Centennial changes in heat waves characteristics in Athens (Greece) from multiple definitions based on climatic and bioclimatic indices

Heat Waves (HWs) have received outstanding scientific attention over the past decades, inherent to their devastating societal and environmental effects. Observed and projected trends in HWs characteristics differ between geographical areas and studies, due to varying regional warming rates but also the adoption of different methods to assign a HW. The present study investigates and compares long-term statistics and trends in HWs characteristics (occurrence, duration and timing) from 1900 to 2019, at a very sensitive to climate change area, the eastern Mediterranean, derived from 15 HW definitions. In addition to broadly used definitions based on air temperature thresholds, the study introduces new definitions based on bioclimatic indices, like the UTCI (Universal Thermal Climate Index) and PET (Physiologically Equivalent Temperature). The analysis revealed coherent results, especially between definitions of the same type, but also marked deviations across different definitions, with respect to the magnitude of observed trends. Statistically significant trends (p < 0.01) in the number of HWs events and frequency of HWs days have been found in the area, irrespective of HWs definition. Changes in the timing of HWs - consistent across definitions- have been also found, resulting in the lengthening of HWs season by up to 7 days/decade since the 1960s.