Contribution of high-frequency sea level oscillations to the sea level extremes along European coasts

Extreme sea levels can result in catastrophic flooding of coastal regions, endangering the lives of residents and destroying coastal infrastructure. Due to climate change they are becoming more frequent and therefore more dangerous. Extreme sea levels occur on different temporal and spatial scales, including sub-hourly scales, which we have only recently been able to assess due to the recent enhancement of the temporal resolution of the tide gauge measurements. To quantify the contribution of sub-hourly sea level oscillations to positive sea level extremes, raw sea level data from 288 tide gauges along the European coasts, with a sampling resolution of less than 20 minutes, were obtained from: (1) the IOC-SLSMF website (263 stations); (2) National agencies (Portugal, Finland, Croatia – 24 stations). Large portions of the raw dataset had numerous data quality issues (i.e., spikes, shifts, drifts), thus quality control procedure was required. Out of range values and spikes were automatically removed, remaining data were visually examined, and spurious data were removed manually. After quality control, all data series were de-tided, and residuals were split into a low-frequency (T > 2 h) and a high-frequency (T < 2 h) component. The five highest positive sea level extremes per year were extracted from the residual series and the high-frequency series. These were defined as residual extremes and high-frequency extremes, respectively. The contribution of the high-frequency sea level oscillations to the total sea level extremes along the European coasts was estimated. The contribution was shown to be significantly geographically and station-dependent, and it is important to take it into account when estimating flooding levels.