Storm-related sea level variations along the North Sea coast: natural variability and anthropogenic change

The influence of a changing wind climate on the sea surface elevations along the North Sea coast was investigated, with a statistical down-scaling technique and with a dynamical model. Firstly, in an analysis of past variability the two models were run for different periods: the numerical model for the winters 1955–1993 and the statistical one for the winters 1899–93. Secondly, a fine-scale time slice experiment for a control run and a scenario for doubled atmospheric carbon dioxide concentration was used (in both the dynamical and the statistical down-scaling model) to assess the sea level related changes due to an (anthropogenic) increase in atmospheric carbon dioxide concentration. Both models agree on the following results: (a) In the past, the winter means of high water levels along the North Sea coast increase on the order of 1–2 mm/yr, on account of only the atmospheric forcing; (b) the high intramonthly percentiles – reduced by the winter averages – show no clear trend; and (c) the British Coast exhibits a slight negative and the continental coast an equally small positive tendency. In the climate scenarios, the effect on the high percentiles follows the same pattern. A slightly larger and everywhere positive difference is diagnosed in the mean water levels along the North Sea coast. Together with the hindcast result, the above interpretations might suggest a continuing increase of mean water levels at the North Sea coast due to an increasing CO2 concentration in the atmosphere; this would occur throughout the second half of this century and, possibly, beyond. The high frequency variability appears to be much less affected. However, the natural variability of the system is too strong to clearly identify such a process, or attribute it to anthropogenic development. This study evaluates the impact of a changing atmospheric forcing only. Additional influences, such as the eustatic and isostatic effects, are not taken into account.