Are heatwaves and drought more likely to occur simultaneously with warming?

Heat waves and droughts are two the most frequent weather extremes in Poland, both excerbated by the ongoing global warming. Very often they occur simulateously. The aim of this presentation is identification of cases of simultaneous occurence of these extreme situations, examination of their spatial extent, frequency of occurrence and long-term variability. Reasons of their occurrence are also analysed. On the basis of data from the period 1966-2020 from about 40 stations in Poland reference evaporation indices will be estimated based on Palmer-Monteith formula and together with precipitation records used to the water balance determination. The results obtained so far indicate on strong increasing trend of evapotranspiration and only very small, statistically insignificant, increase of precipitation. This causes a decrease of the water balance, defined as a difference between precipitation and evaporation. In Poland the water balance is positive in the cool part of the year and negative in the growing season. This causes that a water deficit  in the growing season will be more and more stressful, especially since the injection of water from the snow cover before the growing season also significantly decreases. Heat waves will be  distinguished basing on maximum daily air temperature. Their number, duration and intensity also have increased considerably from the middle of the twentieth century. The standardized precipitation-evaporation index (SPEI) by  Vicente-Serrano and snow cover data from the winter preceding the drought will be used to find out reasons of simultaneous appearance of heat waves and drought. Among others factors will be considered: the North Atlantic Oscillation, the Arctic Oscillation, Arctic sea ice cover. The research is supported from the Polish National Science Centre (grant 2019/33/B/ST10/01136)