First description and classification of the ozone hole over the Arctic in boreal spring 2020

Ozone data derived from the TROPOMI sensor onboard the Sentinel-5 Precursor satellite are showing an atypical ozone hole feature in the polar region of the Northern hemisphere (Arctic) in spring 2020. A persistent ozone hole pattern with minimum total ozone column values around or below 220 Dobson units (DU) was seen for the first time over the Arctic for about 5 weeks in March and early April 2020. Usually an ozone hole with such low total ozone column values has only been observed in the polar Southern hemisphere (Antarctic) in spring over the last 4 decades, but not over the Arctic. The 15 ozone hole pattern was caused by a particularly stable polar vortex in the stratosphere, enabling a persistent cold stratosphere at higher latitudes, a prerequisite for ozone depletion through heterogeneous chemistry. Based on the ERA5 reanalysis from ECMWF, the Northern winter 2019/2020 (from December to March) showed minimum polar cap temperatures consistently below 195 K around 20 km altitude, which enabled enhanced formation of polar stratospheric clouds. The special situation in spring 2020 is compared and discussed in context with two other ozone hole-like features in spring 1997 and 2011 that were 20 showing comparable dynamical conditions in the stratosphere in combination with low total ozone column values. However, during these years total ozone columns below 220 DU over larger areas and over several consecutive days have not been observed. The similarities and differences of the atmospheric conditions of these three events and possible explanations are presented and discussed. It becomes apparent that the monthly mean of the minimum total ozone column value for March 2020 (i.e. 221 DU) was clearly below the respective values found in March 1997 (i.e. 267 DU) and 2011 (i.e. 252 DU), 25 which emphasizes the noteworthiness of the evolution of the polar stratospheric ozone layer in Northern hemisphere spring 2020. These results provide a first description and classification of the development of the Arctic ozone hole in boreal spring 2020 and highlight its peculiarity. https://doi.org/10.5194/acp-2020-746 Preprint. Discussion started: 21 July 2020 c © Author(s) 2020. CC BY 4.0 License.


Introduction
Today's operating satellite instruments produce a reliable picture of the Earth's atmosphere and its chemical composition.
These instruments monitor, for example, the evolution of the stratospheric ozone layer (e.g., Loyola et al., 2009), which is important for life on Earth. An ozone hole, a region with unusually low ozone values, can occur in polar regions if chemical and dynamical processes are interacting in a specific way that allow for strong ozone depletion and hamper meridional 5 transport of ozone rich air from lower latitudes.
The largest concentrations of atmospheric ozone are found in the stratosphere, in the so-called ozone layer, with about 90% of ozone abundance being located at an altitude between 15 and 30 km (e.g., Langematz, 2019). The Dobson unit (DU)named after Gordon Dobson , who devised the first instrument for measuring atmospheric ozone contentis used to describe the total amount of ozone found in the atmosphere above a specific location. Typically an ozone hole is 10 defined as the area where the total ozone column (TOC) decreases to values of less than 220 DU. In the Southern hemisphere polar region (Antarctic) a TOC below 220 DU is about 30% under the expected ozone value in austral spring. Climatological mean TOCs averaged over the Northern polar region (Arctic) in boreal spring are higher (~400-450 DU; e.g., Dameris, 2010), and therefore the decrease of TOC below 220 DU during this period indicates a reduction in the order of 50%.
Due to the prohibition of the production and usage of ozone depleting substances (among others CFCs: chlorofluorocarbons) 15 in response to the international activities to protect the ozone layer (Montreal Protocol: multilateral environmental agreement of the United Nations, signed in 1987, and its amendments) atmospheric concentrations of these chemical substances (particularly CFCs) and their products have been reduced over the last 20 years by about 15% (Chapter 1 in WMO, 2018).
Nevertheless, the current atmospheric content of CFCs is still enhanced as CFCs have lifetimes of several decades (SPARC, 2013). Consequently, the chlorine concentration in the stratosphere is still high. Based on the current scientific 20 understanding, the chlorine content is expected to reach pre-CFC-era conditions (i.e. levels similar to the ones before 1980) around the middle of this century, and therefore we can expect a full recovery of the ozone layer in the next 30 to 40 years (see Chapters 3 and 4 in WMO, 2018).
Notwithstanding the Montreal Protocol and the projected recovery of the ozone layer, very low temperatures in the polar lower stratosphere in any particular year, which are due to a strong and stable polar vortex in winter, can lead to heavy ozone 25 depletion in early spring, not only in the Southern hemisphere, but also in the Northern hemisphere. Exemplarily, in March 2020 very low TOC values were measured in the Arctic although the stratospheric chlorine content in 2020 is known to be clearly lower than in previous years (Chapter 1 in WMO, 2018).
The dynamical conditions of the stratosphere as observed in Northern hemisphere spring 2020 were unusual, showing a stable polar stratospheric vortex with low temperatures. Two other extreme situations have been noted in the literature, 30 indicating comparable dynamical conditions in the Northern stratosphere in spring: 1997 (e.g., Lefèvre et al., 1998;Hansen and Chipperfield, 1999) and 2011 (e.g., Manney et al., 2011). However TOC below 220 DU have not been observed in these https://doi.org /10.5194/acp-2020-746 Preprint. Discussion started: 21 July 2020 c Author(s) 2020. CC BY 4.0 License. two years. Although the dynamical conditions this winter and spring 2019/2020 were unusual, atmospheric researchers have expected the possible occurrence of such conditions because they are in the natural range of stratospheric dynamical fluctuations in Northern winter and early spring. The importance of stratospheric dynamics with respect to low TOC has been discussed in detail in the last decades (e.g., Chapters 4 and 12 in WMO, 1999;Chapter 3 in WMO, 2003;Rex et al., 2004;Tilmes et al., 2006;Kivi et al., 2007;Harris et al., 2010;Chapter 3 in WMO, 2014). 5 Nevertheless, it was unexpected to detect such low TOC valuesfalling even below the typical ozone hole threshold of 220 DU, which was devised for the Southern hemispherein the polar stratosphere in Northern hemisphere spring in 2020 ( Figure 1), despite the reduced chlorine content in the stratosphere. The occurrence of TOC values below 220 DU in March 2020 derived from satellite instrument measurements is confirmed by ground-based measurements at different Northern hemisphere stations, in particular at stations in Canada (for instance Alert, Eureka, and Resolute; current data are available at 10 http://www.temis.nl/uvradiation/UVarchive/stations_uv.html; see van Geffen et al., 2017).
This study provides a descriptive presentation of the recent dynamical situation in northern winter and spring 2019/2020, which led for the first time to an ozone hole with TOC below 220 DU over the Arctic. It allows a classification of the current situation by the comparison of similar dynamical conditions in spring of other years, but which did not show such low TOC values below 220 DU over the polar Northern hemisphere in spring . 15 In the next Section (Sect. 2) the data sets used for our analyses are introduced including a short description of the performed data processing. In Section 3 the special situation in Northern hemisphere winter and spring 2019/2020 is presented in detail and in Section 4 it is compared with two Northern hemisphere winter and spring periods, namely 1996/1997 and 2010/2011, where similar polar stratospheric conditionsincluding low TOC valueshave been observed. The discussion of results and the conclusions are presented in Section 5 and Section 6, respectively. 20

Meteorological data
In this study the presented dynamical analyses are based on meteorological data derived from ECMWF's most recent atmospheric reanalysis, ERA5 (Hersbach et al., 2019(Hersbach et al., , 2020. For our investigations the ERA5 data was downloaded at 0.25°x0.25° resolution. Daily mean data are prepared for the presentations of the respective meteorological situations. They 25 are produced using hourly data on pressure levels (Copernicus Climate Change Service (C3S), 2018) and using the CDO (climate data operators; Schulzweida, 2019) command ("daymean") to produce daily means from the hourly data. Monthly mean values are obtained from the monthly mean data at pressure levels (Copernicus Climate Change Service (C3S), 2019).
The focus is laid on stratospheric zonal winds and polar temperatures. ERA5 (raw) data is publicly available. For details see the data availability section. 30 https://doi.org /10.5194/acp-2020-746 Preprint. Discussion started: 21 July 2020 c Author(s) 2020. CC BY 4.0 License.

Ozone data
Ozone data from July 2019 to April 2020 from the TROPOMI sensor onboard the EU/ESA Copernicus Sentinel-5 Precursor satellite are scientifically used for the first time in combination with the long-term ozone data set from the European satellite data record GOME-type Total Ozone Essential Climate Variable (GTO-ECV) from July 1995 to June 2019 (Coldewey-Egbers et al., 2015). The publicly available (Level 2) TOC for July 2019 to April 2020 are derived from the TROPOMI 5 sensor using the GODFIT algorithm (Lerot et al., 2014). The estimated mean bias of the TROPOMI total ozone compared with ground-based measurements is less than ±1 % with a mean standard deviation of up to ±1.6-2.5 % (Garane et al., 2019).
The TROPOMI TOC images presented first in this study are based on daily mean data regridded to 1°x1° resolution to facilitate the comparison with the GTO-ECV data. For details see the data availability section.
GTO-ECV has been developed in the framework of the European Space Agency's Climate Change Initiative ozone project 10 and is based on observations from the satellite sensors GOME/ERS-2, SCIAMACHY, OMI, and GOME-2 covering the time period from July 1995 to June 2019 (Coldewey-Egbers et al., 2015). The agreement between GTO-ECV and ground-based observations is 0.5%-1.5% peak-to-peak amplitude with a negligible long-term drift in the Northern hemisphere (Garane et al., 2018) and the difference between GTO-ECV and an "adjusted" TOC data set based on reanalysis data is between −0.5±1.7 % and −1.0±1.1 % (for details see Coldewey-Egbers et al., 2020). In particular the excellent temporal stability 15 makes the GTO-ECV data record suitable and useful for applications related to long-term investigations of the ozone layer.
In this study we use the daily mean data product at 1°x1° resolution to analyse minimum ozone columns in the Northern hemisphere polar region during the past 24 years. It must be noted that during polar night the used satellite sensors cannot provide measurements. For instance in December, north of about 70°N no observations are available. With returning sunlight the coverage in the northern high latitude regions improves and global coverage is resumed around March 20. 20

Situation in Northern winter and spring 2019/2020
In the Arctic winter and early spring 2019/2020 the stratospheric polar vortex turned out to be persistent with strong zonal winds from mid-December until early April. Our analysis of ERA5 data at 60°N, 10 hPa (about 30 km altitude) shows strong zonal mean zonal wind speeds (magenta line and dots in Figure 2), which are high with respect to the monthly mean values for the time period from 1979/1980 to 2019/2020 (see grey dots in the figure). There were some smaller dynamical 25 fluctuations in winter 2019/2020, which were caused by planetary wave activity (which is also indicated by variations of meridional heat and momentum fluxes at mid-latitudes, 100 hPa (Newman et al., 2001); not shown here; but see https://acdext.gsfc.nasa.gov/Data_services/met/ann_data.html or https://ozonewatch.gsfc.nasa.gov/). No specific warmings of the polar stratosphere were observed (see below) and the shape of the vortex and its position was not severely deteriorated, except for the period from mid-January to beginning of February 2020, as hinted in Figure 2. The ERA5 monthly mean zonal winds  Figure 5). Further analyses of the temperature field at 50 hPa indicate large areas below 195 K (magenta line in Figure 6). The maximum daily 10 mean area of temperatures below 195 K is 13·10 12 m 2 , which is found end of January. At the end of March the daily cumulative area below 195 K results to about 920·10 12 m 2 . This led to conditions allowing the formation of polar stratospheric clouds (PSC) of type I (Nitric Acid Trihydrate (NAT) particles), for about 3.5 months. When the sun rises in spring, sunlight delivers the energy required for starting a chemical depletion process of ozone (e.g., Dameris, 2010).
Stratospheric ozone can then be destroyed by heterogeneous chemical reactions due to the still enhanced atmospheric 15 chlorine content (caused by CFC emissions in last decades). As a consequence, in spring 2020 an Arctic ozone hole, i.e. a region with TOC values below 220 DU, has developed within the boundaries of the stable polar vortex for eight continuous days from March 12 to 19 ( Figure 1; see also the magenta line in Figure 7). A region of significantly reduced total ozone column values, i.e. an ozone hole-like pattern, was observed over the polar cap from the beginning of March until early April 2020 ( Figure 1). 20 The temporal evolution of minimum TROPOMI TOC values north of 50°N from July 2019 until April 2020 is presented in Figure 7 (magenta line) and compared with historical values from the GOME-type Total Ozone Essential Climate Variable (GTO-ECV) data record (see Section 2 for details). In winter 2019/2020 ozone values were most of the time slightly below mean conditions until the end of February with respect to mean minimum TOC values (Figure 7, magenta line vs. thick black line). But there were several short-term deviations towards even lower TOC, during so-called ozone mini-hole events. The 25 most noteworthy examples occurred in early December 2019 (Dec 3 and Dec 4), beginning of January 2020 (Jan 4 and Jan 5, and Jan 7 and Jan 8), and the end of January (Jan 25 to Jan 27). Ozone mini-holes are synoptic-scale features (with a high pressure system in the troposphere below the stratospheric polar vortex, i.e. a low pressure area) with significantly reduced TOC values, which areto large partsunrelated to (heterogeneous) chemical processes. It is well understood that ozone mini-holes are primarily resulting from dynamical processes (e.g., Millán and Manney, 2017). The positions of the mini-30 holes correlate well with minima of potential vorticity near the tropopause (Peters et al., 1995;James and Peters, 2002). Hoinka et al. (1996) found that about 50% of short-term TOC fluctuations in the Northern hemisphere can be explained by variations of the tropopause pressure. Furthermore, Steinbrecht et al. (1998) showed that an increase of tropopause height by https://doi.org/10.5194/acp-2020-746 Preprint. Discussion started: 21 July 2020 c Author(s) 2020. CC BY 4.0 License. one kilometer is connected with a reduction of TOC by 16 DU. Figure 7 illustrates that such mini-hole events occur regularly (the lower light grey line) during Northern winter. Very commonly the ozone mini-holes are created in the Northern Atlantic region and then they often drift eastward towards Northern Europe within a few days (James, 1998). This was also the case for the three examples seen in winter 2019/2020 with minimum TOC found over Northern Europe (not shown). Since the polar vortex existed already in late November and early December 2019 with lower than usual TOC, for instance the ozone 5 mini-hole on December 3 and 4 showed very low TOC values (170 DU; Figure 7) at 65°N, north-east of UK and west of Scandinavia.
As the polar vortex was stable and strong since the beginning of the winter 2019/2020, an ozone hole-like pattern is expected from January 2020 onwards, but with TOC values above 220 DU inside the vortex. The stable polar vortex with strong zonal winds should have prevented the meridional transport of ozone rich air from lower latitudes towards the Northern polar ozone hole area (with TOC below 220 DU) was determined with 0.9 million km 2 (= 0.9·10 12 m 2 ), which was detected on March 12 (Figure 1). The daily accumulated ozone hole area in March and April was estimated with 4 million km 2 .

Situations in Northern winter and spring 1996/1997 and 2010/2011
There are two other prominent late winter / early spring periods in the Northern hemisphere, which showed similar stable and cold stratospheric polar vortices. In particular, comparable dynamical conditions in the Northern stratosphere were 25 observed in February and March 1997 (e.g., Lefèvre et al., 1998;Hansen and Chipperfield, 1999) and 2011 (e.g., Manney et al., 2011).
In  Table 1, indicating the characteristic of low temperatures in December 2019 and January and February 2020 (see also the colored dots in Figure 5).   1996/1997 201.3 196.5 191.8 192.7 2010/2011 195.6 194.2 191.2 194.7 2019/2020 194.3 190.7 190.8 194.6 Long-term means (1979/1980-2019/2020) 197.0 195.6 199.5 205.5 20 As demonstrated in Figure 6, the daily areas with temperatures below 195 K at the 50 hPa pressure level are obviously larger To summarize, in all three years the temperatures in the lower stratosphere in February and March were in a similar 5 temperature range, showing colder conditions than usual. In addition, December 2019 and January 2020 were also colder than the long-term mean conditions (Table 1). The minimum temperatures in the lower stratosphere in December/January 2019/2020 were lower than in December/January 1996/1997 and 2010/2011. Therefore, the thermal conditions in winter 2019/2020 were enabling enhanced formation of larger PSC fields, starting from the beginning of the winter until early spring. Having permanent presence of polar stratospheric clouds over about four months enabled more efficient chlorine 10 activation. In addition, they should have supported strong denitrification of the lower stratosphere by irreversible removal of total reactive nitrogen (NO y ), especially HNO 3 , due to heterogeneous reaction on the surface of PSCs followed by sedimentation of PSC particles (Fahey et al., 1990). This ultimately enabled a longer than usual period of chemical ozone depletion (e.g., Fahey et al., 1990;Rex et al., 1999, Pommereau et al., 2018. The occurrence of denitrification is very likely for the winter 2019/2020, but so far we cannot present a definite analysis for this hypothesis. 15 The temporal evolution of minimum TOC values north of 50°N between July 1996 and June 1997 (blue line in Figure 7  Dynamical conditions of the Northern stratosphere at higher latitudes in winter can range from a very disturbed polar vortex (i.e. by strong planetary wave activity), which leads to high stratospheric temperatures, to conditions with a persistent stable polar vortex (i.e. with low planetary wave activity), which creates low stratospheric temperatures.
Therefore, on the one hand, it is possible to find extreme situations with strong mixing of air masses in the polar regions (for instance during major stratospheric warmings) in combination with reduced chemically induced ozone depletion, which 5 leads to enhanced TOC values. On the other hand, situations with significantly suppressed meridional air mass exchange and transport into the polar vortex area can be found in combination with enhanced ozone depletion by heterogeneous chemical processes inside the vortex, which causes a clear reduction of TOC. The latter was the case in winter and spring 2019/2020.
Our comparative analysis of 2019/2020 with respect to the last four decades (i.e. the length of the ERA5 data set) indicates an extra-ordinary dynamical situation with a persistent strong and cold polar vortex over the complete season. There is 10 further evidence that a similar dynamical event did not happen in the period from 1955 to 1980, i.e. before the starting point of our dynamical analyses based on ERA5. A look into the historical data set, which was provided by the Stratospheric Research Group at FU Berlin (e.g., Labitzke and Naujokat, 2000), indicates that the winter and spring 1996/1997 was the coldest within the Berlin time series ranging from 1955 to 2000. In combination with our research results this suggests that the dynamical situation of the winter and spring 2019/2020 is outstanding since the beginning of monitoring the stratosphere 15 in the 1950s.
We recall that the stratospheric dynamical conditions were completely different in the Northern winter 2018/2019 (brown line in Figure 2) compared to 2019/2020 (magenta line in Figure 2) showing a sudden major stratospheric warming event, which started in late December 2018. In the first half of January 2019 the direction of the mean zonal wind (60°N, 10 hPa) changed its direction from westerlies to easterlies. This strong disturbance of the polar vortex by planetary waves led to a 20 pronounced warming of the lower stratosphere (e.g., Lee and Butler, 2020), indicating minimum temperatures in the polar cap region, which were clearly above the threshold for the formation of NAT-PSC (195 K) for the complete winter season including early spring (brown line in Figure 5). Consequently, comparatively high TOC values (around the long-term mean) in the Arctic region were found from late winter to early spring (not shown). Since the polar vortex in winter and spring 2019/2020 provided continuous conditions for the formation of PSCs, significant denitrification of the stratosphere should have occurred, i.e., a permanent removal of total reactive nitrogen (NO y , primarily HNO 3 ) by the sedimentation of NAT-PSC particles (Fahey et al., 1990) is expected. In this case, this would have contributed 20 to the five week period of significant TOC reduction by an extended phase of active stratospheric chlorine. As said, so far we cannot underline our suspicion by definite analyses. However, the observed minimum TOC values in March 2020 with new low TOC records for the Northern hemisphere polar cap were pointing to substantial ozone depletion, although the background chlorine content in 2020 was lower than in the years 1997 and 2011. Here we note that 2020 was also starting at lower base values of TOC (inside the polar vortex; see Figure 7). This will also contribute to the fact that the spring TOC 25 values in the Arctic region in 2020 were clearly lower than those found in 1997 and 2011. Beyond that the cold stratosphere in December 2019 and January 2020 as a single event does not point towards climate change due to increasing greenhouse gas concentrations. From our point of view, the Northern hemisphere winter 2019/2020 30 is a perfect showcase that a Northern winter with less planetary wave activity, and therefore a strong and stable vortex with low temperatures is possible. Only if similar conditions would happen more regularly in the next years, then this could be a sign of climate change. Although the stratosphere is more or less cooling steadily due to increasing greenhouse gas https://doi.org /10.5194/acp-2020-746 Preprint. Discussion started: 21 July 2020 c Author(s) 2020. CC BY 4.0 License.
concentrations (Maycock et al., 2018), consequences for stratospheric dynamics particularly in winter and ozone depletion in spring are still under debate (e.g., Pommereau et al., 2018). For instance the empirical quantification of the relation between winter-spring loss of Arctic ozone and changes in stratospheric climate by Rex et al. (2004) showed the possibility that cold (northern) winters are getting colder in future. It is possible that the cooling of the (lower) stratosphere could delay the recovery of the ozone layer (Pommereau et al., 2018). But this statement is in contradiction with results derived from 5 Chemistry-Climate model predictions (e.g., Langematz et al., 2014;Dhomse et al., 2018) indicating that climate change in the Northern hemisphere will accelerate stratospheric ozone recovery instead of delaying it (see also Chapters 3 and 4 in WMO, 2018). On the other hand, more than 20 years ago model calculations by Waibel et al. (1999) showed that higher degrees of Arctic denitrification in future, related to stratospheric cooling by enhanced greenhouse gas concentrations, could lead to larger seasonal ozone depletion despite the projected decline in inorganic chlorine. 10 Finally, based on our current knowledge we deem it unlikely that the observed enhanced CFC-11 emissions in recent years (Montzka et al., 2018) have significantly influenced the ozone depletion in the Northern hemisphere in 2020 (Dameris et al., 2019;Fleming et al., 2020;Keeble et al., 2020). The impact of the additional CFC-11 emissions should be of minor importance.

Conclusions 15
This study aims to present in a first step a consistent description of the Northern winter and spring season 2019/2020 regarding the dynamical situation of the stratosphere and the evolution of the ozone layer in the Arctic region. For the first time an Arctic ozone hole is detected, meaning that TOC values were in the vicinity or below 220 DU over a larger area (up to 0.9 million km 2 ) and for a longer time period (about a five weeks). The 2019/2020 situation is confronted with other years, which are showing similar stratospheric dynamics in spring. We have used most recently available data sets for the 20 preparation of presented analyses, (i) meteorological data from ERA5 and (ii) total ozone column data sets, i.e. GTO-ECV (based on the European satellite sensors GOME/ERS-2, SCIAMACHY, OMI, and GOME-2) in combination with TROPOMI onboard Sentinel 5P. Although the detected Arctic ozone hole is much smaller in comparison to a typical Antarctic ozone holewhich is in the order of about 20 to 25 million km 2 (from early September until mid-October) and TOC values below 220 DU are seen for up to about four months (WMO, 2018)it is an extra-ordinary event because an 25 ozone hole feature with TOC below 220 DU was not observed before. The results of our study pointed out that the persistent strong polar vortex in 2019/2020 (from mid-December to early April) led to particularly cold stratospheric conditions for the complete winter and early spring season, supporting the process of ozone depletion through heterogeneous chemistry. We have demonstrated that the special dynamical situation in winter 2019/2020 is relevant for this significant reduction of the TOC in spring 2020, which occurred despite decrease in the stratospheric chlorine content over the last 2 decades.
If the regulations of the Montreal Protocol regarding the prohibition of CFCs are implemented strictly one can expect a full recovery of the ozone layer including the polar regions by the middle of this century (Chapters 3 and 4 in WMO, 2018). In recent years, the beginning of ozone recovery was already detected (e.g., Solomon et al., 2016;Weber et al., 2018).
However, in the upcoming decades ozone holes will still occur in the Southern hemisphere, but also in the Northern hemisphere under appropriate dynamical conditions with a stable polar vortex yielding a strong cooling of the polar lower 5 stratosphere. Monitoring of the Earth's atmosphere from space is still an important task. The recovery of the ozone layer and its interactions with climate change must be carefully documented, as discussed for instance by Dameris and Loyola (2011).
The data available from the different monitoring instruments enable well founded scientific explanations of special ozone features. For instance it is possible to explain the recent evolution of the ozone layer, in particular the occurrence of the Arctic ozone hole in spring 2020 with new record low total ozone values for this region and period. This capability is crucial 10 to allow a classification of specific events in the light of the Montreal Protocol.  (2017,2018,2019). Neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus information or data it contains. In particular, subsets, i.e. wind and temperature data, from the pressure level data sets of monthly averaged data (Copernicus Climate Change Service (C3S), 2019) and hourly reanalysis data (Copernicus Climate Change Service (C3S), 2018) have been used. We thank the ECMWF for producing ERA5 data and making it available through the CDS. The used data 20 contains modified Copernicus Climate Change Service information, in particular with respect to Figures 2, 3, 4, and 5 and Table 1. Please note, that the data used here may also contain "preliminary" ERA-5 data (cf. Hersbach et al., 2020).