Joanna Wibig

From the beginning of the 21st century, the heat waves in Europe have become more frequent, severe and longer lasting, especially in the southern region. The longer duration of heat waves may be partly attributed to wave‐amplification, high‐amplitude waves in the mid‐latitudes, favouring the longer‐lasting blocking systems. The primary objective of the paper was to examine the long‐term changes of hot days and heat wave events and to identify the atmospheric circulation condition that prevails its occurrence. The other object was to investigate the correlations between hot days' occurrence in Poland and sea‐ice cover and sea surface temperature changes in the Arctic and its surroundings. Over the past few decades, Central and Southern Poland have experienced a considerable rise in maximum temperatures, which has led to an increase in the number of hot days. This increase is particularly prominent during the summer months (June, July and August) but also in May and September, which extends the hot season. Although heat waves have become more frequent and persistent since 2006, their mean intensity has remained unchanged. During all heat waves in Poland, the high‐pressure centre was located over the country or its close neighbourhood, mainly in the eastern sector but also in the south, southwest or over Scandinavia. The blocking systems were mainly related to the extended ridge of the Azores High, but also to strong blocking over the North Pole and Greenland reaching Scandinavia or anomalous high pressure in the East Arctic. The longest‐lasting heat waves were usually preceded by air masses advection from the Atlantic, but the highest potential temperature along the trajectories was observed for the southern direction of inflow. The growing number of hot days is statistically significantly related to the decreasing sea‐ice cover and increasing sea surface temperature in Arctic and Subarctic regions.

Snow cover (SC) is a great indicator of climate change. It is highly related to temperature. Since the Polish climate faces warmer conditions, changes in the wintertime precipitation phase are observed. More frequent rainfall instead of snowfall is noticed at the end of the 20th and beginning of the 21st centuries. This work presents the spatial distribution, its changes, and variabilities of selected parameters that described snow cover in Poland from 1966/1967 to 2020/2021. The snow characteristics used in the study comprise of SC duration (SCD), the first and last day with SC in the season (SCbeg and SCend), the potential duration of SC season (PSCD), SC stability (SCS), average, maximum and accumulated SC depth (HS, HSmax and AHS). The changes in snow cover waves were analysed. Generally, the Polish climate is mild in the west, where the snow cover is finer and occurs relatively rarely and becomes more continental toward the north‐east, where the snow cover has a better condition to accumulate and preserve. The southern parts of the country are covered by mountain ranges of the Carpathians and Sudetes, where the snow cover remains the longest. The spatial distribution of the coefficient of variability is reversely proportional to SC characteristics—the highest in the north‐east (especially for the HS, HSmax and AHS). The most significant changes in SC are related to a decrease in SCD (5–7 days/decade), HSmax (1–2 cm/decade) and AHS (30–60 cm/decade). At the same time, the snow cover season becomes considerably shorter (especially in the western and central parts—about 10 days/decade). The trends in SCS are not yet significant in most of the country. This study reveals that the snow cover in Poland is under constant change, and the negative trends, which were hardly or not visible at the end of the 20th century, in the last decades have become statistically significant in a greater number of Polish stations.