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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., Environmental Research Communications, 4 (2), ISSN:2515-7620

Distribution changes in seasonal mean 2 m temperature are investigated in Central Europe and the extratropical northern hemisphere for the most recent period 1961–2005 and the 21st century. Data from both observations and climate model runs are used. The latter are taken from scenarios prepared for the fourth IPCC assessment report (AR4). All data sets and model runs are scaled by their corresponding interannual variability to facilitate the comparison between the data sets. Time series are detrended to investigate changes in internal variability. For the last 45 years (1961–2005) the strongest changes in mean are found for in the summer mean temperature, both in observations and AR4 runs. With the exception of autumn, changes in the mean are captured reasonably well by the models on the regional as well as hemispheric scale. For Central Europe, estimates for variability changes show a weak increase (decrease) in summer (winter) observations. Both are not statistically significant at the 10% level. For the 21st century all climate scenario runs suggest large relative increases in mean temperature for all seasons. The AR4 model-to-model differences in the mean changes are largest in summer and not substantially smaller than those from the third assessment report (TAR). Model uncertainties are in the same order or even larger than the uncertainty introduced by the different scenarios. Compared to TAR, the AR4 runs show a much more consistent tendency for increases in Central Europe summer temperature variability especially towards the end of the 21st century (for 2070–2099: ~22–47% increase). No clear changes in seasonal mean temperature variability are found for the other seasons and the averaged northern hemisphere land time series.