Long-term changes in the frequency, intensity and duration of extreme storm surge events in southern Europe

Storm surges are one of the major hazards in coastal regions; positive surge events are added to tidal levels, increasing the risk of coastal flooding by extreme water levels. In this study, changes in the frequency (occurrence rate per year), intensity (magnitude of the extremes) and duration of extreme storm surge events from 1948 to 2013 are investigated using a non-stationary statistical model. To fully model extremes, the time-dependent statistical model combines the Generalized Pareto Distribution (GPD) for studying exceedances over the threshold, and the non-homogeneous Poisson (P) process for studying the occurrence rate of these exceedances. Long-term trends and the association between storm surges and the North Atlantic Oscillation (NAO) are represented in the model by allowing the parameters in the GPD-P model to be time-dependent. Different spatial patterns in the three analysed properties of storm surges are found in the Atlantic region and the Mediterranean Sea. The up to now uncharted regional patterns of storm surge duration show completely different values between the Atlantic and the Mediterranean regions, being the duration of storms surges in the Atlantic two times longer than the duration in the Mediterranean. For the last half century, we detect positive and negative spatial trends in terms of intensity of storm surge but only significant decreasing rates, of around 2 %, in the number of extreme events per year. Regarding duration, we find positive trends in certain Mediterranean areas, with durations of extreme events increasing at a rate of 0.5-1.5 h/year. Values for the 50-year return level are also estimated, showing a large spatial variability with relatively higher values along the coast. A clear sensitivity of extreme storm surges to negative NAO index is detected, specifically in the western Mediterranean basin. Results show that negative NAO phases lead to an increase in the number of extreme events and also in their intensity.