Two models of the North Sea are compared with respect to their performance in the assessment of sea level changes in a changing climate: the OPYC model, originally designed as a global ocean model and run in a regional version, and the HAMSOM, which was built as a shelf sea model and applied to an extended region in a simplified version. Both models agree very well in their hindcast skill as measured by correlation and explained variance. The reaction of both models to the '2 〈 CO2' and 'control' runs of the time slice experiments of the DKRZ (German Climate Computing Center) is also very similar and indicates an increase of up to 30 cm of winter mean sea levels and no significant (compared with the variability in the past) impact on the intra-monthly 90% quantiles. It is concluded that sea level is a rather robust parameter that could still be assessed with some confidence if both models were simplified further.

The “spectral nudging” method imposes time-variable large-scale atmospheric states on a regional atmospheric model. It is based on the idea that regional-scale climate statistics are conditioned by the interplay between continental-scale atmospheric conditions and such regional features as marginal seas and mountain ranges. Following this “downscaling” idea, the regional model is forced to satisfy not only boundary conditions, possibly in a boundary sponge region, but also large-scale flow conditions inside the integration area. In the present paper the performance of spectral nudging in an extended climate simulation is examined. Its success in keeping the simulated state close to the driving state at larger scales, while generating smaller-scale features is demonstrated, and it is also shown that the standard boundary forcing technique in current use allows the regional model to develop internal states conflicting with the large-scale state. It is concluded that spectral nudging may be seen as a suboptimal and indirect data assimilation technique.

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.

In this article, the hydrographic conditions are described which prevailed in the German Bight during the three KUSTOS experiments carried out in summer 1994, spring 1995, and winter 1996. It presents the physical background for the companion articles of this volume and provides insight into the complicated and highly variable hydrodynamics of the German Bight. Typically for the German Bight, the distribution of temperature and salinity in the near-bottom layer was found to relate strongly to the topography in all experiments, and variability in the vertical structure showed the expected seasonal pattern. The thermal and haline stratification were found to coincide in most cases, indicating a clear marking of surface and bottom water masses. Between the three KUSTOS experiments, the intensity of salinity fronts varied strongly, with stronger fronts in spring and weaker gradients in summer and winter, but from the limited number of observations this pattern cannot be generalised. The observed temperature and/or salinity patterns differ strongly from the average long-term situation, as expected in a highly variable area. To a large degree, this variability is caused by mesoscale features which - owing to their transient nature - are not well resolved by in-situ observations during the experiments and, therefore, numerical models are used to complement the observations. Finally, the overall mass transport through the German Bight during the KUSTOS experiments, estimated from a North Sea circulation model and a salt water budget and freshwater balance, ranges between 34 (strong cyclonic circulation) and -3 km3/d (weak anticyclonic circulation). North Sea water advection into the German Bight dominates river runoff by a factor of ∼15 (spring 1995) to ∼49 (winter 1996). However, in situations with a low through-flow in combination with a high ratio of freshwater inflow, residence times of river water can be high, thereby aggravating any harmful effects of riverine contaminants.

Three different versions of a baroclinic three-dimensional circulation model of the North Sea are used to obtain information on the wind and density interactions in the North Sea ROFIs (Regions Of Freshwater Influence): the standard version with fully prognostic treatment of salinity and temperature is compared to a barotropic model run on the same grid on the one hand and to an also fully prognostic model run on a four times coarser grid on the other hand. In order to gain knowledge on the wind and density interactions, two opposing wind directions are chosen for investigation, namely a time of strong north wind, 21st–28th April 1982, and a time of strong southwest wind, 22nd–24th May 1982. In the April case the effect of the salinity gradients on the border of the ROFIs of Rhine, Weser, Ems and Elbe, i.e. along the continental shore, is shown to lead to a clear enhancement of the mean surface currents. In May this result is partly disguised by the additional effect of the thermocline in the deeper parts of the North Sea, i.e. in the classical shelf sea regime region. Nevertheless, the same pattern of enhanced mean surface currents along the coast is detected and is of the same order of magnitude as in the April case. It is thus concluded that although the circulation in the North Sea is reversed by the wind, the density induced component of the general circulation is modified only slightly.

Coastal currents that are forced by buoyancy discharges from rivers often show a tendency to become unstable under certain conditions. Two such coastal current systems in the North Sea, the German Bight and the Skagerrak, and their instabilities are investigated and compared here with the help of a fully nonlinear, three‐dimensional, baroclinic circulation model. Differences between the two regions were found with respect to three main aspects. First, in the Skagerrak, instabilities contribute to the kinetic energy on a scale that is comparable to the mean kinetic energy. This does not hold for the German Bight. Second, of the two forms of instabilities fed by baroclinic and barotropic energy conversion, in the Skagerrak, barotropic conversion is as important as baroclinic instability, while in the German Bight, baroclinic conversion clearly dominates. Finally, in the German Bight, instability tends to occur only if supported by wind in a direction that helps establishing a steep front against strong tidal mixing. One explanation for the distinction in the features of the two regions is the water depth (German Bight is <40 m; Skagerrak is ∼300 m) and, consequently, the amount of tidal mixing applied to the near‐surface coastal current front. Several deep and shallow water coastal currents outside the North Sea are found to exhibit features similar to those presented for the Skagerrak and German Bight.

The European project WASA (Waves and Storms in the North Atlantic) has been set up to verify or disprove hypotheses of a worsening storm and wave climate in the northeast Atlantic and its adjacent seas in the present century. Its main conclusion is that the storm and wave climate in most of the northeast Atlantic and in the North Sea has undergone significant variations on timescales of decades; it has indeed roughened in recent decades, but the present intensity of the storm and wave climate seems to be comparable with that at the beginning of this century. Part of this variability is found to be related to the North Atlantic oscillation.An analysis of a high-resolution climate change experiment, mimicking global warming due to increased greenhouse gas concentrations, results in a weak increase of storm activity and (extreme) wave heights in the Bay of Biscay and in the North Sea, while storm action and waves slightly decrease along the Norwegian coast and in most of the remaining North Atlantic area. A weak increase in storm surges in the southern and eastern part of the North Sea is expected. These projected anthropogenic changes at the time of CO2 doubling fall well within the limits of variability observed in the past.A major methodical obstacle for the assessment of changes in the intensity of storm and wave events are inhomogeneities in the observational record, both in terms of local observations and of analyzed products (such as weather maps), which usually produce an artificial increase of extreme winds. This occurs because older analyses were based on fewer observations and with more limited conceptual and numerical models of the dynamical processes than more recent analyses. Therefore the assessment of changes in storminess is based on local observations of air pressure and high-frequency variance at tide gauges. Data of this sort is available for 100 yr and sometimes more. The assessment of changes in the wave climate is achieved using a two-step procedure; first a state-of-the-art wave model is integrated with 40 yr of wind analysis; the results are assumed to be reasonably homogeneous in the area south of 70 degrees N and east of 20 degrees W; then a regression is built that relates monthly mean air pressure distributions to intramonthly percentiles of wave heights at selected locations with the help of the 40-yr simulated data; finally, observed monthly mean air pressure fields from the beginning of this century are fed into the regression model to derive best guesses of wave statistics throughout the century.

Coastal currents that are forced by buoyancy discharges from rivers often show a tendency to become unstable under certain conditions. Two such coastal current systems in the North Sea, the German Bight and the Skagerrak, and their instabilities are investigated and compared here with the help of a fully nonlinear, three-dimensional, baroclinic circulation model. Differences between the two regions were found with respect to three main aspects. First, in the Skagerrak, instabilities contribute to the kinetic energy on a scale that is comparable to the mean kinetic energy. This does not hold for the German Bight. Second, of the two forms of instabilities fed by baroclinic and barotropic energy conversion, in the Skagerrak, barotropic conversion is as important as baroclinic instability, while in the German Bight, baroclinic conversion clearly dominates. Finally, in the German Bight, instability tends to occur only if supported by wind in a direction that helps establishing a steep front against strong tidal mixing. One explanation for the distinction in the features of the two regions is the water depth (German Bight is <40 m; Skagerrak is ,--300 m) and, consequently, the amount of tidal mixing applied to the near-surface coastal current front. Several deep and shallow water coastal currents outside the North Sea are found to exhibit features similar to those presented for the Skagerrak and German Bight.