Abstract
Abstract. Extreme high sea levels (ESLs) caused by storm floods constitute a major hazard for coastal regions. We here quantify their long-term variability in the southern German Bight using simulations covering the last 1000 years. To this end, global earth system model simulations from the PMIP3 past1000 project are dynamically scaled down with a regionally coupled climate system model focusing on the North Sea. This approach provides an unprecedented long high-resolution data record that can extend the knowledge of ESL variability based on observations, and allows for the identification of associated large-scale forcing mechanisms in the climate system. While the statistics of simulated ESLs compare well with observations from the tide gauge record at Cuxhaven, we find that simulated ESLs show large variations on interannual to centennial timescales without preferred oscillation periods. As a result of this high internal variability, ESL variations appear to a large extent decoupled from those of the background sea level, and mask any potential signals from solar or volcanic forcing. Comparison with large-scale climate variability shows that periods of high ESL are associated with a sea level pressure dipole between northeastern Scandinavia and the Gulf of Biscay. While this large-scale circulation regime applies to enhanced ESL in the wider region, it differs from the North Atlantic Oscillation pattern that has often been linked to periods of elevated background sea level. The high internal variability with large multidecadal to centennial variations emphasizes the inherent uncertainties related to traditional extreme value estimates based on short data subsets, which fail to account for such long-term variations. We conclude that ESL variations as well as existing estimates of future changes are likely to be dominated by internal variability rather than climate change signals. Thus, larger ensemble simulations will be required to assess future flood risks.
Highlights
Inundation due to storm floods is one of the major geophysical risks in coastal regions and bears high damage potential for coastal environments, in both natural and socioeconomic terms
While the statistics of simulated Extreme high sea levels (ESLs) compare well with observations from the tide gauge record at Cuxhaven, we find that simulated ESLs show large variations on interannual to centennial timescales without preferred oscillation periods
Setting recent ESL trends from the observational record (5.7 ± 4.3 cm decade−1 for the 99.9th percentile of hourly sea level at Cuxhaven from 1953 to 2008; see Mudersbach et al, 2013) into context with the simulated ESL variability shows that the trends lie within the internal variability obtained from the long-term simulation: using a running trend with a window length similar to the aforementioned observational data (55 years) over the 1000 years of simulated data, a trend of the same or higher magnitude occurs in roughly 10 % of the segments
Summary
Inundation due to storm floods is one of the major geophysical risks in coastal regions and bears high damage potential for coastal environments, in both natural and socioeconomic terms. This is especially important for low-lying regions such as coasts and estuaries of the southern North Sea and, in particular, the German Bight. The great flood in 1962 resulted in a high death toll and vast economic loss along the coastal regions of Germany and in particular the city of Hamburg.
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