The last three decades in Europe have been characterized by rapid air temperature rise and increased frequency of extreme weather events such as droughts, summer heat waves, windstorms, heavy precipitation events, and floods. This great scientific and public interest to extreme events remains especially relevant under the changing climate conditions. Comprehensive research of extreme weather events is particularly important for the middle latitudes, as the inter-annual variability of the climate is evident. Some studies (Rutgersson et al. 2015) have found that the climate warming in Northern Europe is faster than the global mean.
There has been a great deal of interest in cold waves in 2010, when large-scale cold waves were observed in many parts of the Northern Hemisphere (NH), especially in Europe (Christiansen et at. 2018; Guirguis et al. 2011). It is well known that a sudden drop of air temperature can significantly affect human health. Some scientific studies (Gasparrini et al. 2015) have shown that people die from the severe cold several times more often than from heat. For this reason, each region has its official thresholds of negative temperature, beyond which it is recommended to stay at home, especially for vulnerable groups such as elders, children, and people with certain medical conditions. Severe winters have always brought a lot of damage in various areas of human activity. CSs have a negative effect on the transport and communication sector and can also cause serious damage to buildings. During severe cold periods, gas consumption and heating costs increase significantly. Finally, interchangeable occurrence of extremely warm and cold periods may have a negative effect on agriculture, food, and livestock production (McMichael et al. 2007). Considering the negative impact mentioned above, the accurate prediction of severe CSs and WSs is an important and challenging task in sub-seasonal and seasonal weather forecasting.
Despite rapid global warming over the last decades, when the average air temperature during winter was recorded above normal, there were also periods when the air temperature dropped to extremely low values. The recent winters of 2009–2013, 2017–2018 in Europe, and 2013–2015, 2017–2019 in North America had several severe CSs events and aroused scientific and media attention on those costly episodes of extreme temperatures.
Some studies (Kharin et al. 2007; Claud et al. 2007; Kodra et al. 2011; Mori et al. 2014; Vihma 2014) have shown that the probability of significant cold anomalies in the future will persist. Moreover, CSs may be even stronger. Results from the simulation reveal that the recent Arctic sea ice reduction contributes to reoccurrence of cold winters in some mid-latitude continental regions (Cohen et al. 2018; Nakamura et al. 2015). This was explained as a result of the Arctic amplification, which means the stronger warming of the poles to compare with lower latitudes (Vallis et al. 2015). This seesaw Northern hemisphere winter temperature phenomenon also is called “warm Arctic – cold continents pattern” (Overland et al. 2011).
Together with the mentioned amplification, a decrease in the meridional temperature gradient and weakening of the polar jet stream was observed (Francis et al. 2009; Outten and Esau 2012). The eddy heat flux has also robustly weakened over the last four decades (Vallis et al. 2015; Chemke and Polvani 2020). This creates favourable conditions for the formation of cold anomalies, which are often related to a perturbed jet stream (Francis and Vavrus 2012). High meandering of the polar front jet stream causes a deep southward intrusion of cold Arctic air over the continent (Tang et al. 2013; Vihma 2017; Vavrus 2018; Cohen et al. 2020). Due to the Arctic amplification the WSs in the North Pole also become much stronger and more common, in some cases anomalies are as high as 30°C (Graham et al. 2017). The relationship between temperature anomalies in Arctic and mid-latitude continents, and weaker zonal winds, was once again confirmed by Vihma et al. (2020). Thus, the increased intensity of winter CSs in one region can also be linked to increased intensity of WSs in another (Cohen et al. 2018). Recent extended CSs occurred during a period, which is called a “hiatus” period, when the trend of annual global mean surface temperature remained almost steady (Johnson et al. 2018).
Severe CSs in Europe are often associated with blocking processes over the North Atlantic. This situation is characterized by anticyclonic circulation, which creates favourable conditions for the intrusion of cold air from the north. It is important to note, that the specific location of the atmospheric blocking events over the North Atlantic creates different patterns of temperature extremes in Europe. Blocking processes in the western part of the Atlantic are not so important for the formation of cold anomalies in Europe (Sillmann and Croci-Maspoli 2009). It has also been established that the close relationship between anomalous winter temperatures and blocking processes will remain in the future climate (Sillmann et al. 2011).
Winters WSs in Europe received considerably less attention comparing to the number of studies related to cold waves. However, it is very likely that winter WSs are becoming more frequent, longer lasting, and more intense with a rapidly warming climate in NH mid-latitudes. Statistically significant increase in extremely warm days in winter in Northern Europe during 1979–2016 was found in a study by Sui et al. (2020). The increase in wintertime air temperature in Central Europe has been detected by Tomczyk et al. (2019), however most of these changes were statistically insignificant. In this study, large-scale WSs have been associated with the advection of warm air masses and anomalies in geopotential height in the entire troposphere, with the largest anomalies at 250 hPa several days before the extreme event. In the study by Matthes et al. (2015), it was determined that in winter WSs strengthened for most of the European and western Russian stations, with trends of up to 2.5 days per decade and statistically significant trends over Scandinavia. However, in the same research, statistically significant decrease in WSs of up to 2.5 days per decade were found in some stations in Siberia. The decreasing number of winter warm days was also found for Greece (Efthymiadis et al. 2011).
This can play a considerable role in biochemistry, changes in ecosystems, primary production, and dynamics of an agricultural pest species (Ma et al. 2018; Chapman et al. 2020). Extended warm and cold periods, as well as changes in the freeze-thaw cycle, have a negative impact on structure and functioning of ecosystems, seasonal cycles development, and survival rates of different organisms (Jiguet et al. 2011; Ma et al. 2015). In addition, a return of cold winter conditions can lead to plants tissue damage and its further development during spring (Chapman et al. 2020). Thus, detailed studies of WSs are necessary to understand the mechanism of their formation and the further procedures of improving their predictions. Usually, increased reoccurrences and intensity of WSs in the winter is attributed to a long-term global warming trend and CSs of the same winter are often assigned to the state of regional climate patterns (Guirguis et al. 2011). However, it is also possible that they are both a consequence of the same trigger during the winter period.
Currently, the discussion whether the number of extreme temperature events is increasing or decreasing is becoming especially relevant. The first-order hypothesis is that, in a warming climate, WSs are becoming more frequent and long lasting, and CSs are much rarer and less cold. There are many studies related to extreme temperature drivers, but the understanding of long-term tendencies in extreme temperature events is pivotal for setting further guidelines for climate change adaptation. It is also very important to distinguish between different approaches to extreme temperature events and to pay special attention to the importance of selection of the climate norm. These differences may also lead to different interpretations of the results.
In this article, we studied wintertime temperature extremes in the eastern part of the Baltic Sea region over the last 70 years (1951–2020). To evaluate and describe long term cold and warm anomalies, we use 3 different parameters: absolute maximum and minimum seasonal air temperature, 90/10 percentiles, and 10-year return level. We believe that such comprehensive study will provide more clarity on the change of temperature anomalies in a studied region.