Seasonal predictability of coastal risks from climate modes compounded effects

Extreme weather and climate events result from complex interactions between physical processes at different scales. The convergence of multiple factors, including large-scale environmental conditions and local climate variability, can amplify the effects, resulting in significant societal impacts. Coastal regions are particularly vulnerable to sea level rise and changes in coastal water levels (CWL) due to climate variability, ocean circulation, and atmospheric conditions. The El Niño/Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) are key drivers of interannual CWL variability in the Northern hemisphere, influencing storm activity, flooding, and erosion, with ENSO affecting the Pacific and NAO the Atlantic. While studies have extensively analyzed their independent effects, their combined influence on coastal hazards remain underexplored. This study uses diverse observational datasets to assess the modulation of extreme CWL and associated hazards by different phases of ENSO and NAO. We show that the frequent occurrence of La Niña conditions, although relatively weak in terms of severity, and the comparatively rare but exceptionally strong extreme El Niño events make the world's coastlines more vulnerable to flooding overall. However, the picture is different regionally, especially in the Euro-Atlantic sector, where the co-occurrence of El Niño events and different phases of the NAO tends to exacerbate extreme CWL compared to the local NAO variability alone due to the strengthening of the Pacific-Atlantic jet stream teleconnections either in the high or mid latitudes, depending on the ENSO type and the NAO phase. These results highlight the climate modes’ compounded risks to coastal populations that allows us to produce skillful seasonal forecasting of coastal hazards using the newly developed XRO model.