The European summer heatwave 2019 – a regional storyline perspective

Abstract. The number and intensity of heat waves have increased in the recent past, along with anthropogenic climate change. This poses challenges to many communities and raises the need to develop adaptation measures based on more accurate information regarding regional to local changes in temperature extremes and their impacts. While the general increase in global mean temperature is well established, current global climate projections show a large model spread regarding possible future circulation changes. To isolate the more certain thermodynamic response from the less certain dynamical response to anthropogenic climate change, we employ an event-based storyline approach comprising three steps. Firstly, the large-scale circulation in the free troposphere was spectrally nudged to the ERA5-reanalyses in the global coupled climate model AWI-CM-1.1-MR for a recent period (2017–2022), corresponding to +1.4 K global warming, and repeated under pre-industrial, +2 K, +3 K, and +4 K global warming climates. Secondly, the global storylines were dynamically downscaled with the regional ICON-CLM model to the Euro-CORDEX domain with a horizontal resolution of 12 km and, thirdly, to a Central-European (German) domain with a resolution of 3 km. The present study focuses on the 2019 summer heatwaves over Central Europe. We demonstrate the added value of downscaling global storyline integrations, indicating a significant improvement in present-day temperature patterns and a reduced error in daily 2 m temperature relative to observations in Central Europe. The magnitude of the heatwave temperature response significantly exceeds the globally modelled background warming, with a distinct spatial and temporal variation in the regional increments. Our simulations indicate a general linear dependency of the 2 m temperature response to the global warming level: the warming rates during the July 2019 heatwave ranged between factors of 2 and 3 in Central Europe, resulting in an anthropogenic warming of 8 to 12 °C in the +4 K climate. The spatial extent and the duration of the heat wave are also amplified in the warmer climates. With this three-step downscaling approach, we gain new insights into possible future changes in heat extremes in Central Europe, which apparently surpass global warming trends. Along with its scientific value, our method provides ways to facilitate communication of regional climate change information to the users.