Balloon-borne ozonesondes launched weekly from South Pole
Station (1986–2021) measure high-vertical-resolution profiles of ozone and
temperature from the surface to 30–35 km altitude. The launch frequency is
increased in late winter before the onset of rapid stratospheric ozone loss
in September. Ozone hole metrics show that the yearly total column ozone and
14–21 km partial column ozone minimum values and September loss rate trends
have been improving (less severe) since 2001. The 36-year record also shows
interannual variability, especially in recent years (2019–2021). Here we
show additional details of these 3 years by comparing annual minimum
profiles observed on the date when the lowest integrated total column ozone
occurs. We also compare the July–December time series of the 14–21 km
partial column ozone values to the 36-year median with percentile intervals.
The 2019 anomalous vortex breakdown showed stratospheric temperatures began
warming in early September followed by reduced ozone loss. The minimum total
column ozone of 180 Dobson units (DU) was observed on 24 September. This was
followed by two stable and cold polar vortex years during 2020 and 2021 with
total column ozone minimums at 104 DU (1 October) and 102 DU (7 October),
respectively. These years also showed broad near-zero-ozone (loss
saturation) regions within the 14–21 km layer by the end of September which
persisted into October.
Validation of the ozonesonde observations is conducted through the ongoing
comparison of total column ozone measurements with the South Pole
ground-based Dobson spectrophotometer. The ozonesondes show a more positive bias
of 2 ± 3 % (higher) than the Dobson following a thorough
evaluation and homogenization of the long-term ozonesonde record completed in
2018.
National Oceanic and Atmospheric Administrationn/aIntroduction
In 1986, NOAA began launching weekly balloon-borne ozonesondes at
Amundsen–Scott South Pole Station (90∘ S) measuring
high-resolution vertical profiles of ozone and temperature. This same year
numerous field projects were deployed to Antarctica (Anderson et al., 1989;
Tuck et al., 1989) to investigate the discovery of the springtime Antarctic
ozone hole by Farman et al. (1985). Subsequent studies confirmed that the
chlorine catalytic destruction of ozone was enhanced over Antarctica in the
presence of wintertime polar stratospheric clouds (PSCs) (Solomon, 1999; Solomon et al.,
1986; McElroy et al., 1986). The following decade of balloon-borne
profiles and satellite and ground-based measurements showed a broad and
deepening ozone hole that stabilized in its expansion by the early 2000s
(Hofmann et al., 2009). More recently, several analyses of the ongoing
ground-based and satellite measurements indicate that the ozone hole has
been slowly recovering since 2000 (for a list of studies, see Langematz et al. (2018). The current recovery stage and
upward trend in springtime ozone have been linked to the decline in the
concentration of man-made ozone-depleting substances (ODSs) due to the
successful implementation of the Montreal Protocol international guidelines
phasing out the production of ODSs. In 2020, the ODS abundance over
Antarctica was 25 % below the 2001 peak (Montzka et al., 2021). Full
recovery is predicted to occur by around 2056–2070 when ODS levels return to
the 1980 benchmark levels (Newman et al., 2006; Dhomse et al., 2018; Amos et al.,
2020). However, while long-lived ODS concentrations are steadily declining,
the extent of chemical ozone loss may be quite different from year to year
due to meteorological conditions (Newman et al., 2006; Keeble et al., 2014;
de Laat et al., 2017; Tully et al., 2019; Stone et al., 2021).
After polar sunset, the strengthening of circumpolar winds and the
development of a potential vorticity gradient form the polar vortex
boundary region that isolates stratospheric air over Antarctica (Nash et
al., 1996). Near the center of the vortex, over the South Pole, ozonesondes
measure stratospheric temperatures steadily decreasing during the polar
night and remaining well below the -78 ∘C threshold for
PSC formation and growth. PSCs provide the surface reaction sites for
activating stable chlorine species into radicals that rapidly destroy ozone
after sunlight returns in September (WMO, 2018). However, planetary wave
disturbances in late winter may weaken or completely break apart the cold
and stable Antarctic polar vortex, ending the optimum conditions for rapid
ozone loss in September (Schoeberl et al., 1989; Newman et al., 2004; Hassler
et al., 2011a; Salby et al., 2012; Strahan et al., 2016; de Laat et
al., 2017; Strahan et al., 2019; Milinevsky et al., 2020). These conditions
were observed in the warmer and weaker polar vortex conditions in 1986 and
1988 (Stolarski et al., 1990). The most extreme disruptions in the polar
vortex occurred during the stratospheric warming events in 2002 and 2019
(Hoppel et al., 2003; Safieddine et al., 2020; Wargan et al., 2020).
The South Pole ozonesondes play a key role in monitoring ozone and
temperature during all phases of the ozone hole. These unique measurements
are critical after Antarctic sunset when several months of darkness limit
the ground-based Dobson spectrophotometer and solar ultraviolet satellite
optical measurements. Several indices and indicators have been presented in
past analyses of the Antarctic ozonesonde records by Hofmann et al. (1997,
2009); Solomon et al. (2005, 2016); and Hassler et al. (2011a).
This paper is a review of the South Pole ozonesonde observations beginning
with an overview in Sect. 2 of the electrochemical concentration (ECC)
ozonesonde and recent homogenization of the South Pole data record by
Sterling et al. (2018). Section 3 shows a comparison of ozone and
temperature profiles during the last 3 years when the early polar vortex
breakup and weak ozone hole in September 2019 were followed by severe ozone
loss in 2020 and 2021 when cold vortex conditions persisted into early
December (Kramarova et al., 2021, 2022). Section 4 shows the updated 36-year
homogenized ozone time series and ozone hole metrics focusing on the 14–21 km layer column ozone minimums and linear ozone loss rates during September.
In addition, we update ozone mixing ratio loss rates at selected pressure
levels from the analysis by Hassler et al. (2011a) that showed maximum
September loss rates occur in the 33–48 hPa region, while the 89 hPa
was found to be the optimum layer for observing early detection of reduced
ozone loss rates as ODSs decline. Section 5 illustrates the extent of ozone
loss saturation observed each year during the annual minimum-ozone period
from 26 September to 15 October. The near-zero-ozone layers were variable
and narrowing after 2008 but were near maximum extent again in 2020 and
2021. The summary is given in Sect. 6.
ECC ozonesonde overview
The basic design of the electrochemical concentration cell (ECC) ozonesonde
has remained relatively unchanged during the 36-year South Pole record
(Komhyr, 1967). A Teflon piston pump bubbles ambient air into a sensor cell
chamber with a platinum gauze electrode submerged in 3 mL of dilute,
buffered potassium iodide (KI) solution. The ozone–iodide reaction in the
ECC cell generates an electrical signal proportional to the ozone
concentration.
Since about the mid-1990s, the ECC sonde manufacturers have improved the
sensor cell design and the purity of the platinum electrodes, thus reducing
the sensor's current background to approximately 0.02 µA when sampling
no-ozone filtered air (Vömel and Diaz, 2010; Smit and Thompson, 2021). The limit of detection (LOD) is equivalent to 3×0.02 µA background. This converts to an ozone partial pressure of 0.10 mPa or a mixing ratio
of 0.02 ppmv (parts per million by volume) at 50 hPa ambient pressure.
A Styrofoam box houses and insulates the ozonesonde pump and sensor. The
weather radiosonde, attached to the outside of the box, measures and
transmits meteorological and ozone data to the ground-based receiving
equipment during ascent to the balloon-burst altitude of about 34 km.
Consistent burst altitudes are maintained during the dark, cold months at the
South Pole by switching from standard rubber weather balloons to 500 m3
volume polyethylene film balloons during the first week of April and then
returning to rubber balloons by mid-October.
Data homogenization
Each ECC ozonesonde profile represents a new instrument, used only once;
thus, ozonesonde trends may show an offset or sudden bias rather than a slow
drift in the data record when a new ozonesonde design or standard operating
procedure change occurs (Johnson et al., 2002; Smit et al., 2007; Tarasick
et al., 2016; Thompson et al., 2017; Van Malderen et al., 2016; Witte et al., 2017). The ozonesonde
model and standard operating procedures (SOPs) at the South Pole have not
changed since 2006. However, prior to 2006, several dual and triple
ozonesondes were flown to compare new ozonesonde models or adjustments made
in the SOP in order to determine ad hoc corrections to account for these
changes.
A thorough review and homogenization of the ozonesonde record was completed
by Sterling et al. (2018) following homogenization methods that were
formulated from the Assessments of Standard Operating Procedures (ASOPOS)
workshops (Smit and ASOPOS, 2012; Deshler et al., 2017). The ozonesonde
guidelines, presented in the ASOPOS GAW/WMO report no. 268 (2021), are
based on the World Calibration Center Jülich Ozonesonde Intercomparison Experiments (JOSIE). The JOSIE environmental simulation
chamber experiments are the global reference for evaluation of new
ozonesonde designs and SOPs and the foundation for improving long-term
vertical ozone trends determined by ozonesondes with a goal to reduce
uncertainty to ±5 % throughout the profile (Smit et al., 2007;
Thompson et al., 2019).
Additional verification of the ozonesonde record at South Pole Station is
conducted through the ongoing comparison of total column ozone (TCO) with
the NOAA ground-based Dobson spectrophotometer direct-sun (DS) AD wavelength
pair measurements over South Pole Station from 20 October to 20 February
(Komhyr et al., 1997). Globally, the Dobson network is an important
long-term stable reference for ozonesonde sites and useful for identifying
drifts in satellite platforms (McPeters and Komhyr, 1991; Bodeker et al.,
2005; Thompson et al., 2017). Figure 1 shows the homogenized ozonesonde TCO
record is a constant 2 ± 3 % offset compared to the Dobson
observations. The Dobson DS/AD observations are accurate to within ±1 % (Köhler et al., 2018). The ozonesonde TCO includes a residual
value to account for estimated ozone above the balloon-burst altitude by
extrapolating a constant mixing ratio (CMR) from the balloon-burst pressure
(occurring between 20 to 7 hPa) to zero pressure. While using the ozone residual
lookup values from the satellite SBUV (Solar Backscatter Ultraviolet Radiometer) global climatological residual table
from McPeters et al. (1997) and McPeters and Labow (2012) is the recommended procedure for
determining ozonesonde residuals (Smit and Thompson,
2021), we have found that the CMR extrapolation is more consistent when
comparing with the Dobson spectrophotometer TCO at South Pole Station.
Percent difference (100×(sonde - Dobson) / Dobson)
comparing total column ozone of the South Pole ozonesondes and Dobson
spectrophotometer direct-sun AD wavelength measurements. The solid red
diamonds represent the percent differences before the homogenization of the
1986–2006 ozonesonde data. The gray diamonds show a more consistent record
after homogenization. The dashed horizonal line shows the trend in the
offset is relatively stable at 2 %.
Temperature profile validation
Sterling et al. (2018) discuss the details in the transition from three
different radiosonde models at the South Pole from VIZ (1986–1991) to Vaisala
RS80 (1991–2014) and the current GPS-enabled InterMet (Imet) radiosondes
(2015–2021). The Imet measurements added GPS-computed winds and geometric
altitude to the South Pole profile data. For homogenization of the non-GPS
Vaisala RS80 data, the NOAA SkySonde software (Allen Jordan author – see
acknowledgements) was updated to retrieve nearby weather service radiosonde
data. This provided a data source to identify and flag temperature outliers
and to adjust the radiosonde pressure when offsets were >2 hPa
near burst altitude (Sterling et al., 2018).
The ozonesonde radiosonde temperatures were not adjusted in the
homogenization of the data record. However, temperature accuracy for each
flight was validated by comparing it with an additional radiosonde flown by the
Antarctic Meteorological Research Center (AMRC) at the South Pole. For nearly a
decade, the AMRC–South Pole Meteorology Office radiosondes (using Vaisala
RS92 and Vaisala RS41 GPS models) were “piggy-backed” on board the NOAA
ozonesonde package. This is an important collaboration during the winter
months when NOAA switches to the cold-resistant polyethylene balloons to
maintain burst altitudes of 30–34 km. This results in two independent
temperature profiles to monitor the coldest temperatures in the polar
stratosphere where PSCs begin to form. The typical rubber balloon will
fail/burst at 14–15 km over the South Pole under these extreme cold and dark
conditions. The comparisons between the Vaisala radiosonde models (RS80 and
RS92) during the 2012–2013 flights show almost no difference in stratospheric temperature measurement during the summer months and only a slight
0.2 ∘C difference in winter. The Imet radiosonde measures slightly
higher temperatures by about 0.5 ∘C than the Vaisala RS41,
primarily during the summer months. Steinbrecht et al. (2008) observed
similar offsets in campaigns comparing Vaisala RS80 and RS92 radiosondes.
South Pole ozonesonde profiles: 2019–2021
Balloon-borne ozonesondes provide a unique overview of the yearly ozone hole
over the South Pole by comparing vertical profiles measured during winter
(before depletion begins) with the annual minimum-ozone profile typically
observed between 26 September to 15 October. Figure 2 shows the 2019–2021
ozone (upper panels) and temperature (lower panels) profiles representing the
ozone holes of these 3 years. The winter profiles (blue) are an average
of the six to eight vertical profiles measured from 15 June to 15 August, when the
stratospheric temperatures range between -85 to -95∘C and TCO averages 263 ± 17 Dobson units (DU).
Selected ozonesonde profiles from 2019–2021 representing
the ozone hole severity over the South Pole by comparing the average winter
profile before depletion begins (blue line) to the minimum-ozone profile
(red line). The minimum total column ozone (TCO) and 14–21 km (dashed
horizontal line region) partial column ozone values are given in Dobson units (DU). The
gray shaded region represents the 1986–2018 median 30–70th percentiles for
winter and minimum periods. The record low of 92 DU TCO
measured in 2006 is shown as a dotted line in the 2019 graph. The
temperature graphs show the -78∘C PSC threshold as a dashed vertical
line.
The wintertime ozone profiles are similar to the long-term climatology each
year and typically do not provide any insight into how the polar vortex
conditions and the severity of ozone depletion will unfold when rapid
depletion begins by 1 September at the South Pole. However, during the last 2
weeks of August the first signs of ozone loss are occasionally observed
above 21 km, likely from transported air parcels originating near the polar
vortex boundaries where sunrise, Cl2 photolysis, and chemical ozone
destruction begin (Schoeberl and Hartmann, 1991; Lee et al., 2001;
Hassler et al., 2011a; Strahan et al., 2019).
The springtime minimum-TCO profiles (red) average 114 ± 15 Dobson
units (DU), which represents a 55 %–65 % ozone loss when compared to the
wintertime profile. The 2019 minimum TCO of 180 DU was observed on 24
September, the second-highest minimum in the 36-year South Pole record. This
was the third season when the polar vortex was dramatically disrupted, leading
to an early September warming and meridional mixing of ozone-rich air into
the polar region from the mid-latitude stratosphere (Wargan et al., 2020;
Safieddine et al., 2020). Table 1 lists the other 2 years (1988 and 2002)
when similar events occurred. The table also includes the date intervals
when data were excluded in our long-term median calculations since these
extremes in ozone and temperatures were not representative of typical
chemical ozone hole losses.
Years when an early disruption of the polar vortex was
observed over the South Pole and the corresponding period when the profile data
were excluded from the ozonesonde median and percentile climatology.
YearDates excludedEvent198811 Aug–1 DecEarly vortex weakening in August200222 Sep–15 DecSudden stratospheric warming/split vortex (Allen et al., 2003)20195 Sep–20 DecSudden stratospheric warming/vortex shift (Safieddine et al., 2020)
The stable and cold polar vortex conditions in 2020 and 2021 led to severe
ozone holes over the South Pole with minimum-TCO measurements of
104 DU on 1 October and 102 DU on 7 October, respectively. The profiles in
Fig. 2 show the near-complete destruction of ozone within the 14–21 km vertical
layer during those 2 years. The record-low-TCO profile in
2006 (92 DU) on 9 October, shown as the dotted line in Fig. 2, had a 7 km
vertical extent of near-zero ozone (ozone loss saturation) from 14 to 21 km.
Thereafter, the 14–21 km layer became the baseline region for tracking ozone
loss metrics and the severity of the annual ozone hole over the South Pole
(Hofmann et al., 2009). The term “near-zero ozone” from here on will
represent stratospheric-ozone partial-pressure measurements that fall below
0.2 mPa (2 times the LOD). The next section shows all of the
observations of the 14–21 km column ozone during 2019–2021 from
July–December to illustrate the temporal evolution and variability in the
ozone hole.
South Pole ozonesonde 14–21 km time series: 2019–2021
Figure 3 shows the July–December time series of 14–21 km column ozone and
temperature in 2019, 2020, and 2021 compared to the 1986–2021 climatological
median with 30–70th and 10–90th percentiles in gray shading. The median and
percentile values were calculated using a sliding time series bin that is
gradually reduced from ±14 d in July to a ±3 d bin during
the month of September when more frequent ozonesonde launches track the
rapidly decreasing ozone column. The slope of the median 14–21 km column
ozone (black line) decreases linearly during September at a rate of -3.5±0.3 DU d-1. This metric is computed for each individual year and
presented in Sect. 4 to show the ozone loss rate trend. After 1 November
the ozone and temperature percentiles broaden significantly due to variable
dates when the Antarctic polar vortex fully dissipates (Bodeker et al.,
2005; Karpetchko et al., 2005).
Ozonesonde July–December time series in 2019–2021 showing
the 14–21 km column ozone in Dobson units (DU) (a) and
average temperature (b) compared to the 36-year median (black
line) with gray percentile envelopes. The dramatic polar vortex disruption
in September 2019 versus cold and stable conditions in 2020 and 2021 shows
the extreme variability in September to November measurements that
eventually all converge to normal ozone values and temperatures by the end
of December.
The stratospheric warming event in 2019 included a large-scale shift in the
polar vortex towards the tip of South America (Safieddine et al., 2020)
away from the typical position centered near the South Pole. Figure 3 shows
the anomalous high-ozone and temperature breakout in early to mid-September
2019 over the South Pole. The 8 September temperature profile showed the first
sign of this weakening vortex with an abrupt increase of more than
10 ∘C in the 14–21 km layer. However, the column ozone within the
14–21 km layer remained close to the median line until 20 September when the
strong depletion period ended and ozone values leveled off in the 45–50 DU
range. Then on 10 October, it dropped to the minimum for the year at 44 DU when
the polar vortex briefly centered back over the Antarctic continent and
South Pole Station.
The opposite polar vortex conditions were observed in 2020 and 2021 when
cold temperatures and column ozone tracked well below the median near the
lower edge of the 10–90th percentile line from September through December in
Fig. 3. Both years showed severe loss in the 14–21 km column with minimums
of 2 DU (1 October 2020) and 3 DU (1 October 2021). The daily Dobson TCO
observations also tracked the slow return to typical seasonal values. The
latest date the South Pole exceeded the 220 DU ozone hole threshold value
(Stolarski et al., 1990) was 12 December 2020 when 236 DU was measured
nearly 2 months after the South Pole Dobson observed 109 DU in
mid-October. The NASA satellite observations also showed the longest-lived
ozone hole on record in 2020 due to the very weak planetary-scale wave
activity (Kramarova et al., 2021).
Ozonesonde metrics: altitude intervals: 1986–2021
Table 2 lists the altitude layer metrics presented in this study related to
ozone loss during September and the minimum ozone occurring by early
October. While the lowest ozone is a key metric of ozone hole severity each
year, many recovery indices focus on the September observations when the
highest sensitivity and correlation with decreasing ODSs may be ascribed
(Solomon et al., 2016; de Latt et al., 2017; Pazmiño et al., 2018;
Strahan et al., 2019). The South Pole ozonesonde metrics here focus on the
14–21 km layer and include two additional metrics showing an update of the
mixing ratio loss rate profiles at selected pressure levels from Hassler et al. (2011a) and a metric showing the yearly vertical extent of layers with
near-zero ozone.
Altitude layers and metrics updated for the 1986–2021
ozonesonde record at the South Pole.
Altitude layerMetric – plotted data14–21 kmColumn ozone minimums (DU) and September loss rates (DU d-1)Pressure: 119–33 hPaSeptember mixing ratio loss rate profiles (ppmv d-1)10–24 km – curtain plotNear-zero-ozone (mPa) layers during minimum period (26 Sep–15 Oct)
Dobson units (DU), mixing ratio (parts per million by volume – ppmv),
millipascals (mPa), and hectopascals (hPa).
Figure 4a shows an overview of the 36-year time series of selected 14–21 km
integrated column ozone values representing three stages of the ozone hole
over the South Pole. This panel shows the winter average ozone observed before
depletion begins and the spring minimum-ozone series. An additional series
shows the 15 September values when an ozonesonde is launched each year on
this date to track the progress of ozone depletion (Hofmann et al., 2009).
(a) The 1986–2021 yearly observations of 14–21 km column
ozone over the South Pole showing the winter (15 June–15 August average), 15
September, and the spring minimum ozone in Dobson units (DU). The two dashed
lines show the simple regression linear trends and R2
values for all 15 September measurements before and after 2000. (b) The
lower panel shows the difference between the three time series in the upper
panel to illustrate maximum column ozone loss, the loss during the first
half of September (blue open-circle line), and second half (gray diamond
line) of the depletion period after 15 September.
The winter average 14–21 km column ozone (15 June–15 August) has been
relatively constant at 130 ± 10 DU. The spring minimum-ozone series
bottomed out at near-zero ozone from 1993–2001 followed by an upward trend
after 2001. The long-term trends in 14–21 km column ozone are more evident
in the 15 September series when the 1986–1999 period showed ozone decreasing
at a rate of -2.3± 0.6 DU yr-1. This was followed by an upward trend
line at +0.9 ± 0.4 DU yr-1. The simple linear regression lines in
Fig. 4a were computed by the “least squares” method. The uncertainty is
the standard error in the slope.
Both the spring minimum and the 15 September series show year-to-year
variability. However, the three anomalous polar vortex breakup years (red
dots) stand out as peaks in the minimum series, while the 15 September
observations showed almost no signal. This may be attributed to the South Pole
being near the center of the ozone hole during early September, far from the
vortex edge where there is greater dynamical influence on ozone observed
(Hassler et al., 2011b). Also, the first signs of the polar vortex weakening
typically appear as layers of higher ozone and sudden increases in
temperature above 24 km altitude at South Pole Station.
The lower panel (Fig. 4b) shows the result of subtracting selected series in
Fig. 4a in order to illustrate the 14–21 km layer total ozone loss each year
(winter average minus the spring minimum) and the loss that occurred before and
after 15 September. From 1991–2000, there was an increasing trend in the
14–21 km column ozone loss during the first half of September (blue line in
Fig. 4b) reaching a peak of 100 DU loss in 2000. This was followed by a
downward trend with significant variability, until reaching a relatively
stable 60–65 DU after 2014. The loss during the second half of September
depends on the amount of ozone remaining on 15 September and meteorological
conditions governing the stability of the polar vortex. An early vortex
weakening or breakup may result in transport and mixing of high-ozone air
masses with the depleted ozone thus reducing or ending loss before the end
of September. The year 2021 shows the second-highest overall loss on record
at 133 DU. This year began at a slower than average pace with only 59 DU of
ozone loss by 15 September but followed with a record loss of 74 DU for the
second half of September when only 3 DU remained in the 14–21 km layer on 1
October.
September column ozone loss rates: 14–21 km
The South Pole September 14–21 km column ozone loss rate is a key metric
suggested by Hofmann et al. (1997, 2009) for observing changes related to
potential recovery in stratospheric ozone. The metric is useful since the
rapid ozone loss during the month of September follows a nearly linear
decline which can be compared with the 1986–2021 median loss rate of -3.5±0.3 DU d-1 (see Fig. 3).
Figure 5 shows the yearly September ozone column (14–21 km) loss rates from
the ozonesondes launched every 2–4 d during late August until
mid-October. The selection of the start and end day for the ∼30 d loss period is adjusted forward or backward by ±3 d to
obtain the best linear fit to the observations (see method description in
Hassler et al., 2011a). In late September, near the minimum date, the linear
depletion data point selection ends when a sharp increase in ozone either is
observed or, in severe depletion years, drops to near-zero or shifts to a
nonlinear loss rate when approaching ozone loss saturation. The selected
values between the start and end points are used to determine the yearly
loss rate slope and uncertainty by simple linear regression. The 36-year
time series shows that the 14–21 km column ozone loss rate has increased
(lower ozone loss rate) from a minimum of -3.8 DU d-1 during 2002–2007 to
-3.0 DU d-1 in 2016–2021. The sudden stratospheric warming in 2002 (red
dot) showed rapid ozone loss but within a shortened time period ending on
22 September. This was the date when the first sign of the sudden
stratospheric warming (increase in ozone and temperature) began to show at
altitudes above 21 km. The following ozonesonde profile on 25 September 2002
showed substantial ozone increases throughout the 15 to 32 km layer
elevating TCO to 397 DU, the highest ever observed during September and
October over the South Pole. In 2019, the loss rate calculation period was also
shortened to just 2 weeks before the linear ozone decline ended on 15
September. The loss rate data point for 2019 is included in Fig. 5 with
high uncertainty.
Linear loss rates (DU d-1) within the 14–21 km column
during 1–30 September. The yearly data include 1σ
uncertainty bars. The loss rates calculated for the three anomalous vortex
years, shown as red dots, do not include any measurements after the vortex
disruption was observed in middle to late September.
September ozone mixing ratio loss rates: 119–33 hPa
Hassler et al. (2011a) analyzed vertical profiles of ozone mixing ratio loss
rates during September in 20 layers from 200 to 10 hPa showing that peak
loss rates occurred within the 48 and 33 hPa layers. The 89 hPa pressure level was found to be the optimum layer for detecting
significantly lower loss rates based on model estimates of future declining
equivalent effective stratospheric chlorine (EESC) and lower variability in
measured ozone loss at this level. Early detection was estimated to occur
sometime within the 2017–2021 period.
Figure 6 shows the September ozone mixing ratio loss rates (ppmv d-1) for
five selected pressure levels from 119 hPa (13.6 km) to 33 hPa (20.6 km).
Following the analysis method by Hassler et al. (2011a), the ozone mixing
ratios during September (∼ days 235–270) are grouped in 5-year
intervals to reduce the influence of prevailing phases of the quasi-biennial
oscillation (QBO) and other dynamical processes affecting temperature,
meridional transport, and mixing depending on polar vortex conditions.
Figure 6 shows that overall loss rates peaked during 2001–2005. All the
selected pressure levels showed decreases in loss rates by 2016–2020. The 89 hPa loss rate showed improvement (29 % decrease) with the lowest
variability as predicted in the Hassler et al. (2011a) assessment. The
highest altitude layer at 33 hPa showed a substantial 49 % decline
from the 2001–2005 peak loss rate.
September loss rates (ppmv d-1) calculated within 5-year
blocks at five selected pressure levels within the primary depletion layer
from 119 hPa (13.6 km) to 33 hPa (20.6 km) at the South Pole. The linear fit to
the data includes 1σ uncertainty bars.
Figure 7 shows the 1986–2021 temperature time series for the five pressure
levels during the month of September and during winter before sunrise from
15 July to 15 August when stratospheric temperatures remain cold (-90 to -94∘C) and stable. These data indicate that there have been no
systematic winter temperature trends at these altitudes that may affect
stratospheric-cloud particle surface area and heterogeneous ozone
destruction chemistry. However, 2021 was the coldest September observed at
all pressure levels. In addition, Fig. 7 shows that the stable vortex in
2021 remained very cold with almost no temperature difference between winter
and September at pressure levels of 119, 89, and 67 hPa and just 2 to
3 ∘C warmer at 48 and 33 hPa. This was similar to the record-low-ozone year in 2006, but the 2006 temperatures were not as cold as 2021. The
highest temperatures occurred in 2002 after the sudden stratospheric warming
on 22 September when the 30–50 ∘C increase in temperature
observed from 100–20 hPa sent the September temperature average off the scale in
Fig. 7.
Average 30 d winter temperatures from 15 July to 15
August (black diamonds) and during 1–30 September (red circles) from 1986 to
2021. The pressure levels are selected to correspond with the ozone loss
rates shown in Fig. 6 within the primary ozone depletion altitude region
from 119 hPa (13.6 km) to 33 hPa (20.6 km).
Ozone loss saturation: near-zero-ozone layers
The near-complete destruction of stratospheric ozone (loss saturation)
within the 14–21 km layer is a feature of the Antarctic ozone hole that is
observed in detail by high-resolution ozonesondes. Near the end of
September, as the linear decrease in ozone begins to slow, other nonlinear
depletion reactions complete the pathway to near-zero ozone (Grooß et
al., 2011; Kuttippurath et al., 2018; Müller et al., 2018). A reduction
in the vertical extent of the near-zero-ozone layers will be an important
indicator of recovery showing when decreasing EESC is no longer the excess component in the
reactions that destroy ozone (Kuttippurath et al., 2018).
The vertical extent of near-zero-ozone layers observed over the South Pole is
shown in Fig. 8 as a curtain plot of dark to light gray shaded bars
showing only the lowest partial-pressure minimums from 0 to 0.7 mPa. This range of values in the shaded regions represent a 95 %–99 % loss
compared to the 14–16 mPa values observed during the winter before ozone
depletion begins (see Fig. 2). The near-zero-ozone values for each year are
selected from the lowest-partial-pressure ozone out of all seven to nine vertical profiles measured during the ozone hole minimum period (26 September–15 October). All
values greater than 0.7 mPa are not included in order to highlight the
lowest-ozone region. The three early vortex breakup years, listed in Table 2, are shown as light red bars when ozone minimums were >4–5 mPa.
The South Pole 10–24 km curtain plots highlighting the lowest
observed ozone partial pressure of 0 to 0.7 mPa represented by
the dark to light gray shading during the yearly ozone hole minimum period
from 24 September to 15 October. All values greater than 0.7 mPa are
excluded from the scale shown.
Figure 9 shows the average temperature curtain plot during the minimum-ozone
period. The PSC threshold temperature of -78 ∘C is selected as
the break point between cold (blue) and warm (red) temperature scales.
Together, Figs. 8 and 9 show the coincidence of years with cold temperatures
and low ozone. The recurring year-to-year severe depletion from 1991–2001
also shows that the upper boundary of the near-complete ozone loss layer was
extending to higher altitudes each year, eventually peaking at 21 km.
Hofmann et al. (1997) and Hoppel et al. (2003) noted that a reversal of
ozone loss in the upper altitudes may be an important indicator of
stratospheric ozone recovery as ODSs decline.
The average temperature during 24 September to 15 October
(ozone hole minimum period over South Pole Station). The blue to red
transition temperature falls at -78∘C to highlight the
polar stratospheric cloud formation threshold temperature.
Figure 8 shows that after the record low in 2006, the near-zero-ozone
vertical extent appeared to be narrowing and becoming irregular. Then in
2020 and 2021 ozonesondes observed the optimum cold polar vortex conditions
in September to late October along with extensive near-zero ozone within
13.5 to 20.5 km altitude. However, the 7 km near-zero-ozone layer was not
observed in a single profile as it was in the record-low-ozone profile in
2006. For example, in 2021 the near-zero-ozone layer was initially observed
at 15–20.5 km on 1 October and from 13.5–17.5 km on 21 October.
Severe ozone loss extending below the 14–21 km layer is not common during
the minimum-ozone period over the South Pole. However, the near-zero-ozone
region may extend below 14 km following major volcanic eruptions. The
transport of volcanic plumes may eventually bring sulfate aerosol into the
polar stratosphere leading to additional ozone loss through surface
reactions that are similar to PSC heterogeneous chemistry (Hofmann and
Solomon, 1989). For example, there was significant depletion from 12–14 km
in 1992–1994 after the major eruption of Mount Pinatubo (15.1∘ N;
Philippines) in 1991 (Hofmann and Oltmans, 1993; Deshler et al., 1996). The
Calbuco eruption (41.2∘ S; Chile) (Bègue et al., 2020) in
2015 also led to an increase in ozone loss below about 14 km (100 hPa) at the
South Pole (Stone et al., 2017). More recently, the 15 January 2022 Hunga
Tonga–Hunga Ha'apai (20.5∘ S) volcanic eruption
plume in the SW Pacific Ocean reached altitudes of nearly 55 km (Carr et
al., 2022). This recent mass injection and transport of volcanic aerosols,
SO2, and water vapor (Millán et al., 2022; Vömel et al., 2022)
may enhance future stratospheric ozone depletion within the Antarctic polar
vortex. The ongoing ozonesonde observations along with the 36-year ozone
climatology record over the South Pole will assist in identifying any additional
ozone loss layers.
The South Pole yearly ozone hole minimums in total column
and 14–21 km partial column ozone and the September loss rates during the
long-term record (1986–2021) compared to the record low in 2006 and the
extreme variability in the metrics in 2019–2021.
South Pole Station ozonesondes provide essential year-round
high-resolution ozone and temperature profiles tracking all phases of the
Antarctic yearly ozone hole near the core of the polar vortex, monitoring
both the winter development of the vortex and the conditions that lead up to
rapid ozone depletion in September, as well as looking precisely in the region
where the ozone loss is taking place. The 36-year ozonesonde record has been
reviewed, homogenized, and validated by comparing TCO with
the South Pole Dobson spectrophotometer DS TCO
measurements. Ozonesondes show a positive bias with respect to the Dobson
TCO of 2 ± 3 %.
The 2019, 2020, and 2021 South Pole ozonesonde measurements showed the
greatest year-to-year variability in ozone hole conditions ever observed in
the South Pole long-term record. Table 3 shows several of the key metrics
for those years compared to the long-term median and record low in 2006. The
anomalous polar vortex warming in 2019 disrupted ozone depletion in early
September, resulting in the second-weakest ozone hole on record, but the
following 2 years saw cold and persistent vortex conditions with
TCO minimums dropping to the 8th and 12th lowest in the 36-year record. In
addition, the 2020 and 2021 profiles in early to late October showed
near-zero ozone (ozone loss saturation) within the 14–21 km altitude layer
under the persistent cold and stable vortex conditions.
The time series (1986–2021) of 14–21 km column ozone during the winter
months (15 June–15 August) shows no trend, averaging 130 ± 10 DU.
However, rapid ozone loss during 1–30 September at rates of -3.5± 0.3 DU d-1 results in 95 %–99 % loss of ozone in the 14–21 km layer.
Minimums of near-zero ozone (∼ 1–2 DU) were observed every
year from 1993–2001. This was followed by an irregular upward trend from
2002–2021 with the minimum 14–21 km column ozone values ranging from 1–9 DU. The near-zero-ozone minimum years after 2001 include the following: 2003, 2005, 2006,
2011, 2015, 2020, and 2021.
The 15 September 14–21 km column ozone time series indicates a turnaround
year in 2000/2001. The simple linear regression lines show decreasing ozone
from 1986–1999 at -2.3± 0.6 DU yr-1 followed by a reversal to a slight
positive trend at +0.9 ± 0.4 DU yr-1 in 2000–2021. This pattern is
consistent with several Antarctic ozone hole studies focused on the detection of
ozone recovery due to decreasing ODSs (Langematz et al.,
2018; Petropavlovskikh et al., 2019).
The September mixing ratio loss rates at selected pressure levels averaged
within 5-year blocks all showed improvements by 2016–2020 compared to the
peak loss period in 2001–2005. The ozone loss at 33 hPa showed the greatest
improvement with a 49 % reduction in loss rate. The optimum pressure level
(89 hPa) for detecting significantly lower loss rates showed a 26 %
reduction with the lowest variability as predicted by Hassler et al. (2011a).
The long uninterrupted 36-year South Pole ozonesonde record and future
balloon-borne measurements provide unique and vital data for ozone hole
analyses. The continuing year-round ozonesonde observations at South Pole
Station will be beneficial for observing anomalies in the ozone layer driven
by meteorological events disrupting the polar vortex and for identifying
layers where volcanic aerosols influence ozone depletion.
Data availability
The South Pole ozonesonde data records are publicly available from the NOAA Global Monitoring Lab (GML) at https://gml.noaa.gov/aftp/ozwv/Ozonesonde/ (NOAA Global Monitoring Lab, 2023). The South Pole ozonesonde data are also made available at
https://www.ndacc.org (NDACC, 2022).
Author contributions
BJJ analyzed the data and wrote the manuscript. PC and CS reviewed, edited, and homogenized the long-term ozonesonde data. JB carried out several years of collecting data from ozonesondes and ground-based Dobson. GlM reviewed and provided the ground-based Dobson ozone data. BJJ prepared the manuscript with contributions from all co-authors.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Atmospheric ozone and related species in the early 2020s: latest results and trends (ACP/AMT inter-journal SI)”. It is a result of the 2021 Quadrennial Ozone Symposium (QOS) held online on 3–9 October 2021.
Acknowledgements
The authors are indebted to the many personnel who conducted the balloon
flights over the 36-year period at the South Pole, spending full-year
assignments in extreme cold and high-altitude conditions. Without their
dedicated service to the US National Oceanic and Atmospheric Administration
this work would have been impossible. We thank Allen Jordan, the
NOAA/GML/OZWV division programmer/electronics engineer, for the ongoing
development of the extremely valuable balloon-tracking telemetry and data
analysis SkySonde software. Birgit Hassler was supported by the Helmholtz
Society project “Advanced Earth System Model Evaluation for CMIP”
(EVal4CMIP). We also acknowledge the logistics support in Antarctica
provided by the National Science Foundation, Office of Polar Programs.
Finally, we sadly note the passing of coauthor John Booth in June 2021,
several months after being diagnosed with an aggressive form of cancer. In
15 years working with NOAA, John spent 11 of those wintering at the
South Pole consistently carrying out meticulous ozonesonde and Dobson
observations at South Pole Station.
Financial support
This research has been supported by the US National Oceanic and Atmospheric Administration base funding and observatory operations.
Review statement
This paper was edited by Jens-Uwe Grooß and reviewed by two anonymous referees.
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