Geographical diversity in the occurrence of extreme sea levels on the coasts of the Baltic Sea

Fluctuating sea levels are an important element of hydrodynamic processes that occur in coastal regions. Extreme sea levels – i.e. the highest and lowest levels recorded over many years in a given year or in a given storm event – can stimulate currents, sea abrasion, and the accumulation of sedimentary material in various coastal sections. The objective of this study was to examine the geographical diversity of extreme sea levels along the entire Baltic Sea coast based on hourly sea levels from 37 tide gauges between 1960 and 2010. In this study, 8 Baltic Sea sub-basins (water regions) with similar sea level fluctuation patterns were identified using cluster analysis. Based on a quantitative analysis of the number of hours with high and low sea levels and analysing the number of storm surges and regularities in the distribution of extreme sea levels, certain geographical patterns were identified. The final stage of the study was to determine the degree of diversity of the individual sub-basins of the Baltic Sea as a function of the intensity of extreme sea levels during a year. Additionally, this classification considered the specific features of each basin, namely the exposure of coasts to the paths of dangerous cyclones, bathymetric relations, as well as the location of tide gauges relative to the open waters of the Baltic Sea. The results of the analyses indicate that the highest extreme sea levels were most intense and lasted longer in sections of great bays located farthest inland, off the northeastern and eastern coasts of the Baltic Sea – the Gulf of Finland, the Gulf of Riga, and the Bothnia Bay. Sub-basins of the Western Baltic, in particular Mecklenburg and Kiel Bays, were among the areas with the deepest negative storm surges and the most frequent low and very low sea levels. The sub-basins of the Swedish coasts of the Central and Northern Baltic were the least exposed to extreme sea levels (both high and low) and had the lowest number of storm surges per year.