A SOM-based analysis of the atmospheric drivers of major European droughts: a framework for extreme event attribution

Drought is one of the most important meteo-climatic phenomena. In the last decades, European droughts of extreme intensity and duration have had severe impacts on agriculture, economy, ecosystems, and society. Understanding the drivers of these events has become increasingly crucial, especially in the context of anthropogenic climate change, as the whole continent, and, specifically, the Mediterranean region, is recognized as a climate change hotspot. In this study, we apply Self-Organizing Map (SOM) analysis to classify ERA5 500 hPa monthly geopotential height data into dominant circulation types over the Euro-Atlantic region. These patterns were then analyzed in terms of both atmospheric variability and drought-related variables, assessing their relation to Euro-Atlantic teleconnections and evaluating their effects on surface hydrological variables such as the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). Using this framework, we examined three extreme European drought events characterized by extraordinary intensity and duration: the 1974-1975 UK drought, the 2004-2005 Iberian drought, and the most recent Euro-Mediterranean 2022-2023 drought. Our results show that, during these events, the frequency of precipitation-related circulation patterns significantly decreased, while that of dry patterns substantially increased. For instance, in the 2022-2023 case, the frequency of wet patterns dropped as low as 3.7%, whereas dry patterns reached up to 85.2%. This approach provides a methodology that highlights the atmospheric mechanisms driving droughts and their link to large-scale circulation variability. Most importantly, by allowing for a quantitative assessment of the percentage of the dry, wet, and neutral circulation types, it provides a framework for a future attribution studies on the effect of anthropogenic climate change on extreme droughts.