Near-past and future trends of European extreme heat and heat waves from WRF downscaling experiments

In frame of the H2020-EXHAUSTION project, we present estimated heat indicators from WRF (Weather Research and Forecasting model) downscaling simulations for the periods of 1980-2014 and 2015-2049 at 20 km horizontal resolution over the European domain. WRF simulations are forced by the CESM2 global model simulations, using three shared socio-economic pathways (SSP) future scenarios from the Coupled Model Intercomparison Project Phase 6 (CMIP6): SSP1-2.6, SSP2-4.5 and SSP3-7.0, addressing different levels of mitigation and adaptation. For the period of 1980-2014, another WRF simulation forced by ERA5 is used as comparison in model validation. These near-past simulations have been rigorously evaluated with observations and reanalysis data including European Climate Assessment & Dataset (ECA&D), E-OBS, and ERA5-land for the surface air temperatures. The dynamical downscaling showed clear added value on spatial distribution related to the important coastal or orographic aspects widely present over Europe. Two heat wave indicators, the Warm Spell Duration Index (WSDI) and the Heat Wave magnitude Index daily (HWMId), and four extreme heat indicators, annual maximum temperature (TX­x), NOAA heat index (HIx), wet-bulb globe temperature (WBGTx), and universal thermal climate index (UTCIx), are used to study the heat extremes trends in Europe. During the past 35 years, TXx has been estimated to increase 2.5 °C in WRF_CESM2 and 1.4 °C in WRF_ERA5; the increasing trend is estimated to remain or slow down slightly in the next 35 years with estimated smaller increase of 1.5-2.5 °C in three scenarios. The trends of other extreme heat indicators showed very similar trends with TXx. However, future heat wave duration and magnitude present a contrasted pattern. Heat waves have been estimated to increase 11.2 days of duration, and 2.1 of magnitude during 1980-2014, very similar to the observed increase of 9.1-11 days and 1.8-2.1. Whereas in 2015-2049, heat waves duration and magnitude are estimated to increase 12.3-13 days and 2.5-4.6, respectively. These heat wave changes are also not uniform from a spatial point of view. Heat wave duration and magnitude in Southern Europe are both estimated to increase significantly faster than other zones, with rates at 1.4-2.9 times of which for the whole of Europe. Heat wave indicators in future scenarios also showed much larger interannual variations compared with the past, whereas there are no distinct differences among three mitigation scenarios for all heat indicators. In summary, these results suggested that even though the future increase of air temperatures and heat extreme indicators showed a slowing down sign compared with the near-past, whereas the severity of heat waves are estimated to increase even faster than the past under different levels of mitigation. Southern Europe is expected to be the region that needs the most attention in terms of severe future heat waves.