European summer weather regimes 1990-2019: Automatic classification and representation in a small global climate model ensemble

Abstract: In Central Europe, the occurrence of different weather regimes (WRs) plays a major role in spatiotemporal temperature and precipitation patterns. In the context of increasingly extreme summers, this study focuses on European summer WRs (June-August, JJA) over the last three decades (1990-2019), and aims to examine the changing characteristics of these WRs and their potential implications. In addition, based on ERA5 reanalysis data, the WR representation from a carefully preselected, small ensemble of global general circulation models (GCMs) is analyzed. A methodological refinement concerns the combination of Self-Organizing Maps (SOM) with a novel GCM selection technique, which enhances the robustness of the simulated large-scale circulation patterns. WRs are defined using daily sea level pressure (SLP) and wind in the upper troposphere. Results reveal that the SOM captures predominant European summer synoptic patterns, and the salient result is a positive trend in 2 m air temperature across nearly all WRs. The selected GCMs - MPI-ESM1-2-LR r29i1p1f1, CanESM5 r1i1p1f1 and MRI-ESM2-0 r5i1p1f1 - identify WRs correctly and ERA5-based results are always within the range of this small GCM ensemble. Its members clearly show skill in representing large-scale WRs accurately and, thus, serve as valuable tools for studying synoptic weather patterns during summer in Central Europe. Therefore, we can recommend these GCMs for studies on WR-related climate projections of future summer conditions, particularly for those interested in climate impacts in Central Europe from a synoptic-scale perspective.