The Antarctic ozone hole during 2015 and 2016

The Antarctic ozone hole has continued to appear each spring since its first detected appearance in 1979. Although the underlying mechanism of the ozone hole is now considered well understood, recent further work (Douglass et al. 2014; Solomon et al. 2014; Kirner et al. 2015; Solomon et al. 2015; Zhu et al. 2017) has continued to developed the detailed understanding of the formation of polar stratospheric clouds (PSCs), and the relative contribution of the different types of PSCs and chemical processes which lead to chlorine activation, and then prevent chlorine deactivation, during spring. The variability of reactive forms of chlorine inside the vortex from year to year has been identified (Strahan et al. 2014), including that due to quasi-biennial oscillation (QBO)-influenced transport (Strahan et al. 2015). The significant amount of chemical ozone depletion caused by anthropogenic ozone-depleting substances prior to the year 1980 was studied by Langematz et al. (2016), whereas the impact on Antarctic ozone of the eruption of the Chilean volcano Calbuco in April 2015 has also been investigated (Ivy et al. 2017; Stone et al. 2017).

Solomon et al. (2016) used a combination of modelling and observations to claim that a significantly positive trend in September mean ozone of 2.5 ± 1.7 DU/year could be attributed to declining halogen levels, most markedly in the height range between 100 and 50 hPa.

In this paper, we provide a description of the level of Antarctic ozone depletion in 2015 and 2016 and the relationship with prevailing meteorological conditions using a range of Australian data and analyses including measurements and analyses by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and Atmosphere unit, ozone measurements made by the Australian Antarctic Division (AAD) and the Bureau of Meteorology (BoM), and Antarctic ultra-violet measurements from the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) biometer network. Other data from satellite missions and ground-based instruments are also presented. This work complements the analyses of previous Antarctic ozone holes reported by Tully et al. (2008, 2011) and Klekociuk et al. (2011, 2014a, 2014b, 2015), Krummel et al. (2019) and other analyses of Antarctic atmospheric conditions and ozone depletion during 2015 and 2016 provided by the CSIRO (Krummel et al. 2016, 2017; http://www.environment.gov.au/protection/ozone/publications/antarctic-ozone-hole-summary-reports, accessed 21 April 2020), the World Meteorological Organisation (WMO) Antarctic Ozone Bulletins (http://www.wmo.int/pages/prog/arep/gaw/ozone/index.html, accessed 21 April 2020), upper air summaries of the National Climate Data Center (NCDC; http://www.ncdc.noaa.gov/sotc/upper-air, accessed 21 April 2020) and by Weber et al. (2016, 2017; http://www.ncdc.noaa.gov/bams-state-of-the-climate, accessed 21 April 2020).

As in previous reports in this series, we use total column ozone measurements from satellite instruments to obtain metrics of the Antarctic ozone hole for each year (see Klekociuk et al. (2015) for details). Here we use data processed with the version 8.5 TOMS algorithm from the Total Ozone Mapping Spectrometer (TOMS) series of satellite instruments, the Ozone Monitoring Instrument (OMI) on the Aura satellite and the Ozone Mapping Profiler Suite (OMPS) on the Suomi National Polar-orbiting Partnership satellite.

Table 1 contains the ranking for the 37 ozone holes adequately observed by satellite instruments since 1979 using eight metrics that provide different measures of the extent of ozone depletion in each year (see the notes accompanying the table for the definition of each metric). The first seven metrics in Table 1 measure various aspects of the maximum area and depth of the ozone hole. These metrics highlight the contrasting behaviours of the ozone holes of 2015 and 2016. The 2015 ozone hole was ranked between 3rd and 16th in terms of severity across these metrics, with the maximum 15-day averaged area of 27.6 Mkm² (central date 5 October 2015) equalling the value set in 2006 (central date 22 September 2006) which has only been exceeded by the ozone hole of 2000 (28.7 Mkm², central date 6 September 2000). Rankings for the metrics relating to minimum ozone values were lower (12th–16th) than those for area (3rd–4th) or deficit (5th–7th). In contrast, the ozone hole of 2016 was ranked between 17th and 21st for all metrics and exhibited quantitative similarity with ozone holes of the late 1980s-early 1990s and some recent years, particularly 2010 and 2014.

Table 1.  Ranked Antarctic ozone hole metrics obtained from TOMS/OMI/OMPS satellite data
TOMS data are used from 1979 to 2014, OMI data are used for 2005–15 and OMPS data are used for 2016. Data for 2015 and 2016 are highlighted in bold. Note: In previous papers in this series (Tully et al. 2008, 2011; Klekociuk et al. 2011, 2014a, 2014b, 2015), the Antarctic ozone hole metrics for the year 2005 were an average of both TOMS and OMI data, and hence the 2005 rankings in those papers will differ from those quoted below. Rank 1 is the lowest ozone minimum, greatest area, greatest ozone loss etc.; Rank 2 is the second lowest ozone minimum, etc. There was a gap in TOMS coverage during the growth of the 1994 ozone hole; metrics for some parameters for that year are, therefore, undetermined and are left blank. There were no relevant TOMS measurements in 1995. 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) Breakdown date: The final date at which the daily maximum area (metric 2) falls below 0.5 million km². Note that the metrics use 220 DU as the threshold in total column ozone to define the location and occurrence of the ozone hole

Figure 1 shows the time-series of the ozone hole area, minimum polar total column ozone and total ozone deficit within the ozone hole over the latter half of each year from 2009 to 2016. Notable features of the metrics shown in Fig. 1 for 2015 are the relatively delayed development of the ozone hole (which started showing significant growth in late August, 1–2 weeks later than most of the years since 2011) and late peak (which occurred in early October, ~2 weeks later than the long-term averages). Furthermore, the metrics during the declining phase were at or near record levels throughout the period from October to mid-December and exhibited relatively little in the way of week-to-week fluctuations that normally occur in this period due to warming of the polar vortex. The date of breakdown (column 8 in Table 1) for 2015 was one of the latest on record.

Fig. 1.  Estimated daily (a) ozone hole area, (b) ozone hole depth and (c) ozone mass deficit based on OMI satellite data for 2011–16 and OMPS satellite data for 2016. The shaded region and white line show the range and mean respectively over 1979–2015.

In the case of 2016, the metrics shown in Fig. 1 were more typical of the long-term mean and generally intermediate between the behaviours seen in 2013 and 2014. Notably, there was a rapid decline in the area metric (Fig. 1a) through most of October which gave way to a short hiatus (lasting ~2 weeks up to early November) during which there was slight growth. Subsequently, there was again rapid decline in the size of the hole, and the final breakdown occurred on 20 November, which was similar to the behaviour seen in 2012 and 2013, and generally earlier than the long-term average.

Details of the meteorological conditions in 2015 and 2016 influencing these results are presented in Appendix 1.

Figure 2 shows the estimated total annual ozone deficit associated with the Antarctic ozone hole. This metric shows large year-to-year variability which largely reflects meteorological factors, with a disturbed polar vortex or relatively warm stratospheric temperatures producing a relatively low deficit, such as for 2002, 2004 and 2012, and cold stratospheric temperatures or a relatively long-lived polar vortex creating a large deficit, such as for 1998–2001 and 2006. The annual deficit in 2015 of 2197 Mt was the 5th largest observed.

Fig. 2.  Estimated total ozone deficit for each year in millions of tonnes (Mt), based on TOMS (1979–2004), OMI (2005–15) and OMPS (2016) satellite data. The orange line is obtained from a linear regression to EESC (expressed in parts per billion by volume) from Fraser et al. (2014) using a mean age of air of 5 years.

Also shown in Fig. 2 is the estimated level of Antarctic equivalent effective stratospheric chlorine (EESC; orange line) from Fraser et al. (2014), which is a measure of the potential for ozone depletion in the lower stratosphere.

A longer term data record is available from ground-based Dobson spectrophotometer measurements at the British Antarctic Survey’s Halley station (75.6°S and 26.2°W) in Antarctica (BAS 2015). Figure 3 shows mean October total column ozone from 1957 to 2016, again together with Antarctic EESC (Fraser et al. 2014; Klekociuk et al. 2015). The Halley average value for October 2015 shown in Fig. 3 of 139 DU was similar to that measured in years of notably severe ozone loss (Klekociuk et al. 2015); 2000 (137 DU), 2001 (138 DU), 2006 (137 DU) and 2011 (140 DU). The mean October total column ozone value at Halley over 2011–16 (157 ± 18 DU) is higher than that over 1996–2001 (141 ± 4 DU), but the difference is not statistically significant at the 95% confidence threshold (1 standard error in mean values are quoted). Over the years 1996–2016, ignoring the dynamically disturbed years of 2002 and 2004 (Klekociuk et al. 2015), the linear trend is 1.0 ± 1.0 (2σ) DU/year. As studied by Langematz et al. (2016), it is apparent from Fig. 2 that Antarctic ozone depletion was quite significant in the decade from 1970 to 1980 notwithstanding the general use of 1980 as the baseline year for ozone depletion and subsequent recovery.

Fig. 3.  October monthly mean total column ozone values for Halley station for 1957–2014 (green points and line; BAS 2015) and regression to EESC (orange line) from Fraser et al. (2014) using a mean age of air of 5 years. (Dashed orange lines show the 95% uncertainty limits of the regression.)

Figure 4 shows the 12–20 km partial column ozone amount for all available ozonesonde measurements from Australia’s Davis research station in Antarctica (68.6°S and 78.0°E). Ozonesonde flights during 2015 were restricted to approximately monthly intervals for most of the year following logistical difficulties in supplying consumables to Davis.

Fig. 4.  Time-series of partial column ozone for the height interval 12–20 km obtained from ozonesonde measurements at Davis, Antarctica (68.6°S and 78.0°E). Shown are data for all years of measurement, with data for 2015 highlighted with filled magenta triangles and solid line and 2016 highlighted with black-filled squares and solid line. The grey line is a climatological mean from Fortuin and Kelder (1998) interpolated to the location of Davis. Note that at Davis, this height range is almost exclusively above the lapse rate tropopause and generally below the burst height of the ozonesonde balloons in winter.

The rate of ozone decreases between days 240 and 270 (approximately the month of September) has been quite consistent over the 2003–16 period; however, the minimum value eventually reached, as well as the timing and rate of the following increase, varies significantly from year to year. During the months of October (day-of-year 274–304) and November (day-of-year 305–334), the location of Davis at times can lie outside the ozone hole depending on the size and shape of the hole, leading to the observed spikes in 2015 and more particularly, 2016.

Further detail of the course of depletion in 2015 and 2016 compared to previous years can be seen by breaking down the partial column into the following three narrower height ranges, 12–15, 15–18 and 18–21 km (Fig. 5a–c).

Fig. 5.  Partial ozone columns measured by Davis ozonesondes between (a) 12 and 15 km, (b) 15 and 18 km (c) 18 and 21 km.

The 12–15 km partial column ozone was slightly below the long-term record during the period of decline in September in 2015 and slightly above in 2016; however, the more noteworthy feature was the series of very low values recorded in November and December 2015 after the minimum had been reached, well below those previously seen in the Davis record (2003 onwards). In contrast, in 2016 the 12–15 km partial column was never reduced to the very low ozone values observed in many previous years and recovered steadily.

The 15–18 km partial columns decayed during September somewhat later (10 days) than is typical for the Davis record in both 2015 and 2016, as noted earlier with respect to the area of the ozone hole. Values stayed very low for several weeks in October in 2015 and only slowly recovered. In 2016 this height range showed large variability as the position of Davis relative to the vortex shifted from week to week.

In contrast, the 18–21 km partial column did not display exceptionally low values in 2015, and in fact most other years since 2003 have recorded lower annual minima in this height range. In 2016, a single ozonesonde flight (27 September 2016) measured almost zero ozone (0.23 mPa) at 19.7 km (43 hPa); however, values were much higher after this date for the rest of the year.

The difference in behaviour within the respective partial columns is consistent with the finding of Stone et al. (2017) that the impact on ozone of volcanic aerosol transported polewards from Calbuco (Chile) was concentrated between 150 and 100 hPa (approximately corresponding to 12–15 km in altitude).

Annual values of the vortex-average rate-of-change of ozone mixing ratio as a function of temperature, averaged over days 200–260 (19 July–17 September in nonleap years), for isentropic levels of potential temperature (θ) equal to 450 K (~18 km height) and θ = 850 K (~31 km height), are shown in Fig. 6a, b, for years 2004–16. These figures essentially summarise characteristics during the period when the ozone hole is generally growing. The values are obtained from Aura microwave limb sounder (MLS) version 4.2 data as described in Appendix 1.

Fig. 6.  Vortex-average ozone rate of change vs temperature, averaged between days 200 and 260, on isentropic surfaces of (a) θ = 450 K (~18 km altitude) and (b) θ = 850 K (~30 km altitude) obtained from MLS v4.2 swath measurements. The vertical and horizontal bars span ± one standard error in the mean of the deseasonalised daily measurements. The base period used to deseasonalise the daily values is 2004–14. The relevant year is indicated immediately adjacent to each value – years 2015 and 2016 are highlighted in red and blue respectively.

Figure 6a shows annual values for the θ = 450 K isentrope. The growth of the ozone hole in the lower stratosphere is primarily influenced by the amount of chemical processing that has taken place within the vortex over the winter (which is enhanced at lower temperatures by greater polar stratospheric cloud volume), and the amount of the vortex that is illuminated by sunlight after the end of the polar night (which depends on the size and symmetry of the vortex). As discussed by Krummel et al. (2019), the amount of chemical processing in the vortex for any given year appears to be a stronger influence on the scatter in Fig. 6a than the amount of illumination received by the vortex during the formation of the ozone hole. There is a general tendency in Fig. 6a for the ozone loss rate to be positively correlated with temperature (Pearson correlation coefficient r = 0.80, significant at the 95% confidence limit), with enhanced ozone loss occurring at lower temperatures. On the θ = 850 K isentrope (Fig 6b), the correlation between the ozone rate-of-change and temperature is negative (r = −0.75, also significant at the 95% confidence limit). At this level, ozone loss is primarily by gas-phase processes which are more efficient at higher temperatures.

Overall, the behaviour of 2015 and 2016 in Fig. 6a, b appears typical in comparison to years in which the polar vortex during late winter and early spring was relatively undisturbed by dynamical activity. This is consistent with the Davis ozonesonde data (Fig. 4) which show the rate of ozone decline in September was fairly typical in both 2015 and 2016, with only subsequent time periods displaying more distinctive behaviour.

The BoM carries out long-term high-quality measurements of total column ozone at Macquarie Island (54.5°S and 158.9°E) using the Dobson spectrophotometer, continuing a program dating back to 1957. Observations for 2015 and 2016 are shown in red in Fig. 7a, b respectively compared to the 1987–2014 range.

Fig. 7.  Daily mean total column ozone observations made by Dobson spectrophotometer at Macquarie Island in (a) 2015 and (b) 2016 compared to the 1987–2014 range. (The black line shows the smoothed daily median, the dark blue band the 30th–70th percentile range, and the light blue band the 10th–90th percentile range.)

In 2015, total column ozone measurements at Macquarie Island were generally below the long-term mean in winter but returned to more typical values from the middle of October (day 285) onwards. A number of episodes of very low ozone values were observed in September, whereas Macquarie Island was in the vicinity of the polar vortex. On 25 September (day 268), the daily average ozone was 239.5 DU with the lowest single Dobson reading (AD-Direct Sun) of 233 DU. Anomalously low ozone was also recorded during this time as far north as Melbourne (284 DU).

A further episode of low ozone (273 DU) was observed on 27 November (day 331) during the break-up of the ozone hole when Macquarie fell under the influence of the distorted polar vortex.

No episodes of high ozone above the climatological range were observed in 2015.

Compared to 2015, the total ozone measured at Macquarie Island stayed much closer to the long-term seasonal cycle in 2016. Two episodes of low ozone were observed though in August when Macquarie Island was influenced by the polar vortex. On 9 August (day 222), a daily average of 269.9 DU was recorded and 282.8 DU on 25 August (day 238). As was the case in 2015, no episodes of high ozone were observed in spring 2016.

Measurements of biologically effective solar ultraviolet (UV) radiation are made at Casey (66.3°S and 110.5°E), Mawson (67.6°S and 62.9°E) and Davis research stations in Antarctica. Details on the instrumentation and methods used are provided by Tully et al. (2008) and Klekociuk et al. (2015).

Figure 8 shows the September–December mean UV Index for the three stations from 2007 to 2016. The prolonged low values of total column ozone in October and November (Fig. 1b) when the sun is much higher in the sky, resulted in mean UV values being the highest on record at all three stations, exceeding 2010. In 2016, values were significantly lower and broadly consistent with 2013 and 2014. In general, the mean UV values measured each year are strongly rank-correlated with ozone hole breakdown dates as given in column 8 of Table 1, as a longer lasting ozone hole will cause relatively larger UV increases due to the increased solar elevation later in the year.

Fig. 8.  UV index averaged over September–December for 2007–16 from daily measurements at Casey, Davis and Mawson. At Casey in December 2016, only 5 days of measurements were made.

We have examined meteorological conditions and ozone concentrations in the Antarctic atmosphere during 2015 and 2016 using a variety of data sources, including meteorological assimilations, satellite remote sensing measurements, and ground-based instruments and ozonesondes.

The ozone hole of 2015 was one of the most severe on record with respect to maximum area and integrated deficit and was notably long-lasting, with many values above previous extremes in October, November and December. This is attributed to a very strong polar vortex, with the 50 hPa Southern Annular Mode (SAM) at record levels from September to November, and very cold polar temperatures throughout winter and spring – exceeding previous (1979–2014) limits at 100 hPa in October and November. In these conditions, increased concentrations of liquid binary sulfate aerosols resulting from the eruption of the Chilean volcano Calbuco were able to greatly extend the area of the ozone hole equatorward.

In contrast, in 2016, September to November polar temperatures were slightly warmer than the long-term mean in September and October, before warming more markedly in November in conjunction with a weak vortex resulting in an early end to the ozone hole. All assessed metrics for the 2016 ozone hole were at or below their median values.