Augenstein et al. (2025) Influence of the North Atlantic Oscillation on annual spatio-temporal lightning clusters in western and central Europe

Identification

• Journal: Natural hazards and earth system sciences
• Year: 2025
• Date: 2025-12-11
• Authors: Markus Augenstein, Susanna Mohr, Michael Kunz
• DOI: 10.5194/nhess-25-4921-2025

Research Groups

• Institute of Meteorology and Climate Research Troposphere Research (IMKTRO), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
• Center for Disaster Management and Risk Reduction Technology (CEDIM), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

Short Summary

This study investigates the spatio-temporal characteristics and trends of thunderstorm activity in western and central Europe using lightning data from 2001 to 2021, revealing a significant decrease in thunderstorm activity and a shift towards smaller, more separated convective systems, which is linked to an accumulation of negative North Atlantic Oscillation phases.

Objective

• To identify convectively active regions of coherent thunderstorms and analyze their spatio-temporal behavior.
• To present a grid-based, high-resolution climatology and trend analysis of thunderstorm frequency based on direct lightning measurements for western and central Europe over a 21-year period.
• To investigate the temporal distribution of lightning clusters detected by ST-DBSCAN.
• To establish possible connections of the convective clusters with teleconnection patterns (specifically the North Atlantic Oscillation, NAO) and their influence on the temporal variability of the clusters.

Study Configuration

• Spatial Scale: Western and central Europe, ranging from 6° W to 18° E and 42° N to 56° N. Grid-based analyses were performed on a 10 km x 10 km resolution. Convective clustered events (CCEs) can span several hundred kilometers.
• Temporal Scale: 21-year period from May to August (MJJA) between 2001 and 2021.

Methodology and Data

• Models used:
  • Spatio-Temporal Density-Based Spatial Clustering of Applications with Noise (ST-DBSCAN) for identifying convective clustered events (CCEs).
  • Non-parametric linear trend analysis using the Repeated Median (RM) estimator.
  • Mann-Kendall test with Trend-Free Pre-Whitening (TFPW) for trend significance.
  • Concave hull algorithm (alpha shapes method) for determining CCE polygon envelopes.
  • Bagplots for assessing the distribution of CCE polygons in spatial and temporal dimensions.
  • Odds Ratio (OR) and Fisher’s exact test for quantifying correlations between binary variables (e.g., thunderstorm days and NAO phases).
• Data sources:
  • Cloud-to-ground (CG) strokes from the EUropean Cooperation for LIghtning Detection (EUCLID) network.
  • Monthly values of the North Atlantic Oscillation (NAO) index from the US National Oceanic and Atmospheric Administration (NOAA), derived from NCEP/NCAR Reanalysis 1 (500 hPa geopotential height anomalies).

Main Results

• A significant decrease in thunderstorm activity was observed in many regions of western and central Europe, particularly in a large contiguous area of Central France, extending to Belgium, Luxembourg, and western Germany. This area showed negative trends of up to -4 thunderstorm days per decade and up to -80 CG strokes per decade.
• Analysis of Convective Clustered Events (CCEs) indicated a shift towards the occurrence of smaller, more spatially separated thunderstorm activity over time, with larger clusters occurring less frequently. The 90th quantile of CCE size in 2021 (22,900 km²) was reduced by approximately 31% compared to 2001 (33,000 km²).
• The ratio of small (< 1000 km²) to large (> 5000 km²) clusters almost doubled over the study period, suggesting a change in the mesoscale organization of convective systems.
• A correlation was identified between the North Atlantic Oscillation (NAO) and thunderstorm frequency. Specifically, during negative NAO phases, there was a pronounced reduction of thunderstorm activity and an increased number of separated convective systems in France, with the probability of thunderstorm days reduced to almost half (Odds Ratio < 1).
• An unusual accumulation of years with predominantly negative NAO values was observed between 2011 and 2020 (7 out of 10 years), which is hypothesized to be a major contributing factor to the observed negative trends in thunderstorm activity, particularly in France.

Contributions

• Development and application of a novel heuristic for determining optimal density-defining parameters for the ST-DBSCAN algorithm in a 3D spatio-temporal context, enabling objective identification of convective clustered events (CCEs).
• First-time analysis of spatio-temporal characteristics (size, duration, and organizational forms) of CCEs in western and central Europe, moving beyond traditional grid-based assessments of thunderstorm activity.
• Identification of a significant negative trend in lightning activity and thunderstorm days in parts of western and central Europe, which contrasts with some proxy-based estimates, prompting questions about the comparability of different data sources.
• Establishment of a plausible link between observed changes in thunderstorm activity and organization (e.g., shift to smaller, more separated clusters) and large-scale teleconnection patterns, specifically the North Atlantic Oscillation (NAO).

Funding

• "ClimXtreme" project, subproject "VarCLuST", funded by the German Ministry of Education and Research (Bundesministerium für Bildung und Forschung, BMBF) under its "Research for Sustainability" (FONA) strategy.
• Grant no. 01LP1901A.
• Article processing charges covered by the Karlsruhe Institute of Technology (KIT).

Citation

@article{Augenstein2025Influence,
  author = {Augenstein, Markus and Mohr, Susanna and Kunz, Michael},
  title = {Influence of the North Atlantic Oscillation on annual spatio-temporal lightning clusters in western and central Europe},
  journal = {Natural hazards and earth system sciences},
  year = {2025},
  doi = {10.5194/nhess-25-4921-2025},
  url = {https://doi.org/10.5194/nhess-25-4921-2025}
}