Dynamical and chemical processes contributing to ozone loss in exceptional Arctic stratosphere winter-spring of 2020

. The features of dynamical processes and changes in the ozone layer in the Arctic stratosphere during the winter-spring season 2019-2020 are analyzed using ozonesondes, reanalysis data and numerical experiments with a chemistry-transport model (CTM). Using the trajectory model of the Central Aerological Observatory (TRACAO) and the ERA5 10 reanalysis ozone mixing ratio data, a comparative analysis of the evolution of stratospheric ozone averaged along the trajectories in the winter-spring seasons of 2010-2011, 2015-2016, and 2019-2020 was carried out, which demonstrated that the largest ozone loss at altitudes of 18-20 km within stratospheric polar vortex in the Arctic in winter-spring 2019-2020 exceeded the corresponding values of the other two winter-spring seasons 2010-2011 and 2015-2016 with the largest decrease in ozone content in recent year. The total decrease in the column ozone inside the stratospheric polar vortex, 15 calculated using the vertical ozone profiles obtained based on the ozonesondes data, in the 2019-2020 winter-spring season was more than 150 Dobson Units, which repeated the record depletion for the 2010-2011 winter-spring season. At the same time, the maximum ozone loss in winter 2019-2020 was observed at lower levels than in 2010-2011, which is consistent with the results of trajectory analysis and the results of other authors. The results of numerical calculations with the CTM with dynamical parameters specified from the MERRA-2 reanalysis data, carried out according to several scenarios of accounting 20 for the chemical destruction of ozone, indicated that both dynamical and chemical processes make contributions to ozone loss inside the polar vortex. In this case, dynamical processes predominate in the western hemisphere, while in the eastern hemisphere chemical processes make an almost equal contribution with dynamical factors, and the chemical depletion of ozone is determined not only by heterogeneous processes on the surface of the polar stratospheric clouds, but by the gas-phase destruction in nitrogen catalytic cycles as well. and changes and their possible causes in the data identify the relative role of in the et al., 2020), but meteorological fields are not calculated but specified from Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) reanalysis (Gelaro et al., 2017). The RSHU CTM a 5°×4° horizontal resolution in longitude by latitude and 31 vertical sigma levels from the surface up to approximately 60 km. The distributions of the 74 oxygen, hydrogen, nitrogen, chlorine, bromine, carbon and sulfate gases are calculated in the manner described by 185 Smyshlyaev et al. (1998). Polar Stratospheric Clouds (PSCs) formation and evolution is treated as a super cooled ternary solution (H 2 O/HNO 3 /H 2 SO 4 ) based on the Carslaw (1995) parameterization. PSCs surface variability, depending on temperature, pressure and partial pressures of the relevant gases, including denitrification and dehydration through sedimentation is taken into account according Sovde et al., (2007) and Smyshlyaev et al. (2010). the beginning of April (Fig. 1c), the ozone anomaly turned further eastward, shifting towards Greenland, the Norwegian Sea, Svalbard and Franz Josef Land. The minimum values less than 200 DU were observed in the Northern part of Greenland. By mid-April, the ozone anomaly continued to move eastwards, covering most of the Arctic region of the Eastern Hemisphere with minimum values less than 220 DU in the Franz Josef Land archipelago. At the same time, the territory with values below 300 DU also covered a significant part of the North-West Russia. spread ended nearly in two weeks. We assume that the anomalously warm winter 2019-2020 (and especially in February- early March with high positive AO index) contributed to the strengthening of the stratospheric polar vortex due to a decrease in the propagation of wave activity into the stratosphere from the troposphere over the north-eastern Eurasia. It is known that the main source of wave activity 290 propagation into the stratosphere, characterized by the maximum of the vertical component Fz of Plumb's fluxes, (e.g. Jadin 2011) is located over this region. Comparison of two diagrams with Plumb vertical component Fz at 100 hPa averaged over February 7 - March 7, 2020 and corresponding climate mean over 1981-2010 shows that weakening of upward wave activity propagation in the first period was observed over the north-eastern Eurasia (Fig. 4 b, c). In the same time, a strong stratospheric polar vortex in February-March 2020 influenced the troposphere, enhancing the positive AO phase. numerical experiments with a chemical-transport model carried out, in which the dynamic parameters were set from the MEPRA-2 reanalysis data. The use of temperature, wind speeds, surface pressure and air from the reanalysis data made it possible to simulate the effect of atmospheric circulation on the transport of ozone and associated gases close to reality. Variability of specified dynamical parameters determines the dynamical decrease in ozone content, as well as the atmospheric temperature govern the rate of chemical reactions, polar stratospheric clouds formation and the rate of heterogeneous reactions on their surface, which determine the chemical destruction of PSCs surface area and the area covered by PSCs significantly reduced. In addition, it should be noted that the area where the maximum surface areas of PSC are observed do not completely coincide with the zones of minimum values of the column ozone, which suggests that the relationship between the formation of PSCs and ozone destruction is not linear and confirms the theory of several stages of 415 the formation of ozone anomalies. In the polar stratosphere, associated, first, with the formation of PSCs, then with halogen activation on their surface, and only then with ozone destruction after the return of the Sun after the polar night. 24 in the total from DU in early DU the second of and again the end of Analysis of the noCHEMall scenario reveals that fluctuations in the total ozone content due to dynamical factors amount to more than 150 DU, and analysis of the model simulation results for different scenarios of switching off the chemical destruction of ozone demonstrates that the chemical destruction of ozone at Tura station is from 30 to 50 DU, of which about half (about 25 DU) is the destruction of ozone as a result of halogen activation on the surface of polar stratospheric clouds. At the same time, results of noCHEMall scenario comparison with the average climatic data shows that, due to dynamical factors, the decrease in the ozone content at Tura station is on average greater than at the Pechora station 480 and exceeds 100 DU for some periods.


Introduction
The circulation of the Arctic stratosphere in the winter-spring season (hereinafter winter season) is characterized by strong interannual and seasonal variability, which can affect the tropospheric circulation and weather conditions (Kidston et al., 2015;Nath et al., 2016;Peters et al., 2018;King et al., 2019;Matthias et al., 2020), temperature and chemical composition https://doi.org/10.5194/acp-2021-11 Preprint. Discussion started: 29 January 2021 c Author(s) 2021. CC BY 4.0 License. of the total column ozone were less than 220 DU, which makes it possible to speak about the first such long-term ozone anomaly in the Arctic (Dameris et al., 2020). For the first time in all the years of observations, the analysis of data from a 65 number of certain ozonesondes revealed an extremely strong decrease in the ozone content in the Arctic stratosphere in the spring of 2020, amounting to up to 90% (Wohltmann et al., 2020). MLS satellite data also indicate record low ozone concentrations in the polar stratosphere during Arctic 2020 spring, which began to be recorded earlier than in all other years and ended later than in all other years, with the exception of the winter season 2010-2011 (Manney et al., 2020). According to data produced by the Copernicus Atmosphere Monitoring service (CAMS reanalysis) the monthly mean ozone columns in 70 Arctic in March 2020 were up to 180 DU or 40% lower than mean values over 2003-2019 (CAMS climatology) while values for 2011 and 1997 were lower by 31% and 35%, respectively (Innes et al., 2020). Severe ozone depletion led to large increase of solar ultraviolet radiation according to measurements performed at 10 Arctic and subarctic locations between early March and mid-April 2020 (Bernhard et al., 2020). According to OMPS LP satellite observations, the area of the PSCs during the winter 2019-2020 in the Arctic has reached the values typical to Antarctica (deLand et al., 2020). The general 75 conclusion of all studies: the reason for such record strong ozone depletion in the Arctic during spring 2020 is an unusually cold and isolated stratospheric polar vortex.
Model experiments indicated, that one of the reasons for the low wave activity propagation to stratosphere could be the positive phase of the dipole in the Indian Ocean (IOD), defined as the SST gradient between the western and eastern 80 equatorial Pacific Ocean. The positive phase of IOD through the propagation of wave chains to the northeast led to a weakening of the Aleutian low (Hardiman et al., 2020), which, by analogy with the El Niño-South Oscillation effect, weakens the propagation of planetary waves from the troposphere to the stratosphere. The frequency of positive IOD events doubled in the 20th century, and their intensity also increased, and according to model projections, this trend is expected to continue (Abram et al., 2020). 85 The 2019-20 winter was very different from the previous one, with the main SSW and splitting of the stratospheric polar vortex in early January 2019, after which it recovered in the middle stratosphere, in contrast to the lower stratosphere, where above zero temperature anomalies persisted until the final warming in April Rao et al., 2020). The recovery of the polar vortex after SSW in the middle and upper stratosphere could be due to the weak propagation of wave activity 90 from the upper troposphere -lower stratosphere from early January to mid-March and its reflection downward in the first half of January 2019 (Vargin et al., 2020). According to model estimations that take into account the decrease in the content of ozone-depleting substances in the atmosphere (revealed from satellite data since the early 2000s and is a consequence of the implementation of the Montreal Protocol and its amendments) and the continued growth of greenhouse gases, leading to decrease in the stratospheric temperature, by the middle of this century, significant ozone layer anomalies comparable to 95 record depletion in spring 2011 (Langematz et al., 2014). It has been suggested that due to changes in stratospheric climate the cold northern winters are getting colder in coming decades and it will influence the winter loss of Arctic ozone (Rex et https://doi.org/10.5194/acp-2021-11 Preprint. Discussion started: 29 January 2021 c Author(s) 2021. CC BY 4.0 License. al., 2004;Wohltmann et al., 2020). The cooling of the stratosphere could delay the recovery of the ozone layer (Pommereau et al., 2018). However, the other results of model simulations supposed that climate change will accelerate stratospheric ozone recovery instead of delaying it (e.g. Langematz et al., 2014). 100 Assessing the role of climate change in the occurrence of winter seasons with significant depletion of the ozone layer in the stratosphere, both in the Arctic and in the Antarctic, remains an urgent task, which determines the timing of ozone layer recovery (Pommereau et al., 2018;WMO, 2018). By the end of the twentieth century, the increasing anthropogenic impact on the ozone layer led to the formation of a tendency towards a decrease in the thickness of the ozone layer on a global scale 105 and the regular appearance of spring ozone holes in Antarctica and the episodic appearance of a large ozone holes in the Arctic. The unprecedented measures taken by the joint efforts in the framework of Montreal protocol to reduce emissions of ozone-depleting substances containing chlorine and bromine components have led to a decrease in the tendency for an increase in the content of ozone-depleting substances in the stratosphere, and in recent years there have been signs of recovery of the ozone content (WMO 2014). However for most datasets and regions the total ozone trends since the 110 stratospheric halogen reached its maximum (in 1996-2000) are mostly not significantly different from zero (Weber et al., 2018). Moreover multiple satellite measurements showed a decline of lower stratospheric ozone between 60°S and 60°N continuously since 1985 (Ball et al., 2018).
Signs of recovery in ozone levels began to be noted in the polar regions, in particular, a decrease in the depth of the ozone 115 hole and its size in Antarctica. The 2019 ozone hole in Antarctica was one of the lowest in decades. Nevertheless, despite the decrease in the content of halogen-containing ozone-depleting substances in the atmosphere, episodes of low ozone content have been formed in the Arctic in recent years. In addition, the strong interannual variability of the ozone content that has always existed in the Arctic has been increasingly manifested in the Antarctic in recent years. In particular, after the 2019 low Antarctic hole, the 2020 hole is again one of the deepest in recent years Wargan et al., 2020). Taking 120 into account the fact that in the Arctic in 2020 one of the deepest spring ozone anomalies was recorded, understanding the processes affecting the variability of the stratospheric ozone in the polar regions under the conditions of the decrease in the concentration of ozone-depleting substances in the atmosphere controlled by the Montreal Protocol and its amendments requires further clarification.

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The aim of this work is to study the dynamical, chemical processes and ozone layer changes and their possible causes in the Arctic stratosphere during the winter 2019-2020 using numerical modeling, data analysis and to identify the relative role of dynamic and chemical factors in the observed near record reduction in ozone content. The paper is organized in the following manner. The data used and the methodology of applied diagnoses are described in Section 2. Diagnostic results of the ozone layer and dynamical evolution for the Northern Hemisphere stratosphere winter of 2019-2020 are provided and an 130 estimate of the polar chemical ozone loss using trajectory modeling and balloon measurements of ozone are given in Section https://doi.org/10.5194/acp-2021-11 Preprint. Discussion started: 29 January 2021 c Author(s) 2021. CC BY 4.0 License.
3. Results of chemistry-transport modeling experiments performed to reveal the relative role of dynamical and chemical processes in the formation of the Arctic ozone anomaly in the spring 2020 are presented in Section 4. The discussion and conclusion are given in Section 5.

Data and methods of analysis 135
The present study of the dynamical and chemical processes features and variability of the ozone layer in the Arctic stratosphere is carried out using numerical modeling (chemical transport model of the RSHU, trajectory model TRACAO), reanalysis data ERA5 (Hersbach and Dee, 2016), NCEP (Kalnay et al., 1996), JRA (Kobayashi et al., 2015), MERRA2 (Gelaro et al., 2017) and observational data. A brief description of applied methods of analysis is present below.

Arctic stratospheric dynamics analysis 140
The evolution of the temperature of the Arctic lower stratosphere, where the largest reduction of the ozone layer takes place, in the winter 2019-2020 was analyzed by comparing the minimum temperatures at 70 hPa in the region of 70-90° N with other winters with the severe depletion of the ozone layer: 1995-1996, 1996-1997, 2004-2005, 2010-2011, 2015-2016. Temperature anomalies of stratosphere-troposphere are calculated relative to climate means over 1981-2010. The propagation of wave activity was analyzed by using the zonal mean meridional heat flux and three-dimensional Plumb flux 145 (Plumb 1985). It is known as an extended Eliassen-Palm (EP) flux due to the fact that its zonal average is equal to the wellknown EP flux. The Plumb flux vectors are proportional to the group velocity of a planetary wave packet indicating the direction of propagation of the wave activity and are useful to localize regions of wave activity sources and sinks.
The influence of the circulation of the Arctic stratosphere on the troposphere is analyzed through the propagation from the 150 stratosphere to the troposphere of geopotential height anomalies from climatic values in the region of 60-90°N, normalized to the standard deviation. When multiplied by -1, this parameter corresponds to the North Annular Mode (NAM) index.
Following to (Runde et al., 2016) the periods with an uninterrupted downward propagation of NAM index with values above +/-1.5 σ from the middle stratosphere to troposphere were defined as events with a consistent downward propagation of anomalies to the troposphere. 155

Trajectory analysis
To estimate the average behavior of ozone inside the polar vortex at the levels (18-22 km) in the Arctic winters 2010-2011, 2015-2016, and 2019-2020 the Lagrangian approach was applied similar to an analysis of Arctic stratospheric polar vortex in the winter of 2018-2019 (Vargin et al., 2020). An ensemble of forward 120-day trajectories were calculated inside the polar vortex using TRACAO trajectory model (Lukyanov et. al., 2003) and ERA5 reanalysis data. ERA5 ozone mixing ratio data 160 https://doi.org/10.5194/acp-2021-11 Preprint. Discussion started: 29 January 2021 c Author(s) 2021. CC BY 4.0 License.
were interpolated into the points of each trajectory and were ensemble averaged, providing the estimates of the mean ozone loss in descending air masses inside the polar vortex.

Ozonesonde analysis
Ozonesonde data obtained from several Arctic stations during the winter 2019-2020 has been used to estimate vortexaveraged chemical ozone loss inside the polar vortex. It is assumed that inside the well isolated polar vortex the temporal 165 evolution of vortex-averaged ozone volume mixing ratio on an isentropic surface is driven by two processes: 1) the net chemical ozone change due to gas phase and heterogeneous reactions and 2) the dynamical ozone change due to diabatic descent of air mass inside the polar vortex (Braaten, et.al., 1994). Thus, chemical ozone loss rate on any isentropic surface can be derived as the difference between the observed resulting ozone change rate on this surface and the calculated dynamical ozone change due to the diabatic descent. The mean rate of temporal ozone change on different fixed isentropic 170 levels from 350 K to 675 K has been calculated from the ozonesonde volume mixing ratio time series by linear regression method. The diabatic cooling/heating rates were calculated by the radiation transfer model (Chou et al., 1999) using daily temperature profiles from JRA reanalysis. A decrease of total column ozone due to chemical ozone loss can be deduced by integrating of cumulative ozone loss profile ( Fig. 7) between 350 K and 675 K potential temperature levels. Detailed description of the calculation method is given in (Braathen, et. al., 1994, Tsvetkova, et al., 2004. 175

Chemistry-transport modeling
To assess the relative role of chemical and dynamical processes in the ozone anomalies formation, numerical experiments with a chemical transport model were carried out. The temporal evolution of the Arctic stratospheric gases was simulated with the Russian State Hydrometeorological University chemistry-transport model (RSHU CTM) using meteorological fields directly from the reanalysis data (Smyshlyaev et al., 2017). This version of the global RSHU CTM is based on the Institute 180 of Numerical Mathematics and RSHU chemistry-climate model (INM RAS -RSHU CCM) (Galin et al., 2007;Smyshlayev et al., 2020), but meteorological fields are not calculated but specified from Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) reanalysis (Gelaro et al., 2017). The RSHU CTM has a 5°×4° horizontal resolution in longitude by latitude and 31 vertical sigma levels from the surface up to approximately 60 km. The distributions of the 74 oxygen, hydrogen, nitrogen, chlorine, bromine, carbon and sulfate gases are calculated in the manner described by 185 Smyshlyaev et al. (1998). Polar Stratospheric Clouds (PSCs) formation and evolution is treated as a super cooled ternary solution (H 2 O/HNO 3 /H 2 SO 4 ) based on the Carslaw (1995) parameterization. PSCs surface variability, depending on temperature, pressure and partial pressures of the relevant gases, including denitrification and dehydration through sedimentation is taken into account according to Sovde et al., (2007) and Smyshlyaev et al. (2010).

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For a more detailed study of the influence of dynamical and chemical factors on the local variability of the ozone content, additional numerical experiments with the RSHU CTM for two additional scenarios to the baseline (PSC) scenario have https://doi.org/10.5194/acp-2021-11 Preprint. Discussion started: 29 January 2021 c Author(s) 2021. CC BY 4.0 License. been done. The first scenario (noPSCaer) does not take into account the formation of PSCs during the Arctic winter and spring (from November 1 to April 30 north to 64°N), while the second scenario (noCHEMall) does not take into account any chemical ozone destruction in this area during the same period. Comparison of the baseline scenario with these two 195 additional scenarios makes it possible to estimate the periods when the chemical destruction of ozone is most effective after heterogeneous activation on the PSC surface, and when the gas-phase destruction of ozone in nitrogen catalytic cycles is more significant. In addition, based on a comparison of these calculation scenarios, it is possible to assess the comparative role of dynamical and chemical processes of ozone reduction. to -10 K from mid-February to mid-April (Fig. 2b). Two main causes of so cold and stable Arctic polar vortex were suggested: reduced upward wave activity propagation from troposphere and downward refraction of wave activity from upper stratosphere (Lawrence, 2020). The evolution of wave activity propagation during the winter of 2019-2020 is illustrated by variability of zonal mean meridional heat flux (Fig.3 a). Reduced wave activity was observed mainly over two 235 periods: January 5-20 and from early February to early March. The latter period was followed by enhanced wave activity propagation over about 10 days in the middle of March.   : 1995-1996, 1996-1997, 2004-2005, 2010-2011, 2015-2016 (Fig.3 b). The first period (February 7 -March 7) corresponds to strongest weakening of wave 245 activity propagation in 2020, the second: January -February, the third: January -March. It is seen that heat fluxes over all periods are lowest in 2020 with the exception of the third period when it is slightly stronger than in 1997.

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Zonal mean meridional heat flux at 70 hPa averaged over 45-75° N and the following periods: from 7 February to 7 March, from 1 January to 28 February, from 1 January to 31 March of 1996March of , 1997March of , 2005March of , 2011March of , 2016March of , and 2020. Temperature anomalies at 925 hPa averaged over the period from 7 February to 7 March 2020. Latitudes are from 30°N (c).

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At the same time, the winter season of 2019-2020 in most of Northern Eurasia was characterized by anomalously warm weather due to increased westerly winds. The positive temperature anomalies were observed in most of Northern Eurasia, including Siberia, where the absolute maximum of the average temperature in February was reached. The period with strongly reduced wave activity was also characterized by positive anomalies of temperature over the north-eastern Eurasia (Fig 3c). Positive temperature anomalies over the Northern Eurasia of more than 5 K are retained when averaged over 260 January-March 2020 (Lawrence et al., 2020). The positive anomalies up to 6 K were observed over the north-eastern Eurasia https://doi.org/10.5194/acp-2021-11 Preprint. Discussion started: 29 January 2021 c Author(s) 2021. CC BY 4.0 License. during the period of strongly reduced upward wave activity propagation: February 7 -March 7 (Fig. 3c). Notably that described positive temperature anomalies were observed not only near surface but at higher levels in troposphere.
The dominant mode of circulation in the troposphere of extratropical latitudes of the Northern Hemisphere in winter is the 265 Arctic Oscillation (AO). The AO positive phase is characterized by a reduced pressure at the pole and increased in the region of 40-50°N, and the negative one, on the contrary (Thompson and Wallace, 1998). With a positive AO phase, a stronger western zonal transport leads to milder winters, but with more precipitation in Southern Europe. In the negative AO phase, this transfer is weaker; as a result, cold air masses from the Arctic spread more strongly to the territory of Europe, including the European territory of Russia. 270 AO is the result of interaction between the dynamics of the stratosphere and the troposphere. The positive AO phase is associated with a positive anomaly in the intensity of the stratospheric polar vortex, which is facilitated by an increase in the temperature gradient between the heated by the sun and shaded parts of the atmosphere, an increase in the zonal mean flux, and smaller amplitude of planetary waves. The stratospheric polar vortex becomes less sensitive to the effects of waves, due 275 to their refraction towards the equator. Also the strong stratospheric polar vortex is often accompanied by a positive AO phase (Thompson and Wallace, 1998).
In the first half of January and February-March 2020, a positive AO phase was observed with index values of more than 4 in early January and then in February-March. Average monthly values of the AO index were 3.4 in February and 2.6 in March. 280 Thus, the positive AO phase with a reduced propagation of wave activity into the stratosphere could contribute to the enhancement of the Arctic stratospheric polar vortex (Lawrence et al., 2020).
In the second half of February and during most of March 2020, areas with NAM index values above 1.5 σ spread continuously from the middle stratosphere to the lower troposphere, which indicates the influence of the Arctic stratosphere 285 on the troposphere (Fig. 4a). After the SSW event in the mid-March, this spread ended nearly in two weeks.
We assume that the anomalously warm winter 2019-2020 (and especially in February-early March with high positive AO index) contributed to the strengthening of the stratospheric polar vortex due to a decrease in the propagation of wave activity into the stratosphere from the troposphere over the north-eastern Eurasia. It is known that the main source of wave activity  After the period with a strongly weakened propagation of wave activity from the troposphere to the stratosphere (since early February till early March), a sharp increase in such propagation was observed in the upper troposphere -lower stratosphere over northwestern Canada in the middle of March over about 10 days. This is confirmed by the diagram with the vertical 305 component of the Plumb fluxes in the lower stratosphere at 100 hPa during the SSW event onset on March 14-16 (Fig. 5a).
The enhanced propagation of wave activity, which led to the development of SSW event, is apparently associated with the https://doi.org/10.5194/acp-2021-11 Preprint. Discussion started: 29 January 2021 c Author(s) 2021. CC BY 4.0 License.
Rossby wave breaking event in the troposphere over the Gulf of Alaska region (150°W -120°W), and accompanied by a poleward transport of wet and warm air masses with low potential vorticity and formation of anticyclone (Fig. 5 b). Such a poleward transport, leading through a change in the zonal current and redirection of wave activity upward to the stratosphere 310 (instead to the equator), can lead to the development of SSW event, for example, as in January 2006 (Coy et al., 2009).  (Fig. 6 c). Later on the enhanced upward propagation of wave activity in the troposphere and lowermiddle stratosphere is clearly seen on March 14-16, 2020 (Fig. 6 d). Zonal mean zonal wind weakening in lower -middle stratosphere over high latitudes is also observed. Altitude-longitude diagram of Plumb fluxes over the northern high latitudes

Chemistry-transport modeling
For a more detailed study of the degree of dynamical and chemical processes influence on the formation of ozone anomalies 380 during the spring of 2020, numerical experiments with a chemical-transport model were carried out, in which the dynamic parameters were set from the MEPRA-2 reanalysis data. The use of temperature, wind speeds, surface pressure and air humidity from the reanalysis data made it possible to simulate the effect of atmospheric circulation on the transport of ozone and associated gases close to reality. Variability of specified dynamical parameters determines the dynamical decrease in ozone content, as well as the atmospheric temperature govern the rate of chemical reactions, polar stratospheric clouds 385 formation and the rate of heterogeneous reactions on their surface, which determine the chemical destruction of ozone. https://doi.org/10.5194/acp-2021-11 Preprint. Discussion started: 29 January 2021 c Author(s) 2021. CC BY 4.0 License. Fig. 11 demonstrates the coefficient of chemical ozone destruction in the lower stratosphere calculated using CTM. This 425 coefficient is a factor by which the concentration of ozone should be multiplied in order to obtain the rate of its chemical destruction. The coefficient includes all processes of chemical destruction of ozone, including processes involving chlorine, bromine, nitrogen and hydrogen gases. It should be noted that the rate of ozone destruction in March has its maximum values at the boundary of the polar night in the region of the newly returned Sun. In this case, the maximum rate of ozone destruction is a necessary, but not sufficient condition for the formation of a zone of low ozone content in the spring. 430 Dynamical factors also play an important role, in particular, for definition of the zone where the polar vortex is located. In particular, in early March, the rate of destruction of the base is maximum over the entire circle of latitude near the boundary of the polar night, and the minimum values of the ozone content are noted only in the eastern hemisphere (Figs. 8 and 9).
Also in mid-March and April, areas of high ozone depletion cover a wider zone than areas of minimum total ozone.

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For a more detailed study of the influence of dynamical and chemical factors on the local variability of the ozone content for two additional scenarios to the baseline (PSC) scenario. The first scenario (noPSCaer) does not take into account the formation of PSC based on stratospheric sulfate aerosol during the winter season (from November 1 to April 30 North to 66° N), and the second scenario (noCHEMall) does not take into account any chemical processes at this region during the same period. Comparison of the baseline scenario with these two additional scenarios allows us to estimate the periods when chemical destruction of ozone is most effective after heterogeneous activation on the PSC surface, and when gas-phase 445 destruction of ozone in nitrogen catalytic cycles is more significant. In addition, the comparative role of dynamical and chemical processes of ozone reduction can be assessed by comparing these scenarios with each other and to mean climatic values presented at the bottom of these figures.
for different scenarios indicates that the influence of dynamic factors in the Western Hemisphere is more significant than in the Eastern Hemisphere, since for the noCHEMall scenario comparison to climatic data, the total content fluctuates in the order of 100-150 DU. As can be seen from the comparison of the baseline scenario with additional ones, the chemical destruction of ozone also ranges from 70 to 80 DU, which is slightly higher than in the Eastern Hemisphere. At the same time, in April, chemical factors for ozone destruction prevail over dynamic factors at western stations. It should also be noted 495 that there are two peaks of maximum chemical destruction of ozone: in late March and mid-April. At the same time, chemical destruction in the second half of March is superimposed on a dynamic decrease in its content, which leads to a minimum in the seasonal variation of the total ozone content, while in April, when the chemical destruction of ozone is even greater than in March, the polar vortex is already shifting towards the eastern hemisphere ( Fig. 8 and 9), and the total ozone content is higher than in March. 500 Comparison of calculations for different scenarios of accounting for the chemical destruction of ozone depicts that the destruction of ozone over heterogeneous reactions in the western hemisphere exceeds 30 DU, which is more than in the eastern hemisphere, while the gas-phase destruction of ozone in the Western hemisphere is greater than in the Eastern Hemisphere. It should also be noted that in the Western Hemisphere, the minimum values of the ozone content according to 510 satellite measurements in March are lower than the values calculated using the model, while in the Eastern Hemisphere the satellite and model results are closer. This result may be due to relatively coarse model resolution to simulate fine local effects in the western hemisphere.

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(yellow), OMI data (grey), CTM basic scenario (PSC) (blue), CTM scenario without PSC processes included (noPSCaer) (green), CTM scenario with no chemical ozone destruction included (noCHEMall) (red); and column ozone difference between CTM simulation for several scenarios (bottom): noCHEMall and SBUV 1979-2019 averages (yellow), PSC and noCHEMall (red), PSC and noPSCaer (blue) 520 Additional numerical calculations to assess the effect of various catalytic cycles of chemical ozone destruction on a decrease in its content in April-May 2020 revealed that the main increase in the gas-phase ozone destruction occurs in the nitrogen https://doi.org/10.5194/acp-2021-11 Preprint. Discussion started: 29 January 2021 c Author(s) 2021. CC BY 4.0 License. catalytic cycle, in which the chemical reaction with the participation of nitrogen dioxide and atomic oxygen plays a determining role. In the Arctic stratosphere, in contrast to the Antarctic stratosphere, significant denitrification does not occur, and therefore a sufficient amount of nitrogen oxides remains in it, which plays a decisive role in the destruction of 525 stratospheric ozone.

Discussion and conclusions
Motivated by sufficiently rare event in the Arctic: very strong and long-lasting stratospheric polar vortex and near record ozone loss in the winter season 2019-2020 we have analyzed the peculiarities of stratospheric dynamical processes including those that contributed to the SSW event in the middle of March 2020 and interrupted the period of weakened wave activity 530 propagation and strengthening of the stratospheric polar vortex and ozone layer destruction. Further we have estimated the values of chemical ozone loss by trajectory modeling and using ozonosondes observations and finally we have investigated dynamical and chemical processes in the Arctic stratosphere and their role in ozone destruction.
The of SSW event in the middle of March 2020 which, although it did not satisfy the WMO definition of Major SSW event, 535 prevented an even stronger ozone layer destruction that would have observed if this SSW event had not occurred or occurred later. The revealed enhancement of wave activity propagation over the Gulf of Alaska could be important but not a sole factor responsible for the onset of the SSW event in the middle of March 2020 as supposed only 30% / 60% of SSW events are preceded by extreme wave activity episodes at the lower troposphere / lower stratosphere (White et al., 2019). Therefore numerical experiments are desirable to verify a role of this enhanced wave activity propagation in the generation of SSW 540 event in the mid-March 2020.The weakened propagation of wave activity from the troposphere to the stratosphere could be caused in addition to the influence of Indian Ocean Dipole suggested by Hardiman et al., (2020) by the positive temperature anomalies in the troposphere over the north-eastern Eurasia. However, the weakening of wave activity propagation to stratosphere could be not only due to changes in troposphere but in the stratosphere too. Therefore this our speculation needs further research. 545 Overall conclusions of our study are as follows:  The results of numerical calculations with a chemistry-transport model with dynamical parameters specified from the MERRA-2 reanalysis data, carried out according to several scenarios of accounting for the chemical destruction 565 of ozone, reveal that both dynamical and chemical processes make significant contributions to the decrease in the ozone content inside the polar vortex. In this case, the chemical ozone depletion is determined not only by heterogeneous processes on the surface of polar stratospheric clouds, but by gas-phase destruction in nitrogen catalytic cycles as well.
Code and data availability. The data used in this study and IDL code for data reading and plotting can be downloaded from 570 ra.rshu.ru/files/Smyshlyev_et_al_ACP_2020. The model codes are available from the authors upon request.