Polar ozone depletions are formed during the period from late winter to spring inside stable large-scale vortices and stratospheric polar vortices as a result of heterogeneous and photochemical reactions involving chlorine reservoirs (chlorohydrogen and chloronitrate) and polar stratospheric clouds as the “surfaces” for heterogeneous reactions [1–4]. The scale and intensity of ozone destruction during the formation of ozone depletion are determined by the stability of the polar vortex during this period and during the preceding winter months [5, 6]. Polar vortices form annually in the fall above the winter hemisphere due to an increase in the stratospheric meridional temperature gradient; they usually dissipate in the spring, when the temperature gradient decreases (the Arctic polar vortex occasionally disappears in late winter) [7, 8]. In terms of thermodynamics, the winter peak of intensity of the polar vortex is substantiated, because, in the polar region, the temperature decreases during the polar night [9]. In the Southern Hemisphere, the peak of intensity of the polar vortex is usually observed in September, simultaneously with the maximum in temperature variations in subtropics, which is affected by the seasonal temperature variations in the lower subtropical stratosphere. The latter determines to a great extent the stratospheric meridional temperature gradient [9]. However, over the past 30 years, a distinct tendency toward a shift of stability of the Antarctic polar vortex to the late spring and early summer has been observed. It results in prolongation of the period of existence of the Antarctic ozone hole.

To analyze the dynamics of the Antarctic polar vortex and the area of the ozone hole, we used the hourly data of the ERA5 reanalysis [10] with 0.25° × 0.25° horizontal resolution: the speed of the zonal and meridional wind, the geopotential and mass ratio of the ozone mixture at 50 hPa, and the daily average satellite data on the ozone hole (area with a total ozone content below 220 Dobson Units) for the region 40°–90° S over the period 1992 to 2021. The data were obtained from the NASA Goddard Space Flight Center (GSFC, http://ozonewatch.gsfc.nasa.gov). We delineated the vortex using the geopotential values, which were determined by the maximum temperature gradient and the maximum wind speed [11, 12]. A geopotential depends only on pressure and temperature [13]. Thus, it describes the dynamics of the polar vortex well, because a significant decrease in temperature and in pressure is observed inside the vortex and an increase was recorded outside it. In addition, the geopotential is not affected by significant seasonal changes during the existence of the vortex and, accordingly, is well suited for determining the boundaries of the polar vortex. Along the vortex boundary, the maximum temperature gradient and the maximum wind speed are generally observed. The mean value of the geopotential F* near the maximum temperature gradient along the boundary of the Antarctic polar vortex at the level of 50 hPa is F* = (19.30 ± 0.17) × 104 m2/s2 [11, 12]. The vortex area, the average wind speed along the vortex boundary, and the average mass ratio of the ozone mixture inside the vortex were calculated on the basis of the fact that the boundary of the Antarctic polar vortex at the 50 hPa level is determined by the geopotential of 19.3 × 104 m2/s2.

Figure 1 demonstrates the interannual variations in the area of the Antarctic polar vortex, the average wind speed at the vortex boundary, and the area of the Antarctic ozone hole averaged for October 1–15, October 16–31, November 1–15, November 16–30, and December 1–15 for the last 30 years (1992–2021). Figure 1 also illustrates the linear trends corresponding to each time period and their determination coefficients R2 (the closer R2 is to 1, the higher the significance level of the trends). Anomalous data for 2002 and 2019 were excluded from consideration. In these years, the vortex dissipated earlier (in late October) accompanied by splitting in the first case and by strong shifts in the second case (main and minor sudden stratospheric warmings were recorded) [14, 15]. Ozone depletion in the Antarctic usually exists from August to November, reaching the maximum values in September–October. In October and in the first part of November, the trends of changes in the considered characteristics are insignificant (R2 < 0.1, Fig. 1). In the second half of November and, especially, in December, the data are more spaced out due to the episodic earlier destruction of the vortex and have a distinct positive trend. The values for 2006, 2015, and 2020, when the greatest depletion of ozone was observed, are distinguished in all plots of Fig. 1 [16, 17]. Between the changes in the characteristics in 2006, 2015, and 2020, there is also a tendency toward late spring strengthening of the polar vortex, proceeding with an increase in the area of the ozone hole in late spring and early summer (Fig. 1).

Fig. 1.
figure 1

The temporal variations of the area of the Antarctic polar vortex and the average speed of wind along the vortex boundary at 50 hPa, as well as the areas of the Antarctic ozone hole on average for the periods October 1–15, October 16–31, November 1–15, November 16–30, and December 1–15 from 1992 to 2021.

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To consider the dynamics of the Antarctic polar vortex in the years with the greatest ozone depletion in detail, Figs. 2 and 3 illustrate the changes in the vortex characteristics and ozone depletion from July to December 2006, 2015, and 2020. Figure 2 shows intra-annual changes in the area of the Antarctic polar vortex, the average speed of wind along the vortex boundary, and the area of the Antarctic ozone hole in 2006, 2015, and 2020 compared to the 30-year average changes in these characteristics with standard deviations (MSD, ±1 σ). In 2006, the maximum area of the ozone hole was observed in September and, in 2015, in October; however, in 2020, record levels of the anomalous areas of the ozone hole were recorded from mid-November to December compared to the period from 1979 to 2021. A significant increase in the area of the Antarctic polar vortex in late spring and early summer was also recorded in 2015 and 2020, while in 2006, the maximum area of the vortex was observed in August (Fig. 2). In addition, in 2020, the average wind speed along the vortex boundary unusually increased over the entire period of existence of the polar vortex. Actually, the dynamics of the studied characteristics in Fig. 1 and in the years with the strongest vortex in Fig. 2 reflects the prolongation of the stable period of the Antarctic polar vortex in late spring and early summer (in 2006, the vortex intensity peak was observed in September; in 2015, in October, while in 2020 the anomalous intensity was registered in November and December). This tendency is reflected in 2020, when the Antarctic polar vortex existed until the last week of December, which is unprecedented.

Fig. 2.
figure 2

The intra-annual variation in the area of the Antarctic polar vortex, the average speed of wind along the vortex boundary at 50 hPa, and the area of the Antarctic ozone hole from July to December 2006, 2015, and 2020 against the average values for 1992 to 2021 with MSD (±1 σ).

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Fig. 3.
figure 3

The fields of the geopotential, wind speed, and mass ratio of the ozone mixture at 50 hPa over Antarctica from July to December 2006, 2015, and 2020.

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One of the dynamic characteristics of the polar vortex is the presence of a dynamic barrier [12]. Figure 3 shows the fields of geopotential, speed of wind, and mass ratio of the ozone mixture for the 1st and 15th dates from July to December 2006, 2015, and 2020. They reflect the dynamics of the polar vortex in the years considered. The values of 19.3 × 104 m2/s2, characterizing (according to the vortex delineation method) the Antarctic polar vortex boundaries, are connected by a line on the geopotential fields. On the wind speed fields, the values of 20 m/s, reflecting the presence of a dynamic barrier, are connected by a line. For all the years considered, a very strong polar vortex with deep ozone destruction in the spring period was observed. In September 2006, the polar vortex was most symmetrical and large-scale, while beginning in October, it elongated and gradually weakened. In 2015, the stability of the vortex increased from October to November. In turn, in 2020, the vortex stability increased in late spring and early summer, starting in early November. In the fields for December 15, 2020 (in contrast to 2006 and 2015), the polar vortex is still traceable and is characterized by the presence of a dynamic barrier.

Therefore, this study, using the delineation method, shows the trend for prolongation of the stability period of the Antarctic polar vortex in the lower stratosphere in the late spring and early summer periods observed over the last 30 years. This is manifested both in the dynamics of the main characteristics of the polar vortex (the vortex area and the average wind speed along the vortex boundary) and in the area of the Antarctic ozone hole. The trend of the late spring intensification of the polar vortex is also observed in the years with the strongest vortices: 2006, 2015, and 2020. It was reflected in the increase in the area of the ozone hole. In 2020, anomalously high values of the average wind speed along the vortex boundary during the entire period of its existence and anomalously high values of the vortex area from mid-November were observed. The Antarctic polar vortex in 2020 existed until the last week of December, which is an unprecedented case. The tendency toward prolongation of the period of stability of the polar vortex in the lower stratosphere is reflected in the vortex dynamics in the middle and upper stratosphere in spite of the fact that the weakening of the vortex at these heights occurs earlier. The stability of the Antarctic polar vortex in the lower stratosphere in the late spring period can increase under the conditions of increasing temperature in the lower subtropical stratosphere, as was observed in 1987, 1998, 1999, 2001, 2006, 2011, and 2015 [18].