Increasing compound warm spells and droughts during the growing season in the Mediterranean Basin

The Mediterranean Basin is known as a hot spot of climate change and therefore especially prone to increasing frequencies of warm spells and droughts. Investigating these events in isolation neglects their interactions, which illustrates the need to account for such compound events in a holistic manner. We analysed during which months the frequency of compound warm spells and droughts increased most over the 40-year period from 1979 – 2018. Warm spells and droughts were detected using daily maximum air temperature, precipitation and potential evaporation data from ERA 5. The two drought indices Standardised Precipitation Index (SPI) and Standardised Precipitation-Evapotranspiration Index (SPEI) were calculated.

Our results show the number of compound events increases substantially for almost the entire Mediterranean indicating that novel climatic conditions are occurring. The increases in compound events are predominantly driven by the rising number of warm spells, whereas SPI droughts remain almost constant. However, the rising temperatures lead to higher evapotranspiration, which alters the water balance in the Mediterranean. Therefore, the SPEI droughts shows significant increases in contrast to the SPI, indicating that even though the amount of precipitation does not decrease, the Mediterranean Basin is likely facing drier conditions due to increasing evapotranspiration. The highest changes in the number of compound warm spells and droughts occur in the time span from late winter to early summer. This finding is particularly relevant for Mediterranean ecosystems because this period encompasses the main growing season, and therefore ecosystem productivity and carbon sequestration might be reduced.