Beyond localized hotspots: hazard connectivity in spatially compound rainfall-coastal storm events

Abstract Compound flooding is commonly assessed through the co-occurrence of multiple hazards at a single location. However, extreme weather systems can also generate simultaneous impacts across geographically separated regions, creating systemic challenges for emergency response and risk management. Here, we investigate spatially compound rainfall-coastal storm events along the ∼1600 km Spanish Mediterranean coastline to identify the atmospheric mechanisms governing their spatial organization. Using rainfall observations, wave reanalysis, and ERA5 atmospheric fields for 1980–2022, we show that spatially compound events emerge through two distinct modes of hazard connectivity. The first is a spatially coherent Catalonia-Valencia corridor extending over more than 600 km of coastline, where localized extreme rainfall occurs together with widespread coastal storms. The second is a remote Andalusia-Catalonia linkage, in which concurrent impacts occur in geographically distant regions. These contrasting connectivity modes arise from distinct synoptic-scale circulation regimes that differentially control rainfall and coastal storm development. A northern-blocking pattern, characterized by a cut-off low over the western Mediterranean and high pressure over northern Europe, simultaneously enhances moisture transport towards eastern Iberia and generates persistent easterly winds, producing the connected Catalonia-Valencia corridor. In contrast, Atlantic low-pressure systems generate heavy rainfall in southwestern Iberia while inducing southerly wave storms in the northwestern Mediterranean, explaining the remote linkage. Our results demonstrate that compound flood risk is governed not only by hazard interactions but also by hazard connectivity across space, providing a transferable framework for regional hazard assessment, early-warning systems, and climate adaptation.