Hydrometeorological Persistence and Compound Extremes in the Azores: Observational Evidence and Synoptic Context from ERA5 Reanalysis

Hydrometeorological extremes in oceanic island environments are influenced not only by rainfall intensity but also by wet-condition persistence and the interaction of multi-ple atmospheric drivers. This study investigates extreme, persistent, and compound hydrometeorological events at the Chã de Macela station (São Miguel Island, Azores) using a quality-controlled daily dataset for 2011–2023. Compound events were defined as days simultaneously exceeding the precipitation P95 threshold (95 th percentile), wind-speed P90 threshold (90 th percentile), and relative-humidity P90 threshold. Eight compound events were identified. Results revealed a strong relationship between persistence and hydrometeorological severity, with 75% of compound events and nearly half of all P99 (99 th percentile) precipitation events occurring within persistent wet spells. This indicates that the most severe hydrometeorological conditions gener-ally develop within pre-existing humid regimes rather than as isolated rainfall epi-sodes. ERA5 reanalysis data were used to analyse mean sea level pressure, total column water vapour, and synoptic circulation patterns. Extreme precipitation events were associ-ated with significantly lower pressure and higher atmospheric moisture than ordinary conditions (Mann-Whitney test, p < 0.001). Synoptic analysis revealed multiple at-mospheric pathways, including cyclonic systems, strong meridional pressure gradi-ents, and developing large-scale disturbances. These findings demonstrate that com-pound hydrometeorological extremes in the Azores arise through different synoptic mechanisms but are consistently linked to strong atmospheric forcing over the North Atlantic.