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Recent and projected marine storm climate in the North Sea

Recent and projected marine storm climate in the North Sea. Katja Woth, Ralf Weisse, Frauke Feser, Hans von Storch Institute for Coastal Research GKSS. Victoria, 16/17. Oct. 2003. Outline of presentation. Part I Introduction (storms, storm surges, waves)

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Recent and projected marine storm climate in the North Sea

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  1. Recent and projected marine storm climatein the North Sea Katja Woth, Ralf Weisse, Frauke Feser, Hans von StorchInstitute for Coastal Research GKSS Victoria, 16/17. Oct. 2003

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

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

  4. Extratropical storms / natural conditions in the German bight for surges N Typical tracking direction of Northern hemisphere storms leads to west, north-west or south-west winds: North Sea,German bight  water is pushed to the coast water builds up in the shallow water zone / coast -> surge Hamburg Andreas Pluess: Section of the Nordseemodel /BAW http://www.hamburg.baw.de/scn/sc4-99a/node41.htm

  5. 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) tide Water-level 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

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

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

  8. In discussion: are extratropical storms increasing? (WASA Update / Alexandersson) Time series from 1880 until 2002  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 WASA-Update: Storm index after Alexandersson (2000) based on geostrophic wind speeds obtained from pressure triangles. Time series of normalized intra-annual 99% percentile of daily geostrophic winds for a series of triangles over the British Isles, The Norwegian Sea.

  9. IPCC statement due to changing storminess... 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 ...”

  10. Summary: part I • Point of interest in the context of “climate change” : • >> Is there a systematic change in a certain variable? • here: wind and low pressuresystems 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

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

  12. Example dynamical downscaling for the North Sea (HIPOCAS) NCEP Global Reanalysis 1958 - 2001 REMO Wind speed and direction 21.02.1993 12 UTC WAM Wave height and direction 21.02.1993 12 UTC Resolution: 210 km Resolution: 210 km Resolution: 50 km Resolution: 5km Resolution: 5km

  13. Validation: Individual events (HIPOCAS) Wave height and direction Wind speed and direction m m/s deg deg Weisse, pers. comm. K13-Alpha (53oN13´04´´;003oE13´13´´) (green - model, black - observations)

  14. Comparison of significant wave height from ERS (Meteomer) and HIPOCAS hindcast for the Southern North Sea for 1993. Estimated extreme values of wind speed at platform K13 (southern North Sea) simulated January 1980-January 1997 observed Weisse, pers. comm.

  15. Downscaling as projection for possible future climate conditions >> One of the objectives in the EU funded PRUDENCEproject 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.

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

  17. Methods: dynamical downscaling of surge Forcing of the surge model (TRIM): SLP and near surface windcomponents (from different RCMs) 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 (REMO Wind speed and direction, snap shoot)

  18. Scenarios and projections: The TRIM-GEO Model 6’ meridional direction 10’ zonal direction  ~ 10 km * 10 km • 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 Model bathymetry and selected coastal grid-cells for certain statistical analysis (pink-points)

  19. Scenarios and projections: first statistical analysis Comparisons between 30 year CTL-simulation (1961 - 1990) and 30 year A2 projections (2071 - 2100) / only winter month: • 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

  20. 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)

  21. Projections for the future : statistical significance 99-%-tile / CTL andA2 (solid lines) +/- 1 standard deviation from inter-annual 99-%-tile / CTL (dotted line) Continental North Sea coast East coast of Great Britain HIRHAM

  22. Projections for the future :return values 25 year return value / CTL andA2 and 90 % confidence limits based on 1000 Monte Carlo simulations: Continental North Sea coast East coast of Great Britain HIRHAM (fitted on GEV / Zwiers & Kharin J. Climate 1997)

  23. Summary: part II • in general: only very few observed variables due to marine climate • dynamical downscaling allowsto 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.,extremevalue 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)

  24. 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

  25. Validation: extreme events:storm surge/TRIM storm flood: 16/17 Feb. 1962 observations Cuxhaven: 3.9 m above predicted tide model: max. 3.65 m problem: external surge

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