Ralf Weisse

Das Projekt EXTREMENESS verfolgte das Ziel, extreme Nordseesturmfluten zu identifizieren und zu beschreiben, die zum einen extrem unwahrscheinlich, zum anderen aber noch physikalisch plausibel sind. Darauf aufbauend wurden in einem transdisziplinären Ansatz mögliche Auswirkungen untersucht und diskutiert, welche Maßnahmen und Handlungsoptionen im Umgang mit solchen Extremereignissen existieren. EXTREMENESS leistet damit wichtige Beiträge zur Diskussion über Anpassung, Formen und Notwendigkeiten eines zukünftigen Küstenschutzes und Risikomanagements.

The paper describes the initial efforts in a project whose objective is to obtain a 40-year hindcast of wind, sea level and wave climatology for European waters. The 40-year global atmospheric re-analysis carried out by the National Centre for Environmental Prediction, Washington, USA (NCEP) and the National Centre for Atmospheric Research, Boulder, Colorado, USA (NCAR) will be used as forcing of limited area atmospheric models. The fine grid atmospheric fields will be used to force state-of-the-art wave models (WAM) and sea level models (HAMSOM and TELEMAC) in regional areas around Europe so as to produce climatic information on waves, sea levels, and currents in a very large extend of the European waters, including the Mediterranean, North East Atlantic and North Sea. The available satellite data, including wind, wave and sea-level data, will be collected and will be used to be compared with the hindcast results, so as to yield uncertainty measures related to the data. Statistical...

A multi-decadal medium-resolution met-ocean hindcast for the North Sea and parts of the Northeast Atlantic is presented. The hindcast is based on a dynamical downscaling of the global NCEP/NCAR weather re-analyses using some simple data assimilation techniques. It is shown that the reconstructed wind, wave and storm surge climate agree reasonably with available in-situ observations. Analysis of the wind, wave and storm surge climate based on hindcast data reveals that they have undergone considerable variations from year to year and on longer time scales. An increase in storm activity from the beginning of the hindcast period has levelled off later and was replaced by a downward trend over the northeast North Atlantic. This behaviour closely corresponds to that based on the analysis of proxies for storm activity. Changes in extreme wave and storm surge conditions show a similar pattern over much of the North Sea area.

<p>Assessing past storm activity provides valuable knowledge for economic and ecological sectors, such as the renewable energy sector, insurances, or health and safety. However, long time series of wind speed measurements are often not available as they are usually hampered by inhomogeneities due to changes in the surroundings of a measurement site, station relocations, and changes in the instrumentation. On the contrary, air pressure measurements provide mostly homogeneous time series as the air pressure is usually unaffected by such factors.</p><p>Therefore, we perform statistical analyses on historical pressure data measured at several locations within the German Bight (southeastern North Sea) between 1897 and 2018. We calculate geostrophic wind speeds from triplets of mean sea level pressure observations that form triangles over the German Bight. We then investigate the evolution of German Bight storminess from 1897 to 2018 through analyzing upper quantiles of geostrophic wind speeds, which act as a proxy for past storm activity. The derivation of storm activity is achieved by enhancing the established triangle proxy method via combining and merging storminess time series from numerous partially overlapping triangles in an ensemble-like manner. The utilized approach allows for the construction of robust, long-term and subdaily German Bight storminess time series. Further, the method provides insights into the underlying uncertainty of the time series.</p><p>The results show that storm activity over the German Bight is subject to multidecadal variability. The latest decades are characterized by an increase in activity from the 1960s to the 1990s, followed by a decline lasting into the 2000s and below-average activity up until present. The results are backed through a comparison with reanalysis products from four datasets, which provide high-resolution wind and pressure data starting in 1979 and offshore wind speed measurements taken from the FINO-WIND project. This study also finds that German Bight storminess positively correlates with storminess in the North-East Atlantic in general. In certain years, however, notably different levels of storm activity in the two regions can be found, which likely result from shifted large-scale circulation patterns.</p>

Anthropogenic climate change may cause long-term changes in the storm conditions of the North Sea. This could lead to an increased endangerment of human safety and activities. To estimate possible future wind-induced changes of the water levels, the effects of four transient climate realizations for 2001-2100 are analysed in comparison to control simulations presenting conditions for 1960-2000. Regionalized pressure and

Im Rahmen des Verbundprojekts „KLIWAS - Auswirkungen des Klimawandels auf Wasserstraßen und Schifffahrt – Entwicklung von Anpassungsoptionen“ des Bundesministeriums für Verkehr, Bau und Stadtentwicklung (BMVBS) wurden im Rahmen einer Kooperation zwischen dem Helmholtz-Zentrum Geesthacht Zentrum für Material und Küstenforschung GmbH (HZG) und dem Bundesamt für Seeschifffahrt und Hydrographie (BSH) Seegangsszenarien für die Nordsee für unterschiedliche Klimaprojektionen erstellt. Eine genaue Abschätzung von möglichen Seegangsänderungen in der Nordsee durch Klimaänderung ist wichtig, um mögliche Auswirkungen sowohl für den Offshore- als auch für den Küstenbereich frühzeitig zu erkennen und gegebenenfalls Anpassungsoptionen einleiten zu können. Die Untersuchungen wurden in drei Abschnitten durchgeführt: Im ersten Abschnitt wurden detaillierte Seegangsberechnungen bis 2100 mit Hilfe der Version 4.5.3 des Wellenmodells WAM durchgeführt. Dabei wurden Windfelder aus zwei unterschiedlichen r...

Changes in global wave climate and its impacts received only minimal attention in the IPCC Fourth Assessment Report (AR4). Discussions provided are mostly restricted to the long-term variability of the significant wave height based on visual estimates from voluntary observing ships while other components of the wave climate are mostly ignored. Despite some attempts in studying the impact of a warmer climate on the global wave field based on statistical projections, and some recent regional dynamical projections using regional climate models to force wave models, a coherent global modelling study of the future changes in the global wave climate is still lacking. Tropical and extra-tropical cyclones are the main generating forces behind the global wave field. Recent studies, based on runs with the high resolution (T213; 63 km) version of the ECHAM5 global climate model revealed that in a warmer climate extra-tropical storms will not necessarily get more intense. On the other hand a po...

Wind-generated waves at the sea surface are of outstanding importance for both their practical relevance in many aspects, such as coastal erosion, protection, or safety of navigation, and for their scientific relevance in modifying fluxes at the air–sea interface. So far, long-term changes in ocean wave climate have been studied mostly from a regional perspective with global dynamical studies emerging only recently. Here a global wave climate study is presented, in which a global wave model [Wave Ocean Model (WAM)] is driven by atmospheric forcing from a global climate model (ECHAM5) for present-day and potential future climate conditions represented by the Intergovernmental Panel for Climate Change (IPCC) A1B emission scenario. It is found that changes in mean and extreme wave climate toward the end of the twenty-first century are small to moderate, with the largest signals being a poleward shift in the annual mean and extreme significant wave heights in the midlatitudes of both he...

Global atmospheric reanalyses have become a common tool for both validation of climate models and diagnostic studies, such as assessing climate variability and long-term trends. Presently, the Twentieth Century Reanalysis (20CR), which assimilates only surface pressure reports, sea ice, and sea surface temperature distributions, represents the longest global reanalysis dataset available covering the period from 1871 to the present. Currently the 20CR dataset is extensively used for the assessment of climate variability and trends. Here, the authors compare the variability and long-term trends in northeast Atlantic storminess derived from 20CR and from observations. A well-established storm index derived from pressure observations over a relatively densely monitored marine area is used. It is found that both variability and long-term trends derived from 20CR and from observations are inconsistent. In particular, both time series show opposing trends during the first half of the twent...

A compilation of coastal weather analyses and climate change scenarios for the future for Northern Europe from various sources is presented. They contain no direct measurements but results from numerical models that have been driven either by observed data in order to achieve the best possible representation of observed past conditions or by climate change scenarios for the near future. A comparison with the limited number of observational data points to the good quality of the model data in terms of long-term statistics such as multi-year return values of wind speed and wave heights. These model data provide a unique combination of consistent atmospheric, oceanic, sea state and other parameters at high spatial and temporal detail, even for places and variables for which no measurements have been made. In addition, coastal scenarios for the near-future complement the numerical analyses of past conditions in a consistent way. The backbones of the data are regional wind, wave and stor...

Abstract. Storm surges represent a major threat to many low-lying coastal areas in the world. While most places can cope with or are more or less adapted to present-day risks, future risks may increase from factors such as sea level rise, subsidence, or changes in storm activity. This may require further or alternative adaptation and strategies. For most places, both forecasts and real-time observations are available. However, analyses of long-term changes or recent severe extremes that are important for decision-making are usually only available sporadically or with substantial delay. In this paper, we propose to contextualize real-time data with long-term statistics to make such information publicly available in near real-time. We implement and demonstrate the concept of a ”storm surge monitor” for tide gauges along the German North Sea and Baltic Sea coasts. It provides automated near real-time assessments of the course and severity of the ongoing storm surge season and its singl...

The effect of large scale atmospheric pressure changes on regional mean sea level projections in the German Bight in the 21 century are considered. A developed statistical model is applied to climate model data of sea level pressure for the 21 century to assess the potential contribution of large scale atmospheric changes to future sea level changes in the German Bight. Using 78 experiments an ensemble mean of 1.4 cm rise in regional mean sea level is estimated until the end of the 21 century. Changes are somewhat higher for realisations of the special report on emission scenarios (SRES) A1B and A2, but generally do not exceed a few centimeters. This is considerably smaller than changes expected from steric and self-gravitational effects. Large scale changes in sea level pressure are thus not expected to provide a substantial contribution to 21 century sea level changes in the German Bight.

Project: coastDat-1 - CoastDat (http://www.coastdat.de/about_us/index.php) comprises a compilaton of coastal analyses (that is hindcasts and reconstructions) and scenarios for the future obtained from numerical models. The objective is to provide a consistent meteorological-marine data set. Summary: The data are from multi-decadal hindcast simulation with the wave model WAM 4.5 covering the entire Baltic Sea using a grid size of about 5.5x5.5 km (0.05 degrees latitude x 0.10 degrees longitude). The hindcast covers the period 1958-2002. Integrated parameter derived from 2D spectra are available every hour; the wave spectra are available with a 3-hour time step. Atmospheric forcing was obtained from an atmospheric hindcast with SN-REMO (http://dx.doi.org/10.1594/WDCC/coastDat-1_SN-REMO) driven by the NCEP/NCAR Reanalysis 1 data set (Kalnay et al.,1996). Lateral boundary conditions were obtained from corresponding hindcast for the southern North Sea driven by the same atmospheric forci...

Storms represent a major environmental threat. At sea, wind pushes water masses towards the coast where water levels may become dangerously high eventually overwhelming coastal defences and inundating low-lying coastal areas. Also, the sea-surface is affected where wind waves and swell are generated and obviously represent a threat to offshore navigation, shipping and coastal defence. We review a number of questions related to windstorms in the Northeast Atlantic and the North Sea, in particular: (1) How to determine decadal and longer variations in the storm climate? The methodological problem is that many variables suffer from inhomogeneities or are available for too short periods only. Proxies are suggested from which useful information can be derived. (2) How has the storm climate developed in the recent past? It is shown that an increase from about the 1960s has replaced a downward trend since about 1890. Storm activity is decreasing again since about 19901995. (3) How did stor...

. Based on the Baltic Earth Assessment Reports of this thematic issue in Earth System Dynamics and recent peer-reviewed literature, current knowledge about the effects of global warming on past and future changes in climate of the Baltic Sea region is summarized and assessed. The study is an update of the Second Assessment of Climate Change (BACC II) published in 2015 and focusses on the atmosphere, land, cryosphere, ocean, sediments and the terrestrial and marine biosphere. Based on the summaries of the recent knowledge gained in paleo-, historical and future regional climate research, we find that the main conclusions from earlier assessments remain still valid. However, new long-term, homogenous observational records, e.g. for Scandinavian glacier inventories, sea-level driven saltwater inflows, so-called Major Baltic Inflows, and phytoplankton species distribution and new scenario simulations with improved models, e.g. for glaciers, lake ice and marine food web, have become available. In many cases, uncertainties can now be better estimated than before, because more models can be included in the ensembles, especially for the Baltic Sea. With the help of coupled models, feedbacks between several components of the Earth System have been studied and multiple driver studies were performed, e.g. projections of the food web that include fisheries, eutrophication and climate change. New data sets and projections have led to a revised understanding of changes in some variables such as salinity. Furthermore, it has become evident that natural variability, in particular for the ocean on multidecadal time scales, is greater than previously estimated, challenging our ability to detect observed and projected changes in climate. In this context, the first paleoclimate simulations regionalized for the Baltic Sea region are instructive. Hence, estimated uncertainties for the projections of many variables increased. In addition to the well-known influence of the North Atlantic Oscillation, it was found that also other low-frequency modes of internal variability, such as the Atlantic Multidecadal Variability, have profound effects on the climate of the Baltic Sea region. Challenges were also identified, such as the systematic discrepancy between future cloudiness trends in global and regional models and the difficulty of confidently attributing large observed changes in marine ecosystems to climate change. Finally, we compare our results with other coastal sea assessments, such as the North Sea Region Climate Change Assessment (NOSCCA) and find that the effects of climate change on the Baltic Sea differ from those on the North Sea, since Baltic Sea oceanography and ecosystems are very different from other coastal seas such as the North Sea. While the North Sea dynamics is dominated by tides, the Baltic Sea is characterized by brackish water, a perennial vertical stratification in the southern sub-basins and a seasonal sea ice cover in the northern sub-basins.

Long and consistent wave data are important for analysing wave climate variability and change. Moreover, such statistics are also needed in coastal and offshore design and for addressing safety-related issues at sea. Using the third-generation spectral wave model WAM a multi-decadal wind-wave hindcast for the North Sea covering the period 1949–2014 was produced. The hindcast is part of the coastDat database representing a consistent and homogenous met-ocean data set. It is shown that despite not being perfect, data from the wave hindcast are generally suitable for wave climate analysis. In particular comparisons of hindcast data with in situ and satellite observations show on average a reasonable agreement while a tendency towards overestimation of the highest waves could be inferred. Despite these limitations, the wave hindcast still provides useful data for assessing wave climate variability and change as well as for risk analysis, in particular when conservative estimates a...

Geostrophic wind speeds calculated from mean sea level pressure readings are used to derive time series of northeast Atlantic storminess. The technique of geostrophic wind speed triangles provides relatively homogeneous long-term storm activity data and is thus suited for statistical analyses. This study makes use of historical air pressure data available from the International Surface Pressure Databank (ISPD) complemented with data from the Danish and Norwegian Meteorological Institutes. For the first time, the time series of northeast Atlantic storminess is extended until the most recent year available, that is, 2016. A multidecadal increasing trend in storm activity starting in the mid-1960s and lasting until the 1990s, whose high storminess levels are comparable to those found in the late nineteenth century, initiated debate over whether this would already be a sign of climate change. This study confirms that long-term storminess levels have returned to average values in recent ...

... jaw@pol.ac.uk) Val Swail (val.swail@ec.gc.ca) Natacha B. Bernier (natacha.bernier@phys.ocean. dal.ca) Sergey Gulev (gul@sail.msk.ru) Kevin J. Horsburgh (kevinh@pol.ac.uk) Alakkat S.Unnikrishnan (unni@nio.org) John R. Hunter (john.hunter@utas.edu.au) Ralf Weisse ...

In the following we describe some basic concepts that are fundamental for understanding (marine) climate and climate variability. We begin with a brief historical review on earlier and more modern concepts of climate and subsequently define marine weather and marine climate as used throughout the remainder of this book (Section 1.2). In Section 1.3 an overview of the present understanding of the global climate system is provided. In particular, the components of the climate system and the general circulations of the atmosphere and the oceans are addressed. We further show that the planetary-scale features of atmospheric circulation may be determined without any knowledge on regional details. In Section 1.4 concepts for understanding observed climate variability are discussed. Here emphasis is put on internally driven climate variability, in particular on the concept of stochastic climate models. This concept is considered to be fundamental for understanding observed climate variability ranging from several months to hundreds of years. We conclude with a discussion of the interplay between large-scale1 climate and regional-scale climate. It is shown that there are some large-scale constraints that may be used to describe regional climate variations in terms of large-scale changes. Also, feedbacks of the regional on the large scale are discussed. It is demonstrated that in general the details of regional climate are unimportant, while its statistics indeed do matter for realistically modeling the observed climate.

So far we have reviewed the dynamics of the global climate system, the marine weather phenomena this book is about—in particular, storms, wind waves, and storm surges—and how to mathematically describe these phenomena. In this chapter, we address the question on how to determine long-term changes in the statistics of marine weather phenomena.

In this chapter we introduce and describe some of the marine weather phenomena that may cause high impacts at sea or in coastal areas. Naturally, high wind speeds play a crucial role and they are associated with all the phenomena discussed. We start with a description of mid-latitude cyclones and storm tracks (Section 2.2). Mid-latitude cyclones form along the polar front in both hemispheres and preferably propagate eastward. The regions that, on average, experience high mid-latitude cyclone activity are referred to as storm tracks. Mid-latitude or extra-tropical cyclones are to be distinguished from tropical cyclones that preferably form over the tropical oceans within a latitude band ranging from about 5° to 20° in both hemispheres (Section 2.3). Both mid-latitude and tropical cyclones are associated with high wind speeds that are responsible for high-impact variations of sea surface height. The latter comprise wind-generated waves at the sea surface (Section 2.4) and storm surges (Section 2.5). Changes in mean sea level and tides are also addressed in Section 2.5. Although they are not related to high wind speeds, their effects may add to wind-induced variations of sea surface height and thus may significantly enhance the risk of flooding in coastal areas.

Models are widely used in environmental sciences. However, the word model covers a much broader range than usually recognized by the user. In different areas of science, different meanings prevail and are considered to be correct. These differences can cause much confusion and problems for interdisciplinary cooperation. What a model or even a good model constitutes is a matter of social or cultural agreements within a wider or broader scientific field.1 When referring to models a large variety of different concepts is generally meant, ranging from simple analogs like maps, to idealizations, conceptualizations, huge miniaturizations and, particularly in climate science, to a mathematically constructed substitute reality (von Storch, 2001).

In this chapter we review present knowledge about past and potential future changes and variability of marine weather phenomena. The review provides a snapshot of knowledge as of early 2009 when this chapter was completed. Conclusions, numbers, and interpretations may change when new observational evidence becomes available or when some of the identified shortcomings have been addressed.

Long-term changes in storm surge conditions of the North Sea may occur as a possible consequence of anthropogenic climate change and may further endanger human safety and activities. Four transient future climate projections of storm surge conditions in the North Sea for 2001 to 2100 were compared to reference simulations for 1961 to 2000 to estimate possible future wind-induced changes in case of anthropogenic climate change. These climate projections incorporate two different scenarios of future emissions (IPCC A1B and B1) and two initial conditions of the driving climate model. In this study we will present the wind-induced surge changes for the outer Elbe estuary (southeastern North Sea). Special emphasis is given to the dependence of the climate signals to specific wind sectors. Generally, the time series of severe wind speed and surge (given by the 99.5 percentiles) in the outer Elbe estuary show strong fluctuations on time scales of decades for the four future climate realizations. Superimposed there is a tendency towards an increase of severe surge up to the end of the 21st century which appears to be small compared to the variability between and within the different climate projections. Severe surge occurs for strong winds blowing from westerly sectors. There is a general tendency that stronger winds come more frequent from the westerly sectors. Changes in frequency distributions of wind directions show also decadal variations. In detail, the four future climate realizations show significant differences in magnitude and patterns of the climate change signals. These uncertainties with respect to emission scenarios and models will be discussed for the whole time series as well as for 30-year time slices 1961-1990 and 2071-2100. Restricting to these time slices, the ensemble of future climate projections will be enlarged by previous simulations for the IPCC scenarios A2 and B2 obtained with different global and regional circulation models.

Regional climate models, which are constrained by large scale information (spectral nudging) provided by re-analyses, allow for the construction of a mostly homogeneous description of regional weather statistics since about 1950. The potential of this approach has been demonstrated for Northern Europe. That data set, named CoastDat, does not only contain hourly data on atmospheric variables, in particular wind, but also on marine weather, i.e., short term water level, current and sea state variations. Another example is the multi-decadal variability of Polar Lows in the subarctic waters. The utility of such data sets is broad, from risk assessments related to coastal wind and wave conditions, assessment of determining the causes for regional climate change, a-posteriori analysis of the efficiency of environmental legislation (example: lead). In the paper, the methodology is outlined, examples are provided and the utility of the product discussed.

A challenging question is whether there is a systematic climate change in the North Sea and which physical processes play an important role. To study this numerical model runs are done for the period of 1948 till 2007. For the simulations the 3-dimensional barocline shelf sea model HAMSOM with a spatial resolution of 20' x 12' and 19 levels is applied. The simulation is forced with the NCEP/NCAR Reanalysis 1 data set which included air temperature, humidity, cloud cover, precipitation, sea level pressure and wind components. The boundary conditions for the open seas are from a climatological data set (Levitus, 1982) and also eight partial tides are included. Model results show good agreement with long time measurements. Results and trends for temperature and heat content will be discussed.

The changes of global mean sea level and its potential accelerating rise is of great interest in both science and public. Since mean sea level changes are depended on regional conditions, investigations of the different regions of the world are neccessary for dike and land use planners and of course for people living near the coast. The changes of the regional mean sea level in the German Bight are analysed by constructing one single time series out of the tide gauge data of 15 locations with two different methods. The first method is based on the mean of the available locations for each time step and the second on an empirical orthogonal function analysis of the tide gauge data. Both methods show similar resulting time series for the regional mean sea level but anyhow some differences in the decadal trends can be seen. Based on our analysis we estimate that the regional mean sea level rose between 1.64mm/yr and 1.74mm/yr for the time period from 1924 to 2008. The longest data used is Cuxhaven which goes back to 1843. The linear trend of the tide gauge Cuxhaven for the period 1924 to 2008 is 1.93mm/yr and thus somewhat higher than the one of the whole German Bight. It is analysed to what extent this single tide gauge is representative for the whole German Bight and shown that this is not the case at least from the year 1924 on. We are not able say anything about the time before 1924, which is especially a factor of uncertainty in analysing the accelerating rise of the regional mean sea level. This is done by analysing 20- and 37-year trends to see decadal changes. We find that all trends are somewhat higher in the end. However, these trends are not extraordinary high but such high trends and even higher ones have occured before. The detailled analysis of the two different methods and of the single tide gauge Cuxhaven is a basis to give more reliable estimates for the sea level changes of the recent past. As three different timeseries are compared and analysed the range of change given is supposed to reduce and dominant uncertainties which arise from limited data and different methods. These analyses of the recent past are a neccessary condition to assess sea level changes in the future.

Sea Level Rise (SLR) is one of the major consequences we are facing in times of a warming climate and it is obvious that a higher sea level influences the heights of occurring storm surges and thus results in a higher risk of inundation for the affected coastal areas. Therefore, regional and global sea level rise are subjects to many recent scientific publications. In contrast, the mean sea level (MSL) and its variability over the last centuries in the German North Sea area have not been analysed in detail up to now. This contribution focuses on presenting the results from analysing high quality time series of relative mean sea level (RMSL) from 13 tide gauges covering the entire German North Sea coastline. One single time series, which is assumed to be representative for the German Bight is constructed and parametric fitting approaches as well as non-parametric data adaptive filters are applied to analyse long-term behaviour and decadal variability of the time series. The results indicate a weak negative acceleration of SLR since 1844 with a strong positive acceleration at the end of the 19th century, to a period of almost no SLR around the 1970s with subsequent positive acceleration and to recent rates, which are high but not unusual taking the last 166 years of observations into account. The comparison between the German North Sea and a global sea level reconstruction clearly reveals the existence of different patterns of SLR. A stronger SLR in the German North Sea area is detected for a period covering some decades starting at the end of the 19th century and for another period covering the last ten to fifteen years and decadal correlations are not significant on the 95%-significance level. From these findings the question arises to what extend global sea level projections published by the Intergovernmental Panel on Climate Change are suggestive to be considered for regional coastal planning strategies along the German North Sea coastline.

A challenging question is whether there is a systematic temperature change in the North Sea and which physical processes play an important role. To study this topic numerical model runs are done for the period from 1948 to 2007. For the simulations the 3-dimensional baroclinic shelf sea model HAMSOM with a spatial resolution of 20' x 12' and 19 levels is applied. The simulation is forced with the NCEP/NCAR Reanalysis 1 data set which included air temperature, humidity, cloud cover, precipitation, sea level pressure and wind components. The boundary conditions for the open seas are provided by a Northwest-European Shelf model, which is driven also by NCEP winds and a climatological T/S-data set (Levitus, 1982), also eight partial tides are included. Model results show good agreement with long-term measurements. Results and trends for temperature and heat content will be discussed. Model results show a positive trend for temperature and heat content for the entire North Sea over the last 25 years. The impact of changing atmospheric forcing parameter on the temperature and the heat content in the North Sea is investigated by specifically designed experiments. They show that air temperature has the dominant influence on the heat content.

Hindcasts as well as climate change projections with GCMs can be downscaled by regional climate models (RCMs; "dynamical downscaling") to obtain greater horizontal information. Here, the hindcast surface marine wind speed fields from the RCMs REMO and CLM are investigated with regard to their added value in comparison to the driving wind field from the global NCEP/NCAR reanalysis (NRA). To do so in a first step wind speed measurements from buoys, light ships and platforms in the eastern North Atlantic are considered as "truth". Wind speed fields from NRA, REMO and CLM are bilinearily interpolated to measurement locations. Added value from the RCMs is obtained when correspondence with both the measured statistical distribution and instantaneous wind speeds is higher than that of the reanalysis. The results show that dynamical downscaling does not add value to NRA wind speed in open ocean areas, while it does for complex coastal areas. However, the results are based on a relatively small amount of short in-situ measurements irregularly distributed in space. Using wind speed retrievals from satellites enables an easier and regularly spaced assessment of added value and is invaluable in remote areas far offshore where in-situ wind speeds are rarely measured. Thus, in a second step the added value of the dynamically downscaled wind is assessed by replacing the in- situ data with QuikSCAT Level 2B 12.5 km (L2B12) wind speed retrievals as "truth". After validating the L2B12 data with buoy winds in the eastern North Atlantic (RMSE: 1.7 m/s), L2B12, REMO and NRA data are co-located for the years 1999-2007. Co-location criteria between REMO and L2B12 data are within 0.1° and 0.06° in longitudinal and latitudinal distance from REMO model grid points and within 20 minutes, leading to a high quality L2B12 gridded wind speed data set with REMO grid dimensions. NRA data is interpolated in time and space onto the REMO grid. The results of the first step are confirmed for a wide area including the eastern North Atlantic, the Baltic Sea and the Mediterranean: dynamical downscaling does not add value to NRA wind speed in open ocean areas, while it does for complex coastal areas.

Anthropogenic climate change may cause long-term changes in the storm conditions of the North Sea. This could lead to an increased endangerment of human safety and activities. To estimate possible future wind-induced changes of the water levels, the effects of four transient climate realizations for 2001-2100 are analysed in comparison to control simulations presenting conditions for 1960-2000. Regionalized pressure and wind fields (regional climate model CLM) from the global control and four climate realizations (global circulation model ECHAM5/MPIOM), which consist of combinations of two scenarios of future emissions (IPCC A1B and B1) and of two different initial conditions, are used to force the circulation model TRIM for the North Sea. The analysis of future wind-induced changes of the water levels is focussed on extreme values. Special emphasis is given to the German Bight (SE North Sea). Comparing the 30-years averages of the annual 99 percentiles of the wind-induced water levels of the four climate realizations for 2071-2100 to the values of the two control climates for 1961-1990, a small tendency towards an increase is inferred for all climate change realizations. These results are consistent with those of previous investigations e.g. by Woth et al. (2006) or Woth (2005) using other emission scenario/model combinations. Furthermore, our study suggests that, considering the whole time series (1960-2100) for selected areas, this tendency is superimposed with strong fluctuations on time scales of decades. Uncertainties related to emission scenarios but also to initial conditions will be discussed.