The role of preconditioning for extreme storm surges in the western Baltic Sea

Abstract. When natural hazards interact in compound events, they may reinforce each other. This is a concern today and in the light of climate change. In the case of coastal flooding, sea-level variability due to tides, seasonal to inter-annual salinity and temperature variations or larger--scale wind conditions modify the development and ramifications of extreme sea levels. Here, we explore how prior conditions influence peak water levels for the devastating coastal flooding event in the western Baltic Sea in 1872. By imposing a range of antecedent conditions in numerical ocean model simulations, we quantify the change in peak water levels that arise due to alternative preconditioning of the sea level before the storm surge. Our results show that different preconditioning could have generated even more catastrophic impacts. As an example, a simulated increase of 36 cm compared to the 1872 event was seen in Køge just south of the Danish capital region – a region that was already severely impacted. The increased water levels caused by the alternative water mass distributions propagate until encountering shallow and narrow straits, thereafter the effect vastly decreases. Adding artificial increases in wind speeds to each study point location reveals a near-linear relationship with peak water levels for all Western Baltic locations highlighting the need for good assessments of future wind extremes. Our research indicates that a more hybrid approach to analysing compound events, and readjusting our present warning system to a more contextualised framework, might provide a firmer foundation for climate adaptation and disaster risk management.