Trends and drivers of recent summer drying in Switzerland

The Alpine region recently experienced several dry summers with important and adverse impacts on economy, society and ecology. Here, we analyse drought indicators, evapotranspiration and meteorological data from point observations, reanalyses and regional climate model data to assess trends and drivers of summer drought in Switzerland in the period 1981-2020. The drought indicators from station observations and ERA5-Land and ERA5 reanalyses show a tendency towards drier summer half-years (climatic water balance: -39 mm decade(-1), 0-1 m integrated soil water content: -5 to -7 mm decade(-1)) with a drying in most months from March to October. Both, increasing evapotranspiration (potential evapotranspiration: +21 mm decade(-1) or +7% K-1 warming; actual evapotranspiration: +8 to +15 mm decade(-1)) and a non-significant precipitation decrease of 17 mm decade(-1) are identified as important and roughly equivalent drivers. The reanalyses show considerable differences for soil water and actual evapotranspiration, especially in drought summers. The ERA5 soil is clearly drier than the one in ERA5-Land. ERA5 evapotranspiration is smallest and partly soil moisture-limited in drought years while evapotranspiration in ERA5-Land is highest, still mainly energy-limited and scales well with temperature (+4% K-1 warming). ERA5-Land seems to better match with in situ measurements of soil water and evapotranspiration than ERA5, but considerable differences with in situ measurements remain. Variability and trends of the drought drivers temperature and precipitation are also investigated in the EURO-CORDEX regional climate model ensemble. Most simulations considerably underestimate the recent warming and the ensemble shows a large possible range of precipitation changes with a mean change near zero. The summer precipitation-temperature scaling and the correlation between summer temperature and precipitation on the interannual time scale are mostly overestimated. Our results highlight that the analysis of Central European summer drought evolution and its drivers remains challenging especially with regional climate model data, but considerable uncertainties also exist in reanalyses.

Automated summary

The Alpine region has been experiencing increasingly dry summers, which have had significant negative impacts on the economy, society, and ecology. This study analyzes drought indicators, evapotranspiration, and meteorological data to assess the trends and drivers of summer drought in Switzerland from 1981 to 2020. The results suggest that both increasing evapotranspiration and a non-significant decrease in precipitation are important drivers of summer drought in the region.