Exceptionally Cold and Warm Spring Months in Kraków Against the Background of Atmospheric Circulation (1874–2022)

In the changing climate, exceptionally warm (EWMs) and dry spring months are increasingly observed. At the same time, exceptionally cold months (ECMs) are less frequent, although their impact on a warming climate becomes significant. Due to the role that such climatic anomalies play in the environment and their effects on human activity, it is very important to explain the causes of their occurrence. For this reason, in this study, the authors have attempted to determine the circulation conditions favourable to the occurrence of extremely cold (ECM) and warm (EWM) spring months in Kraków in the years 1874–2022. The study used the average temperature of individual spring months (March–May), as well as types of atmospheric circulation and air masses from the daily Calendar of Atmospheric Circulation Types for southern Poland. A distinct increase in spring air temperature (0.181 °C/10 years) and its individual months (0.162–0.191 °C/10 years) was confirmed. It was accompanied by a significant increase in the occurrence of EWM and a decrease in ECM. It was also found that the direction of air advection and the related temperature characteristics of air masses have the greatest impact on the occurrence of exceptionally cold or warm months. A slight positive effect of zonal circulation on the temperature increase at the beginning of the spring season and the advection of air from the south in April and east in May was found. In the case of the coldest months (ECMs), low temperatures most often developed in the presence of advection from the NW-N-NE directions.


Introduction
Recent decades have observed a significant increase in air temperatures which are particularly strongly marked on the European continent (IPCC, 2013;Krauskopf & Huth, 2020;Kundzewicz et al., 2020;Twardosz et al., 2021).One of the manifestations of climate warming in Europe is an increasing frequency and intensity of heat waves, as well as prolonged month periods and even entire seasons with extremely high air temperatures, deviating considerably from the long-term average (Twardosz & Kossowska-Cezak, 2021).Among these extreme climatic phenomena, the exceptionally hot summers of 2003 and 2010 have gone down in the history of Europe's climate.The anomalies triggered a range of serious consequences, such as a heavy death toll in western Europe, especially in France in 2003 (e.g.Campbella et al., 2018), and extensive forest fires in eastern Europe in 2010 (Zvyagintsev et al., 2011).With the memory of these extreme heatwaves in Europe still fresh, there came August 2015, which proved similarly hot, with record-breaking anomalies over large areas of the central part of the continent (Hoy et al., 2017;Krzy_ zewska & Dyer, 2018;Luterbacher et al., 2016;Muthers et al., 2017;Wypych et al., 2017).The summer of 2022 should also be considered exceptionally hot.In large areas of the northern hemisphere (including Western Europe and China), the air temperature exceeded 40 °C, many tropical nights and the occurrence of heat waves and droughts were recorded (Met Office, 2022, Jiang 2023, Lu et al., 2023).Since the beginning of the twenty-first century, the attention of researchers has been drawn to increasingly frequent exceptionally warm spring months, including two record warm and very dry months: April and May 2018 (Sinclair et al., 2019;Twardosz, 2019).In many areas of central Europe, they were the hottest months in the history of temperature monitoring (Bła_ zejczyk et al., 2022).The observable general increase in the frequency of exceptionally warm months is also due to the prolongation of the period over which they occur during the year, mainly their earlier onset, i.e. as early as in April and/or May (Twardosz & Kossowska-Cezak, 2021).As a result, spring has become the fastest warming season in Europe with the increase reaching 0.061 °C/year in the years 1985-2020 (Twardosz et al., 2021).
Such early emergence of unusually warm and dry weather after a mild and snowless winter aggravates drought at the beginning of the growing season, leading to adverse consequences in agriculture and many other areas of the economy (Peters et al., 2020;Sinclair et al., 2019;Twardosz, 2019).Notably, the emergence of prolonged anomalous hot weather in spring is also extremely onerous for the human body, which has not yet adapted to heat after the winter (Muthers et al., 2017).
The awareness of the above facts has prompted the authors to pose a question about changes in the frequency and timing of such thermally anomalous spring months in the long term.Above all, it has become an inspiration to explain the underlying reasons by studying them against the backdrop of circulation conditions.The authors' previous research on changes in the occurrence of exceptionally cold and warm months over time focused on the European continent.The research covered different periods and various numbers of meteorological stations (Twardosz & Kossowska-Cezak, 2021;Skrzyn ´ska and Twardosz, 2023), which means that they concerned a macro-scale.The authors sought to explain the overall circulation-related factors behind such extreme climatic events by analysing the distribution of the average pressure across Europe (Twardosz & Bielec-Ba ˛kowska, 2022).
Obviously, in order to explain fully the impact of circulation on thermal conditions in a given area, it is necessary to use a classification of circulation types at a mesoscale that corresponds to the area under investigation.Krako ´w is perfectly suited for such synoptic and climatological studies since it has a homogeneous series of air temperatures measured since 1792 and a mesoscale typology of atmospheric circulation types for southern Poland since 1874, which has been prepared by Niedz ´wiedz (1981,2022).This being the case, the aim of the present study is to gain insight into the variability of occurrence of exceptionally cold and exceptionally warm spring months in Krako ´w in the years 1874-2022 and to describe the underlying circulatory conditions.The inclusion of anomalously cold months in the research, which are less and less frequent, is motivated by the fact that they continue to pose a serious problem in agriculture, especially for novel crops, which are ever more willingly grown in Poland's warming climate (e.g.vine).The findings of the study will contribute to understanding the recent warming, which is a complex and spatially diverse process (Ji et al., 2014;Hegerl et al., 2018;Krauskopf & Huth, 2020;Skrzyn ´ska and Twardosz, 2023) and which requires continuous monitoring in order to assess its potential social, economic and environmental implications.

Data and Study Methods
Two sets of data are used in the article.The first set consists of average monthly air temperature values from March to May from a meteorological station located in the centre of Krako ´w (50°04 0 N, 19°58 0 E, 206 m a.s.l.; Fig. 1) which is operated by the Department of Climatology of the Jagiellonian University.This station began operating in 1792, and since 1826, measurements have been made without any interruptions.Constant meteorological observations were made at the station, even during the world wars.This means that the temperature series from Krakow maintains the uniformity of time and place of observation.Measurements have been made in the same location since the beginning of the station's existence, i.e. in the former Astronomical Observatory of the Jagiellonian University located in the Botanical Garden of the Jagiellonian University (''historical station'', weather instrument shelters by the north window of the second floor of the building, 12 m above ground level), thanks to which Krakow has one of the longest measurement series in Poland and Europe (Trepin ´ska, 1997).During the period of operation of the station, the thermometers changed for obvious reasons, but this did not affect the uniformity of the temperature in any way (Ustrnul, 1997).The homogeneity of the series has also been verified in many studies on temperature changes in Krako ´w.
Data for the 3 spring months from 1874-2022 is used.In order to identify the spring months and entire spring seasons in which the average temperature diverged significantly from the long-term average, use was made of criteria for identifying the occurrence of extreme events.According to them the frequency of extreme events should not be higher (lower) than the 10th, 5th or even 1st percentile (90th, 95th or 99th percentile, respectively) of all the cases analysed (Beniston et al. 2007, IPCC, 2007;Labajo et al., 2008).In this way, exceptionally cold and exceptionally warm months (ECMs -with a temperature B 10th percentile of all values; EWMswith a temperature C 90th percentile), as well as exceptionally cold and exceptionally warm springs (ECSs/EWSs) from the entire 149-year period 1874-2022 were identified.In addition, those with values below the 5th (ECM/S 5 ) or 1st percentile (ECM/S 1 ) or above the 95th (EWM/S 95 ) or 99th percentile (EWM/S 99 ) were distinguished.
The second data set is the daily Calendar of Atmospheric Circulation Types for southern Poland compiled by Niedz ´wiedz ´(1981Niedz ´wiedz ´( , 2022)).
The calendar defines 20 types of circulation, 16 of which are types describing the advection of air over the area concerned.These are described with a code denoting the direction of air advection and the associated type of pressure system ('a' for anticyclonic systems: Na, NEa, Ea, SEa, Sa, SWa, Wa and NWa or 'c' for cyclonic systems: Nc, NEc, Ec, SEc, Sc, SWc, Wc and NWc).The remaining 4 types describe the following situations: Ca -central anticyclonic situation, Ka -anticyclonic wedge, Cc -central cyclonic situation, centre of low pressure, Bccyclonic trough, and X (number 21) unclassified situations (Niedz ´wiedz 1981;Twardosz, 2010).
In addition, the calendar is used as a basis for complex atmospheric circulation indices: Wi-western circulation zonal index, Si-southern circulation  Murray and Lewis (1966) for the British Isles.In this paper, the index values were calculated for the spring months and the entire spring season by assigning appropriate weights to particular types of circulation (? 2, ? 1, 0, -1, -2).A detailed description of how these indices are calculated has been presented in several previous publications (e.g.Niedz ´wiedz ´, 2000; Niedz ´wiedz ´& Ustrnul, 2021).Given the different numbers of days in individual months, in their final version the indices are converted into percentage values so that they fall within the range from -100 to ?100.For example, the value ? 100 of the Wi index would mean the inflow of air from the west on all days of the month, while -100 would denote advection from the east.The same applies to the Si index (?100-advection from the south; -100 from the north) and the Ci index (?100-centre of low pressure or cyclonic trough over the area on each day, -100-centre of high pressure or anticyclonic wedge).
In order to conduct a detailed analysis of the ECMs and EWMs identified, use is also made of the typology of air masses developed by Niedz ´wiedz (1981,2022), which is employed to supplement the calendar referred to above.The author identified six air masses: A-Arctic, mPf-polar maritime (fresh), mPw-polar maritime warm, mPo-polar maritime old (transformed), cP-polar continental, T-tropical.There is also a separate type for various air masses during a day-vAm.
In the study, trends in air temperature changes were determined using an equation of linear regression, whereas trend significances were calculated using the Mann-Kendall test (Kendall, 1975;Mann, 1945).While examining the correlation of temperature changes in individual months and throughout the spring season and between temperature and circulation indices, the r-Pearson correlation analysis was used.

General Characteristics of the Variability of Spring Temperatures Against the Background of the Atmospheric Circulation Indices
The study investigates changes of temperature throughout the spring season and in individual spring months (March-May).The average spring temperature in the entire study period was 8.8 °C and ranged from 3.3 °C in March to 14.2 °C in May (Table 1).The lowest average monthly temperatures were typically above 0 °C, with negative values only recorded in March in 16 out of the 149 years.Four of these occurred in the nineteenth century, ten were recorded in 1917-1969, and the last two were in 1987 and 1996 (Fig. 2).For both spring months and spring as a whole, they occurred in the early years of the study period (in the nineteenth century and, in April, in 1929), while the highest value was in the second decade of the twenty-first century (Table 1).This reveals a discernible upward trend in air temperature for spring as a whole and for individual spring months.The highest increase is observable for March, but it is the least stable (0.191 ± 0.045 °C/ 10 years; Table 2) as a result of the high temperature variability in this month (Table 1).The smallest rise in temperature is displayed by May (0.162 ± 0.030 °C/10 years), but it turns out to be the most stable due to lower temperature variations (Tables 1 and 2, Fig. 2).The existing trends in average temperature changes are statistically highly significant (Table 2).
Spring temperature changes are mainly attributable to changes in March.The correlation coefficient between these temperature values was 0.744, meaning that 55% of the spring temperature changes can be explained by changes in March.The Statistically significant as they are, the correlation coefficients that were calculated between the average temperature values in spring months are small (about 0.250), which is manifest proof of the non-synchronic occurrence of extreme air temperatures over time (Fig. 2).This is reflected by an analysis of exceptionally cold and exceptionally warm spring months and springs (ECMs/Ss and EWMs/Ss; Fig. 2).
The strong increase in temperature is also reflected by the timing of ECMs and EWMs.The former (ECMs) would mainly occur in the first half of the multi-year period, while the latter (EWMs) occurred after 1950.Another notable fact is that, in the case of very cold or warm springs, temperature values of at least one month were also identified to represent the exceptionally low or high category, respectively.In the most manifest cases, such values were usually observed for two months (mostly March and April).Examples include the Marches and Aprils of the coolest springs (ECSs) that occurred in 1875 and 1883, as well as the April and May of the very warm spring of 2018 and all three months of the slightly cooler season of 2007.In the analyzed period, there were also 4 cases in which none of the months was classified as ECMs or EWMs, but the entire season was exceptionally cold (3 times) or warm (once).There were also years when two months were counted as ECMs or EWMs, but the whole spring was not classified as the warmest or coldest.An example may be the years 1931 and 1958, in which March and April were cold months (ECM 10 ), and May was warm (respectively: EWM 95 and EWM 90 ) and the year  In order to identify the causes behind the trends in spring temperatures identified, an investigation was made of their relationship with atmospheric circulation.In the first step, the temperature rises were compared to the complex Wi, Si, and Ci circulation indices.
The analysis found that long-term changes in the indices over the period under consideration were not significant except for the Ci index in May (Table 3; Fig. 3).One consequence of this lack of clear trends is a weak correlation between the temperature  4).The above applies primarily to the relationships between temperature and the types of pressure systems (Ci index) that shaped the weather over the study area.This stems from the fact that in the transitional and thermally highly diverse season that is spring, the type and thermal characteristics of incoming air masses are determined by the direction of advection rather than by the pressure system.This is confirmed by the (statistically significant) relationship between the increased frequency of zonal circulation (Wi) and an increase in temperature in March (correlation coefficient r equal to 0.351).There is also an indication that advection of air masses from the western sector (Wi) causes a drop in temperature in May (r = -0.146;statistically insignificant).This results from the warming effect of air masses from the Atlantic in early spring and the similar, or sometimes lower than in Krako ´w, air temperature inflowing from the ocean in May.A stronger effect on temperature is found to have been produced by air advection from the south (Si), both for the entire spring (r = 0.282) and April and May (r = 0.521 and 0.603).
Given the rather weak relationship between longterm changes in the circulation indices and changes of temperature, further research focused on selected seasons (ECSs and EWSs) and months (ECMs and EWMs) rather than including the entire series of observations.All in all, 16-18 such cases were identified for the springs and individual months alike.The most thermally exceptional months (ECMs 5/1 and EWMs 95/99 ) and springs (ECSs 5/1 and EWCs 95/ 99 ) are presented in Table 5.
A detailed analysis of the circulation indices (Wi, Si and Ci) confirms the previously described effect of particular forms of circulation on monthly and seasonal temperatures (Table 5).However, in many cases, the exceptionally high or low average monthly or seasonal temperatures are attributable to the specific configuration of circulation types in a given period and the thermal characteristics of the prevailing air masses.One example is March 2014, which saw the highest average monthly air temperature in the study period.The values of the circulation indices reveal a low frequency of inflow of air masses from the west (Wi = -12.9%)and a prevalence of air masses inflowing from the north (Si = -9.7%)and of anticyclonic systems, which shaped the weather over the period (Ci = -9.7%).
It was also found when analysing the exceptional values of the indices (Wi, Si and Ci; B 10 or C 90 percentile of all the values) that in more than half of the cases such values occurred in years that did not qualify as exceptional in temperature terms (ECMs or EWMs).
The difficulties in conclusive interpretation of the effect of particular forms of circulation (as described by the indices employed) are largely related to the change in the 'relative' thermal characteristics of the air masses (warmer/cooler) that arrived over the study area in spring.In this transitional season of the year, one time air masses that flow from a given direction may entail advection of warm air and another of cold air.This depends both on whether it is early, mid-or late season and on the weather conditions in the areas where the air masses originate in the individual years.It should also be remembered that the thermal conditions over the area under consideration in March The results they had obtained earlier prompted the authors to conduct an in-depth analysis, taking into account the types of circulation that prevailed over the study area in the timespan being investigated.The analysis found a weak correlation between the occurrence of higher or lower temperature values in 12.3 10.1 13.6 -4.4 -6.7 -8.3 -9.1 -1.6 -1.5 -4.4 1.9 0.2 -2.0 0.0 4.5 -2.7 ECMs 5 1.5 7.9 2.8 -7.1 -2.4 2.4 0.0 2.2 -1.0 -3.8 -0.3 -1.4 -0.9 0.0 -6.8 7.5 ECMs 10 3.0 3.2 -3.0 -2.0 -2.7 -3.9 -1.1 1.3 -0.3 4.5 0.7 -0.6 0.8 0.0 -1.5 0.5 3360 Z. Bielec-Ba ˛kowska and R. Twardosz Pure Appl.Geophys.
a given month and the type of baric system that controlled the inflow of air masses to south-eastern Poland.However, it can be seen that exceptionally cold and warm Marches (ECMs and EWMs respectively) were accompanied by an increased frequency of anticyclonic systems.This association was strongest in the months that were most exceptional in temperature terms (ECMs 1 and EWM 99 ), when it was even about 20% higher than in the long-term study period (Tables 6, 7, 8).This was largely caused by a large decrease in the frequency of cyclonic troughs (Bc-by as much as 8.2%).A slight predominance of cyclonic situations was visible in April, although it mainly concerned EWMs, with one exception, namely EWMs 99 , which saw about 10% more highpressure systems than in the entire period under review.By contrast, it is very difficult to identify any clear regularities regarding the relationship between ECM and EWM temperatures and pressure systems in May.

Occurrence of ECMs and Their Relationship with Air Advection Directions
When analysing the directions of advection of air masses in spring ECMs, it is clear that, against the background of the entire study period, air masses arriving from the northern and eastern directions were more frequent while those incoming from the south and west showed a lower frequency (Table 6).The trends described differed slightly depending on the month concerned.In March, advection from the N-NE-E-SE directions was more frequent than the long-term average (in total, they accounted for 44.8% of all cases), and advection was less frequent from the S-SW-W-NW directions (34.1%; the directions marked in bold saw particularly high differences).In April, inflow of air masses exclusively from the S and SW directions was recorded less frequently (less than half as frequently, representing only 7.4% of the cases studied).Among the remaining advection situations (which accounted for a total of 71.9%), a significant increase in frequency (by almost 6%) was observed in the case of advection from the NW.In May, air masses from the NW-N-NE-E directions were recorded more often than the average over the timespan under study (a total of 48.0%), as opposed to air from the SE-S-SW-W directions, which was much less frequent (26.4%).An analysis that took into account the monthly temperature value (ECMs 1- 10 ) found the same trends (Table 7).However, in each of the additionally distinguished groups, as well as in each month (March-May), the differences in the frequency of individual directions of advection relative to the long-term average varied slightly, with no easily identifiable pattern behind them.However, Table 8 The difference in the frequency of advection directions of air masses and weak-advection situations in the EWMs and in entire 1874-2020 period over southern Poland (explanations in the text)

Period
The direction of advection Weak-advection situations Sum A c

Occurrence of EWMs and Their Relationship with Directions of Air Advection
As the study shows, EWMs were likewise under the influence of two groups of directions that were more and less favourable to higher values of air temperature.In March, advection of air masses from the west (SW-W-NW) was much more frequent than the long-term average, accounting for 51.2% of all cases in total (Table 6).This was 12% more than the long-term average and about 46% more than the average for exceptionally cold months.The influx of masses from other directions accounted for only 26%.In April, temperatures would be higher in the presence of advection from the south sector (SE-S-36.2% of all cases), exceeding the average for the long-term and for the cold months by approx.11% and 18%, respectively.In May, the directions of advection of the warmest air masses shifted towards the easterly sector (NE-E-SE-S), and their aggregate frequency reached 43.2% and was 10% above the long-term average and the average for the cold months.The trends described differ slightly depending on the value of monthly temperature, effects mainly noted in the warmest months (EWMs 99 ; Table 8), which is not only due to the thermal conditions of the air masses themselves, but also to the small number of months assigned to the individual groups.The change in the thermal characteristics of air masses arriving from a given direction in the individual months (March-May) was also much more distinct, notably in the case of westerly advection (W).Its markedly increased frequency in March adds to the increase in temperatures, while in April and May, days observing such advection in the EWMs are much less frequent than the long-term average.

Occurrence of ECMs and EWMs and Their Relationship with Types of Atmospheric Circulation
The study has also analysed the prevalence of all types of circulation in ECMs and EWMs.In addition to confirming the above-described trends, it has found several features that are typical of the individual groups of cold (ECMs 1-10 ) and warm months (EWMs 90-99 ), and changes in these features by month (March-May).
In ECMs in March, an increased frequency of advection from the N-NE-E-SE directions, especially of anticyclonic types, and a discernible decrease in the frequency of NWa, Wc and Bc are clearly visible (Fig. 4a).It is also worth noting the increase in the occurrence of a central cyclone (Cc), which is most likely associated with a higher frequency of northerly advection.In April, these differences tend to be smaller.However, the share of advection from S and SW, both in the presence of anticyclonic and cyclonic systems, declines significantly, while the inflow of cool air from SEc, NWc and NWa increases in frequency.May saw no such increase, but instead a clear decrease in the frequency of SEa types was observed.Among the EWMs, March stands out for a substantial increase in the frequency of inflow from the Wa and Wc and NWa and NWc directions, as well as for the frequent occurrence of an anticyclonic wedge (Ka) and occasionally of a cyclonic trough (Bc; Fig. 4b).In April, there was a greater number of days with influx of air from the SE-S-SW directions in connection with low-pressure systems that brought in warmer air from southern and western Europe.This characteristic was slightly less pronounced in May, which also saw more frequent advection of air masses from NEa, SEa and Sa, and a very low frequency of advection from the western sector.
Such large variations in the occurrence of the circulation types in spring ECMs and EWMs prompted the authors to check whether the number of the types recorded in each of them changed significantly depending on monthly temperature.In a pattern that emerged a vast majority of months fell within a range of 10 to 15 circulation types per month (Table 9), while the exceptions with either fewer than 10 types (ECMs: 9 in April and 8 in May, and EWMs: 6 and 9 in March and 8 in May) or a more than 15 types (ECMs: 16 and 19 types in March) were few and far between.An attempt at identifying months with a marked dominance of one or several types of circulation yielded inconclusive results, as the study period included only seven months, in which one type reoccurred 10 or more times (Table 9).
For ECMs, the greatest frequency of one type of circulation-as many as 15 occurrences-was recorded in March 1996.The situation was of the SEa type (Fig. 5), which, as the Calendar of Atmospheric Circulation Types for Southern Poland indicates, was associated with the influx of very frosty continental polar air masses (cP).A similar situation took place in April 1884 and 1907, when the SEc type (Fig. 5) prevailed 11 and 10 times, respectively.In EWMs, an exceptionally high frequency of one type of circulation was recorded four times.Two of these cases occurred in March 1961 and 1994.Both years recorded equally often, 13 and 11 times respectively, the Wc type (Fig. 5), during which warmer polar marine air (PPm) flowed over the study area.Notably, 1961 also recorded 7 days with advection of air from the north-west (NWc) and 6 from the west (Wa; Fig. 5).In April, it was the Wc type that marked its presence most frequently (10 times in 1939), while in May the inflow of cool air from the east (Ec; Fig. 5) occurred 11 times in 1889.Such a great variety of circulation types provides evidence of the fact that the thermal characteristics of an air mass arriving over a given area are determined by the direction of advection rather than by the associated pressure system.It should also be remembered that the direction of incoming air masses rarely changes in a radical way from one day to another.Usually the directions are similarly translated into similar thermal and humidity characteristics of the incoming air mass.Therefore, when seeking to identify the relationships between temperature changes over longer spring periods (e.g.months), it seems advisable to also investigate the sequence of circulation types.This is exemplified by the selected ECMs and EWMs presented in Fig. 6.As can be seen, in some cases, exceptionally low/high monthly temperatures followed from a sequence or series of days with inflow of air from 1-4 similar directions (e.g.ECM 10 -March 1996 and ECM 1 -May 1919).In other months, very low/high monthly temperatures were associated with a much greater diversity of synoptic situations (ECM 1 -May 1874 or EWM 99 -March 2014).It is also worth highlighting that even the occurrence of a very long sequence of similar types of circulation, accompanied by air masses clearly indicative of an influx of very cool/warm air, did not go hand in hand with the occurrence of extreme monthly temperature values.One example is March 1996, which recorded 17 days with advection from the SEa and Ea directions which were associated with the influx of polar-continental air masses for 16 days (Fig. 6).In addition, for the first 5 days and the last 4 days of the month, arctic air (PA) was transported into the area examined.Nevertheless, although this month qualified as exceptionally cold, its temperature was the highest of all the ECMs considered.

Discussion and Conclusions
Climate change is inherently associated with changes in the temperature and length of seasons which have been described in the literature (Twardosz et al., 2021;Wang et al., 2021).Some of the most significant of these are observable in the spring season.In particular, they have caused the growing season to extend and have changed the start dates of the season itself and of the individual vegetation phases (Buermann et al., 2018;Maignan et al., 2008;Wang et al., 2011Wang et al., , 2021;;Yu et al., 2010).This prompted the present study, which investigates the trends of these changes in Poland (using the example of Krako ´w) and seeks to understand their underlying factors relating to circulation.Rather than taking into account entire seasons or extreme heat events, which have been widely researched, the study investigates the temperature in the individual months in spring.In addition, it uses the mesoscale Calendar of Atmospheric Circulation Types for southern Poland compiled by Niedz ´wiedz ´(1981Niedz ´wiedz ´( , 2022) ) in order to analyse the relationship between atmospheric circulation and temperature.
Based on this, it was found that the area under study also saw a distinct increase in air temperature in spring and the individual spring months over the 1874-2022 period investigated.The increase ranged from 0.162 °C/10 years in May to 0.191 °C/10 years in March, and amounted to as much 0.181 °C/ 10 years for spring as a whole.It was the temperature in March, in which there are winter-like conditions, that had the greatest effect on the spring temperature.A strong increase in temperature is also confirmed by the variability in occurrence of ECMs and EWMs over time, with the former more frequent in the first half of the study period and the latter in the post-1950 period.The above trend also entails a noticeable increase/decrease in the number of warm/cold spells observable from the mid-twentieth century (Brunner et al., 2017;Sulikowska et al., 2022).Another notable finding is that the changes in monthly air temperature values were largely asynchronous, as a result of which it was very rare for more than one month to qualify as an ECM or EWM during an exceptionally warm or cold spring.
The spring temperature changes in the northern hemisphere are considered to have been caused, inter alia, by changes in the surface area of sea ice in the Arctic, which causes distinct changes in the macroscale circulation in the atmosphere over Eurasia.These developments translate into an increased frequency of advection of warmer air masses, thereby influencing both spring air temperatures themselves and the frequency of spring extreme heat events (Ding & Wu, 2020;Sun et al., 2022).The above implies that these changes should be reflected by changes in circulation over various areas, primarily in the frequency of prevalence of directions of air advection and in the presence and positioning of blocking systems (Brunner et al., 2017).
In order to find relationships between the changes in air temperature referred to above and changes in atmospheric circulation, the former were compared both with complex circulation indices (Wi, Si, and Ci) and with the direction of inflowing air masses and individual types of circulation.The findings imply a weak correlation between changes in temperature and changes in the values of the said indices, in particular with regard to the type of pressure system.Slightly stronger relationships are noticeable as regards the positive effect on temperature of zonal flow in early spring and the advection of air from the south in April, May and throughout the spring season.The above relationships are partially confirmed for the thermally exceptional seasons (ECSs and EWSs) and months (ECMs and EWMs).However, in some cases, the exceptional thermal conditions of those months/ seasons rather developed under the influence of a specific configuration of circulation types that prevailed at that time.
A more detailed analysis shows that the occurrence of exceptional monthly temperatures in March, and in the case of EWMs also in May, is associated with the occurrence of anticyclonic systems, which is partly confirmed by Brunner et al. (2017).However the temperature was much more often determined by the direction of air advection and the related temperature characteristics of air masses.However, here it is also difficult to identify unambiguous regularities concerning all months and distinguished groups of months with extreme temperatures (ECMs 1-10 and EWMs 90-99 ).Generally, in the case of the coldest months (ECMs), low temperatures most frequently developed in the presence of advection from the NW-N-NE-E directions.Meanwhile, for exceptionally warm months (EWMs), the prevalent direction of advection of warm air masses would change from westerly in March, through southerly, to south-easterly in April and to easterly in May.In parallel, the frequency of warm air masses associated with cyclonic systems declined in favour of anticyclonic systems.These regularities are confirmed by the findings of Sulikowska et al. (2022), who have identified similar developments for thermal extremes in spring.It is also worth noting that both Poland and Europe see noticeable changes in the occurrence of air advection from western directions, which may have a major effect on air temperature.In spring, advection from the west declines only slightly (Bielec-Ba ˛kowska, 2022), but in summer a significant increase in advection from the E is recorded and forecast, which implies that increase in temperature is already possible in late spring.Meanwhile, advection from the west becomes more frequent in winter, which in turn may increase the temperature at the beginning of March (Herrera-Lormendez et al., 2022;Niedz ´wiedz ´& Ustrnul, 2021).
The regularities described above have been confirmed through an analysis of the frequency of all the types of atmospheric circulation occurring over the area concerned.However, again there were significant differences in their occurrence depending on the month (March-May) and monthly air temperature (ECM 1-10 and EWM 90-99 ).In addition, discernible changes in the number of situations that involved weak advection have been identified (Ca, Ka, Cc and Bc).There were also only a few situations when an exceptional temperature in a month was caused by one or several types of circulation and the accompanying air masses.The study highlights a need for in-depth analysis of the effects of sequences of individual types of circulation.A focus should be on the types that precede the influx of cool air masses, including the presence of blocking systems that are conducive to the occurrence of very low temperatures (mainly in early spring) or high temperatures (in May; Brunner et al., 2017).
The great variety of circulatory conditions that underlie ECMs and EWMs means there is great unpredictability and a lack of sufficient repeatability to identify clear circulation patterns that add to the occurrence of such long (monthly) periods with exceptionally high or low temperatures.The occurrence of ECMs or EWMs is rather linked to the presence of one or two longer periods with air temperature significantly different from the long-term average.The great variety of the considered circulation conditions is related to Poland's location in the centre of Europe, which sees a confrontation of the key features of European climate (maritime v continental or subtropical v subarctic), which also applies to the presence of the dominant directions of advection and the role of pressure systems over the period of a year (Brunner et al., 2017;Cahynova ´& Huth, 2009;Mellado-Cano et al., 2020).This is confirmed by the identification of a separate type of climate for Poland in many climate classifications and by the frequently stressed transitional nature of the climate (Martyn, 1992).In addition, spring itself is a transitional season, when the climate characteristics of winter (most conspicuous in March) and of summer (in May) clash.It must also be remembered that researchers tend to define seasons by dividing them into months-long periods, which do not usually correspond to thermal and circulatory seasons (Nowosad, 2000;Piotrowicz, 2010).Other previous studies have also found that changes in climate conditions are not necessarily correlated with changes in the frequency of particular types of circulation, but rather with changes in the thermal and humidity characteristics of the associated air masses (Bartoszek & Kaszewski, 2022;Bartoszek & Matuszko, 2021;Brunner et al., 2017;Cahynova ´& Huth, 2009;Herrera-Lormendez et al., 2022).

Figure 1
Figure 1 Location of study area

Figure 2
Figure 2 Average monthly air temperature (°C) in spring and spring months in Krako ´w in the years 1874-2022

Figure 3
Figure 3 Average monthly values of circulation indices (%) in spring over southern Poland in the years 1874-2022

Figure 4
Figure 4 Frequency (%) of circulation types in exceptionally a) cold (ECMs) and b) warm (EWMs) spring months compared to their frequency in the multiannual period in Krako ´w in the years 1874-2022

Figure 5
Figure 5Selected examples of distinguished atmospheric circulation types on synoptic maps of Europe(Niedz ´wiedz ´, 1981)

Figure 6
Figure 6Types of circulation occurring in selected ECMs and EWMs in spring in Krakow in the years 1874-2022.In addition, the accompanying types of air masses determined according to the Calendar of Atmospheric Circulation Types for southern Poland were added (for the years 1951-2022)

Table 2
Coefficient of linear regression (slope) average mean air temperature

Table 3
Statistical characteristics of Wi, Si and Ci circulation indices in the spring and in the spring months over southern Poland in the years 1874-2022

Table 4
Correlation coefficients of air temperature in Krako´w with circulation indices over southern Poland (Wi, Si and Ci) in the spring and in the spring months in the years 1874-2022

Table 5
Selected values of air temperature (°C) and circulation indices (%) (Wi, Si and Ci) over southern Poland in spring (ECSs

Table 6
Frequency (%) of the occurrence of advection directions of air masses and weak-advection situations in ECMs and EWMs and in the entire 1874-2020 period over southern Poland (explanations in the text)The difference in the frequency (%) of advection directions of air masses and weak-advection situations in the ECMs and in the entire 1874-2020 period over southern Poland (explanations in the text)

Table 9
The number of circulation types and the maximum number of days with the same type of circulation (Max) in ECMs and EWMs in Krakow in the years 1874-2020