Simulated extreme sea levels at Helsinki

We present an assessment of the exceedance probabilities of sea levels at Helsinki, on the coast of the Gulf of Finland in the Baltic Sea, based on an 850-year numerical sea-level simulation. The internal sea-level variations in the Baltic Sea are calculated with a two-dimensional hydrodynamic model, whereas the variations in the Baltic Sea water volume are evaluated using a statistical model based on the geostrophic wind speeds near Bornholm. The atmospheric data used for the sea-level simulation is taken from downscaled regional climate scenario simulations. The simulated sea-level extremes at Helsinki in the current climate are slightly smaller than the previous estimates which were based on measured data only. The extrapolation of simulated data gives an estimate of sea level 227 cm at Helsinki for the exceedance frequency of 10–4 events per year. The sum of the maxima of the model components during a short 30-year verification period indicates that sea level 225 cm is possible at Helsinki if all the components simultaneously attain their maxima.