Understanding coastal-pluvial compound floods associated with extra-tropical cyclones in Denmark

Compound events leading to significant coastal flooding have become a major concern in recent years. Storm surges caused by extra-tropical cyclones and coastal precipitation are key drivers of such events. While previous research has developed various methods for storm tracking, these have not sufficiently focused on impact-relevant storm tracking that directly addresses coastal flood risks. Motivated by this research gap, we initiated our analysis by identifying storm surges from 1991 to 2021 and tracked associated low-pressure systems and the associated impact-related compound dynamics. This approach not only enables the understanding of storm impacts but also offers potential for application to downscaled regional climate-scale products.

As a case study, we use Denmark, known for its complex ocean-circulation patterns due the North-Sea/Baltic Sea interface, narrow straits and fjords, and diverse coastline orientation. We clustered 32 sea level stations in Denmark by analyzing the co-occurrence of extreme storm surge events in the period 1990-2023. We then tracked extra-tropical cyclones over Northern Europe using the CERRA mean sea level pressure dataset, by identifying minimas at each time interval and reconstructing tracks by minimizing the distance between candidate points, through the use of Mixed Integer Programming. Finally, we investigate coastal precipitation in different regions of Denmark, as defined by the clustering of sea water level stations, with precipitation estimates from the CERRA-Land dataset.

Our analysis successfully identified storm tracks associated with extreme storm surge events, which were categorized into four distinct clusters. Similarly, Danish water level stations were grouped into three clusters based on the co-occurrence of extreme surge events: (1) the West coast of Jutland, (2) Kattegat and Inner Danish water, and (3) Baltic sea coastlines. By examining the dominant storm track types and station clusters, we revealed significant differences in impacted regions associated with different storm tracks.

We conclude that storm tracks have markedly different impacts on the occurrence of storm surge events across the Danish sub-regions. Precipitation levels associated with these storm surge events, and type of storm track, can uncover the need to consider both storm track characteristics and regional vulnerabilities when assessing compound and multi-variate coastal flood risks as opposed to storm surges in isolation.