Long-term changes in heat wave parameters in the eastern Baltic region

Temporal and spatial variabilities of heat waves in the three eastern Baltic countries – Lithuania, Latvia and Estonia – in 1951–2021 were studied. Three heat wave definitions – daily maximum temperature (CTX), daily minimum temperature (CTN) and excess heat factor (EHF) at 90 and 95 percentile levels – were used for the calculation of five heat wave indices – heat wave factor (HWF), heat wave number (HWN), heat wave duration (HWD), heat wave amplitude (HWA) and heat wave magnitude (HWM) – at 40 stations. Additionally, numbers of extremely warm days (Tmax > 27 °C) were analysed. Linear regression and Mann-Kendall tests were applied for trend analysis. The mean spatial distribution of the number of extremely warm days has clear regularities. Lower values are typical for the coastal zone and for the northern part of the study area while higher values are located in the continental region and in the southern part. Extremely warm days are mostly concentrated to the mid-summer period, from 6 July to 9 August. The results of the trend analysis indicate the presence of statistically significant increasing trends in the majority of heat wave indices. The number of extremely warm days has approximately doubled during 1951–2021 while the trend has been statistically significant at all stations. The strongest increasing trends were revealed in the case of HWF (total number of heat wave days). This increase has nearly doubled while the trend was significant at all stations. The increase is stronger also for other heat wave indices measured in days (number of heat waves HWN, duration of heat waves HWD). The indices showing temperature anomalies (HWA, HWM) have notably slower growth trends, which are mostly insignificant.