Spatio-temporal evolution of compound hot and dry extremes synchronizations in Europe

The probability of incidence of compound extreme climate events is increasing due to human-induced climate change: in particular, there is high confidence that concurrent hot and dry extremes will become more frequent with increased global warming. In this context, Europe is no exception. Understanding the aggregated impact of synchronized compound hot and dry events at different locations is a pressing issue, especially when it comes to predicting these extremes. We use concepts and methods derived from network theory to highlight hotspot regions in Europe where these spatially compound extremes are increasing and analyse the atmospheric precursors driving these anomalous conditions. Using ERA5 reanalysis data and focusing on the summer and winter seasons of the period 1941-2020, we construct evolving networks constituted by 51 consecutive blocks, encoding the spatial synchronization structure of compound hot and dry events. Next, we highlight the regional and seasonal differences of compounds occurrences and synchronizations, unraveling the main changes in the graphs structure, identifying hotspot regions and, finally, describing the atmospheric conditions behind compound events. The increase of compounds frequency and spatial synchronizations do not always match: synchronizations increase in Southeastern Europe during winter and in some locations in Finland, north of Poland and the Baltic states in summer, although, in both cases, we do not detect a corresponding trend in compounds occurrences. Moreover, we show that most of the synchronizations evolution can be explained by atmospheric pressure dynamical changes, including NAO and SNAO intensity. This work brings out key aspects concerning the underlying spatio-temporal dynamics of concurrent hot and dry events.