Observation and modeling of high-Be events in Northern Europe associated with the instability of the Arctic polar vortex in early 2003

Events of very high concentrations of Be cosmogenic radionuclide have been recorded in the subpolar 15 regions of Europe during the cold season. With an aim to investigate the mechanisms responsible for those peak Be events, and in particular to verify if they are associated with the fast descent of stratospheric air masses occurring during sudden stratospheric warming (SSWs), we analyse Be observations at six sampling sites in Scandinavia during January-March 2003 when very high Be concentrations were observed and the Arctic vortex was relatively unstable as a consequence of several SSWs. We use the GEOS-Chem chemistry and transport model 20 driven by the MERRA-2 meteorological reanalysis to simulate tropospheric Be over Northern Europe. We show that the model reasonably reproduces the temporal evolution of surface Be concentrations observed at the six sampling sites. Our analysis of model simulations, surface Be observations, as well as atmospheric soundings of ozone and temperature indicates that the Be peak observed in late February 2003 (between 20 and 28 February 2003) at the six sampling sites in Scandinavia was associated with downward transport of stratospheric vortex air 25 originated during SSW that occurred a few days before the peak (18-21 February 2003).

Be events, and in particular to verify if they are associated with the fast descent of stratospheric air masses occurring during sudden stratospheric warming (SSWs), we analyse 7 Be observations at six sampling sites in Scandinavia during January-March 2003 when very high 7 Be concentrations were observed and the Arctic vortex was relatively unstable as a consequence of several SSWs. We use the GEOS-Chem chemistry and transport model 20 driven by the MERRA-2 meteorological reanalysis to simulate tropospheric 7 Be over Northern Europe. We show that the model reasonably reproduces the temporal evolution of surface 7 Be concentrations observed at the six sampling sites. Our analysis of model simulations, surface 7 Be observations, as well as atmospheric soundings of ozone and temperature indicates that the 7 Be peak observed in late February 2003 (between 20 and28 February 2003) at the six sampling sites in Scandinavia was associated with downward transport of stratospheric vortex air 25 originated during SSW that occurred a few days before the peak (18-21 February 2003).
The spatial and temporal variability of the 7 Be surface concentrations in Europe and their relationship with meteorological variables was previously analysed in many studies (e.g., Piñero García et al., 2012;Błażej and 40 Mietelski, 2014). The impact of the 11-year solar modulation on the 7 Be concentrations in the air is well established (e.g., Leppänen et al., 2010). The distinctive spring/summer maximum of 7 Be concentrations is widely described and mainly linked with the increased downward transport from the upper troposphere resulting from the intense convection and higher tropopause height typical of the warm season (Cristofanelli et al., 2006;Gerasopoulos et al., 2001Gerasopoulos et al., , 2003. In addition, cases of high 7 Be surface concentration, some of which occurring over the autumn/winter 45 season, have been analysed in Europe, e.g., over the Iberian Peninsula  and at high-altitude stations in the Alps and the Apennines Cristofanelli et al., 2006Cristofanelli et al., , 2009. The spring/summer maximum was originally observed with fission products injected into the stratosphere during atmospheric nuclear tests (Dutkiewicz and Husain, 1985;Cristofanelli et al., 2018). A study of Salminen-Paatero et al. (2019) using potential vorticity analysis indicated that the transfer of stratospheric air into the troposphere 50 was at its maximum in March followed by gradual movement into the ground-level air during spring and early summer.
Further, over the last decade many studies have investigated the 7 Be records in Northern Europe (Leppänen et al., 2010(Leppänen et al., , 2012Leppänen and Paatero, 2013;Sarvan et al., 2017;Leppänen, 2019), one of the three regions in Europe identified with a distinct 7 Be behaviour in the surface air (Ajtić et al., 2017;Hernández-Ceballos et al. 2015, 55 2016b). Among these studies, Ajtić et al. (2016) analysed the 7 Be concentration measured in Helsinki, Finland, over 25 years , and pointed out a relatively high number of 7 Be extremes occurring over autumn and winter: more specifically, 10 % of the highest 7 Be concentrations (above 95 th percentile) were observed in the cold season (October-March). Furthermore, recent studies have also indicated that the polar vortices can have a notable influence on the wintertime 7 Be surface concentrations in both the Northern (Ajtić et al., 2018;Bianchi et al., 2019;60 Terzi and Kalinowski, 2017) and Southern Hemispheres (Pacini et al., 2015). In particular, Ajtić et al. (2018) and Bianchi et al. (2019) employed two different methodologies to identify episodes of extremely high 7 Be surface concentrations in the autumn and winter seasons, pointing out a large number of cases over the October-March period of the years investigated. The comparison of the dates identified in both analyses showed an overlap with the events of the so-called Sudden Stratospheric Warming (SSW) of the 65 Arctic vortex, i.e., a sudden rise in the polar temperatures that leads to a highly irregular shape of the vortex and its misalignment from the pole. Ajtić et al. (2018) also noted cases of extremely high 7 Be concentrations occurring right after a very low 7 Be concentration over the Scandinavian Peninsula during autumn and winter. Overall, this relationship between the SSW of the Artic vortex and high 7 Be surface concentrations is likely linked to the perturbed stratosphere-troposphere interactions associated with SSWs, which could favor a fast descent of: 1) 70 midlatitude air rich in 7 Be, thus increasing this radionuclide's surface abundance, and 2) aged vortex air wherein 7 Be is subjected to radioactive decay and not transported from outside the vortex, thus decreasing the 7 Be surface abundance.
The atmospheric circulation in the Arctic is dominated by the presence of two distinct polar vortices, one in the troposphere and one in the stratosphere (Waugh et al., 2017). The two vortices present well distinct features: 75 first, the vortex in the troposphere is much more extended than the stratospheric one; and second, while the tropospheric vortex is present all-year round, the stratospheric polar vortex exists only from fall to spring (Waugh et al., 2017). The stratospheric polar vortex in winter stems from the large-scale temperature gradients between the midlatitudes and the poles. Therefore, the stratospheric polar vortex begins to form in autumn as a result of the decreasing solar heating in the polar regions; it strengthens during winter and then breaks down in spring when 80 solar radiation returns to the polar region. Larger topographic and land-sea contrasts and the resulting stronger upward-propagating waves in the Northern Hemisphere, make the northern stratospheric vortex, or the Arctic vortex, weaker and more distorted than its Southern Hemisphere counterpart, the Antarctic vortex. A higher temporal variability of the Arctic vortex includes the SSW. A major SSW can even cause the stratospheric vortex to break down during midwinter (Waugh et al., 2017). 85 While the initial scientific interest over the stratospheric polar vortex was especially linked to the stratospheric ozone depletion and formation of the ozone holes over the poles, it is now recognized that the vortices might affect the processes in the troposphere and surface weather (e.g., Mitchell et al., 2013). The present work aims at investigating in detail the atmospheric processes responsible for high 7 Be activities recorded in the cold season over the Scandinavian Peninsula and its relationship with the Arctic polar vortex through model simulations. 90 For this purpose, we conduct 7 Be simulations for the period of January-March 2003 using the GEOS-Chem global 3-D chemical and transport (CTM) model. The period was selected because of the large number of events with https://doi.org/10.5194/acp-2020-1121 Preprint. Discussion started: 17 February 2021 c Author(s) 2021. CC BY 4.0 License. extremely high 7 Be concentrations at surface in Scandinavia; some of these events were preceded by very low surface concentrations in the lower troposphere (< 10 th percentile). This period thus offers the opportunity to test the hypothesis that SSWs ease a fast descent of not only the midlatitude but also vortex air (Ajtić et al., 2018.). To 95 achieve this goal, our analysis will therefore focus on: • Investigating the processes responsible for the variability of 7 Be concentrations in surface air in Northern Europe; • Better understanding whether and how SSW and the Arctic vortex winter-time instability influence the surface concentrations of 7 Be in Northern Europe;

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• Quantifying the rate of air subsidence on the inner and outer side of the vortex during the period of its instability.
To analyse the influence of SSW and of the Arctic polar vortex on 7 Be concentrations, we first assess the performance of the GEOS-Chem model in reproducing the observed 7 Be variability. We then use model simulations together with other supporting measurements from soundings and meteorological datasets to examine the processes 105 responsible for the variability in the 7 Be concentrations over the period of January-March 2003.
As opposite to the cosmogenic origin of 7 Be, 210 Pb (half-life 22.3 years) is a nuclide of crustal origin derived from decay of 222 Rn (half-life 3.8 days), which is emitted from soils by decay of 226 Ra. Owing to the contrasting natural origins of the two nuclides, the 7 Be/ 210 Pb ratio is often regarded as indicative of vertical transport processes and convective activity in the atmosphere (e.g., Koch et al., 1996;Tositti et al., 2004;Brattich et al., 2017a,b). 110 After being produced by contrasting physical mechanisms, both 7 Be and 210 Pb rapidly attach to ambient submicronsized particles (e.g., Gaffney et al., 2004) and are removed by wet (mainly) and dry (secondarily) deposition processes of their carrier aerosol. The bias in the simulated 7 Be/ 210 Pb ratio due to uncertainties in the model deposition schemes is thus reduced. For this reason, besides 7 Be, the 7 Be/ 210 Pb ratio was also analysed to gain further insights into vertical transport processes during the study period. 115 The rest of this paper is organized as follows. Section 2 describes the radioactivity ( 7 Be and 210 Pb) and meteorological data used. Section 3 provides a brief description of GEOS-Chem, the HYSPLIT trajectory model, and statistical parameters used to assess the model's performance in reproducing the observations. Section 4 presents an overview of the 7 Be observations made in Northern Europe in the boreal winter 2003. Section 5 analyses the precipitation and transport pattern in the study region, while Section 6 assesses how the GEOS-Chem model 120 performs in reproducing the observed variability in the monthly mean surface 7 Be concentrations during the study period. Section 7 further evaluates the performance of the model in reproducing the short-time variability of 7 Be in https://doi.org/10.5194/acp-2020-1121 Preprint. Discussion started: 17 February 2021 c Author(s) 2021. CC BY 4.0 License. Northern Europe, followed by interpreting the observed variability using model simulations and additional meteorological observations in section 8. Finally, summary and conclusions are given in Section 9.

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In this section we briefly describe the 7 Be and 210 Pb radioactivity data as well as the meteorological datasets analysed in this work.

210 Pb data
Daily aerosol samples were collected in Helsinki on the roof of the Finnish Meteorological Institute's main building (60°10'N, 24°57'E). Filters (Munktell MGA, diameter Ø = 240 mm) were changed every day at 06 UTC.
The air volume was about 3500 m³/day. The filters were assayed for 210 Pb by alpha counting of the in-grown daughter nuclide 210 Po (Mattsson et al., 1996). 145

Meteorological data
The 7 Be variability is tightly linked to horizontal and vertical transport of the carrier aerosol, and to precipitation that leads to the radionuclide's removal from the atmosphere. Here we use the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) meteorological reanalysis (Gelaro et al., 2017) to assist in the data analysis and to drive the GEOS-Chem model simulations. MERRA-2 is produced 150 with version 5.12.4 of the Goddard Earth Observing System (GEOS) atmospheric data assimilation system. It assimilates modern observations of the atmosphere, ocean, land, and chemistry, and includes assimilation of aerosol remote sensing data.

Methods
In this section we give a brief description of the GEOS-Chem and HYSPLIT models and the statistical parameters used to indicate the model performances.

GEOS-Chem model
GEOS-Chem (http://www.geos-chem.org) is a global 3-D CTM that has been widely used to study 165 atmospheric composition and processes (e.g., Bey et al., 2001;Park et al., 2004;Eastham et al., 2014). In this study, we use the GEOS-Chem v11-01f to simulate 7 Be and 210 Pb and assist in interpreting the observations. GEOS-Chem includes a radionuclide simulation option ( 222 Rn-210 Pb-7 Be), which simulates the emission, transport (advection, convection, boundary layer mixing), deposition and decay of the radionuclide tracers (Jacob et al., 1997;Liu et al., 2001;Yu et al., 2018). 170 We use the 7 Be production rates recommended by Lal and Peters (1967) for a maximum solar activity year (1958), which has been shown to produce the best results compared to aircraft 7 Be observations (Koch et al., 1996;Liu et al., 2001). The production rate is formulated as a function of latitude and pressure without seasonal variation (Koch et al., 1996). About two thirds of atmospheric 7 Be is generated in the stratosphere. 222  flux rate is a function of latitude, longitude, and month. 7 Be and 210 Pb are assumed to behave like aerosols once formed in the atmosphere and subject to dry and wet deposition (Liu et al., 2001). Both wet and dry deposition for 222 Rn are neglected due to its inert nature.
GEOS-Chem simulations in this work are driven by the MERRA-2 meteorological reanalysis. The native 180 resolution of MERRA-2 is 0.667° longitude by 0.5° latitude, with 72 vertical layers (top at 0.01hPa). The meteorological fields are regridded into 2.5° longitude by 2° latitude for the GEOS-Chem simulations in this work.
GEOS-Chem uses the TPCORE advection algorithm of Lin and Rood (1996). Convective transport is calculated using archived convective mass fluxes (Wu et al., 2007). Boundary-layer mixing is based on the non-local scheme implemented by Lin and McElroy (2010). The wet deposition scheme follows that of Liu et al. (2001) and includes 185 rainout (in-cloud scavenging) due to stratiform and anvil precipitation, scavenging in convective updrafts (Mari et al., 2000), and washout (below-cloud scavenging) by precipitation (Wang et al., 2011). Precipitation formation and evaporation fields are archived in MERRA-2 and used directly by the model wet deposition scheme. Dry deposition is based on the resistance-in-series scheme of Wesely (1989).
In addition to the standard model simulations of 7 Be and 210 Pb, we also separately transport 7 Be produced 190 in the stratosphere to quantify the stratospheric contribution to 7

HYSPLIT
The were selected to help us to understand the behaviour of the airflows circulating in the Atmospheric Boundary Layer (ABL), just above the ABL, and in the free troposphere. We used the cluster methodology implemented in the HYSPLIT model to group the calculated trajectories according to their length and curvature, and thus identify the airflow patterns over the whole period of the analysis. It is worth mentioning that clusters, as well as trajectories, indicate an estimation of the general airflow rather than the exact pathway of an air parcel (e.g., Jorba et al., 2004;205 Salvador et al., 2008).

Evaluation of the model output
The performance of the model in reproducing observed activity concentrations is evaluated by calculating some basic statistical parameters, such as the mean and standard deviation and other indicators, according to the methodology developed by Hanna (1993) and summarized later by Chang and Hanna (2004). Specifically, the 210 performance of the CTM was evaluated using the following set of indicators, proposed by Carruthers et al. (2004): • The mean bias (MB), a measure of the mean difference between the modelled and observed concentrations: where Cm is modelled concentration and Co is observed concentration.

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• The normalized mean square error (NMSE), a measure of the mean difference between matched pairs of modelled and observed concentrations: • The fraction of modelled concentrations within a factor of 2 of observations (FA2), i.e., for which 0.5 < < 2 ⁄ 220 • The Pearson's correlation coefficient (R), a measure of the extent of a linear relationship between the modelled and observed concentrations: (3) A perfect model has MB and NMSE values equal to 0 and FA2 value equal to 1, while the R results range from −1 (perfect negative relationship) to + 1 (perfect positive relationship), where 0 implies no relationship 225 between the variables. To better understand the quantitative differences between observations and simulations, scatter plots were used.

Boreal winter 2002/2003
As indicated by Ajtić et al. (2018) and Bianchi et al. (2019), the winter of 2003 offers a good opportunity to investigate a possible link between SSWs and extreme surface concentrations of 7 Be detected in Northern 230 Europe. This period is sufficiently covered by the 7 Be activity concentration measurements at all six monitoring sites.
In particular, very high 7 Be activity concentrations, above the 90 th percentile simultaneously at most of the Scandinavian Peninsula sampling sites, were recorded around 23-24 February 2003 (Ajtić et al., , 2018 ( Figure 1b). During the 2002/2003 boreal winter, the Arctic vortex was relatively unstable, with six SSWs taking 235 place over the whole season (Peters et al., 2010). Two very pronounced episodes, which were both associated with the vortex splitting and fast SSW recovery, occurred in January and February, respectively (Günther et al., 2008).
The evolution of the vortex caused vortex filamentation and vigorous mixing of the vortex and midlatitude stratospheric air (Günther et al., 2008;Müller et al., 2003). Several balloon flights inside the Arctic polar vortex in early 2003 observed unusual trace gas distributions connected to an intrusion of mesospheric air down to altitudes 240 of about 25 km (Engel et al., 2006;Huret et al., 2006;Müller et al., 2007). Since such disturbances around the pole are expected to affect the troposphere, i.e., on weather conditions (Baldwin and Dunkerton, 2001), and air composition (Hsu, 1980;Limpasuvan et al., 2004). Hence, the high 7 Be concentrations that were measured in  precipitation throughout the period, high precipitation over the North Atlantic region during winter, and Europe where the seasonal pattern of precipitation is similar to that in the North Atlantic region.
To assess the capability of the model to correctly capture the trend in precipitation during the observation period at the sampling sites, we examined the normalized differences between the MERRA-2 and the observed precipitation, calculated as a difference between the MERRA-2 and the observed values, normalized over the 265 observed value (Table 1).
Overall, the MERRA-2 precipitation tends to be generally higher than that of GPCP at all sampling sites (Table 1) (Table 1). Figure 3 shows that winter circulation in the Scandinavian Peninsula is dominated by SW and W winds (Chen, 2000;Linderson, 2001). The analysis of the main circulation in the three months in Figure 3 reveals low wind speeds from S-SW in the study area and period. A region of strong wind speeds, possibly corresponding to 275 the Arctic vortex, is clearly visible at surface level to the west of the study area in all the three months. In addition, there appears to be a convergence area (opposite wind directions) between 60 and 75°N. Model-simulated 7 Be/ 210 Pb ratios and fraction of stratospheric 7 Be increase over the three months period and peak in March, suggesting increasing stratospheric influence, subsidence, or convective mixing in the study region.  Table 2 reports the statistical parameters and the normalized differences that indicate the performance of the GEOS-Chem model in reproducing the observed 7 Be monthly means.
In general, the model well simulates the month-to-month trend in 7 Be concentrations measured at the 285 sampling sites, as indicated by the fact that all the values fall within the 95% confidence levels (Figure 4a) and the high positive correlation coefficients (> 0.7) except for Ivalo and the low MB and NMSE values (Table 2). In fact, the normalized differences are not very high (generally <1), except at Risoe. The bias between the model and the observations is partly attributed to the coarse resolution of the model. Overall, the simulations underestimate the observed values, likely due to uncertainties associated with the deposition schemes and/or precipitation as discussed 290 earlier.
The use of the 7 Be production rate of Lal and Peters (1967) for a solar maximum year (1958) may also partly explain the tendency of simulated 7 Be to be lower than observed. The sunspot number in 2003 (99.3) was rather low (slowly decreasing from 2000, a solar maximum year, and reaching minimum in 2008) if compared to the 1958 value of 184.8. As known, the galactic cosmic-ray intensity, largely responsible for the production of 295 cosmogenic radionuclides, at the Earth's orbit is inversely related to solar activity (O'Brien, 1979), leading to the well-known phase opposition between sunspot number and 7 Be concentration (e.g., Hernández-Ceballos et al., 2015). Sunspot number data herein used were extracted from the World Data Center for the production, preservation and dissemination of the international sunspot number (Sunspot Index and Long-term Solar Observation, SILSO, Royal Observatory of Belgium, Brussels, http://www.sidc.be/silso/datafiles -total). 300

Variations of the Be weekly and daily mean surface concentrations in the Arctic region: observations vs. model simulations
After analysing the model's performance in reproducing 7 Be monthly mean observations in the previous Section, we compare here in Figure 4b simulated and observed weekly (daily in the case of Helsinki) 7 Be activity concentrations at the six sampling sites. Table 3 shows the corresponding parameters that indicate the performance 305 of the GEOS-Chem model in reproducing observations. The weekly evolution of simulated versus observed 7 Be concentrations at these sites is shown in Figure 5.
As with the monthly means, the model generally represents adequately the temporal pattern but not the magnitude of weekly mean concentrations, which tend to be lower than those observed (Table 3, Figures 4b and   5). This can arise from the higher precipitation in the model than in the observations and/or from errors in the 310 deposition schemes. However, the correct reproduction of the 7 Be temporal pattern, as indicated by the high correlation values at all sampling sites with the exception of Risoe, suggests that the model captures the transport processes leading to the peak in 7 Be concentrations at the end of February 2003 and the preceding very low concentration values. In addition, the low MB and NMSE values calculated at all sites and especially at Ivalo and Umea suggest that the model reproduces adequately the observed values ( Figure 5). 315 As for the 7 Be/ 210 Pb ratio in Helsinki (Table 3), the model tends to underestimate the observed ratio, which could be due to the model underestimating 7 Be and overestimating 210 Pb. Nevertheless, the relatively high  (Peters et al., 2010), and two very pronounced SSWs, with the vortex splitting and re-330 establishing within a few days, occurred in January and February (Günther et al., 2008). On one hand, this evolution of the vortex caused vortex filamentation and vigorous mixing of the vortex and midlatitude stratospheric air (Günther et al., 2008;Müller et al., 2003). On the other hand, such disturbances around the pole were also expected to propagate through the troposphere, i.e., on weather conditions (Baldwin and Dunkerton, 2001), and air composition (Hsu, 1980;Limpasuvan et al., 2004). Hence, the high 7 Be concentrations measured in Scandinavia 335 around 24 February 2003 could be the result of downward transport of midlatitude stratospheric air. Interestingly, prior to this episode, very low (below the 10 th percentile for each site) surface concentrations of 7 Be were measured in Risoe, Kista and Ivalo on 3, 10 and 16 February 2003, respectively (Figure 1b), as connected to precipitation scavenging that occurred the days before, as discussed earlier.  (Tilmes et al., 2006), obviously lower-stratospheric ozone was still enhanced relative to tropospheric ozone.
Simultaneously, the analysis of maps of surface transport and relative humidity (Figure 9) highlights the different winds and relative humidity values in the two periods, with low relative humidity values (~40-50%) 365 suggesting subsidence from 18 to 21 February, and the transition from a clockwise circulation to the fast and complex wind system typical of the second period corresponding to the high 7 Be peak and the SSW. Interestingly, the parcel of the lowest relative humidity values occurs during the 18-21 February period, i.e., a couple of days before the dates of the 7 Be peaks in the measurements and those peaks in the simulated 7 Be/ 210 Pb ratio and stratospheric 7 Be fraction (Figure 1b and Figure 6). Together with enhanced ozone concentrations observed in the 370 lower troposphere on 19 th February, this suggests that the downward transport from the UT-LS was triggered by the SSW occurring a few days before.
To better constrain the stratospheric origin of the air masses arriving at the sampling sites during the two periods, we further analyzed the potential vorticity data from ECMWF during the month of latitude where values are higher than 1.6 PVU, a value considered as a threshold for stratospheric air in the lower troposphere especially when in conjunction with low relative humidity, high 7 Be/ 210 Pb ratios and ozone (Cristofanelli et al., 2006) were observed.
The low relative humidity (Figure 9) and high potential vorticity (Figure 10) corresponded to high 7 Be 380 descending to lower atmospheric levels, as simulated by the model (Figure 11). The descending vertical motion from the upper vertical levels during the period is clearly visible in the MERRA-2 pressure vertical velocity (omega) fields sampled at the six sampling sites for the month of February 2003, especially for the northernmost sites ( Figure 12). Further evidence of this is seen from the maps of pressure vertical velocity ( Figure 13) and of the stratospheric fraction of 7 Be originated in the stratosphere ( Figure 14) at 940 hPa in the boundary layer. 385 Analysis and comparison of 4-day back-trajectories at each sampling station allowed a reconstruction of two distinct atmospheric circulation patterns in the two periods. Figure 15 shows the clustering results for three sampling sites (Ivalo, Harku, Risoe) during both periods. The stations are ordered as a function of decreasing latitude from high (upper panels) to low (lower panels). Only results at 1000 m are shown and cluster results for other altitudes in the lower troposphere are similar. While the first period (with low 7 Be values) presents the 390 dominance of westerly winds (air masses flowing eastward), as typical of these latitudes (Zanis et al., 1999), the second period is characterized by a clockwise displacement of airflows with origins at higher altitudes ( Figure 15). This pattern in the second period is better established at lower latitude stations (Helsinki, Harku, Kista and Risoe) compared to higher latitude ones (Ivalo, Umea). It likely results from the aged vortex (Günther et al., 2008) and the SSW at the end of February, corresponding to a decrease in the MERRA-2 daily average height of the thermal 395 tropopause on 21 st -22 nd February at the Sodankylä station in Finland ( Figure S3) during the SSW (Peethani et al., 2014;Wargan and Coy, 2016). Associated with these processes is the downward transport of stratospheric air previously identified with an independent approach. Before using the model's output to investigate the processes responsible for 7 Be variability in Northern Europe over the period, we evaluated the MERRA-2 precipitation fields against the GPCP satellite and surface observations. A generally good agreement was found both at regional scale and at the six sampling sites. Analysis of the wind fields in the study period indicates low wind speeds from S-SW in agreement with the major circulation patterns over the Scandinavian Peninsula in winter, and the presence of a region of strong wind speeds to the west 410 of the study area, likely in connection with the Arctic polar vortex.

Summary and conclusions
The model reproduces efficiently the 7 Be and 7 Be/ 210 Pb temporal (i.e., monthly and weekly) patterns at the six sampling sites in the study period, even though it tends to underestimate the observed surface 7 Be concentrations. The lower modelled values are likely due to its coarse resolution (2.5° longitude by 2° latitude), lack of year-to-year variation in 7 Be production rates, and uncertainties associated with precipitation scavenging. 415 In order to investigate the processes responsible for 7 Be variability at the six sampling sites during the study period, and in particular to test whether the peak 7  Additionally, low relative humidity and a change in the circulation pattern from slow, clockwise to fast, swirling winds occurred over the study area. The change in the circulation pattern and the downward transport of stratospheric air was verified by the analysis of the clusters of back trajectories during the periods of low and high 7 Be concentrations, which showed a change from westerlies to airflows from upper vertical levels.

Data availability
The 7 Be and 210 Pb observational data are described in Section 2.1 and 2.2, respectively. The 222 Rn and 7 Be emission data used in this paper is described in Section 3.1. 7

Competing interests
The authors declare that they have no conflict of interest.