Compound coastal flooding in high-latitude Baltic Sea estuaries of Finland: Patterns, seasonality and trends

Abstract

<p><strong>Study region</strong></p><p>Baltic Sea estuaries of Finland, 60–66° N.</p><p><strong>Study focus</strong></p><p>Joint sea level–streamflow peaks may cause severe and underestimated flood hazard in estuaries, and their likelihood is modified by climate change. Information is urgently needed on compound flood potential in the high-latitude estuaries, and its latitudinal, seasonal, and decadal variation. We address this by comparing sea level and streamflow data for 11 sites along the Finnish coast in 1971–2024.</p><p><strong>New hydrological insights for the region</strong></p><p>The co-variability between sea level and streamflow is low in northernmost Finland and highest in the south-west. Compound flood events are most frequent in south-west and their return periods increase northward and eastward towards the Baltic Sea gulfs. In the north, sea level and streamflow peaks are seasonally offset, occurring during autumn and winter, and during spring snowmelt season, respectively. Potentially, more joint peaks occur in the south because catchments often remain unfrozen and receive some of the precipitation as rain during the winter months. The hydrological winter is beginning increasingly late in the north, which may have led to more frequent compound flood events. While the inferred patterns are moderated by regional factors (Baltic Sea oceanography and climatology), we hypothesize that the compound flood probability in the high-latitude estuaries is impacted by the state of precipitation during the storm season, and therefore highly sensitive to the changing climate.</p><p><br></p>