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 alon g the North Sea coast was investigated, with a statistical down-scaling tec hnique and with a dynamical model. Firstly, in an analysis of past variabil ity 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, Seco ndly, a fine-scale time slice experiment for a control run and a scenario f or doubled atmospheric carbon dioxide concentration was used (in both the d ynamical and the statistical down-scaling model) to assess the sea level re lated changes due to an (anthropogenic) increase in atmospheric carbon diox ide concentration. Both models agree on the following results: (a) In the p ast, the winter means of high water levels along the North Sea coast increa se 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 - sho w no clear trend; and (c) the British Coast exhibits a slight negative and the continental coast an equally small positive tendency. In the climate sc enarios, the effect on the high percentiles follows the same pattern. A sli ghtly larger and everywhere positive difference is diagnosed in the mean wa ter levels along the North Sea coast. Together with the hindcast result, th e above interpretations might suggest a continuing increase of mean water l evels 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 c learly identify such a process, or attribute it to anthropogenic developmen t. 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. (C) 1999 Elsevier Science Ltd. All rights reserved.