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

Abstract 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 CO 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

A possible indication of anthropogenic climate change in the wave climate in the central North Sea

In den letzten Jahrzehnten konnte in der zentralen Nordsee eine Zunahme der Haeufigkeit ostwaerts laufender Wellen beobachtet werden. Mit Hilfe eines statistischen Modells wurde die Wellenstatistik des 20. Jahrhunderts rekonstruiert. Der monatliche mittlere Bodenluftdruck ueber dem Nordatlantik und Westeuropa wurde mit einer linearen multivariaten Technik auf die monatliche Verteilung der Wellenrichtungen regionalisiert. Der Vergleich der juengsten Aenderungen mit dem gesamten Jahrhundert zeigt, dass diese Aenderung im Wellenklima signifikant ist (5-%-Grenze). Zur Untersuchung der Ursachen dieser Veraenderung wurden diese mit den regionalisierten Ergebnissen aus einem transienten Klimaszenario und einem hochaufgeloesten Zeitscheibenexperiment verglichen. Beide Szenarien produzieren bei ansteigenden Treibhausgasen und Aerosolen eine qualitativ konsistente Aenderung. Die Zunahme der oestlich laufenden Wellen koennte somit ein lokaler Hinweis auf eine globale Klimaaenderung sein. (orig.)In the central North Sea we observe an increase in the frequency of eastward propagating waves in the recent four decades. To assess the significance of this change the wave statistic for this century was reconstructed with a statistical model. With a linear multivariate technique (redundancy analysis) monthly mean air pressure fields over the North Atlantic and Western Europe were downscaled on the intramonthly frequency of directional wave propagation. When compared against this reference, the recent change appears significant at the 5% level. In order to investigate the reason for this local climatic change, reconstruction was compared with the downscaled results of a transient GCM scenario (ECHAMA-OPYC3) and with results obtained in a high resolution time slice experiment with increased concentrations of greenhouse gases and aerosols. Both estimates are qualitatively consistent with the changes observed in the last decades. We suggest that the recent increase of eastward propagation is a local manifestation of anthropogenic global climate change. (orig.)SIGLEAvailable from TIB Hannover: RA 3251(2000/3) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Natural variability and anthropogenic change of storm related sea level variations along the North Sea coast

Der Einfluss eines sich aendernden Windklimas auf die Wasserstaende entlang der Nordseekueste wurde mit einer statistischen Technik ('downscaling') und mit einem dynamischen Modell untersucht. Beide Modelle ergeben die folgenden Resultate: ausschliesslich aufgrund veraenderter atmosphaerischer Bedingungen stieg das mittlere Tidehochwasser in den vergangenen vier Dekaden um 1-2 mm/Jahr. Im gleichen Zeitraum zeigten die intra-monatlichen Quantile -abzueglich des Wintermittels - an der Britischen Kueste einen leichten negativen und an der kontinentalen Kueste einen schwach positiven Trend. Unter der Bedingung eines verdoppelten Kohlendioxidgehaltes in der Atmosphaere ergeben sich fuer die Entwicklung der hohen Quantile und des mittleren Tidehochwassers aehnliche Resultate. Diese Untersuchung beruecksichtigt nur den Einfluss der atmosphaerischen Zirkulation auf die Wasserstaende, andere Effekte wie beispielsweise die Eustatik und Isostatik sind nicht enthalten. (orig.)The influence of a changing wind climate on the sea surface elevations along the North Sea coast was investigated with a statistical downscaling technique and with a dynamical model. Both models agree on the following results: in the past, the winter means of high water levels along the North Sea coast increase on the order of 1-2 mm/year, on account of only the atmospheric forcing, whereas the high intra-monthly percentiles - reduced by the winter averages - show no clear trend: 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. This study evaluates the impact of a changing atmospheric forcing, only. Additional influences, such as the eustatic and isostatic effects are not accounted for. (orig.)Available from TIB Hannover: RA 3251(97/E/48) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Danish Climate Centre

Introduction and scientific background ............................................................................................ 3 2. Statistical analysis of T106 data (Task 1) ......................................................................................... 6 3. Baroclinic developments (Task 2) .................................................................................................... 8 4. Polar lows (Task 3) ........................................................................................................................... 9 5. Mediterranean systems (Task 4) ..................................................................................................... 10 6. Surge statistics based on 30 year T106 time slices: North-western European shelf seas (Task 5) ........................................................................................................................................... 12 7. Surge statistics based on 30 year T106 time slice