equivalent Effective Stratospheric Chlorine (EESC) using a mean age of air of 5.5 years; the dashed green lines show the estimated 95 per cent confidence limit of the regression.

below 0.5 M km2. Figure 2 further highlights the unusual nature of 2012, showing that the recovery date was the earliest for most years since the late 1980s. The two other outlying years evident in Fig. 2 are 2002 and 1988.

The panels of Fig. 3(a–c) show time series of metrics three, one and seven respectively, together with regressions against Equivalent Effective Stratospheric Chlorine (EESC, Fraser et al., 2014) evaluated for a mean age of air of 5.5 years. The method used to calculate each regression curve is described in Klekociuk et al. (2014); the EESC value used is evaluated for the middle of each calendar year. Figure 4, which shows the time series of October monthly average total ozone column values from Halley, Antarctica, is updated from Fig. 3 of Klekociuk et al. (2015) for 2012. The regression curves in Figs 3 and 4 all show a reversal in trend after approximately 2000. However, as can be gauged by the relatively large confidence intervals shown in Fig. 3(a–c), only low statistical confidence can be ascribed to the reality of the trend reversals. For example, in the case of the ozone hole area time series shown in Fig. 3(b), a two-tailed t test indicates that the regression value for 2012 is not statistically different to regression values after approximately 1985.

Figure 5 shows the distribution of September to November (SON) average total column ozone from TOMS and OMI measurements. The three-month averaging period highlights features that are relatively persistent relative to synoptic time scales. In Fig. 5(a), the mean position and size of the ozone hole (as shown by the white contour) and the phase of the wave one pattern as a function of longitude (as shown by the red curve) in the panel for 2012 appear similar to the multi-year climatological mean shown in the panel at bottom right. The peak strength of the high ozone ridge between Australia and the Antarctic coast was generally more intense than most years since the 1990s, with the exception of 1991, 1996 and 2005. The anomaly of the mean SON ozone distribution compared with the climatological mean for years since 2004 is shown in Fig. 5(b), in which the zonal asymmetries in the ozone distribution for 2012 are somewhat similar to the situation in 2005 but which are generally distinctly different to the other years shown.

Fig 4. October monthly mean total column ozone values for Halley station for 1957–2012 (green points and line) and regression to EESC (orange line) using a mean age of air of 5.5 years.


Fig. 5. Analysis of total column ozone data for September to November (SON) from the TOMS Earth-Probe and Nimbus-7 in-struments (1979–2004) and the OMI instrument (2005–2012). Monthly mean data processed with the version 8.5 TOMS algorithm have been used. (a) Mean annual SON values for 1979–2012 (data are not available for 1992–1994) shown for individual years and as the climatological mean (lower right panel). The longitude of maximum of a fitted wave one sinu-soid on each latitude circle poleward from 45°S is shown by red line. The lower most meridian is the Greenwich Meridian. Latitude circles are plotted at 15° intervals from 90°S, with the 60°S latitude circle highlighted in solid black. The 220 DU contour is highlighted by a thin white line. (b) SON annual anomalies (observed minus climatology 1979–2012 (for 2004 to 2012). Note that TOMS and OMI are restricted to viewing the sunlit portion of the earth, and as a result, measurements are not possible near the pole for a period in September. This produces a small artefact poleward of approximately 80°S that can be most clearly seen in the panels of (b).


Vertically resolved ozone measurements

MLS partial columnsIn Fig. 6(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 in 2011 and 2012. 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. Note that the ‘mid’ and ‘low’ regions essentially cover the height range where the distribution of polar ozone is primarily influenced by chemical cycles promoted by heterogeneous reactions as well as large-scale transport.

The ‘full’ partial column area metrics for 2011 and 2012 presented in Fig. 6(a) show similar values and variability to the corresponding OMI data presented in Fig. 1(a). For the ‘low’ and ‘mid’ partial column metrics, the values shown in Fig. 6(a) for 2012 were appreciably smaller than for corresponding dates in 2011. The growth of these metrics during the development of the ozone hole in 2012 occurred approximately 1–2 weeks later compared with 2011, and the final dates of recovery for 2012 were approximately three to four weeks earlier than in 2011.

Figures 6(b) and 6(c) show time series of the total ozone mass in the ‘mid’ and ‘low’ partial columns, respectively, over the polar cap poleward of 40°S obtained from MLS measurements for 2004–2012. The latitudinal range of the polar cap region considered here includes both the ozone hole and the majority of circumpolar ozone ridge. We describe the seasonal variations in Fig. 6(b) and 6(c) as follows. The ‘mid’ partial column time series shown in Fig. 6(b) generally shows a downward progression from the beginning of the year until April, when the upper part of the polar vortex forms. From April until early July, the ozone mass within the polar cap normally remains relatively stable. During this period, the vortex steadily grows in volume, although there is relatively little change in the ozone mass within the vortex. Outside the vortex, there is a strengthening of the meridional gradient in ozone, but the influence of the growth in the vortex does not significantly change the ozone mass across the polar cap. During August, the photolytic destruction of ozone begins as the vortex starts to receive solar illumination. At the same time, poleward transport outside the vortex strengthens the circumpolar ridge, and by early September, the ozone in the ridge starts to become the dominant contributor to the net mass within the polar cap region. The action of transport peaks around November, at which time the breakdown in the vortex and the subsequent reversal in the zonal stratospheric winds at the beginning of summer produce a gradual decline in ozone mass across the polar cap. Figure 6(b) shows that the behaviour of the ‘mid’ partial column polar cap ozone mass in 2012 up to July was generally similar to the other

Fig. 6. Analysis of Aura MLS version 3.3 ozone data (Froidevaux et al., 2008). All daily swath data pass-ing the recommended MLS quality checks have inter-polated to a grid spacing of 1.5° in longitude and 1° in latitude. (a) Ozone hole area metrics for 2011 and 2012 for partial columns: 146–68 hPa (red, ‘low’ col-umn), 46–10 hPa (blue, ‘mid’ column) and 464–0.1 hPa (black, ‘full’ column), for which the ozone hole thresh-olds 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) Time series of the total ozone mass in the ‘mid’ partial column south of 40°S from 2004 to 2012, with 2012 highlighted in black. (c) as for (b), but for the ‘low’ partial column south of 40°S.


years shown. During July and August there is divergence in the behaviour over the years shown. From examination of daily partial column maps (not shown), this appears related to inter-annual variability in the date at which significant ozone destruction begins within the vortex and the average concentration of ozone within the vortex at this time, and the strength and latitudinal extent of the circumpolar ridge. In 2006 and 2008, ozone destruction began relatively early and the circumpolar ridge was weak. In 2009, ozone destruction also began relatively early, and while the ridge was stronger than in 2006 and 2008, its poleward edge was generally situated outside of the equatorward edge of the integration region (40°S) until mid-August. For 2012, the strength of the circumpolar ridge and its poleward extent combined with relatively modest ozone destruction within the vortex were the main factors that resulted in the daily mass values shown in Fig. 5(a) being near or exceeding the climatological range from July onwards.

The polar cap ozone mass for the ‘low’ region (Fig. 6(c)) has a somewhat different seasonal progression to that of the ‘mid’ region, which we attribute as follows. In the ‘low’ region, the ozone mass in January and February is relatively stable, and then steadily increases up to the end of July as ozone accumulates within the dynamical barriers imposed by the tropopause and the polar vortex under the influence of both downward and poleward transport associated with the descending branch of the Brewer-Dobson circulation. Ozone destruction outweighs the influence of inflow from transport from early August through to early October, after which, the breakdown of the vortex allows the polar cap to be replenished. The behaviour of 2012 in Fig. 6(c) was similar to most other years at least up to early August, and then attained upper bound values for most of the remainder of the year. Overall, the partial column time series presented in Fig. 6(a–c) further highlight that relatively moderate ozone loss occurred over the polar cap in 2012.

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 a polar-orbiting satellite, launched in 2001, with typical latitude coverage (in the orbit plane) of ~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, the southern hemisphere data are available from late August until early March each year. The detailed instrument description and validation of OSIRIS stratospheric ozone retrievals are given in (Llewellyn et al., 2004; Petelina et al., 2004; Roth et al., 2007).

Time-height sections of daily zonal mean OSIRIS ozone number density between September and February in the height range between 14 km and 36.5 km are shown in the panels of Fig. 7. Figures 7(a) and 7(b) show time-height section

Fig. 7. OSIRIS mean daily zonal mean stratospheric ozone profiles for (a) latitudes 60°S–83°S for September to February 2002–2012, (b) 60°S–83°S for September 2012 to February 2013 and (c) 60°S–70°S for September 2012 to February 2013. White gaps indicate time periods when no measurements were performed. Vertical banding in panel (a) is an artefact due to missing data in some years.


averages for the latitude range 60°S to 83°S; the climatological daily mean for 2002–2012 is shown Fig. 7(a), and daily values for 2012–2013 are shown in Fig. 7(b). The climatological mean (Fig. 7(a)) shows the influence of ozone depletion and effects associated with the breakdown of the polar vortex, which are manifest as decreasing ozone densities through September and October that are followed by recovery which persists into January at the lowest heights. In comparison, ozone densities during the latter part of 2012 (Fig. 7(b)) were noticeably higher than the climatological mean, particularly below heights of approximately 30 km from October 2012 to January 2013.

In the outer region of the polar cap, the effects of ozone depletion are not clearly evident, particularly in comparison with the same seasonal interval for 2011–12 shown in Fig. 5(c) of Klekociuk et al (2014).

Overall, the OSIRIS measurements provide additional evidence of the relatively high ozone levels that occurred over the polar cap in the latter part of 2012.

Ozonesonde measurements at DavisAustralian measurements of Antarctic ozone have been made at Davis station (68.6°S, 78.0°E) since 2003 under a collaborative program between AAD and the Bureau. Throughout 2012, ozone sondes were launched at weekly intervals. A summary of the Davis ozone sonde data is presented in Fig. 8. In Fig. 8(a), the time-height evolution of ozone partial pressure is shown. The ozone partial pressures in the height range 15–25 km were generally higher in 2012 than in 2011 (comparing Fig. 8(a) with Fig 6(a) in Klekociuk et al., 2015), and this is consistent with the expectation based on the different phases of the QBO in the respective periods of the two years (polar ozone shows a zero-lag anti-correlation with QBO phase, and vice versa, as shown in Tully et al., 2013). The relatively high ozone concentration in 2012 can also be seen by examining Fig. 8(b), where the partial column in the height range 12–20 km for 2012 (red squares) are predominantly above values for most other years, particularly during spring and early summer (after approximately mid-September or day 260). The time and altitude ranges of low ozone concentrations associated with the ozone hole were relatively restricted in 2012 compared with other years. The 12–20 km partial column value was at its lowest level from mid- to late October. At this time, while the ozone partial pressure at 15 km was at its lowest level for the year, there was a significant overburden of ozone above approximately 22 km, resulting in estimated total column values of 279 DU on 18 October, 319 DU on October 26 and 287 DU on October 31. Figure 8(b) shows large positive excursions during the spring of 2012 on days 273 (29 September) and 287 (13 October). The 12–20 km partial column values on these dates were the largest values observed for the corresponding time of year since measurements began in 2003, and were associated with the strong distortion of the polar vortex by mid-latitude planetary wave activity noted above. A map of the ozone content above the mid-troposphere is shown in Fig. 9 for 13 October, showing that Davis was exposed to the

mid-latitude ozone ridge. Also evident in this figure is that the polar vortex was strongly displaced off the pole on this day, with a distinct break in the circumpolar ridge from the most equatorward region of the ozone hole northwestward towards and across South America.

Antarctic ultraviolet radiationMeasurements of biologically effective solar ultraviolet radiation (UVR) have been made at the Australian Antarctic stations by ARPANSA since 1996 at Casey (66.3°S, 110.5°E) and Davis (68.6°S, 78.0°E) and since 2002 at Mawson (67.6°S,

Fig. 8. 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 in the 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 2012 highlighted in red. The grey line is a climatological mean from Fortuin and Kelder (1998) interpolated to the location of Davis.


62.9°E). Broadband ultraviolet radiation (UVR) detectors (UVBiometer Model 501, Solar Light Co., Philadelphia USA) are deployed, installed and maintained at the stations with AAD assistance. The data are transmitted back to ARPANSA for analysis. The UVR measurements provide data to compare to model-generated UVR which incorporate the effects of cloud cover, ozone levels and other environmental factors. A detailed description of the detectors and calibration methods used by ARPANSA in Antarctica have been published previously (Tully et al., 2008).

The UV Index (WHO, 2002) is a measure of the intensity of solar UVR at earth’s surface taking into account its biological effects on human skin. 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 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 10(a), 10(b) and 10(c) show daily peak values of the measured UV Index for Casey, Davis and Mawson, respectively, over the 2012 season. Also shown in these figures is the daily total column ozone value derived from OMI satellite observations. The annual variation in measured UV Index exhibits a regular seasonal dependence due to variations in the minimum solar zenith angle (apparent height of the sun in the sky at solar noon). Local weather conditions (cloud cover) and varying stratospheric ozone levels also exert a major influence on the measured solar UVR. For clear-sky days there is a strong anti-correlation between the measured UV Index and total column ozone, where low ozone results in higher measured UV index. UV Index levels recorded at the Australian stations on the edge of the Antarctic continent occasionally reach extreme levels when clear skies coincide

Fig 9. Ozone partial column map for 13 October 2012 de-rived from Aura MLS version 3.3 data for pressure levels between 464 hPa and 0.01 hPa.

Fig. 10. Total column ozone derived from OMI 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).


Table 2. Number of ozone hole days (defined as less than 220 DU) by month for 2007–2012 over each of the Australian Antarctic stations.

MonthCasey Davis Mawson

2007 2008 2009 2010 2011 2012 2007 2008 2009 2010 2011 2012 2007 2008 2009 2010 2011 2012

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

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

Sep 3 11 9 10 12 0 15 21 20 20 23 3 17 21 21 18 27 4

Oct 0 7 9 7 3 0 20 17 26 22 9 1 23 20 29 23 8 4

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

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

All 9 27 30 22 20 0 53 49 54 62 40 5 59 59 59 58 47 10


2007 2008 2009 2010 2011 2012 2007 2008 2009 2010 2011 2012 2007 2008 2009 2010 2011 2012

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 0.0 0.0 0.1 0.0 0.0 0.0

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

Sep 1.0 2.3 1.6 1.9 1.6 1.0 1.6 2.0 1.5 1.5 1.6 1.3 1.5 2.3 1.8 1.9 2.1 1.4

Oct 3.2 4.1 4.4 4.6 4.1 2.6 4.5 4.4 6.0 5.3 4.0 2.8 5.2 5.8 7.1 6.0 4.3 3.8

Nov 6.3 5.9 5.9 7.2 5.0 4.4 7.2 7.0 4.9 7.7 5.2 4.6 6.3 7.6 5.2 8.5 6.3 4.8

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

All 2.7 3.3 3.1 3.5 3.0 2.5 3.4 3.4 3.2 3.6 3.2 2.7 2.6 4.1 3.6 4.1 3.6 2.9


2007 2008 2009 2010 2011 2012 2007 2008 2009 2010 2011 2012 2007 2008 2009 2010 2011 2012

Low 91 87 101 87 93 110 90 84 93 90 96 96 88 76 87 82 85 94

Moderate 21 46 36 43 44 38 38 41 41 34 39 41 33 33 34 27 36 50

High 19 28 29 22 27 17 24 35 34 28 19 28 21 31 30 31 27 28

Very high 13 21 17 26 13 15 23 23 16 27 23 5 9 38 30 30 27 10

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

All 144 183 184 184 177 180 179 184 184 184 182 170 151 181 182 184 178 182

Table 3. Monthly mean UV Index for 2007–2012 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 2007–2012 (July to December) measured at each of the Australian Antarctic stations.

with the presence of the ozone hole during the Antarctic spring and on occasions during summer. Figure 10(a) shows a strong anti-correlation in late October for Casey, where the ozone levels dropped to 225 DU and the measured UV Index reached eight, with similar occurrences shown in Fig. 10(b) for Davis and to a lesser extent at Mawson. Defining total column ozone values reported by the OMI satellite of less than 220 DU as an ozone hole event, Table 2 shows the number of days for which this occurred for each of the Australian stations in Antarctica over the past six years. No ozone hole events were recorded for Casey station and far fewer than previous seasons were observed at both Davis and Mawson.

The monthly mean of the (daily peak) UV Index recorded at each of the Australian stations in Antarctica over the past

six years is listed in Table 3. The October and November monthly mean UV Index was the lowest observed over the past six years for all stations.

Table 4 shows for each of the past six 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. No extreme days were recorded this season and there was a sharp reduction in the number of very high days at both Davis and Mawson. The fact that the 2012 ozone hole was one of the smallest on record according to most of the metrics (maximum hole area, ozone hole minima, estimated ozone deficit) means that it has resulted in the lowest measured solar UVR at the Australian Antarctic stations in comparison to recent years.


was generally positive up to October, and was close to the maximum value in the period since 1979 in July as discussed further below.

As shown in Fig. 11(b), the 50 hPa SAM and QBO were both negative in March and April, and again from August to December. Based on the discussion above, this situation would be expected to favour a weak and disturbed vortex, and this was potentially a factor responsible for the tendency towards positive stratospheric temperature anomalies from July onwards.

The panels of Fig. 11 also show monthly values for 1988 and 2002 as black crosses and open grey circles, respectively. Both of these years were unusual in having anomalous planetary wave activity during spring (Grytsai et al., 2008)—in particular, a unique major sudden stratospheric warming occurred over Antarctica in September 2002. In Fig. 11(a), the progression of the temperature anomalies during 2012 show some similarities with 1988 and 2002, transitioning from negative to positive for the 10 hPa and 50 hPa levels around July. In contrast, at 100 hPa this transition also took place around July for 1988 and 2002, but in October for 2012. The magnitude of the anomalies in spring and early summer show some similarities for the three years; at 10 hPa the warm anomalies gave way to cold anomalies in November, while the peak warm anomalies at 50 hPa and 100 hPa occurred in October and November, respectively. A notable difference between 2012 and the other two years shown relates to the phase of the QBO. As can be seen in the top panel of Fig. 11(b), the QBO throughout 2012 was in a negative (westward), which favours a weak polar vortex. However, the situation in 2002 was opposite; the QBO was in a positive or eastward phase, which would normally be expected to favour a strong vortex, although contrastingly, the vortex was considerably disturbed in spring (Scaife et al., 2005). The QBO phase in 1988 was weakly positive, which again would be expected to generally favour less significant disturbance of the polar vortex.

In terms of the behaviour of the surface SAM index (middle panel of 11(b)), this was strongly negative in September and October of both 1988 and 2002, and in contrast, weakly positive in the same months of 2012. In the stratosphere, the SAM index at 50 hPa was negative for all three years from August. As can be seen in Fig. 11(b), several of the extreme negative values of the stratospheric SAM index were set in 2002 (May and September–November), although values in November and December of 2012 were close to the lowest extremes for 1979–2012 (October and November 2012 were each ranked 3rd lowest behind 1988 and then 2002, and December 2012 was ranked 2nd lowest behind 1991). Overall, the behaviour of the stratospheric SAM index in the latter part of 2012 shows similarities with the unusual years of 1988 and 2002, with anomalously negative pressure gradients between mid- and high latitudes.

A further representation of NCEP Reanalysis-2 temperatures is shown in Fig. 12, which shows the pressure-time structure of the climatological zonal mean anomaly for the latitude band 55°S–75°S. Up to early July,

Discussion

Polar temperatures and atmospheric indicesMonthly mean temperatures in the lower stratosphere above Antarctica during 2012 were generally close to or slightly below normal from January to July, but showed marked warming from mid-spring to early summer, particularly at and below the 50 hPa level. Figure 11(a) shows monthly temperature anomalies for the latitude range 90°S to 65°S from the National Centers for Environmental Prediction (NCEP) Reanalysis-2 data (Kanamitsu et al., 2002) with respect to the base period 1979–2011 for three pressure levels. Temperatures at 10 hPa were consistently below the climatological mean from January to July, with the May, November and December averages being the lowest of the preceding decade. At the 50 hPa and 100 hPa levels, the warmest temperatures of the preceding decade were reached in both October and November, while December at 100 hPa was similarly anomalously warm.

During 2012, the NCEP standardised 30 hPa Quasi-Biennial Oscillation (QBO) index (www.cpc.ncep.noaa.gov/data/indices/qbo.u30.index) was consistently negative (westward) (Fig. 1(b), top panel). The QBO modulates the ability of upward propagating planetary waves to influence extratropical latitudes in the winter hemisphere. A weaker and more disturbed polar vortex is preferentially observed during the negative (westward) QBO index or phase (Baldwin and Dunkerton, 1998; Watson and Gray, 2014).

The surface standardised Southern Annular Mode (SAM) index (Marshall, 2003 and www.antarctica.ac.uk/met/gjma/sam.html) (Fig. 11(b), middle panel) was mostly positive during the year (the exceptions being February, November and December when the SAM index was negative). 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 exists. Stratospheric and tropospheric SAM variations are sometimes independent, but as noted in Baldwin and Dunkerton (2001), SAM anomalies in the lowermost stratosphere tend to favour tropospheric SAM anomalies of the same sign. The bottom panel of Fig. 11(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 50 hPa SAM index was generally weak in 2012, except from October through December when it was strongly negative. In contrast, the surface SAM index


Fig. 11. (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 1979–2011 at pressure levels of 10 hPa (top), 50 hPa (middle) and 100 hPa (bottom). Coloured bars show monthly anomalies for 2012, and diamonds connected by solid lines show maximum and minimum anomalies for 1979–2012. Numbers at the top of each panel are the rank of 2012 relative to years 2003–2012 (1 [10] = most positive [most negative] anomaly), and numbers at the bottom of each panel are values (K) of the monthly anomalies for 2012. Values for 1988 and 2002, years in which the ozone hole exhibited anomalously disturbed behaviour in spring, are shown as black crosses and grey open circles, respectively. (b) (top) NCEP standardised 30 hPa Quasi-Biennial Oscillation (QBO) index, (middle) standardised surface Southern Annular Mode (SAM) index (Marshall, 2003), and (bottom) stan-dardised SAM index evaluated at 50 hPa (see text for details). The indices are expressed in standard deviations relative to base period of 1983–2012 (for QBO) and 1979–2000 (for SAM). Diamonds connected by solid lines show maximum and minimum anomalies for each index over the period 1979–2012. Values for 1988 and 2002 are shown as black crosses and grey open circles, respectively.

Fig. 12. Pressure-time cross section of the NCEP Reanalysis-2 zonal mean temperature anomaly in the lower stratosphere for the latitude range 55°S–75°S with respect to the 1979–2012 climatology. Red colours indicate warmer than climatological con-ditions, blue colours indicate cooler than climatological conditions.


In comparison with a similar time-height section for 2011 shown in Fig. 12(a) of Klekociuk et al. (2014), it can be seen that peak heat flux amplitudes in this region were larger by about a factor of two or more in 2012 compared with 2011. Indeed, the dynamical events in 2012 episodically penetrated into the polar cap region from July through October, as evident by comparing Figs. 14(a) and 14(b). An interesting characteristic shown in Fig. 14(b) is the apparent general decrease in height of the heat flux events (i.e. the lowest height of the -50 K m/s contour) between June and October, which suggests that a gradual weakening of the polar vortex took place over this time. Additionally, the onset of the rapid decrease in the ‘mid’ partial column that began at the beginning of October in Fig. 6(a) coincided with the last significant heat flux anomaly shown in Fig. 14(b).

The polar vortexThe structure of the southern hemisphere stratospheric polar vortex during 2012 is shown in Fig. 15 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° equivalent latitude from May (when the vortex formed) until mid-June, and then moved equatorward by approximately 10° through to early August. After this time, the centre of the vortex edge gradually retreated poleward to reach -70° equivalent latitude by the beginning of November, after which the rapid breakdown of the vortex took place. Davis station in Antarctica was generally located well inside the vortex edge for the majority of winter and spring of 2012 (as shown by the red contour in Fig. 15).

the anomalously cold temperatures noted in Fig 11(a) are apparent, particularly in the upper levels. A warming begins in the upper levels in July, and generally propagates to lower levels through the remainder of the year.

This warming pattern is also very evident in daily measurements by the Microwave Limb Sounder (MLS) on the Aura spacecraft averaged over the Antarctic region (Fig. 13). A marked feature of the lower panel of this figure is the band of anomalously warm temperatures that appears near the stratopause in early July and descended to reach the tropopause region (near 10 km altitude) by the end of the year (Fig. 12 essentially shows a similar situation for heights between 15 and 30 km). Significant cold anomalies are particularly apparent immediately below the stratopause from April through June (Fig. 13, lower panel), and above stratospheric warm anomalies after July. Note that the mesopause region (near 90 km altitude) exhibited anomalously warm episodes over most of the year, particularly from April.

Wave activityThe 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 14 shows the evolution of heat flux (measured by the product of the zonal anomalies in temperature and meridional wind speed) during 2012 using assimilated meteorological data from the United Kingdom Meteorological Office (UKMO). Outbreaks of poleward heat transport in the mid and upper stratosphere (pressure levels 10–0.2 hPa) occurred from July through to late October at mid-latitudes (Fig. 14(a)).

Fig. 13. (a) Daily time-height section of zonal average air temperature for latitudes 85°S to 65°S from Aura Microwave Limb Sounder (MLS) quality controlled version 3.3 data for 2012 (Schwartz et al., 2008). The solid black line marks the height of the warm-point stratopause in 2012, 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 2011. The white bar marks missing data. (b) Daily temperature anomaly obtained by subtracting the average daily temperature for 2004–2011 (the climatology) from the 2012 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 2012. Crossed diagonal hatching marks anomalies at the daily maximum or minimum value for all measurements up to and including those of 2012.


Comparing to 2011 (Fig. 13 of Klekociuk et al. (2014)), it can be seen that the vortex edges in 2012 were generally situated a few degrees further poleward. This resulted in Davis lying generally closer to the polar vortex edge region in 2012 relative to 2011. Excursions of the core of the vortex close to or poleward of Davis occurred during mid-August and mid-October (red contour situated close to or equatorward of the central edge of the vortex in Fig. 15).

Time series of proxies for the areal extent of the stratospheric polar vortex are shown in Fig. 16 for the 450 K and 850 K isentropic surfaces. For the 450 K isentrope (Fig. 16(a)), the vortex area was mainly close to the climatological mean up to mid-October, after which the area decreased abruptly to lie near or below the climatological 95th percentile, attaining the earliest breakdown date for this particular form of analysis. The progression of the vortex area at 850 K was generally the climatological mean up to mid- to late September (except for a period of above average size in late June and early July), after which the decline was more rapid than the median behaviour. Note that for both isentropes, the growth phase (early April for 450 K and early May for 850 K) took place somewhat later than the climatological mean (by approximately 1–2 weeks).

The size of the stratospheric vortex for these two isentropes is compared for separate months over the years since 1992 in Fig. 17. For 2012, the vortex in the lower stratosphere broke down relatively early, as shown in Fig 17(a) by the small area seen in November for the 450 K isentrope compared with other years. Note that the value for 2012 in Fig. 17(a) was similar to that for 2002, which followed the unprecedented vortex splitting event that took place in September of that year. As shown in Fig. 17(b), the size of the vortex in the mid-stratosphere during late winter was not atypical of the other years shown.

Fig. 14. Daily eddy heat flux averaged between latitudes of (a) 55°S to 35°S and (b) 85°S and 65°S as a function of pressure evalu-ated from UKMO Stratospheric Assimilated Data (Swinbank and O’Neill, 1994). Negative values indicate poleward trans-port of heat. The zero contour is outlined in white.

Fig. 15. Potential vorticity gradient (expressed in Potential Vorticity Units (PVU) per degree of equivalent lati-tude, where 1 PVU = 10-6 K m2 kg-1 s-1) 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. Equiva-lent latitude is derived using the method of Nash et al. (1996). The red contour denotes the equivalent lati-tude of Davis station (68.6°S, 78.0°E). The thick black contour shows the mean location of the ‘central’ region of the vortex edge as defined by Nash et al. (1996), while the upper and lower thin black contours show the location of the ‘inner’ and ‘outer’ vortex edge regions, respectively.

Overall, the meteorological data reviewed in this section indicates that polar stratospheric temperatures in 2012, which were generally below average in the early winter, markedly increased from early July. The warming progressed downward in altitude through to early summer and was associated with distinct episodes of poleward heat transport centred around mid-September and early to mid-October. These warmings aided the relatively rapid breakdown of the polar vortex and restricted the overall severity of ozone depletion.


by dynamical activity consistent with the vortex being in a weakened and disturbed state. Overall, the ozone hole was amongst the least severe on record, being ranked in the lowest third of the 33 well characterised ozone holes in the terms of maximum area, minimum ozone level and total ozone deficit.

The dynamical conditions promoted by the negative phase of the QBO favoured a weak and disturbed polar vortex throughout the winter and spring, and this situation likely helped limit chemical changes during winter and ozone loss in spring. The generally higher levels of stratospheric ozone in spring over Antarctica compared with recent years resulted in UVR levels being generally reduced. A notable feature of 2012 was the general and consistent downward movement of temperature anomalies from the mid- to lower stratosphere over Antarctica during the winter and spring period, suggestive of consistent weakening of the polar vortex. The breakdown date of the vortex, as measured using ozone as a diagnostic, was amongst the earliest observed since the late 1980s.

Conclusions

We have examined meteorological conditions and ozone concentrations in the Antarctic atmosphere during 2012 using a variety of data sources, including meteorological assimilations, satellite remote sensing measurements, and ground-based instruments and ozone sondes.

Polar stratospheric temperatures in 2012 were mostly below average in winter, but were generally above average in spring. The warm spring temperatures were influenced

Fig. 16. Southern hemisphere vortex area evaluated on po-tential temperature (θ) surfaces of (a) 450 K (~18 km height) and (b) 850 K (~31 km height). The time series for 2012 is shown in black; the blue time-series is the mean for 1992–2012, while the lower and upper red time-series in each graph show the 5th and 95th per-centiles, respectively, for 1992–2012. The vortex area is evaluated using data from the United Kingdom Meteorological Office (UKMO) stratospheric assimi-lation, and represents the surface area enclosed by potential vorticity contours of (a) -30 PVU and (b) -600 PVU.

Fig. 17. Monthly averages of daily southern hemisphere vor-tex area on potential temperature (θ) surfaces of (a) 450 K for November and (b) 850 K for August, obtained using data and methods described in Fig. 16.


The ozone hole metrics of 2012 showed similar anomalous behaviour to 1988 and 2002 which were years having unusually disturbed polar stratospheric conditions. Interestingly, the QBO index for these three years were in distinctly different states; in 2012 the index was strongly negative which favours a weak vortex, while in 1988 and 2002 the index was weakly and strongly positive, respectively. In general, the relationship between stratospheric conditions and QBO phase in 2012 was more in line with expectations than in the two other years (Butchart and Austin, 1996; Baldwin and Dunkerton, 1998; Scaife et al., 2005).

The behaviour of the ozone hole of 2012 contrasted with recent years. In 2011, the ozone hole metrics were amongst the lower third in terms of severity, while the other holes during the preceding decade (with the exception of 2004 and 2010) were also ranked near or below the lower third. In particular, the estimated integrated ozone mass effectively removed within the ozone hole of 2012 was approximately 720 Mt, which is the 12th smallest deficit on record and 28 per cent of the peak deficit observed in 2006. This aspect further demonstrates that ozone metrics have significant inter-annual variability, which limits our current ability to track ozone recovery.

Acknowledgments

We acknowledge the Department of Environment 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, BoM observers for collecting upper air measurements, expeditioners of the British Antarctic Survey for collecting the Halley measurements, and the staff at the Cape Grim Baseline Station, Tasmania, for the collection of ODS data. 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. 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 Arndt, D.S., Eds., 2013: State of the Climate in 2012. Bull. Amer. Meteor. Soc., 94 (8), S1–S238.

Butchart, N. and Austin, J. 1996. On the relationship between the quasi-biennial oscillation, total chlorine and the severity of the Antarctic ozone hole. Q. J. R. Meteorol. Soc., 122, 183–217.

Calvo, N., Garcia, R.R., Marsh, D.R., Mills, M.J., Kinnison, D.E. and Young, P.Y. 2012, Reconciling modeled and observed temperature trends over Antarctica, Geophys. Res. Lett., 39, L16803, doi:10.1029/2012GL052526.

Canziani, P. O., O’Neill, A., Schofield, R., Raphael, M., Marshall, G. and Redaelli, R. 2014, World Climate Research Program Special Workshop on Climatic Effects of Ozone Depletion in the Southern Hemisphere, Bull. Meteorol. Soc., doi:10.1175/BAMS-D-13-00143.1.

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 Substances and Synthetic Greenhouse Gases, Report prepared for Department of the Environment, CSIRO Marine and Atmospheric Research, Centre for Australian Weather and Climate Research, Aspendale, Australia, iv, 42 pp..

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.

Fu, Q., Solomon, S. and Lin, P. 2010. On the seasonal dependence of tropical lower-stratospheric temperature trends, Atmos. Chem. Phys., 10, 2643–2653, doi:10.5194/acp-10-2643-2010.

Gonzalez, P.L.M., Polvani, L.M., Seager R. and Correa, G.J.P. 2013. Stratospheric ozone depletion: a key driver of recent precipitation trends in South Eastern South America, Clim. Dyn., doi:10.1007/s00382-013-1777-x.

Grise, K. M., Polvani, L.M., Tselioudis, G., Wu, Y. and M. D. Zelinka, M.D. 2013. The ozone hole indirect effect: Cloud-radiative anomalies accompanying the poleward shift of the eddy-driven jet in the Southern Hemisphere, Geophys. Res. Lett., 40, doi:10.1002/grl.50675.

Grytsai, A.V., Evtushevsky, O.M. and Milinevsky, G.P. 2008. Anomalous quasi-stationary planetary waves over the Antarctic region in 1988 and 2002. Ann. Geophys., 26, 1101–1108.

Hendon, H.H., Thompson, D.W.J., and Wheeler, M.C. 2007. Australian rainfall and surface temperature variations associated with the Southern Hemisphere Annular Mode. J. Clim., 20, 2452–2467. doi: http://dx.doi.org/10.1175/JCLI4134.1.

Kang, S. M., Polvani, L.M., Fyfe, J.C. Son, S.-W., Sigmond, M. and Correa, G.J.P. 2013. Modeling evidence that ozone depletion has impacted extreme precipitation in the austral summer, Geophys. Res. Lett., 40, 4054–4059, doi:10.1002/grl.50769.

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). Bulletin of the American Meteorological Society, 83(11), 1631–1643.

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., Krummel, P.B., Petelina, S.V., Shanklin, J.D., Siddaway, J.M. and Stone, K.A. 2011. The Antarctic Ozone Hole during 2010. Aust. Meteorol. Oceanogr. J., 61, 253–267.


Tully, M.B., Klekociuk, A.R. and Rhodes, S.K. 2013. Trends and variability in total ozone from a mid-latitude Southern Hemisphere site: The Melbourne Dobson record 1978–2012. Atmosphere-Ocean, 1–8, doi:10.1080/07055900.2013.869192.

Turner, J., Barrand, N.E., Bracegirdle, T.J., Convey, P., Hodgson, D.A., Jarvis, M., Jenkins, A., Marshall, G., Meredith, M.P., Roscoe, H., Shanklin, J., French, J., Goosse, H., Guglielmin, M., Gutt, J., Jacobs, S., Kennicutt II, M.C., Masson-Delmotte, V., Mayewski, P., Navarro, F., Robinson, S., Scambos, T., Sparrow, M., Summerhayes, C., Speer, K., and Klepikov, A. Antarctic climate change and the environment: an update. Polar Record, doi:10.1017/S0032247413000296.

Watson, P.A.G. and Gray, L.G. 2014. How does the Quasi-Biennial Oscillation affect the stratospheric polar vortex? J. Atmos. Sci., 71, 391–409. doi: http://dx.doi.org/10.1175/JAS-D-13-096.1

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

Klekociuk, A.R., Tully, M.B., Krummel, P.B., Gies, H.P., Petelina, S.V., Alexander, S.P., Dargaville, R.J., Deschamps, L.L., Fraser, P.J., Henderson, S.I., Javorniczky, J., Shanklin, J.D., Siddaway, J.M. and Stone, K.A. 2014. The Antarctic Ozone Hole during 2011. Aust. Meteorol. Oceanogr. J., 64, 291–310

Lin, P., Fu, Q., Solomon, S. and Wallace, J.M. 2009. Temperature trend patterns in Southern Hemisphere high latitudes: Novel indicators of stratospheric change. J. Clim., 22, 6325–6341. doi: http://dx.doi.org/10.1175/2009JCLI2971.1

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., McLin¬den, 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., Rich¬ardson, E.H., King, R.A., Wevers, I., McCreath, W., Kyrola, E., Oikar¬inen, 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. and Nash, E.R. 2005. The unusual Southern Hemisphere stratosphere winter of 2002. J. Atmos. Sci., 62, 614–628. doi: http://dx.doi.org/10.1175/JAS-3323.1

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., Ev¬ans, 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.

Previdi, M. and Polvani, L.M. 2013. Climate system response to stratospheric ozone depletion and recovery, Q. J. R. Meteorol. Soc, 140, 2401–2419. doi: 10.1002/qj.2330.

Purich, A. and Son, S.-W. 2012. Impact of Antarctic Ozone Depletion and Recovery on Southern Hemisphere Precipitation, Evaporation, and Extreme Changes. J. Clim., 25, 3145–3154. doi: http://dx.doi.org/10.1175/JCLI-D-11-00383.1

Roth, C.Z., Degenstein, D.A., Bourassa, A.E. and Llewellyn, E.J. 2007. The retrieval of vertical profiles of the ozone number density using Chap¬puis band absorption information and a multiplicative algebraic re¬construction technique, Can. J. Phys., 85(11), 1225–43.

Scaife, A. A., Jackson, D. R., Swinbank, R., Butchart, N., Thornton, H. E., Keil, M. and Henderson, L. 2005, Stratospheric vacillations and the major warming over Antarctica in 2002. J. Atmos. Sci., 62, 629–639. doi: http://dx.doi.org/10.1175/JAS-3334.1

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.

Swinbank, R., and O’Neill, A.A.1994. Stratosphere-troposphere data assimilation system. Mont. 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.

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