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

Hydrometeorological extremes reflect rainfall intensity, persistence, and interactions among atmospheric variables. Quality-controlled daily observations from Chã de Macela (São Miguel Island, Azores; 2011–2023) were used to examine precipitation extremes, wet and dry spells, and compound events. The primary same-day definition (precipitation ≥ P95, wind speed ≥ P90, and relative humidity ≥ P90) identified eight events. Six events (75%), 92 of 233 P95 days (39.5%), and 22 of 47 P99 days (46.8%) occurred within persistent wet spells. Sensitivity tests identified four same-day events using wet-day precipitation percentiles; allowing ±1-day alignment yielded 15 and six events under full-record and wet-day precipitation thresholds, respectively. Moving-block bootstrap comparisons of ERA5 data showed that P95 days had mean sea-level pressure 5.45 hPa lower and total column water vapour 3.69 kg m−2 higher than non-extreme wet days. Within the P95 set, compound days had pressure 4.49 hPa lower than non-compound days (95% CI: −9.04 to −0.69), whereas the water-vapour contrast was inconclusive. Visual examination suggested three descriptive configurations: persistent, vertically coherent cyclonic systems; pronounced meridional gradients in surface pressure and geopotential height; and a developing large-scale disturbance. These results describe persistence and synoptic context but do not establish predictive compound-event signatures.