Investigation of North Sea high-frequency sea-level extremes using long-term sea-level measurements

Extreme sea-level events occur across a range of temporal and spatial scales, including high-frequency oscillations (periods T < 2 h). However, most research uses hourly or daily data and neglects higher-frequency processes such as seiches and meteotsunamis. This study investigates the characteristics of high-frequency (HF) sea-level extremes along the North Sea coast. Long-term (1993-2025) measurements were analysed from 29 tide gauges. After quality control, the astronomical tide was removed to isolate residuals, which were decomposed into low-frequency (T > 2 h) and high-frequency (T < 2 h) components. Next, extremes were extracted from the HF signal alone (HF extremes) and from the residual (residual extremes). Clustering techniques (K-medoids with Dynamic-Time-Warping and Euclidean distances) were applied to HF extremes to classify event types and identify regional patterns. HF extremes were grouped into six event types, and selected stations yielded five spatial clusters. Each event type was characterised by period, intensity, and its relationship to residual extremes. HF extremes were generally low compared with tidal ranges, and compound events (i.e., events in which an HF extreme coincided with a residual extreme) were infrequent (12.2% of HF extremes on average). Nevertheless, during compound events, alignment of HF and residual peaks substantially amplified sea levels. Spatial patterns highlighted the most intense events along the Dutch coast (IJmuiden Buitenhaven, Scheveningen, Brouwershavensche Gat, and Terschelling). These results were influenced by differences in local environmental conditions and sampling. Therefore, this study advances understanding of HF sea-level variability and compound events along the North Sea coast, supporting improved coastal hazard assessments. Supplementary Information: The online version contains supplementary material available at 10.1007/s11069-026-08343-y.