key: cord-1031061-rlwjbbyk authors: Sanchez-Lorenzo, A.; Vaquero-Martínez, J.; Calbó, J.; Wild, M.; Santurtún, A.; Lopez-Bustins, J.A.; Vaquero, J.M.; Folini, D.; Antón, M. title: Did anomalous atmospheric circulation favor the spread of COVID-19 in Europe? date: 2020-12-17 journal: Environ Res DOI: 10.1016/j.envres.2020.110626 sha: 35e64a463d32b279278303b045d2454fda2e521e doc_id: 1031061 cord_uid: rlwjbbyk The current pandemic of coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is having negative health, social and economic consequences worldwide. In Europe, the pandemic started to develop strongly at the end of February and beginning of March 2020. Subsequently, it spread over the continent, with special virulence in northern Italy and inland Spain. In this study we show that an unusual persistent anticyclonic situation prevailing in southwestern Europe during February 2020 (i.e. anomalously strong positive phase of the North Atlantic and Arctic Oscillations) could have resulted in favorable conditions, e.g., in terms of air temperature and humidity among other factors, in Italy and Spain for a quicker spread of the virus compared with the rest of the European countries. It seems plausible that the strong atmospheric stability and associated dry conditions that dominated in these regions may have favored the virus propagation, both outdoors and especially indoors, by short-range droplet and aerosol (airborne) transmission, or/and by changing social contact patterns. Later recent atmospheric circulation conditions in Europe (July 2020) and the U.S. (October 2020) seem to support our hypothesis, although further research is needed in order to evaluate other confounding variables. Interestingly, the atmospheric conditions during the Spanish flu pandemic in 1918 seem to resemble at some stage with the current COVID-19 pandemic. The world is currently undergoing a pandemic associated with the severe acute respiratory first outbreaks worldwide occurred during periods with temperatures around 5-11ºC, never 45 falling below 0ºC, and specific humidity of 3-6 g kg -1 approximately (Sajadi et al., 2020) . 46 Nevertheless, there are still some uncertainties about the role of climate variability in 47 a teleconnection pattern linked to NAO, showed in February 2020 the strongest positive 119 value during 1950-2020 (Fig. S3) . The AO reflects the northern polar vortex variability at 120 surface level (Baldwin et al., 2003) , and it consists of a low-pressure center located over the 121 Norwegian sea and the Arctic ocean and a high-pressure belt between 40 and 50ºN, forming 122 an annular-like structure. Positive values of the AO index mean a strong polar vortex, and 123 the anomalous positive phase experienced during early 2020 has been linked with the out-124 standing ozone loss registered over the Arctic region during March 2020 (Witze, 2020) . In 125 a separate study, we have hypothesized that this strong AO positive phase could have 126 played a non-negligible role in the first steps of the disease worldwide. Specifically, it is 127 worth remembering that the COVID-19 pandemic started to develop strongly by the end of 128 January, first in China with subsequent rapid spread to other countries concentrated mainly 129 within the 30-50ºN latitudinal regions. This feature seems to be in line with unusual persis-130 tent anticyclonic situation prevailing at latitudes around 40ºN, which was observed on 131 global scale due to the strong positive phase of the AO described above. This atypical situa-132 tion could have helped to provide favourable meteorological conditions for a quicker spread 133 of the virus (for more details, see Sanchez-Lorenzo et al., 2020, Fig. S4 ). 134 Back to Europe, we argue that this spatial configuration of the atmospheric circulation ( Fig. 135 1) might have played a non-negligible role in the modulation of the early spread of the 136 COVID-19 outbreaks over Europe. It is known that some cases were reported already in 137 mid-January in France, with subsequent cases in Germany and other countries (Spiteri et 138 al., 2020) . Thus, the SARS-CoV-2 virus was already in Europe in early 2020, but maybe it 139 started to extend rapidly only when suitable atmospheric conditions for its spread were 140 reached. It is possible that these proper conditions were met in February, mainly in Italy 141 and Spain, due to the meteorological conditions previously mentioned. 142 J o u r n a l P r e -p r o o f The link between the COVID-19 spread and atmospheric circulation has been tested as fol-143 lows. We have extracted monthly anomalies of sea level pressure (SLP) and 500 hPa geo-144 potential height for February 2020 over each grid point of the 15 capitals of the European 145 countries (Fig. S5) with the highest number of COVID-19 cases reported on late March 146 (see Section 2). Fig. 3 shows that there is a clear relationship between the anomalies of the 147 500 hPa and the total cases per population, which is given by a statistically significant 148 (R 2 =0.481, p<0.05) second order polynomial fit. Italy, Spain, and Switzerland, which are 149 the only countries with more than 1,000 cases/million inhabitants in our dataset, clustered 150 together in regions with very large positive anomalies of 500 hPa geopotential heights. 151 Similar results are obtained using SLP fields (not shown). 152 These results evidence that it seems plausible that the positive phase of the NAO, and the 153 atmospheric conditions associated with it, provided optimal conditions for the spread of the 154 COVID-19 in southern European countries like Spain and Italy, where the start of the out-155 break in Europe was located. To test this hypothesis further we have also analyzed the rela-156 tionship between the disease and meteorological data within Spain (see Section 2 and Fig. 157 S6). The results show that mean temperature and specific humidity variables have the 158 strongest relation with infections and deaths of COVID-19 and fit with an exponential func-159 tion (Fig. 4) . They indicate that those meteorological conditions given by lower mean tem-160 peratures (i.e., average of around 8-11ºC) and lower specific humidity (e.g., <6 g kg -1 ) are 161 related to a higher number of cases and deaths in Spain. Nevertheless, it is worth mention-162 ing that both meteorological variables are highly correlated (R 2 =0.838, p<0.05) and are not 163 independent of each other. The temperatures as low as 8-10ºC are only reached in a few 164 regions such as Madrid, Navarra, La Rioja, Aragon, Castilla and Leon and Castilla-La 165 Mancha. These areas are mainly located in inland Spain where drier conditions were re-166 J o u r n a l P r e -p r o o f ported the weeks before the outbreak. The rest of Spain experienced higher temperatures 167 and consequently were out of the areas of higher potential for the spread of the virus, as 168 reported so far in the literature. In addition, higher levels of humidity also seemed to limit 169 the impact of the disease (Barcelo, 2020), and therefore the coastal areas seem to benefit 170 from lower rates of infection. Thus, the southern regions of Spain (all of them with more 171 than 13ºC and higher levels of specific humidity) reported lower rates of infection and de-172 ceases. This is in line with the spatial pattern in Italy, with the most (least) affected regions 173 by COVID-19 mainly located in the North (South). In contrast, when the whole of Europe 174 is considered on a country by country basis (see above and Fig. 3 ), the opposite is found, a 175 clear gradient with more severity from North to South as commented previously. 176 The spatial pattern of COVID-19 described above has some intriguing resemblances with 177 the 1918 influenza pandemic, which is the latest deadly pandemic in modern history of Eu- France were the countries with the highest detected 14-days COVID-19 incidence in Eu-244 rope (Fig. S7) , which seems to be in line with the results reported above for the first wave 245 of virus infection in winter-spring. 246 In addition to Europe, Fig. 8 shows the anomaly 500 hPa field over North America for Oc-247 tober 2020, which was characterized by anticyclonic conditions over the Atlantic and Pacif-248 ic coastal regions of the U.S., whereas a very low pressure center in central-eastern Canada 249 enhanced a northwesterly flow circulation over the northern and central inland U.S. This 250 atmospheric circulation is associated with lower temperature and very low specific humidi-251 ty in these regions. The 7-days COVID-19 cases incidence map in early November over the 252 U.S. (Fig. S8) shows that most of the central and northern states reported the highest num-253 ber of cases, which seems to be aligned with the areas that experienced the northwestern 254 wind flows during October. It is interesting to note that several atmospheric conditions 255 might drive large outbreaks (i.e., not only anomalous anticyclonic conditions could trigger 256 COVID-19 outbreaks), which should be taken into account in further studies as we can ex-257 pect that these atmospheric patterns can be different along the year and also highly geo-258 graphical dependent, i.e., mid-latitudes vs tropical regions (Lowen and Palese, 2009) . The results presented in this study could involve some health policy implications, as the lag 287 between large atmospheric circulation anomalies and the COVID-19 outbreaks could be 288 used for implementing early alert protocols using weather and seasonal forecasting models 289 that can predict atmospheric circulation patterns several days/weeks in advance. 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Image generated with the Web-based Reanalysis Intercomparison Tool provided 601 by the NOAA/ESRL Physical Sciences Laboratory 1 m distance) and the smaller airborne particles spreading in sus-615 pension for longer time, and reaching longer distances, especially in dry and stable indoor 616 conditions as compared to wet environments. It is also possible that a resuspension of aero-617 sol particles can eventually happen due to human activities (e.g., walking, cleaning, etc.) or 618 air flows Anomaly pattern of 500 hPa geopotential height (m) for July 2020 over Europe as 631 compared to the climatology mean (1981-2010 period). Image generated with the Web-632 based Reanalysis Intercomparison Tool provided by the NOAA/ESRL Anomaly pattern of 500 hPa geopotential height (m) for Image generated with 644 the Web-based Reanalysis Intercomparison Tool provided by the NOAA/ESRL Physical 645