Changing storminess? An analysis of long-term sea level data sets

This paper considers the analysis of long-term observational sea level data sets within the context of a possible change in storminess over North-West Europe. Sea level variations are studied as a proxy for storminess, i.e, in the synoptic frequency band. In that band, a proxy correspondence is assumed between sea levels and wind stress, although with some damping of the highest frequencies. Trends and variability over the past 100 yr are studied. However, a complicating factor is that many data sets include astronomical tidal influences (e.g. spring tide -neap tide cycle), which mask the signal that is of most interest in this study. Sea level data sets from stations in the coastal zones of North-West Europe were collected, homogenised and made free of auto-correlation. To analyse these data sets, the 'quantile analysis method' is presented. This method involves an advanced analysis technique which, on the basis of a frequency analysis, determines the roughness/smoothness of succeeding decades in relation to the complete time period of a data set. Possible storm-related trends and variations in this decadal quantity are defined by means of linear regression. Besides this main method, a time-shift variant of the quantile analysis method is applied in order to discuss the sensitivity of the obtained results. This sensitivity is also investigated with respect to the length of the time period in which a complete data set is split up. Although the analysis results show considerable natural variability on relatively short (decadal) time scales, no sign of a significant increase in storminess over North-West Europe is detected over the complete time period of the data sets. The results indicate a distinction between stations in the German Eight and stations in the southern part of the North Sea. In the latter area, the natural variability is more moderate and there appears to be a tendency towards a weakening of the storm activity over the past 100 yr (not significant). Stations in the German Eight show a more enhanced natural variability on relatively short (decadal) time scales, with no indication of a weakening of the storm climate.