The co-occurrence of heat-related climate indices in Hungary during the 21st century using raw and bias-corrected EURO-CORDEX simulations

A substantial increase in the frequency and intensity of extreme climatic events is clear evidence of climate change, which can be observed across South-Eastern Europe, including the Carpathian Basin and Hungary. This trend constitutes an increasing risk to human health, as populations are progressively exposed to environmental stressors with amplified recurrence and severity. Heatwave days, defined as daily mean temperature of at least 25 °C and tropical nights, when temperature does not drop below 20 °C at night are among those weather related phenomena that pose serious health risks. Due to heat stress, mortality rises during time periods characterized by such temperature extremes. 

In our study the co-occurrence of heatwave days and tropical nights is analysed for the 21st century. In addition, we investigate the appearance of a second level heat alert (when the daily mean temperature is at least 27 °C) combined with higher minimum temperatures up to 25 °C. For this research five regional climate models (RCMs) of the EURO-CORDEX initiative were selected (CCLM, HIRHAM, RACMO, RCA, REMO), all available at a horizontal resolution of 0.11° and driven by two different representative concentration pathway scenarios (RCP4.5 and RCP8.5, respectively). In addition to the raw simulations, bias-corrected versions of these RCMs were also investigated: the RCM projections available from the EURO-CORDEX project using MESAN as reference data, the FORESEE-HUN database and BC-HUCLIM, a bias-corrected set of simulations specifically established for this research. For BC-HUCLIM, bias-correction was performed using the percentile-based quantile mapping method with the quality controlled HuClim data serving as reference. The bias-correction was carried out separately for each raw simulation on a monthly basis.

The results are presented for the regions of two Hungarian cities: Budapest, the capital city of Hungary and Szeged, located in the south-eastern part of the country, which often experiences increased heat advection from the Mediterranean. The co-occurrence of the climate indices are shown throughout the year for the past (1976-2005), for the near future (2021-2050) and for the end of the 21st century (2070-2099). The investigation focuses on the question of (i) how many RCM simulations project the occurrence of the events at least once in 30 years on a given day of the year (ii) in which part of the year can these temperature extremes occur.