Recent and projected marine storm climate in the North Sea Victoria, 16/17. Oct Katja Woth, Ralf Weisse, Frauke Feser, Hans von Storch Institute for Coastal Research GKSS

Outline of presentation Part I Ø Introduction (storms, storm surges, waves) Ø Changing storms and weather extremes? Part II Ø Reconstruction (recent changes) - hindcast of North Sea wave climate Ø Scenarios / Projections of possible future climate conditions due to North Sea storm surge climate

Why we are interested in marine storm climate: Storm- and wave-climate are important parts of the coastal climate with impacts on e.g.: Ø coastal geomorphology Ø security of dikes Ø sea and ocean traffic Ø security and operation of offshore constructions Questions we are interested in for the future are: Ø Is there an intensification of extra-tropical storm climate under projected future climate conditions? Ø Can we confirm systematic changes due to more, or more intense wind, wave- and storm surge extremes?

Extratropical storms / natural conditions in the German bight for surges Andreas Pluess: Section of the Nordseemodel /BAW N water is pushed to the coast water builds up in the shallow water zone / coast -> surge Hamburg Typical tracking direction of Northern hemisphere storms leads to west, north-west or south-west winds: North Sea, German bight

Introduction: Storm surges Storm surge: Fluctuations in the water level due to meteorological factors Surge caused by : wind stress & inverse barometric effect inverse barometric effect: 1 cm / 1 hPa Storm Vivian ~ 950hPa core pressure increase of 63 cm (1013 hPa) Water-level tide surge Water level: composition of Tide + Surge North Sea (surges of several metres) wind stress usually more important Surges at High Water are particularly dangerous

Changing storms and weather extremes? More storms More record high waters, flooding More human tragedies We are right in the middle of it

In discussion: are extratropical storms increasing? Articel in nature: Wave highs from 1960 to 1990 were used to fit a linear trend

In discussion: are extratropical storms increasing? WASA-Update: Storm index after Alexandersson (2000) based on geostrophic wind speeds obtained from pressure triangles. (WASA Update / Alexandersson) Time series of normalized intra-annual 99% percentile of daily geostrophic winds for a series of triangles over the British Isles, The Norwegian Sea. To calculate a trend in the data, it is essential to have a long enough time-series, which include the long term variability of the phenomenon Time series from 1880 until 2002

p.33:...There is no compelling evidence to indicate that the characteristics of tropical and extra- tropical storms have changed......Owing to incomplete data and conflicting analyses, it is uncertain as to whether there have been any long-term and large-scale increases in the intensity and frequency of extra-tropical cyclones in the Northern Hemisphere... IPCC statement due to changing storminess...

Point of interest in the context of climate change : >> Is there a systematic change in a certain variable? here: wind and low pressure systems linked with waves and surge. Problem: >> observational data base is often limited To fill this gap: >> different downscaling methods are used e.g. dynamical downscaling for wind, waves and surges with numerical models (reconstructions/hindcasts and projections for different future scenarios) part II of presentation Summary: part I

Part I Ø Introduction (storms, storm surges, waves) Ø Changing storms and weather extremes? Part II Ø Reconstruction (recent changes) - hindcast of North Sea wave climate Ø Scenarios / Projections of possible future climate conditions due to North Sea storm surge climate Outline of presentation

Resolution: 210 km Resolution: 5km Resolution: 50 km REMO Wind speed and direction UTC NCEP Global Reanalysis Example dynamical downscaling for the North Sea (HIPOCAS) WAM Wave height and direction UTC Resolution: 5km Resolution: 210 km

Wave height and directionWind speed and direction K13-Alpha (53 o N13´04´´;003 o E13´13´´) (green - model, black - observations) mm/s deg Validation: Individual events (HIPOCAS) Weisse, pers. comm.

January January 1997 Estimated extreme values of wind speed at platform K13 (southern North Sea) Comparison of significant wave height from ERS (Meteomer) and HIPOCAS hindcast for the Southern North Sea for simulated observed

>> One of the objectives in the EU funded PRUDENCE project is an assessment of possible changes in North Sea storm surges in a future climate and of the uncertainty due to the driving model formulation. Downscaling as projection for possible future climate conditions

Outline of presentation Part I Ø Introduction (storms, storm surges, waves) Ø Changing storms and weather extremes? Part II Ø Reconstruction (recent changes) - hindcast of North Sea wave climate Ø Scenarios / Projections of possible future climate conditions due to North Sea storm surge climate

(REMO Wind speed and direction, snap shoot) Results of the dynamical downscaling: a high resolved data set in time and space of barotropic current velocity and water level for the integrated area Forcing of the surge model (TRIM): SLP and near surface wind components (from different RCMs) Methods: dynamical downscaling of surge

6 meridional direction 10 zonal direction ~ 10 km * 10 km Scenarios and projections: The TRIM-GEO Model Model bathymetry and selected coastal grid-cells for certain statistical analysis (pink-points) Only one vertical level (barotropic run) changes in water level at the open boundaries occur only due to the tides without taking account of the increase in mean sea level up to now without taking account of external surges coming from Atlantic

Scenarios and projections: first statistical analysis Storm surge model forcing: RCM HIRHAM from DMI (Danish meteor. Institute) For each experiment: wind_SLP_tide: 1 model run driven by both atmospheric and tidal forcing only_tide: 1 model run only driven by tidal forcing SURGE = wind_SLP_tide - only_tide Comparisons between 30 year CTL-simulation ( ) and 30 year A2 projections ( ) / only winter month:

Projections for the future / surge meteorological forcing: HIRHAM Differences in inter-annual percentiles of surge / A2 - CTL (HIRHAM) no changes in mean regionally located shift in the high percentiles. -> German bight: increase of storm surge highs up to 25/30 cm (only moderate increase in high percentiles of wind speed)

Projections for the future : statistical significance 99-%-tile / CTL and A2 (solid lines) +/- 1 standard deviation from inter-annual 99-%-tile / CTL (dotted line) HIRHAM East coast of Great Britain Continental North Sea coast

Projections for the future : return values 25 year return value / CTL and A2 and 90 % confidence limits based on 1000 Monte Carlo simulations: (fitted on GEV / Zwiers & Kharin J. Climate 1997) East coast of Great Britain Continental North Sea coast HIRHAM

Summary: part II in general: only very few observed variables due to marine climate dynamical downscaling allows to create * consistent wind, wave, surge data set for European Coastal Waters * homogeneous, high-resolution data set, which is * applicable for coastal and offshore applications (either directly [e.g., extreme value statistics] or indirectly [boundary conditions]) coastal protection: Changing of wave energy affecting coastal structures and morphodynamics; erosion operation time of e.g. windfarms are dependant on wave height operation time of offshore constructions (oil platform) and ferry service also derivable: proxies for water quality (mixing)

Summary: part II With respect to the questions: » How will extremes of surge change in a perturbed climate » How certain can we be about our predictions >> up to 20% increase in high percentiles, no change in mean >> return values show an increase up to ~ 50/60 cm (25 y retval - regionally limited on the German bight, Netherlands, Denmark >> future distribution seems to be partially significant different from CTL simulations, based on bootstrap re-sampling OUTLOOK: extend the statistical analysis by statements about the statistical certainty. Extend the database by running the surge model with a series of RCMs, driven by same GCM and different GCMs

Validation: extreme events: storm surge/TRIM storm flood: 16/17 Feb observations Cuxhaven: 3.9 m above predicted tide model: max m problem: external surge