Katja Woth & Hans von Storch Climate change and North Sea storm surge extremes - an ensemble study (PRUDENCE) Katja Woth & Hans von Storch Institute for Coastal Research GKSS Research Center Geesthacht, Germany Abstract Reference Number: 251 Abstract Title: Climate change and North Sea storm surge extremes - an ensemble study (PRUDENCE) Paper Number: A51F-01 Presentation Type: Oral Presentation Date: Friday, 17 December Location: 3016 Starting Time: 08:04 Time allotted: Time Allotted: 12 minutes Climate change and North Sea storm surge extremes - an ensemble study (PRUDENCE) Hans von Storch & Katja Woth Possible changes in North Sea storm surge climate are studied in a systematic manner. Following up on previous studies, we use a tide-surge model to derive storm surge climate and extremes from atmospheric conditions under present-day and enhanced greenhouse gas conditions. Results for modeled storm surges obtained by using regional model output from four RCMs, namely CLM (GKSS), RCAO (SMHI), REMO5 (MPI) and HIRHAM (DMI) are presented. The atmospheric regional simulations were prepared within the EU project PRUDENCE. The research strategy of PRUDENCE is to compare simulations of different regional models (RCMs) driven by the same global control and climate change simulations. These global conditions, representative for 1961-1990 and 2071-2100 were prepared by the Hadley Center based on the IPCC A2 SRES scenario. The effect on windiness of the enhanced greenhouse gas conditions, projected by these four regional climate models was in all cases similar, namely a moderate increase of high wind speeds in most parts of the North Sea during winter. These simulated surface wind and pressure data have been used to run a storm surge model. We show the expected storm-related changes in different storm surge parameters. For instance, the largest increase of high water levels, defined as the 99.5%ile during winter sampled every half hour would have to be expected along the southern and eastern North Sea coast, with maximum values of around 30 cm, which is beyond the range of normal year-toyear variations. Similar results can be found for all four experiments. Together with the expected rise of mean water levels of 40 cm by IPCC (2001), the total increase is 70 cm at the end of the 21st century under the assumptions of the rather severe A2 scenario. If an ECHAMscenario A2 is used, quantitatively similar results are obtained, but in that case there are also significant increases along parts of the UK coast.

Institute for Coastal Research GKSS, Germany Assessing ongoing change of coastal climate. Deriving scenarios of plausible, possible futures of coastal climates. Special emphasis on wind-related aspects, i.e., wind force, storm surges and ocean waves. Special emphasis on North Sea and Baltic Sea. Participant in HIPOCAS, PRUDENCE and ENSEMBLES.

NCEP Globale Reanalysen ( 210 km x 210 km ) 1958 - 2002 BAW - TELEMAC 2D Wasserstand und barotrope Strömung 21.02.1993 12 UTC REMO Windgeschwindigkeit und Richtung 21.02.1993 12 UTC HIPOCAS: Für Rekonstruktionen verwendete Modellkette Gebiet hier: Nordsee und östlicher Nordatlantik WAM sig. Wellenhöhe und Richtung 21.02.1993 12 UTC Rekonstruktionen am Beispiel einer neueren Arbeit, dem HIPOCAS Hindcast Auflösung etwa 50 x 50 km Auflösung zwischen etwa 100 m und 5km Auflösung etwa 5 x 5 km

Climate risk assessments done with these models Reconstruction of past and ongoing state and change PCPnP futures (PCPnP = plausible, consistent, possible but not necessarily probable = scenarios)

Comparison of wind speed statistics with in-situ data: RCM is skillful in describing marine wind statistics Not assimilated into NCEP (Ionic Sea) These plots are the quantile-quantile diagrams (REMO & NCEP Vs Observations) for 10-m wind speed at 2 buoys station. The first one is an Atlantic offshore buoy (ZBGSO, located at 48.7N,12.40W), already assimilated by NCEP. The second one shows results from a Mediterranean buoy (ZATOS, located at 39.96N, 24.72E, Aegean Sea), whose data have NOT been previously assimilated by NCEP.

External Forcing – Future Scenarios THE IPCC…scenarios corresponding to different storylines assume diff. Emission scenarios. The main scenario characteristics are described in the Box. A2 assumes..… B2 assumes… with ….and … producing therefore lower emissions and less future warming than A2 External Forcing – Future Scenarios

EU-Project PRUDENCE (Prediction of Regional scenarios and Uncertainties for Defining EuropeaN Climate change risks and Effects) GCM RCMs Regional Climate Models: CLM - REMO - HIRHAM - RCAO Global Climate Model (HadAM3) IPCC A2 SRES Scenario (1961-1990 / 2071-2100) today today Storm Surge Model for the North Sea: - TRIM 3D Impact model Impact scenarios scenario scenario global local scale

A2 - CTL: changes in 99 % - iles of wind speed (6 hourly, DJF): west wind sector selected (247.5 to 292.5 deg) HIRHAM RCAO REMO5 CLM

Projections for the future / surge meteorological forcing: HIRHAM / RCA Differences in inter-annual percentiles of surge / A2 - CTL: HIRHAM Differences in inter-annual percentiles of surge / A2 - CTL: RCA

Mean change of winter 95%ile surge levels Near coastal model cells

SREs emission scenarios: Impact on global sea level According to scenario A2 and the specific set of GCM / RCMs, water level may rise in extreme situations by as much as 40 cm + 30 cm = 70 cm along the German North Sea coast. IPCC, 2001 SREs emission scenarios: Impact on global sea level

Conclusions Dynamical models have been demonstrated to be a useful tool to describe the statistics, and changes thereof, of wind-related phenomena in coastal seas – wind force, storm surge, coastal currents and ocean waves. An ensemble of regionalizations, derived from one global A2 scenario, results in rather similar changes of regional wind over the North sea and the associated change of surge levels. Along the German North Sea coast, water levels may rise by up to 70 cm - of which 40 cm are due to mean sea level rise and 30 cm to different storm patterns.