Influence of the North Atlantic Oscillation on annual spatio-temporal lightning clusters in western and central Europe

Abstract

Based on lightning measurements in western and central Europe from 2001 to 2021 (May-August), a grid-based climatology and trend analysis of thunderstorm activity has been developed. The results indicate a significant decrease in thunderstorm activity in many regions. Extending the analysis beyond a purely grid-based approach, areas with spatio-temporal intense lightning (convective clustered events, CCEs) were identified in a second step by applying a clustering algorithm (Spatio-Temporal Density-Based Spatial Clustering of Applications with Noise, ST-DBSCAN). For this purpose, a methodology is presented which seeks out to determine an appropriate density definition, as required by ST-DBSCAN. An analysis of the characteristics of the CCEs indicates a slight increase of smaller, more separated clusters, while larger clusters occur less frequently over time. This suggests a shift in the mesoscale organization of convective systems. Furthermore, a correlation between the North Atlantic Oscillation (NAO) and thunderstorm frequency has been identified. Notably, there was a pronounced reduction of thunderstorm activity, as well as an increased number of separated convective systems during negative NAO phases in France. This, in conjunction with a documented accumulation of years with predominantly negative NAO values between 2011 and 2020, is likely a contributing factor to the aforementioned negative trends.