Spatiotemporal variability of blockings in the Euro-Atlantic region and their impact on the occurrence of heat waves and cold spells in Poland

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Tomczyk This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5094045/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 23 Dec, 2024 Read the published version in Theoretical and Applied Climatology → Version 1 posted 9 You are reading this latest preprint version Abstract The spatiotemporal variability and trends of atmospheric blockings over the Euro-Atlantic region and their influence on the occurrence of the persisting extreme temperature conditions in Poland namely heat waves (HWs) and cold spells (CSs) during the period 1978–2023 were analyzed. Blockings were identified at 500 hPa geopotential level, using the meridional geopotential gradient method, supplemented with the quantile filter and persistence filter, using reanalysis data from the National Oceanic and Atmospheric Administration Physical Science Laboratory (NCEP-DOE AMIP-II R-2). HWs and CSs were defined as sequences of at least 3 days with the maximum air temperature above 30°C or below − 10°C, respectively based on data obtained from the Institute of Meteorology and Water Management – National Research Institute (IMGW – PIB) for the period 1978–2022 across 37 stations in Poland. The climatology of Euro-Atlantic blocking occurrence in the zonal belt between 45 and 75 degrees in the northern hemisphere exhibits high spatiotemporal variability. Blocking structures are most frequent in the spring, particularly in May. A secondary peak of frequency is observed in July when the Ural blocking exhibits 15% frequency. Patterns of trends in blocking occurrence are variable and the strongest signals of changes are observed in spring. The occurrence of HWs in Poland is constantly accompanied by blocking situations, most often located northeast of Poland, while the winter CSs are associated with the blockings located over the North Atlantic and northern Scandinavia. Heat waves cold spells atmospheric blocking Euro-Atlantic region Poland Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction In the context of significant contemporary climate change, extreme weather events have become a prominent focus in climatology, largely due to their substantial economic and social risks. The observed increase in air temperatures, which has accelerated particularly in the early 21st century (NOAA 2023; IPCC 2021), has led to a rise in the frequency of heat waves (HWs), considered among the most lethal natural hazards (e.g., Kovats and Kristie 2006; Mohleji and Pielke 2014). Additionally, HWs are frequently linked with droughts that exacerbate the risk of forest fires and cause substantial damage to agriculture, thus amplifying the associated hazards (e.g., Christian et al. 2020). According to Hansen et al. (2012), the area of the Earth’s surface experiencing air temperatures classified as extreme heat increased significantly from less than 1% during 1951–1980 to over 10% during 2006–2011. In recent decades, Europe has witnessed several severe HWs. Matzarakis et al. (2020) demonstrated that 13 of the top 23 most intense HWs over the last 70 years occurred within the last 12 years, whereas only seven were recorded in the 50 years before 2000. Russo et al. (2015) identified notable heat waves in 1994, 2003, 2006, 2010, and 2015 as some of the most severe in Europe since 1950. Similar trends have been observed in Poland (Krzyżewska and Dyer 2018; Wibig 2018, 2021; Tomczyk et al. 2019, 2020). Krzyżewska and Dyer (2018) highlighted the HWs of 1963, 1994, and 2015 as among the most intense in Poland. The events in 1994 and 2015 were the longest HWs in recent decades, affecting much of Poland and central Europe (Krzyżewska and Dyer 2018; Wibig 2018; Tomczyk et al. 2019, 2020). These events resulted in increased mortality rates in Poland (Kuchcik 2001; Graczyk et al. 2019) and in neighboring countries (Kyselý and Huth 2004; Gabriel and Endlicher 2011; Muthers et al. 2017). Despite significant climate warming, several-day periods of very low air temperatures, known as cold spells (CSs), have been observed during numerous winter seasons in recent years across various regions of Europe. Notable examples of such cold episodes occurred in Poland in 2016, 2017 (Report of the Government Centre for Security, 2016, 2017), and 2018 (Tomczyk and Bednorz, 2020). Similar situations have been recorded in other regions as well, such as the Balkans, where significant CSs were observed in 2012 (Tolika et al., 2014; Unkašević and Tošić, 2015) and in 2017 (Anagnostopoulou et al., 2017; Demirtaş, 2022). The occurrence of both HWs and CSs is often associated with a common feature: large-scale pressure patterns known as atmospheric blocking. Despite being relatively rare, atmospheric blockings significantly influence atmospheric dynamics in midlatitudes, and they have been extensively studied in climatological literature, including various reviews and systematizing papers (e.g., Barnes et al., 2012; Lupo, 2021; Kautz et al., 2022). Blocking systems are characterized as persistent, quasi-stationary disturbances with high pressure at the surface, typically forming in middle and high latitudes (e.g., Lupo, 2021). The fundamental concept of atmospheric blocking involves a transition from the typical zonal flow in midlatitudes to a meridional flow, often associated with a prolonged, quasi-stationary anticyclone and the bifurcation of the jet stream around the high-pressure system (e.g., Liu, 1994; Sousa et al., 2021). Nakamura and Huang (2018) provided a mathematical description of blocking mechanisms, likening them to a 'traffic jam' in the jet stream. According to Sousa et al. (2021), atmospheric blockings tend to form in specific regions characterized by enhanced subtropical ridge activity. In the Northern Hemisphere, these blockings are most frequent in mid- to high-latitude sectors between Greenland and Europe, across northeastern Asia and Alaska, and over the eastern parts of oceans and western edges of continents. In the Euroatlantic area, high-latitude blockings often develop north of the jet stream, particularly over Greenland, and are reinforced by cyclonic wave breaking associated with the negative phase of the North Atlantic Oscillation. Greenland blocks, which result in a northerly flow, significantly influence weather conditions in Europe. Over the continent, blockings driven by anticyclonic wave breaking typically occur in Scandinavia, extending sometimes to the Azores, and eastward to the Ural region (e.g., Barriopedro et al. 2006, Tyrlis and Hoskins 2008, Davini et al. 2020, Cheung et al. 2013, Mokhov et al. 2013, Small et al. 2013, Lupo et al. 2019). Atlantic and European blockings are more frequent from winter to early spring and less common in summer and early autumn (e.g., Lupo and Smith 1995, Barriopedro et al. 2006). According to Lupo et al. (2019), atmospheric blockings have become more frequent since the late 20th century. A significant amount of interest in atmospheric blockings, as highlighted in numerous studies not fully covered in preceding paragraphs, stems from their association with extreme weather events (Matsueda 2009, Kautz et al. 2022). Thermal extremes are often linked to blockings, primarily due to clear-sky conditions within anticyclones. Additionally, temperature extremes are influenced by shifts in wind direction from westerly to northerly/easterly in winter and southerly in summer. During summer, radiative forcing plays a crucial role in temperature increases, whereas in winter, changes in circulation associated with blockings are more influential in causing temperature drops (Buehler et al. 2011, Sillmann et al. 2011, Lupo et al. 2012, Pfahl and Wernli 2012, Porebska and Zdunek 2013, Brunner et al. 2017, Luo et al. 2017, Quandt et al. 2017, Yao et al. 2017, Brunner et al. 2018, Sousa et al. 2018, Woollings et al. 2018, Chan et al. 2019). The emergence of extreme weather in Central Europe is primarily attributed to atmospheric circulation patterns influenced by pressure systems forming over the Euro-Atlantic region. Despite the documented strong impact of blocking patterns on weather extremes, there remains a significant gap in detailed regional studies on this topic. Specifically, understanding the specific geographical locations and seasonal characteristics of blocking anticyclones driving thermal extremes in Poland is crucially lacking. Therefore, this study aims to address these gaps with two primary objectives: first, to distinguish blocking structures over the Euroatlantic region and elaborate on their climatology, including their spatial and temporal variability and; and second, to assess the impact of these blockings—considering their location and persistence—on the occurrence of summer HWs and winter CSs in Poland from 1978 to 2022. 2. Data and methods There is currently no universally accepted definition of atmospheric blocking, leading to diverse criteria and detection methods in climatological research (Barriopedro et al. 2010). Various meteorological variables are employed to construct blocking indices, with the 500 hPa (Z500) geopotential height being the most commonly used. This approach typically considers anomalies in Z500 or reversals in meridional gradients (Dole and Gordon 1983, Tibaldi and Molteni 1990, Barriopedro et al. 2006, Diao et al. 2006, Scherrer et al. 2006, Barriopedro et al. 2010, Schalge et al. 2011, Porebska and Zdunek 2013). The geopotential height gradient method employed in this study was first introduced by Lejenäs and Økland (1983) and has since undergone refinement in subsequent studies, being now the most widely used method (e.g., Tibaldi and Molteni 1990, Scherrer et al. 2006, Schalge et al. 2011, Barnes et al. 2012; Pinheiro et al. 2019). The Z500 reanalysis data were obtained from the National Oceanic and Atmospheric Administration Physical Science Laboratory [NCEP-DOE AMIP-II Reanalysis (R-2), Kanamitsu et al. 2002]. This dataset covers the period from 1978 to the present day with a temporal resolution of 1 day and a spatial resolution of 2.5°x2.5°, which is adequate for this study. Following modifications by Scherrer et al. (2006) to the methodology of Tibaldi and Molteni (1990), the blocking index was computed for each grid point in the Euroatlantic sector between latitudes 35° and 75°N, using latitudes φ 0+15 , φ 0 , and φ 0−15 . Geopotential height gradient thresholds were set at > 0 m/°lat for the southern gradient (Z500φ 0 – Z500φ 0−15 ) and < -10 m/°lat for the northern gradient (Z500φ 0+15 – Z500φ 0 ). Additionally, a persistence filter of 3 days duration and a quantile filter set at 0.5 for each latitude were applied following the approach of Tibaldi and Molteni (1990). Monthly frequencies of blockings and a rate of changes for each month in every grid point were computed and shown on maps. HWs and CSs were defined as sequences lasting at least 3 days with maximum air temperatures above 30°C or below − 10°C, respectively, based on data obtained from the Institute of Meteorology and Water Management – National Research Institute (IMGW – PIB) for the period 1978–2022 across 37 stations in Poland. The adopted definitions of HW and CS are based on bioclimatic preconditions and have been used in many earlier studies concerning extreme events in Poland (e.g. Wibig et al., 2009a,b; Porębska and Zdunek, 2013; Wibig, 2018; Tomczyk et al., 2019) Kuchcik (2006) has supplied a detailed description of the thresholds and definitions of prolonged extreme temperature events commonly applied in climatological research. 3. Results 3.1. Spatiotemporal variability of Euro-Atlantic blocking The occurrence of blocking in the Euro-Atlantic region exhibits considerable spatiotemporal variability (Fig. 1 ), with distinct spatial patterns of blocking frequency varying from month to month. During winter (Dec-Feb), two areas of high blocking frequency are notable, with one being the Greenland High. The highest frequency of the winter blocking is observed along the western coast of Europe, extending into southern Scandinavia in February. Towards the end of winter and into spring, blocking structures propagate eastward along the 60°N latitude, with the highest frequency occurring in late spring, particularly in May. During this period, blockings appear most often (> 10%) in an arched belt extending from Greenland through the North Sea, southern Scandinavia, to western Russia, where their frequency exceeds 15%. In the summer season (Jun-Aug), blocking events become less frequent overall, except for a pronounced peak in the northeastern edge of the study area where the frequency exceeds 15% in July. Towards the end of summer (August) and into autumn, the frequency of blocking events decreases further, generally not exceeding 10%. The occurrence of atmospheric blocking in the Euro-Atlantic region exhibits significant but highly variable trends in both space and time, more so than the overall blocking frequency. In winter, positive trends are observed in the northeast (January) and the northwestern Atlantic (December). In contrast, February shows negative, statistically significant trends over continental Europe, particularly north of 60° latitude. A strong positive trend has been observed in early spring over the British Isles, with an increase of 1.5 blocking days per decade in March. However, eastward across continental Europe, the trend is negative. In April, positive signals appear over the northern Atlantic, while both April and May show a decrease in blocking structures over Europe between 50° and 60° latitude. During summer, positive trends emerge in June over central Europe and in August over the eastern part of the continent. In contrast, July and August see a reduction in blocking days in the northeastern part of the study area, and blocking events related to the Azores High over the southwestern Atlantic have also become less frequent. Autumn trends are generally weaker, with positive signals in the north (October and November) and negative trends in the mid-latitudes (November). 3.2. Occurrence of heat waves (HWs) and cold spells (CSs) in Poland Although the mean annual number of HW days in Poland varies considerably from year to year, a significant increasing trend is observed, with a rate of increase of approximately 4.5 days per decade (Fig. 3 ). Since 2000, seven summers have experienced more than 20 HW days, and in the past decade alone, more than 30 HW days per season were recorded in the summers of 2015, 2019, and 2022. Notably, only two summers – 1980 and 2009 – did not witness any 3-day periods with Tmax exceeding 30°C. HWs are most frequent in July, August, and July, however, they may also occur sporadically in September, May, or even April. The frequency of HW days in Poland exhibits significant spatial variability across different regions. In the southern mountainous areas and northern Poland, the average is approximately one HW day per year. In contrast, the central and western parts of the country experience an average of more than four HW days per year (Fig. 4 ). However, during exceptionally hot summers, such as in 2015, the number of HW days in southern Poland surged to over 20. CSs appear in Poland mostly in January and February and are less frequent in December. Only sporadically were observed in March. Only two seasons in the analyzed 44-year period experienced more than 20 CS days (1984/1985 and 1986/87). Despite observed warming, CSs were observed during half of the winters in the 21st century, with as much as > 10 CS days in seasons 2002/2003, 2009/2010, and 2011/2012. On average, only one CS day is observed in most of Poland, and only in the northeast, there are approximately two CS days per season. Only during specific seasons, such as the winter of 1986/1987, did the number of CS days exceed 10, and this was observed exclusively in northeastern Poland (Figs. 3 and 5 ). 3.3. Location of blockings associated with HWs in Poland The occurrence of HWs in Poland is consistently associated with atmospheric blocking. During every HW event recorded between 1978 and 2022, an atmospheric blocking pattern was present in some parts of the Euro-Atlantic region. These blocks are most frequently centered northeast of Poland, where their frequency exceeds 20%, which is significantly higher than the typical summer frequency (Fig. 6 ). The overall frequency of blocking structures along 30°E longitude surpasses 10% during Polish HWs, compared to a general summer frequency of approximately 6% (Fig. 6 c). Notably, a significant increase in blocking frequency is observed in July in the region extending from eastern Poland to Belarus and western Russia (Fig. 2 ), likely contributing to the rising number of HW days in Poland. Blocking systems, positioned northeast of Poland, induce meridional circulation patterns over the country, facilitating the advection of warm air. Additionally, the downward movement of air and adiabatic subsidence under stable anticyclonic conditions can further increase surface temperatures. This process is amplified by cloud-free weather, allowing a high influx of solar energy. The increased solar radiation enhances the surface energy balance, leading to elevated temperatures. Location of blockings associated with CSs in Poland Polish winter CSs are also strongly associated with blockings, primarily located over the North Atlantic and the North Sea region, where their frequency during Polish CSs exceeds 30% – significantly higher than the usual winter frequency in this area. A secondary region of high blocking frequency associated with Polish CSs is located over northern Scandinavia, with frequencies exceeding 30% (Fig. 6 ). Between latitudes 65–70°N, the frequency of blocking situations during Polish CSs is four times higher than normal winter conditions. Longitudinally, the maximum blocking frequency during CSs occurs between 10°W and 10°E, more than twice the usual winter frequency (Fig. 6 c and d). The frequency of winter blocking events in the zonal belt to the north and northeast of Poland exibits a substantial decreasing trend by approximately 0.5 to 1 day per decade in February (Fig. 2 ). This reduction, along with the overall trend of climate warming, diminishes the likelihood of CSs occurrences in Poland. Anticyclonic blocking patterns inhibit zonal airflow and intensify meridional flow. In winter, this results in the advection of the Arctic or polar continental air masses into central Europe. Additionally, anticyclones contribute to cloud-free conditions and strong radiative cooling. During winter, the significant radiation of heat from the ground, coupled with low or no cloud cover typical of anticyclonic weather, leads to a decrease in surface air temperature. 4. Discussion This study, along with numerous previous studies, has demonstrated that in a warming world, HWs are becoming more frequent, with this trend being particularly strong in central Europe, including Poland (i.e., Wibig 2018, 2021, Tomczyk and Bednorz 2019, Ustrnul et al 2020). Hot periods exert significant stress on humans, with thermal stress intensifying as temperatures rise (Owczarek and Filipiak 2016, Owczarek 2019, Tomczyk and Owczarek 2020, Tomczyk et al. 2019, Tomczyk and Bednorz 2023). Although CSs are becoming less frequent in midlatitudes and are therefore less popular in scientific research, they still have a severe negative impact on human health, causing cold stress as well as socioeconomic losses and disruptions (Owczarek and Tomczyk 2022, Tomczyk and Bednorz 2023). Because of this, there is a significant interest in understanding the synoptic conditions and causes of extreme temperature periods. It is widely agreed, that the prolonged summer heat and winter cold conditions in Poland are associated with high-pressure systems (i.e., Tomczyk and Bednorz 2016, Wibig 2018, 2021, Tomczyk et al. 2019). Anticyclones contribute to cloud-free weather and strong energy fluxes. During summer, increased solar irradiation enhances the surface energy balance, raising temperatures. Conversely, in winter, the strong radiative cooling of the ground due to low or no cloud cover, characteristic of anticyclonic weather, decreases surface air temperatures. The extreme thermal conditions have been less frequently analyzed in relation to midlatitude atmospheric blockings, which inhibit the zonal airflow and intensify meridional flow, resulting in the presence of Arctic or polar continental air masses in winter and tropical air masses in summer in Central Europe. Porębska and Zdunek (2013) investigated the occurrence of extreme temperature events in Central Europe in conjunction with high-pressure blocking situations during the decade 2001–2011. They found cold waves were associated with blockings located over the Atlantic Ocean, while heat waves most often occurred under eastern European blocks. In this study, it was specified that CSs are most often accompanied by blockings located over the North Atlantic and the North Sea region, whereas HWs occur under blockings located northeast of Poland, over western Russia. Kautz et al. (2022) in their review of the atmospheric blocking impact on weather extremes point out that while European HWs are associated with high-pressure anomalies, cold anomalies are typically not located directly beneath the blocking anticyclone but rather downstream or to the south of it. This rule was also documented in our study. Most research indicated that the highest frequency of atmospheric blocking over the North Atlantic and European sections occurs in spring, with the lowest frequency in summer. However, Barriopedro (2006) noted, that while these structures are rare over the ocean in summer, they are much more frequent over the continent. Lupo (2021) demonstrated that summer blocks have similar durations to those in spring and winter, but of much lower intensity as measured by Z500 gradient). Porębska and Zdunek (2013) identified May as the month with the highest number of blocking events in the Euro-Atlantic region, with a secondary peak in July. This observation aligns precisely with the findings on blocking frequency presented in this study. Understanding the physical mechanisms and processes leading to thermal extremes can improve the predictability of these weather events, which is crucial to society and policymakers. Furthermore, the predictability of extreme events associated with blocking should be analyzed in the context of climate change, as blocking is projected to remain a significant circulation feature initiating European heat waves under future climate conditions (Brunner et al., 2018; Schaller et al., 2018, Lupo 2021). This study identified a statistically significant decreasing trend in winter blocking frequency over European midlatitudes, and most models project a further decrease in blocking frequency in both winter and summer (Sillmann and Croci-Maspoli, 2009; Davini and D’Andrea, 2020). However, there are exceptions to the decreasing trend, one of them being the Ural region, with summer blocking frequency projected to increase, a trend also was observed in the analyzed period 1978–2022. 5. Conclusions It has been demonstrated that the occurrence of blocking in the Euro-Atlantic region exhibits high spatiotemporal variability. Blocking events are most frequent in the spring, particularly in May, occurring with a 10% frequency in the arched belt extending from the west of the British Islands through the North Sea, south Scandinavia, to the east of the Baltic Sea. A secondary peak in blocking frequency is observed in July, with Ural blockings reaching a 15% frequency. The trends in atmospheric blocking occurrence across the Euro-Atlantic region show notable spatial and temporal variability. Significant negative trends were detected over Europe at latitudes between 60° and 70°N, while strong positive trends emerged locally during spring and summer. The mean annual number of HW days in Poland has been increasing in the period 1978–2022 and spatially it ranges from one day in the southern mountainous regions and northern Poland to more than four days in the central and western parts. CSs have become rarer, with the average seasonal number of CS days amounting to 1–2 days in recent decades. Only in singular seasons, more than 10 days fulfilled the criteria of CS in northeastern Poland. Heatwaves (HWs) and cold spells (CSs) in Poland are consistently accompanied by blockings that have distinct locations. Summer HWs are most often associated with blocks located northeast of Poland (over western Russia), while winter CSs most often occur under blockings located over the zonal belt between 60°N and 70°N extending from the North Atlantic to northern Scandinavia. Declarations Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding This work was supported by the National Science Centre, Poland (grant number UMO-2020/37/B/ST10/00217). Author Contribution E.B.: Conceptualization; Data curation; Methodology; Formal analysis; Investigation; Methodology; Resources; Software; Supervision; Validation; Visualization; Roles/Writing - original draft; Writing - review & editing. A. M.T.: Data curation; Methodology; Funding acquisition; Project administration; Roles/Writing – original draft; Writing – review & editing. Data availability Data will be made available from the corresponding author on request. References Anagnostopoulou C, Tolika K, Lazoglou G, Maheras P (2017) The exceptionally cold January of 2017 over the Balkan Peninsula: A climatological and synoptic analysis. Atmosphere 8(12):252. https://doi.org/10.3390/atmos8120252 Barnes EA, Slingo J, Woollings T (2012) A methodology for the comparison of blocking climatologies across indices, models and climate scenarios. 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Adv Meteorol 2011(1):717812. https://doi.org/10.1155/2011/717812 Schaller N, Sillmann J, Anstey J, Fischer EM, Grams CM, Russo S (2018) Influence of blocking on Northern European and Western Russian heatwaves in large climate model ensembles, Environ Res Lett 13:054015. https://doi.org/10.1088/1748- 9326/aaba55 Scherrer SC, Croci-Maspoli M, Schwierz C, Appenzeller C (2006) Two‐dimensional indices of atmospheric blocking and their statistical relationship with winter climate patterns in the Euro‐Atlantic region. Int J Climatol 26(2):233–249. https://doi.org/10.1002/joc.1250 Sillmann J, Croci-Maspoli M (2009) Present and future atmospheric blocking and its impact on European mean and extreme climate, Geophys Res Lett 36:L10702. https://doi.org/10.1029/2009GL038259 Sillmann J, Croci-Maspoli M, Kallache M, Katz RW (2011) Extreme cold winter temperatures in Europe under the influence of North Atlantic atmospheric blocking. J Clim 24(22):5899–5913. https://doi.org/10.1175/2011JCLI4075.1 Small D, Atallah E, Gyakum JR (2013) An objectively determined blocking index and its Northern Hemisphere climatology. J Clim 27(8):2948–2970. https://doi.org/10.1175/JCLI-D-13-00374.1 Sousa PM, Barriopedro D, García-Herrera R, Woollings T, Trigo RM (2021) A new combined detection algorithm for blocking and subtropical ridges. J Clim 34(18):7735–7758. https://doi.org/10.1175/JCLI-D-20-0658.1 Sousa PM, Trigo RM, Barriopedro D, Soares PM, Santos JA (2018) European temperature responses to blocking and ridge regional patterns. Clim Dyn 50:457–477. https://doi.org/10.1007/s00382-017-3620-2 Tibaldi S, Molteni F (1990) On the operational predictability of blocking. Tellus A 42(3):343–365. https://doi.org/10.1034/j.1600-0870.1990.t01-2-00003.x Tolika K, Maheras P, Pytharoulis I, Anagnostopoulou C (2014) The anomalous low and high temperatures of 2012 over Greece – an explanation from a meteorological and climatological perspective. Nat. Hazards. Earth Syst Sci 14:501–507. https://doi.org/10.5194/nhess-14-501-2014 Tomczyk AM, Bednorz E (2023) Thermal stress during heat waves and cold spells in Poland. Weather Climate Extremes 42:100612. https://doi.org/10.1016/j.wace.2023.100612 Tomczyk AM, Bednorz E (2019) Heat waves in Central Europe and tropospheric anomalies of temperature and geopotential heights. Int J Climatol 39(11):4189–4205. https://doi.org/10.1002/joc.6067 Tomczyk AM, Bednorz E (2020) The extreme year – analysis of thermal conditions in Poland in 2018. Theor Appl Climatol 139:251–260. https://doi.org/10.1007/s00704-019-02968-9 Tomczyk AM, Bednorz E, Półrolniczak M, Kolendowicz L (2019) Strong heat and cold waves in Poland in relation with the large-scale atmospheric circulation. Theor Appl Climatol 137 (3–4):1909–1923. https://doi.org/10.1007/s00704-018-2715-y Tomczyk AM, Owczarek M (2020) Occurrence of strong and very strong heat stress in Poland and its circulation conditions. Theor Appl Climatol 139:893–905. https://doi.org/10.1007/s00704-019-02998-3 Tyrlis E, Hoskins BJ (2008) The morphology of Northern Hemisphere blocking. J Atmos Sci 65(5):1653–1665. https://doi.org/10.1175/2007JAS2338.1 Unkašević M, Tošić I (2015) Seasonal analysis of cold and heat waves in Serbia during the period 1949–2012. Theor Appl Climatol 120:29–40. https://doi.org/10.1007/s00704-014-1154-7 Ustrnul Z, Czekierda D, Wypych A (2010) Extreme values of air temperature in Poland according to different atmospheric circulation classifications. Phys Chem Earth 35:429–436. https://doi.org/10.1016/j.pce.2009.12.012 Wibig J (2018) Heat waves in Poland in the period 1951–2015: trends, patterns and driving factors. Meteorology Hydrology and Water Management. Research and Operational Applications 6(1):37–45. Wibig J (2021) Hot days and heat waves in Poland in the period 1951–2019 and the circulation factors favoring the most extreme of them. Atmosphere 12(3):340. https://doi.org/10.3390/atmos12030340 Wibig J (2018) Heat waves in Poland in the period 1951–2015: trends, patterns and driving factors. Meteorol Hydrol Water Management 6 (1):37–45. https://doi.org/10.26491/mhwm/78420 Wibig J (2021) Hot Days and heat waves in Poland in the period 1951–2019 and the circulation factors favoring the most extreme of them. Atmosphere 12(3):340. https://doi.org/10.3390/atmos12030340 Wibig J, Podstawczyńska A, Rrzepa M, Piotrowski P (2009a) Heatwaves in Poland–Frequency, trends and relationships with atmospheric circulation. Geogr Pol 81:33–46. Wibig J, Podstawczyńska A, Rzepa M, Piotrowski P (2009b) Coldwaves in Poland–Frequency, trends and relationships with atmospheric circulation. Geogr Pol 82:47–59. Woollings T, Barriopedro D, Methven J, Son SW, Martius O, Harvey B, Sillmann J, Lupo AR, Seneviratne S (2018) Blocking and its response to climate change. Curr Clim Change Rep 4:287–300. https://doi.org/10.1007/s40641-018-0108-z Yao Y, Luo D, Dai A, Simmonds I (2017) Increased quasi stationarity and persistence of winter Ural blocking and Eurasian extreme cold events in response to Arctic warming. Part I: Insights from observational analyses. J Clim 30(10):3549–3568. https://doi.org/10.1175/JCLI-D-16-0261.1 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 23 Dec, 2024 Read the published version in Theoretical and Applied Climatology → Version 1 posted Editorial decision: Revision requested 26 Oct, 2024 Reviews received at journal 24 Oct, 2024 Reviews received at journal 02 Oct, 2024 Reviewers agreed at journal 02 Oct, 2024 Reviewers agreed at journal 01 Oct, 2024 Reviewers invited by journal 30 Sep, 2024 Editor assigned by journal 16 Sep, 2024 Submission checks completed at journal 16 Sep, 2024 First submitted to journal 15 Sep, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5094045","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":370685836,"identity":"68e825f2-3f51-47f3-9c2c-40e3a191ee54","order_by":0,"name":"Ewa Bednorz","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0klEQVRIiWNgGAWjYHACNjDih3CYQbiBOC2SDXAtjERqMThArBZ+ieRnDz6U3ZEzPn46TYKhwjqxQboRvxbJGWnmhjPOPTM2O5O7TYLhTHpig8xB/FoMbieYSfO2HU7cdgCohRHIaJBIxK/F/nb6N+m/QJWb+98CtfwjQouBdI6ZNMjwDRIgWxqI0CJx/025Yc+5w8YSN95utkg4lm7cRsgv/D3Htz34UXZYjr8/d+ONDzXWsv3SzQfwakEFCUDMJkGCBphbSdcyCkbBKBgFwxsAAF9iSvAZ9Yt1AAAAAElFTkSuQmCC","orcid":"","institution":"Adam Mickiewicz University in Poznań","correspondingAuthor":true,"prefix":"","firstName":"Ewa","middleName":"","lastName":"Bednorz","suffix":""},{"id":370685837,"identity":"120ccef6-c234-455b-9ebb-47e6e79801f6","order_by":1,"name":"Arkadiusz M. Tomczyk","email":"","orcid":"","institution":"Adam Mickiewicz University in Poznań","correspondingAuthor":false,"prefix":"","firstName":"Arkadiusz","middleName":"M.","lastName":"Tomczyk","suffix":""}],"badges":[],"createdAt":"2024-09-15 19:44:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5094045/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5094045/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00704-024-05253-6","type":"published","date":"2024-12-23T15:56:57+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":68276937,"identity":"ca536103-ad85-40b1-8546-1c0eda265d7c","added_by":"auto","created_at":"2024-11-05 14:42:26","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":470999,"visible":true,"origin":"","legend":"\u003cp\u003eFrequency [%] of blocking by months\u003c/p\u003e","description":"","filename":"image1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5094045/v1/416319d71ddd6a4bde0a89b7.jpeg"},{"id":68275059,"identity":"30127fa7-82d2-41d7-aa32-f9e36e2bf23f","added_by":"auto","created_at":"2024-11-05 14:26:26","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":421377,"visible":true,"origin":"","legend":"\u003cp\u003eTrends in number of days [days/10 years] with blocking by months\u003c/p\u003e","description":"","filename":"image2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5094045/v1/414f7e73d67b80816423eec1.jpeg"},{"id":68275330,"identity":"f6681970-fff8-410a-9909-d9f635842ade","added_by":"auto","created_at":"2024-11-05 14:34:26","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":99713,"visible":true,"origin":"","legend":"\u003cp\u003eAnnual (a) and multiannual (b) course of the number of HW (red) and CS (blue) days in Poland with a trend line and equation for the multiannual trend of HW days\u003c/p\u003e","description":"","filename":"image3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5094045/v1/0f7ed611b8d0b661b564742a.jpeg"},{"id":68276938,"identity":"c74a65d5-bd50-40c0-ab12-6c8c8fa03a2f","added_by":"auto","created_at":"2024-11-05 14:42:26","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":169972,"visible":true,"origin":"","legend":"\u003cp\u003eMean (left) and maximum in summer 2015 (right) number of HW days in Poland\u003c/p\u003e","description":"","filename":"image4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5094045/v1/49f28ca82ab75e17bf4663db.jpeg"},{"id":68275063,"identity":"3cd30252-ff45-4d29-af86-47100bfb12c1","added_by":"auto","created_at":"2024-11-05 14:26:26","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":145198,"visible":true,"origin":"","legend":"\u003cp\u003eMean (left) and maximum in winter 1986/1987 (right) number of CS days \u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;in Poland\u003c/p\u003e","description":"","filename":"image5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5094045/v1/3daaa01b924ca8cde94c7127.jpeg"},{"id":68277419,"identity":"7848df3a-3d5c-40c7-8fdd-3fbd9e0d2507","added_by":"auto","created_at":"2024-11-05 14:50:29","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":292592,"visible":true,"origin":"","legend":"\u003cp\u003eMaps – frequency [%] of blockings in summer (JJA) (a) and during HWs in Poland (b); graphs – frequency [%] of blocking in summer (solid line) and during summer HWs in Poland (dashed line) at given longitudes (c) and latitudes (d) in the study area\u003c/p\u003e","description":"","filename":"image6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5094045/v1/798c63baf1c43d091d8b1004.jpeg"},{"id":68275331,"identity":"9b6a042d-13dc-4fb1-b867-c05fb6c2973f","added_by":"auto","created_at":"2024-11-05 14:34:26","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":269895,"visible":true,"origin":"","legend":"\u003cp\u003eMaps – frequency [%] of blockings in winter (DJF) (a) and during winter CSs in Poland (b); graphs – frequency [%] of blocking in winter (solid line) and during winter CSs in Poland (dashed line) at given longitudes (c) and latitudes (d)\u003c/p\u003e","description":"","filename":"image7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5094045/v1/b55c9c1875baa6fce5d8e671.jpeg"},{"id":72640368,"identity":"d5f4dda9-ee56-4b0d-a193-149cbef05d34","added_by":"auto","created_at":"2024-12-30 16:04:57","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2229466,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5094045/v1/b5378202-e7d9-4d99-929e-92fc4f47591d.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Spatiotemporal variability of blockings in the Euro-Atlantic region and their impact on the occurrence of heat waves and cold spells in Poland","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn the context of significant contemporary climate change, extreme weather events have become a prominent focus in climatology, largely due to their substantial economic and social risks. The observed increase in air temperatures, which has accelerated particularly in the early 21st century (NOAA 2023; IPCC 2021), has led to a rise in the frequency of heat waves (HWs), considered among the most lethal natural hazards (e.g., Kovats and Kristie 2006; Mohleji and Pielke 2014). Additionally, HWs are frequently linked with droughts that exacerbate the risk of forest fires and cause substantial damage to agriculture, thus amplifying the associated hazards (e.g., Christian et al. 2020).\u003c/p\u003e \u003cp\u003eAccording to Hansen et al. (2012), the area of the Earth\u0026rsquo;s surface experiencing air temperatures classified as extreme heat increased significantly from less than 1% during 1951\u0026ndash;1980 to over 10% during 2006\u0026ndash;2011. In recent decades, Europe has witnessed several severe HWs. Matzarakis et al. (2020) demonstrated that 13 of the top 23 most intense HWs over the last 70 years occurred within the last 12 years, whereas only seven were recorded in the 50 years before 2000. Russo et al. (2015) identified notable heat waves in 1994, 2003, 2006, 2010, and 2015 as some of the most severe in Europe since 1950. Similar trends have been observed in Poland (Krzyżewska and Dyer 2018; Wibig 2018, 2021; Tomczyk et al. 2019, 2020). Krzyżewska and Dyer (2018) highlighted the HWs of 1963, 1994, and 2015 as among the most intense in Poland. The events in 1994 and 2015 were the longest HWs in recent decades, affecting much of Poland and central Europe (Krzyżewska and Dyer 2018; Wibig 2018; Tomczyk et al. 2019, 2020). These events resulted in increased mortality rates in Poland (Kuchcik 2001; Graczyk et al. 2019) and in neighboring countries (Kysel\u0026yacute; and Huth 2004; Gabriel and Endlicher 2011; Muthers et al. 2017).\u003c/p\u003e \u003cp\u003eDespite significant climate warming, several-day periods of very low air temperatures, known as cold spells (CSs), have been observed during numerous winter seasons in recent years across various regions of Europe. Notable examples of such cold episodes occurred in Poland in 2016, 2017 (Report of the Government Centre for Security, 2016, 2017), and 2018 (Tomczyk and Bednorz, 2020). Similar situations have been recorded in other regions as well, such as the Balkans, where significant CSs were observed in 2012 (Tolika et al., 2014; Unkašević and Tošić, 2015) and in 2017 (Anagnostopoulou et al., 2017; Demirtaş, 2022).\u003c/p\u003e \u003cp\u003eThe occurrence of both HWs and CSs is often associated with a common feature: large-scale pressure patterns known as atmospheric blocking. Despite being relatively rare, atmospheric blockings significantly influence atmospheric dynamics in midlatitudes, and they have been extensively studied in climatological literature, including various reviews and systematizing papers (e.g., Barnes et al., 2012; Lupo, 2021; Kautz et al., 2022). Blocking systems are characterized as persistent, quasi-stationary disturbances with high pressure at the surface, typically forming in middle and high latitudes (e.g., Lupo, 2021). The fundamental concept of atmospheric blocking involves a transition from the typical zonal flow in midlatitudes to a meridional flow, often associated with a prolonged, quasi-stationary anticyclone and the bifurcation of the jet stream around the high-pressure system (e.g., Liu, 1994; Sousa et al., 2021). Nakamura and Huang (2018) provided a mathematical description of blocking mechanisms, likening them to a 'traffic jam' in the jet stream.\u003c/p\u003e \u003cp\u003eAccording to Sousa et al. (2021), atmospheric blockings tend to form in specific regions characterized by enhanced subtropical ridge activity. In the Northern Hemisphere, these blockings are most frequent in mid- to high-latitude sectors between Greenland and Europe, across northeastern Asia and Alaska, and over the eastern parts of oceans and western edges of continents. In the Euroatlantic area, high-latitude blockings often develop north of the jet stream, particularly over Greenland, and are reinforced by cyclonic wave breaking associated with the negative phase of the North Atlantic Oscillation. Greenland blocks, which result in a northerly flow, significantly influence weather conditions in Europe. Over the continent, blockings driven by anticyclonic wave breaking typically occur in Scandinavia, extending sometimes to the Azores, and eastward to the Ural region (e.g., Barriopedro et al. 2006, Tyrlis and Hoskins 2008, Davini et al. 2020, Cheung et al. 2013, Mokhov et al. 2013, Small et al. 2013, Lupo et al. 2019). Atlantic and European blockings are more frequent from winter to early spring and less common in summer and early autumn (e.g., Lupo and Smith 1995, Barriopedro et al. 2006). According to Lupo et al. (2019), atmospheric blockings have become more frequent since the late 20th century.\u003c/p\u003e \u003cp\u003eA significant amount of interest in atmospheric blockings, as highlighted in numerous studies not fully covered in preceding paragraphs, stems from their association with extreme weather events (Matsueda 2009, Kautz et al. 2022). Thermal extremes are often linked to blockings, primarily due to clear-sky conditions within anticyclones. Additionally, temperature extremes are influenced by shifts in wind direction from westerly to northerly/easterly in winter and southerly in summer. During summer, radiative forcing plays a crucial role in temperature increases, whereas in winter, changes in circulation associated with blockings are more influential in causing temperature drops (Buehler et al. 2011, Sillmann et al. 2011, Lupo et al. 2012, Pfahl and Wernli 2012, Porebska and Zdunek 2013, Brunner et al. 2017, Luo et al. 2017, Quandt et al. 2017, Yao et al. 2017, Brunner et al. 2018, Sousa et al. 2018, Woollings et al. 2018, Chan et al. 2019).\u003c/p\u003e \u003cp\u003eThe emergence of extreme weather in Central Europe is primarily attributed to atmospheric circulation patterns influenced by pressure systems forming over the Euro-Atlantic region. Despite the documented strong impact of blocking patterns on weather extremes, there remains a significant gap in detailed regional studies on this topic. Specifically, understanding the specific geographical locations and seasonal characteristics of blocking anticyclones driving thermal extremes in Poland is crucially lacking. Therefore, this study aims to address these gaps with two primary objectives: first, to distinguish blocking structures over the Euroatlantic region and elaborate on their climatology, including their spatial and temporal variability and; and second, to assess the impact of these blockings\u0026mdash;considering their location and persistence\u0026mdash;on the occurrence of summer HWs and winter CSs in Poland from 1978 to 2022.\u003c/p\u003e"},{"header":"2. Data and methods","content":"\u003cp\u003eThere is currently no universally accepted definition of atmospheric blocking, leading to diverse criteria and detection methods in climatological research (Barriopedro et al. 2010). Various meteorological variables are employed to construct blocking indices, with the 500 hPa (Z500) geopotential height being the most commonly used. This approach typically considers anomalies in Z500 or reversals in meridional gradients (Dole and Gordon 1983, Tibaldi and Molteni 1990, Barriopedro et al. 2006, Diao et al. 2006, Scherrer et al. 2006, Barriopedro et al. 2010, Schalge et al. 2011, Porebska and Zdunek 2013). The geopotential height gradient method employed in this study was first introduced by Lejen\u0026auml;s and \u0026Oslash;kland (1983) and has since undergone refinement in subsequent studies, being now the most widely used method (e.g., Tibaldi and Molteni 1990, Scherrer et al. 2006, Schalge et al. 2011, Barnes et al. 2012; Pinheiro et al. 2019).\u003c/p\u003e \u003cp\u003eThe Z500 reanalysis data were obtained from the National Oceanic and Atmospheric Administration Physical Science Laboratory [NCEP-DOE AMIP-II Reanalysis (R-2), Kanamitsu et al. 2002]. This dataset covers the period from 1978 to the present day with a temporal resolution of 1 day and a spatial resolution of 2.5\u0026deg;x2.5\u0026deg;, which is adequate for this study. Following modifications by Scherrer et al. (2006) to the methodology of Tibaldi and Molteni (1990), the blocking index was computed for each grid point in the Euroatlantic sector between latitudes 35\u0026deg; and 75\u0026deg;N, using latitudes φ\u003csub\u003e0+15\u003c/sub\u003e, φ\u003csub\u003e0\u003c/sub\u003e, and φ\u003csub\u003e0\u0026minus;15\u003c/sub\u003e. Geopotential height gradient thresholds were set at \u0026gt;\u0026thinsp;0 m/\u0026deg;lat for the southern gradient (Z500φ\u003csub\u003e0\u003c/sub\u003e \u0026ndash; Z500φ\u003csub\u003e0\u0026minus;15\u003c/sub\u003e) and \u0026lt; -10 m/\u0026deg;lat for the northern gradient (Z500φ\u003csub\u003e0+15\u003c/sub\u003e \u0026ndash; Z500φ\u003csub\u003e0\u003c/sub\u003e). Additionally, a persistence filter of 3 days duration and a quantile filter set at 0.5 for each latitude were applied following the approach of Tibaldi and Molteni (1990). Monthly frequencies of blockings and a rate of changes for each month in every grid point were computed and shown on maps.\u003c/p\u003e \u003cp\u003eHWs and CSs were defined as sequences lasting at least 3 days with maximum air temperatures above 30\u0026deg;C or below \u0026minus;\u0026thinsp;10\u0026deg;C, respectively, based on data obtained from the Institute of Meteorology and Water Management \u0026ndash; National Research Institute (IMGW \u0026ndash; PIB) for the period 1978\u0026ndash;2022 across 37 stations in Poland. The adopted definitions of HW and CS are based on bioclimatic preconditions and have been used in many earlier studies concerning extreme events in Poland (e.g. Wibig et al., 2009a,b; Porębska and Zdunek, 2013; Wibig, 2018; Tomczyk et al., 2019) Kuchcik (2006) has supplied a detailed description of the thresholds and definitions of prolonged extreme temperature events commonly applied in climatological research.\u003c/p\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Spatiotemporal variability of Euro-Atlantic blocking\u003c/h2\u003e \u003cp\u003eThe occurrence of blocking in the Euro-Atlantic region exhibits considerable spatiotemporal variability (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), with distinct spatial patterns of blocking frequency varying from month to month. During winter (Dec-Feb), two areas of high blocking frequency are notable, with one being the Greenland High. The highest frequency of the winter blocking is observed along the western coast of Europe, extending into southern Scandinavia in February. Towards the end of winter and into spring, blocking structures propagate eastward along the 60\u0026deg;N latitude, with the highest frequency occurring in late spring, particularly in May. During this period, blockings appear most often (\u0026gt;\u0026thinsp;10%) in an arched belt extending from Greenland through the North Sea, southern Scandinavia, to western Russia, where their frequency exceeds 15%.\u003c/p\u003e \u003cp\u003eIn the summer season (Jun-Aug), blocking events become less frequent overall, except for a pronounced peak in the northeastern edge of the study area where the frequency exceeds 15% in July. Towards the end of summer (August) and into autumn, the frequency of blocking events decreases further, generally not exceeding 10%.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe occurrence of atmospheric blocking in the Euro-Atlantic region exhibits significant but highly variable trends in both space and time, more so than the overall blocking frequency. In winter, positive trends are observed in the northeast (January) and the northwestern Atlantic (December). In contrast, February shows negative, statistically significant trends over continental Europe, particularly north of 60\u0026deg; latitude. A strong positive trend has been observed in early spring over the British Isles, with an increase of 1.5 blocking days per decade in March. However, eastward across continental Europe, the trend is negative. In April, positive signals appear over the northern Atlantic, while both April and May show a decrease in blocking structures over Europe between 50\u0026deg; and 60\u0026deg; latitude. During summer, positive trends emerge in June over central Europe and in August over the eastern part of the continent. In contrast, July and August see a reduction in blocking days in the northeastern part of the study area, and blocking events related to the Azores High over the southwestern Atlantic have also become less frequent. Autumn trends are generally weaker, with positive signals in the north (October and November) and negative trends in the mid-latitudes (November).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Occurrence of heat waves (HWs) and cold spells (CSs) in Poland\u003c/h2\u003e \u003cp\u003eAlthough the mean annual number of HW days in Poland varies considerably from year to year, a significant increasing trend is observed, with a rate of increase of approximately 4.5 days per decade (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Since 2000, seven summers have experienced more than 20 HW days, and in the past decade alone, more than 30 HW days per season were recorded in the summers of 2015, 2019, and 2022. Notably, only two summers \u0026ndash; 1980 and 2009 \u0026ndash; did not witness any 3-day periods with Tmax exceeding 30\u0026deg;C. HWs are most frequent in July, August, and July, however, they may also occur sporadically in September, May, or even April.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe frequency of HW days in Poland exhibits significant spatial variability across different regions. In the southern mountainous areas and northern Poland, the average is approximately one HW day per year. In contrast, the central and western parts of the country experience an average of more than four HW days per year (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). However, during exceptionally hot summers, such as in 2015, the number of HW days in southern Poland surged to over 20.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCSs appear in Poland mostly in January and February and are less frequent in December. Only sporadically were observed in March. Only two seasons in the analyzed 44-year period experienced more than 20 CS days (1984/1985 and 1986/87). Despite observed warming, CSs were observed during half of the winters in the 21st century, with as much as \u0026gt;\u0026thinsp;10 CS days in seasons 2002/2003, 2009/2010, and 2011/2012. On average, only one CS day is observed in most of Poland, and only in the northeast, there are approximately two CS days per season. Only during specific seasons, such as the winter of 1986/1987, did the number of CS days exceed 10, and this was observed exclusively in northeastern Poland (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Location of blockings associated with HWs in Poland\u003c/h2\u003e \u003cp\u003eThe occurrence of HWs in Poland is consistently associated with atmospheric blocking. During every HW event recorded between 1978 and 2022, an atmospheric blocking pattern was present in some parts of the Euro-Atlantic region. These blocks are most frequently centered northeast of Poland, where their frequency exceeds 20%, which is significantly higher than the typical summer frequency (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The overall frequency of blocking structures along 30\u0026deg;E longitude surpasses 10% during Polish HWs, compared to a general summer frequency of approximately 6% (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec). Notably, a significant increase in blocking frequency is observed in July in the region extending from eastern Poland to Belarus and western Russia (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), likely contributing to the rising number of HW days in Poland.\u003c/p\u003e \u003cp\u003eBlocking systems, positioned northeast of Poland, induce meridional circulation patterns over the country, facilitating the advection of warm air. Additionally, the downward movement of air and adiabatic subsidence under stable anticyclonic conditions can further increase surface temperatures. This process is amplified by cloud-free weather, allowing a high influx of solar energy. The increased solar radiation enhances the surface energy balance, leading to elevated temperatures.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eLocation of blockings associated with CSs in Poland\u003c/em\u003e \u003c/p\u003e \u003cp\u003ePolish winter CSs are also strongly associated with blockings, primarily located over the North Atlantic and the North Sea region, where their frequency during Polish CSs exceeds 30% \u0026ndash; significantly higher than the usual winter frequency in this area. A secondary region of high blocking frequency associated with Polish CSs is located over northern Scandinavia, with frequencies exceeding 30% (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Between latitudes 65\u0026ndash;70\u0026deg;N, the frequency of blocking situations during Polish CSs is four times higher than normal winter conditions. Longitudinally, the maximum blocking frequency during CSs occurs between 10\u0026deg;W and 10\u0026deg;E, more than twice the usual winter frequency (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and d). The frequency of winter blocking events in the zonal belt to the north and northeast of Poland exibits a substantial decreasing trend by approximately 0.5 to 1 day per decade in February (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This reduction, along with the overall trend of climate warming, diminishes the likelihood of CSs occurrences in Poland.\u003c/p\u003e \u003cp\u003eAnticyclonic blocking patterns inhibit zonal airflow and intensify meridional flow. In winter, this results in the advection of the Arctic or polar continental air masses into central Europe. Additionally, anticyclones contribute to cloud-free conditions and strong radiative cooling. During winter, the significant radiation of heat from the ground, coupled with low or no cloud cover typical of anticyclonic weather, leads to a decrease in surface air temperature.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThis study, along with numerous previous studies, has demonstrated that in a warming world, HWs are becoming more frequent, with this trend being particularly strong in central Europe, including Poland (i.e., Wibig 2018, 2021, Tomczyk and Bednorz 2019, Ustrnul et al 2020). Hot periods exert significant stress on humans, with thermal stress intensifying as temperatures rise (Owczarek and Filipiak 2016, Owczarek 2019, Tomczyk and Owczarek 2020, Tomczyk et al. 2019, Tomczyk and Bednorz 2023). Although CSs are becoming less frequent in midlatitudes and are therefore less popular in scientific research, they still have a severe negative impact on human health, causing cold stress as well as socioeconomic losses and disruptions (Owczarek and Tomczyk 2022, Tomczyk and Bednorz 2023). Because of this, there is a significant interest in understanding the synoptic conditions and causes of extreme temperature periods.\u003c/p\u003e \u003cp\u003eIt is widely agreed, that the prolonged summer heat and winter cold conditions in Poland are associated with high-pressure systems (i.e., Tomczyk and Bednorz 2016, Wibig 2018, 2021, Tomczyk et al. 2019). Anticyclones contribute to cloud-free weather and strong energy fluxes. During summer, increased solar irradiation enhances the surface energy balance, raising temperatures. Conversely, in winter, the strong radiative cooling of the ground due to low or no cloud cover, characteristic of anticyclonic weather, decreases surface air temperatures. The extreme thermal conditions have been less frequently analyzed in relation to midlatitude atmospheric blockings, which inhibit the zonal airflow and intensify meridional flow, resulting in the presence of Arctic or polar continental air masses in winter and tropical air masses in summer in Central Europe. Porębska and Zdunek (2013) investigated the occurrence of extreme temperature events in Central Europe in conjunction with high-pressure blocking situations during the decade 2001\u0026ndash;2011. They found cold waves were associated with blockings located over the Atlantic Ocean, while heat waves most often occurred under eastern European blocks. In this study, it was specified that CSs are most often accompanied by blockings located over the North Atlantic and the North Sea region, whereas HWs occur under blockings located northeast of Poland, over western Russia. Kautz et al. (2022) in their review of the atmospheric blocking impact on weather extremes point out that while European HWs are associated with high-pressure anomalies, cold anomalies are typically not located directly beneath the blocking anticyclone but rather downstream or to the south of it. This rule was also documented in our study.\u003c/p\u003e \u003cp\u003eMost research indicated that the highest frequency of atmospheric blocking over the North Atlantic and European sections occurs in spring, with the lowest frequency in summer. However, Barriopedro (2006) noted, that while these structures are rare over the ocean in summer, they are much more frequent over the continent. Lupo (2021) demonstrated that summer blocks have similar durations to those in spring and winter, but of much lower intensity as measured by Z500 gradient). Porębska and Zdunek (2013) identified May as the month with the highest number of blocking events in the Euro-Atlantic region, with a secondary peak in July. This observation aligns precisely with the findings on blocking frequency presented in this study.\u003c/p\u003e \u003cp\u003eUnderstanding the physical mechanisms and processes leading to thermal extremes can improve the predictability of these weather events, which is crucial to society and policymakers. Furthermore, the predictability of extreme events associated with blocking should be analyzed in the context of climate change, as blocking is projected to remain a significant circulation feature initiating European heat waves under future climate conditions (Brunner et al., 2018; Schaller et al., 2018, Lupo 2021). This study identified a statistically significant decreasing trend in winter blocking frequency over European midlatitudes, and most models project a further decrease in blocking frequency in both winter and summer (Sillmann and Croci-Maspoli, 2009; Davini and D\u0026rsquo;Andrea, 2020). However, there are exceptions to the decreasing trend, one of them being the Ural region, with summer blocking frequency projected to increase, a trend also was observed in the analyzed period 1978\u0026ndash;2022.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eIt has been demonstrated that the occurrence of blocking in the Euro-Atlantic region exhibits high spatiotemporal variability. Blocking events are most frequent in the spring, particularly in May, occurring with a 10% frequency in the arched belt extending from the west of the British Islands through the North Sea, south Scandinavia, to the east of the Baltic Sea. A secondary peak in blocking frequency is observed in July, with Ural blockings reaching a 15% frequency. The trends in atmospheric blocking occurrence across the Euro-Atlantic region show notable spatial and temporal variability. Significant negative trends were detected over Europe at latitudes between 60\u0026deg; and 70\u0026deg;N, while strong positive trends emerged locally during spring and summer.\u003c/p\u003e \u003cp\u003eThe mean annual number of HW days in Poland has been increasing in the period 1978\u0026ndash;2022 and spatially it ranges from one day in the southern mountainous regions and northern Poland to more than four days in the central and western parts. CSs have become rarer, with the average seasonal number of CS days amounting to 1\u0026ndash;2 days in recent decades. Only in singular seasons, more than 10 days fulfilled the criteria of CS in northeastern Poland.\u003c/p\u003e \u003cp\u003eHeatwaves (HWs) and cold spells (CSs) in Poland are consistently accompanied by blockings that have distinct locations. Summer HWs are most often associated with blocks located northeast of Poland (over western Russia), while winter CSs most often occur under blockings located over the zonal belt between 60\u0026deg;N and 70\u0026deg;N extending from the North Atlantic to northern Scandinavia.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting Interests\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was supported by the National Science Centre, Poland (grant number UMO-2020/37/B/ST10/00217).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eE.B.: Conceptualization; Data curation; Methodology; Formal analysis; Investigation; Methodology; Resources; Software; Supervision; Validation; Visualization; Roles/Writing - original draft; Writing - review \u0026amp; editing. A. M.T.: Data curation; Methodology; Funding acquisition; Project administration; Roles/Writing \u0026ndash; original draft; Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eData will be made available from the corresponding author on request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAnagnostopoulou C, Tolika K, Lazoglou G, Maheras P (2017) The exceptionally cold January of 2017 over the Balkan Peninsula: A climatological and synoptic analysis. Atmosphere 8(12):252. https://doi.org/10.3390/atmos8120252\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarnes EA, Slingo J, Woollings T (2012) A methodology for the comparison of blocking climatologies across indices, models and climate scenarios. 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Geogr Pol 82:47\u0026ndash;59.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWoollings T, Barriopedro D, Methven J, Son SW, Martius O, Harvey B, Sillmann J, Lupo AR, Seneviratne S (2018) Blocking and its response to climate change. Curr Clim Change Rep 4:287\u0026ndash;300. https://doi.org/10.1007/s40641-018-0108-z\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYao Y, Luo D, Dai A, Simmonds I (2017) Increased quasi stationarity and persistence of winter Ural blocking and Eurasian extreme cold events in response to Arctic warming. Part I: Insights from observational analyses. J Clim 30(10):3549\u0026ndash;3568. https://doi.org/10.1175/JCLI-D-16-0261.1\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"theoretical-and-applied-climatology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"taac","sideBox":"Learn more about [Theoretical and Applied Climatology](https://www.springer.com/journal/704)","snPcode":"704","submissionUrl":"https://submission.nature.com/new-submission/704/3","title":"Theoretical and Applied Climatology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Heat waves, cold spells, atmospheric blocking, Euro-Atlantic region, Poland","lastPublishedDoi":"10.21203/rs.3.rs-5094045/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5094045/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe spatiotemporal variability and trends of atmospheric blockings over the Euro-Atlantic region and their influence on the occurrence of the persisting extreme temperature conditions in Poland namely heat waves (HWs) and cold spells (CSs) during the period 1978\u0026ndash;2023 were analyzed. Blockings were identified at 500 hPa geopotential level, using the meridional geopotential gradient method, supplemented with the quantile filter and persistence filter, using reanalysis data from the National Oceanic and Atmospheric Administration Physical Science Laboratory (NCEP-DOE AMIP-II R-2). HWs and CSs were defined as sequences of at least 3 days with the maximum air temperature above 30\u0026deg;C or below \u0026minus;\u0026thinsp;10\u0026deg;C, respectively based on data obtained from the Institute of Meteorology and Water Management \u0026ndash; National Research Institute (IMGW \u0026ndash; PIB) for the period 1978\u0026ndash;2022 across 37 stations in Poland. The climatology of Euro-Atlantic blocking occurrence in the zonal belt between 45 and 75 degrees in the northern hemisphere exhibits high spatiotemporal variability. Blocking structures are most frequent in the spring, particularly in May. A secondary peak of frequency is observed in July when the Ural blocking exhibits 15% frequency. Patterns of trends in blocking occurrence are variable and the strongest signals of changes are observed in spring. The occurrence of HWs in Poland is constantly accompanied by blocking situations, most often located northeast of Poland, while the winter CSs are associated with the blockings located over the North Atlantic and northern Scandinavia.\u003c/p\u003e","manuscriptTitle":"Spatiotemporal variability of blockings in the Euro-Atlantic region and their impact on the occurrence of heat waves and cold spells in Poland","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-05 14:26:21","doi":"10.21203/rs.3.rs-5094045/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-10-26T12:33:14+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-24T12:25:30+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-02T07:33:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"247050693411170650736895809291581864971","date":"2024-10-02T05:21:38+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"116990155279292424555396651800042210699","date":"2024-10-01T10:35:31+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-09-30T08:11:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-09-16T22:27:42+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-09-16T22:26:19+00:00","index":"","fulltext":""},{"type":"submitted","content":"Theoretical and Applied Climatology","date":"2024-09-15T19:43:14+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"theoretical-and-applied-climatology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"taac","sideBox":"Learn more about [Theoretical and Applied Climatology](https://www.springer.com/journal/704)","snPcode":"704","submissionUrl":"https://submission.nature.com/new-submission/704/3","title":"Theoretical and Applied Climatology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"9990ba37-a489-48c3-8bba-78f68dd26b6b","owner":[],"postedDate":"November 5th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-12-30T15:58:36+00:00","versionOfRecord":{"articleIdentity":"rs-5094045","link":"https://doi.org/10.1007/s00704-024-05253-6","journal":{"identity":"theoretical-and-applied-climatology","isVorOnly":false,"title":"Theoretical and Applied Climatology"},"publishedOn":"2024-12-23 15:56:57","publishedOnDateReadable":"December 23rd, 2024"},"versionCreatedAt":"2024-11-05 14:26:21","video":"","vorDoi":"10.1007/s00704-024-05253-6","vorDoiUrl":"https://doi.org/10.1007/s00704-024-05253-6","workflowStages":[]},"version":"v1","identity":"rs-5094045","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5094045","identity":"rs-5094045","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}