The stratospheric polar vortex is a seasonal low-pressure system characterized by a strong wind belt that isolates polar air from lower latitudes. It appears due to the seasonal cooling associated with the decrease of solar radiation above the pole . As the incident solar energy decreases, and the temperature gradient between the pole and the tropics becomes stronger, the strength of the stratospheric westerly winds increases. When the winds reach a critical value, a large-scale vortex is formed, which extends from the lowermost stratosphere to the stratopause. Depending on the altitude, the maximum area encompassed by the polar vortex exceeds millions of square kilometers . Above an altitude of about 14 km, the vortex edge region is stable and constitutes a powerful barrier, preventing mixing of cold polar air with warmer air masses from lower latitudes. Over Antarctica, the polar vortex is generally present from April until December with a large variability in the breakup dates resulting from the year-to-year variability of dynamical processes in the stratosphere (; ). Conversely, the less stable Arctic polar vortex has more year-to-year variability (e.g., ; ; ). It forms in November and lasts until the end of February or early April, depending on the year. The stratospheric polar vortex has been the subject of studies linked to the ozone layer depletion, which started in the late 1970 (; ). Ozone loss occurs in both hemispheres. This loss is variable in the Northern Hemisphere, as many studies have shown (; ; ; ; ). In the Southern Hemisphere, the ozone hole, defined as an area with total ozone values less than 220 DU, has become a recurring seasonal phenomenon. Ozone destruction begins in late winter, close to the edge region of the polar vortex, as solar radiation increases over the Pole. The destruction of ozone inside the southern vortex accelerates from late August until late September or early October, reaching an almost complete destruction of ozone in the lower stratosphere. The depletion of the ozone layer is caused by anthropogenic emission of ozone-depleting substances (ODS, mainly chlorofluorocarbons and halons and their industrial substitutes), which enhances ozone destruction cycles by halogen compounds. This depletion is largest in the polar vortex due to the activation of chlorine species through heterogeneous reactions that take place at the surface of polar stratospheric clouds (PSCs) which form in the cold polar vortex . The increase in solar radiation over the Pole at the end of winter triggers rapid chemical cycles which quickly destroy ozone, leading to the appearance of the well-known ozone hole over Antarctica (e.g., ). By the end of spring, stratospheric temperatures increase, the polar vortex breaks up, and ozone-depleted air masses dilute into the Southern Hemisphere. From one year to the next, the severity of the ozone hole depends on the strength of the polar vortex, its minimum temperatures, and its duration. The future recovery of the ozone layer and disappearance of the ozone hole depend on the evolution of the polar vortex under the influence of both the decrease in ODS abundance in the stratosphere and the increase in greenhouse gases (GHG) as both phenomena impact radiative, dynamical, and chemical processes in the stratosphere. Many studies document this phenomenon (e.g., and references therein). The polar vortex also has an impact on the climate surface in both hemispheres. Indeed, studies have shown an effect of the stratospheric polar vortex displacements on cold spells in the Northern Hemisphere, in North America . In the Southern Hemisphere, other studies have shown that a weak vortex can have an influence on the surface climate in Australia. highlighted that selected years of lower vortex intensity result in higher temperatures and less precipitation over eastern Australia. The dramatic weakening of the Antarctic vortex in 2019 had a large impact on meteorological conditions over the country that resulted in the severe Australian fires at the turn of the year 2019/2020.

The inner vortex is characterized by high absolute values of potential vorticity (PV). As this parameter is conserved on isentropic surfaces for weeks, PV maps on such surfaces represent one of the primary diagnostic tools for analysis of the dynamical processes in the stratosphere and inside the polar vortex. first represented daily PV global maps of isentropic surfaces, demonstrating a material separation in the stratosphere between the main vortex, characterized by high absolute PV values, the surf zone, characterized by weak absolute PV values, and a zone of strong meridional PV gradient in between: the so-called vortex boundary or vortex edge, which is an area of low mixing representing a dynamical barrier to air mass exchanges. Numerous studies on the vortex boundary definition have been conducted. defined the vortex edge as the location of the maximum PV gradient as a function of equivalent latitude (EL), weighted by the mean wind speed. EL defines the latitude limit of the polar area which exceeds a certain PV value (maximum PV is then given at EL = 90∘, e.g., ). The mean wind speed is the mean of the wind values around an equivalent latitude contour. A PV field sorted by EL will then make the polar vortex concentric around the pole. This is the method used in this study. has developed the effective diffusivity diagnostic, which is applied on tracers to identify transport barriers and mixing regions. used this method to quantify the transport of polar vortex air to mid-latitudes, as well as to evaluate the polar vortex barrier intensity. The method of elliptical diagnostics of a contour used by consists in fitting an ellipse to the contour of a parameter. It subsequently determines several variables of this ellipse, for example, latitude and longitude of the center, the equivalent latitude, or its orientation. It is possible to calculate the elliptical diagnostics of a contour of conservative tracers such as PV or long-lived chemical species around the polar vortex edge region . The vortex forms in autumn, intensifies throughout the winter, and disappears in spring/summer. Its overall strength is variable from one year to the next. Different studies have analyzed the interannual variability of the polar vortex induced by forcings such as the solar flux (SF), quasi-biennial oscillation (QBO) and El Niño–Southern Oscillation (ENSO), particularly in the Northern Hemisphere. QBO is a quasi-periodic oscillation of the equatorial zonal wind between easterlies and westerlies. made a composite study of zonal wind in the Northern Hemisphere at 50 hPa from 1962 to 1977 based on the different QBO phases. They showed that the vortex is less disturbed during the west phase of the QBO (wQBO) at 50 hPa than during the east phase (eQBO). evaluated the temperature and strength of the Arctic polar vortex according to the solar cycle and the QBO. They found that the vortex is warm and weak during solar maxima/eQBO phases, and cold and strong during solar minima/wQBO phases at 50 hPa. support this finding that the state of the Northern Hemisphere polar stratosphere is less perturbed during solar cycle minimum and westerly QBO phases. Then, showed, over a period of 18 years, that the Antarctic polar vortex at 10 hPa is slightly colder during wQBO. ENSO is an irregular oscillation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting the climate of the tropics and subtropics. It also influences other climatic parameters such as precipitations worldwide and ozone levels in the lower stratosphere . indicated that the El Niño events are associated with a warming and weakening of the polar vortex in the polar stratosphere in both hemispheres, and showed that early breakup of the southern polar vortex occurs during El Niño events. By contrast, did not find a significant impact of the canonical ENSO index on the Southern Hemisphere polar vortex both in observations and in modeling studies. With indices of Niño-3 and Niño-4 regions, reported that during a “warm pool event” (positive SST in Niño-4 regions) the heat flux is higher and the Antarctic vortex breaks up earlier. Several methods have been suggested in order to determine the onset and breakup dates of the polar vortex. They are based on a minimum area computed from equivalent latitudes (; ) or mean wind speed thresholds along the edge (e.g., ). The latter is used in to calculate the dates on which the Arctic and Antarctic polar vortex breaks each spring.

The objective of this paper is to analyze the long-term evolution of the intensity, position, and duration of the southern polar vortex edge as a function of equivalent latitude over several decades (1979–2020). ERA-Interim reanalyses and operational data from the European Centre for Medium-Range Weather Forecast (ECMWF) are used for the study, which includes an evaluation of the onset and breakup dates of the polar vortex during this period. At an interannual scale, the signature of the 11-year solar cycle, QBO and ENSO, is evaluated on the vortex edge evolution. This is the first study of the variability of the Antarctic stratospheric polar vortex edge and its persistence over a long period (42 years).

The paper is organized as follows. Section 2 presents the ECMWF dataset and the data sources of the forcings (SF, QBO, and ENSO) used for the analysis of interannual variability of the polar vortex edge. Section 3 describes the methods used in the study, such as the MIMOSA (Modélisation Isentrope du transport Méso-échelle de l’Ozone Stratosphérique par Advection) model , which is used to construct the PV maps as a function of potential temperature and equivalent latitude. The methods used for the vortex edge characterization and for determining the onset and breakup dates of the polar vortex are also discussed in this section. Section 4 presents the statistical analysis of the annual evolution of the vortex edge over the study period as well as its interannual evolution, related to the SF, QBO, and ENSO forcings, while results on the interannual evolution of the vortex onset and breakup dates are given in Sect. 5. A further discussion of the results and perspectives of the study are presented in Sect. 6.

The evolution of the onset dates of the polar vortex during winter from 1979 to 2020 at 675, 550, and 475 K isentropic levels is displayed in Fig. . It represents the day of the year when the polar vortex is well formed, e.g., when the horizontal mean wind speed at the vortex edge is above the threshold values of 15.2, 20, and 25 m s-1, as suggested by .

Due to the stronger radiative processes in the upper stratosphere, the temperature contrast between the polar region and mid-latitudes is stronger and the polar vortex forms more rapidly with a faster wind. Thus the vortex forms earlier at the highest levels: the average day of the year the onset date occurs for all thresholds combined is on days 90, 98, and 108 at 675, 550, and 475 K, respectively. Also, the onset date occurs later for the larger threshold values as the wind strength increases in autumn in the polar stratosphere. The differences between onset dates according to the different threshold values decreases with altitude. At 475 K, the mean values of the onset dates are days 93, 109, and 125 for the 15.2, 20, and 25 m s-1 thresholds, respectively. However, some years show a large difference between the onset dates according to the different threshold values, which can exceed 1 month (e.g., in 2002, 1.5 months between 15.2 and 25 m s-1). For example, the winter of 2002 was characterized by a difference of 1.5 months between the two extreme threshold values, as the wind at the beginning of the winter was weaker compared to other winters. This is actually the first winter in which an SSW was observed, as mentioned previously. Due to the slower and less stable wind at 475 K, the vortex forms slowly and there is an important interannual variability of onset dates with an average difference of 32.9 d between 15.2 and 25 m s-1 during the whole period. There are some outstanding late onset dates at 475 K, particularly for the 25 m s-1 threshold, e.g., on day 152 in 2002 and day 149 in 2014. By contrast, the year 1992 was characterized by an early onset on day 73 for the 15.2 m s-1 threshold. The 550 and 675 K levels show comparatively less variability in the onset dates for the various threshold values and the difference between the onset dates for the largest and lowest threshold values is of the order of 10 d on average (21 and 17.2 d at 550 and 675 K, respectively, between the 25 and 15.2 m s-1 threshold values). This difference in interannual variability in the onset dates among the levels is further confirmed from the average standard deviation of the three thresholds curves after subtracting a 3∘ polynomial. This standard deviation amounts to ±8.2 d at 475 K, which is almost 2 times larger than the values of the 675 and 550 K levels (±4.8 and ±3.7 d, respectively).

Some long-term variability in the evolution of the onset dates is also observed at the different levels. At 675 K, a decreasing trend is visible between 2010 and 2018 for the 15.2 m s-1 threshold, with a slightly higher interannual variability during this last decade. At 550 K, a similar decrease in the onset date from 2011 is observed, most pronounced for the 15.2 m s-1 threshold. At 475 K, the most prominent feature is a significant decline of the onset dates between 1980 and 1999 for the 25 m s-1 threshold value of about 29 d in 19 years, corresponding to a decline of 1.5 d yr-1. It is also noteworthy that later onset days in 2002, 2012, and 2014, correspond to years with smaller ozone holes (e.g., ).

Figure  shows the day when the polar vortex breaks up in spring at 475, 550, and 675 K isentropic levels. As mentioned in , when the vortex is weakening between early and late spring, the winds at the vortex edge also weaken, leading to the final vortex breakup. The vortex breakup is given when the horizontal mean wind speed along the vortex edge falls below the 15.2, 20, or 25 m s-1 threshold values.

The vortex forms earlier at the highest levels and it also breaks earlier: the average breakup dates for the different threshold values are days 340, 334, and 325 at 475, 550, and 675 K, respectively. found that the average Southern Hemisphere stratospheric final warming at 50 hPa occurs around 2 December with JRA-55 reanalyses, which is consistent with our results at 475 K (on 5 December). We notice some early breakup of the polar vortex: for example, in 1988 (the vortex broke up 13 d before the mean breakup date at 675 K, 20 d at 550 K, and 21 d at 475 K). In 2002, the breakup occurred 18, 9, and 8 d before the mean breakup date at 475, 550, and 675 K, respectively. Some late breakups are observed during the last two decades particularly at 15.2 m s-1. The year 1999 is clearly distinguishable at 475 and 550 K with 21 and 27 d, respectively, after the mean breakup date. The years 2007, 2008, 2010 and 2015 also stand out for the three levels: around 14, 15, and 14 d after the mean breakup date at 475, 550, and 675 K, respectively. Finally, the year 2020 is noteworthy for its exceptionally late breakup date, with a breakup date occurring 20, 21, and 29 d after the mean threshold dates at 475, 550, and 675 K, respectively. The value at 675 K sets a record over the whole study period.

Figure 10 shows that the difference between the breakup dates for the various threshold values is much smaller than for the onset dates. The average difference between breakup dates for 15.2 and 25 m s-1 is equal to 11.5, 8.9, and 8.2 d at 475, 550, and 675 K, respectively, compared to ±32.9, ±21, and ±17.2 d, respectively, for the onset dates. The smaller differences can be explained by the important role of dynamical processes in the vortex breakup while the vortex formation is mainly controlled by radiative processes that are less variable from one year to the next. A larger interannual variability is observed for the breakup dates at the various levels and threshold values. Similarly, as for the onset dates, we calculated the standard deviation over the period after averaging the different curves of the different threshold means and after removing the long-term trend by a 3∘ polynomial. The standard deviation is equal to 10.6, 10.2, and 10.4 d at 475, 550, and 675 K, respectively, compared to 8.2, 4.8, and 3.7 d for onset dates.

An increasing trend of the breakup dates between 1979 and 1999 is seen at all levels, which is more pronounced at 475 K. It corresponds to 35, 30, and 15 d over 21 years at 475, 550, and 675 K, respectively, if we average the different threshold values at the various levels. Just after 1999 the vortex breaks up earlier. Then we observe again a later breakup of the vortex between the mid-2000s and 2010. Finally, we observe again that the vortex breaks up earlier, ending with the very long duration of the 2020 vortex. For all levels, a decrease in the breakup dates after 2000 is observed (apart from the extreme years like 2020).

We analyzed the seasonal evolution of the stratospheric polar vortex edge intensity and position in equivalent latitude in the Southern Hemisphere at three isentropic levels, using ECMWF ERA-Interim data over the period 1979–2020. The interannual evolution of the vortex edge intensity and position, as well as the onset and breakup dates at these three isentropic levels, was evaluated. The parameters studied here display long-term and short-term variations over the period that were analyzed using well-known proxies of atmospheric variability in the stratosphere such as the solar cycle, the QBO, and ENSO. Among the main results of our study, the influence of the increasing ozone hole during the 1980s and 1990s on the studied parameters was clearly noticeable, confirming the results of . This influence is mostly pronounced on the maximum intensity of the vortex edge, with an increase of 0.38 PV units per ∘ EL yr-1 at 550 K and 0.30 PV units per ∘ EL yr-1 at 475 K between 1980 and 1996. The vortex breakup dates show an increasing trend of 1.75, 1.5, and 0.75 d yr-1 at 475, 550, and 675 K levels, respectively, over the 1979–1999 period. We also find a decreasing trend during the same period for the onset dates but in this case only at 475 K and for the 25 m s-1 threshold value (1.5 d yr-1 between 1980 and 1999). We see a decreasing trend in the breakup dates after 2010 but this decrease was halted by the very long vortex duration in 2020, which set a record at the 675 K level, and also by the late breakup in 2021.

The solar cycle and to a lower extent the QBO and ENSO modulate the interannual evolution of the maximum intensity of the vortex edge and the breakup dates. Stronger vortex edge intensity is observed in years of solar minimum. QBO and ENSO further modulate the influence of the solar cycle on the vortex edge, especially at 475 and 550 K. During wQBO phases, the difference between vortex edge intensity for minSC and maxSC years is smaller than during eQBO phases. The polar vortex edge is stronger and lasts longer for maxSC/wQBO than for maxSC/eQBO. Regarding ENSO, which has a lower impact than the QBO, the vortex edge intensity is somewhat stronger during cENSO phases for both minSC and maxSC. During this phase, the difference between minSC and maxSC medians is larger.

These results are mainly in agreement with the literature. found that the strongest influence of the QBO on the southern polar vortex occurs in late spring (November) when the final warming happens. From temperature composites at 10 hPa, they found that the vortex is slightly colder during the western phase of the QBO throughout the winter. Later, found that the southern stratospheric polar vortex breaks down later for combined maxSC/wQBO and minSC/eQBO years. The last 2 years of the study (2019 and 2020) stand out in our analysis. In 2019, the vortex maximum area was particularly small, especially at 475 and 675 K and the vortex broke up quite early. The breakup date at 475 and 550 K for the 15.2 m s-1 threshold is the lowest on record (day 323 at 475 K and 319 at 550 K). In 2020, the vortex area was not particularly large and the vortex edge not particularly strong but its duration set a record at 675 K. This very long-lasting vortex was also characterized by a strong ozone destruction . It will be interesting to see how the southern polar vortex evolves in the coming years.

A major perspective of our study is to extend the period analysis, using ERA5 reanalyses which cover a longer period (from 1950) and with a higher resolution (https://www.ecmwf.int, last access: 15 January 2022) (31 km grid for ERA5 versus 79 km for ERA-Interim). The same parameters for the more widely studied Arctic polar vortex are currently being studied for comparison between the two hemispheres. Other factors, which particularly influence the Northern Hemisphere, such as the Arctic Oscillation/Northern Annular Mode, will be included in the future study.