Increasing trends in the nocturnal and compound hot extremes in Eastern Mediterranean cities (1960–2022)

Abstract The study of hot extremes has traditionally been grounded in the analysis of maximum air temperature, often overlooking the effect of minimum air temperature but also the combined effect of daytime and nighttime thermal conditions (compound hot extremes). The present study examines climatic indices related to nighttime conditions like tropical nights (TRN), compound hot extreme days (CHD) but also three types of heat waves (HWs) (nighttime, daytime, compound) at large cities of the eastern Mediterranean, over the period 1960–2022. Three sub-periods (1960–1980, 1981–2001, 2002–2022) are also examined in the study. To this end, long-term observational data from one meteorological station with sufficiently long records in each city were used. The analysis reveals increasing trends in the number of TRN (+ 2.8 to + 6.2 tropical nights/decade) but also in their intensity (+ 0.11 to + 0.31 °C/decade), more prominent at the most highly urbanized cities. The frequency of CHD over the last two decades is almost twofold to threefold times the 1980s and 1990s levels, in all cities. Daytime HWs are more frequent than nighttime HWs in all cities except in the city with the largest urban area (Athens), where nighttime HWs dominate. The number of HWs during the last sub-period 2002–2002 is much higher compared to the previous one 1981–2001 (1.8–3.7 times higher for nighttime and 1.4–2.6 times higher for daytime HWs, depending on the city). The coupling between intensity and duration of daytime HWs is strong and apparent in all cities, indicating that the longer the duration of daytime HWs, the higher is their intensity on average, while the coupling for nighttime HWs seems to be weaker.