Regional storm climate and related marine hazards in the North Sea Hans von Storch and many colleagues Institute for Coastal Research, GKSS, Germany Gerbrand Komen Colloquium, Utrecht, 19. October 2006

1.How to determine decadal and longer variations in the storm climate? 2.How has the NE Atlantic storm climate developed in the last few decades and last few centuries? 3.How is the extratropical storm climate variability linked to hemispheric temperature variations? 4.How did wind storm-impact on storm surges and ocean waves develop in the past decades, and what may happen in the expected course of anthropogenic climate change?

1.How to determine decadal and longer variations in the storm climate? 2.How has the NE Atlantic storm climate developed in the last few decades and last few centuries? 3.How is the extratropical storm climate variability linked to hemispheric temperature variations? 4.How did wind storm-impact on storm surges and ocean waves develop in the past decades?

Changes of the wind climate Climate = statistics of weather, as given by distributions or parameters thereof, such as means, percentiles etc. Changes of wind stats difficult to determine, because of changing observation practices. Earlier: visual assessment, nowadays: instrumental. Another difficulty is that the recorded values depend on the immediate environment of the location where the observation is made. This environment is subject to gradual and abrupt changes. Almost all long record of wind observations are inhomogeneous, i.e., they do not only reflect changes of the wind statistics but also other factors, such as observation method, practice, location, analysis method …. Inhomogeneity is a key constraint, which is usually overseen by non-experts. Improved instruments and analysis introduces into data records such inhomogeneities (and thus, false signals); therefore satellite data are in most cases unsuitable for the assessment multi-decadal change.

10-yearly sum of events with winds stronger than 7 Bft in Hamburg

Pressure based proxies Pressure readings are usually homogenous Geostrophic winds may be derived for a long period. Annual percentiles of geostrophic wind and “real” wind are linearly related. Annual percentiles of geostrophic wind (e.g., 95 or 99%iles) Annual frequency of events with geostrophic wind equal or larger than 25 m/s Annual frequency of 24 hourly local pressure change of 16 hPa in a year Annual frequency of pressure readings less than 980 hPa in a year

Storm indicators Annual statistics of air pressure readings (incl. geostrophic wind speeds). Variance of local water levels relative to annual mean (high tide) water level. Repair costs of dikes in historical times. Sailing times of ships on historical routes.

1.How to determine decadal and longer variations in the storm climate? 2.How has the NE Atlantic storm climate developed in the last few decades and last few centuries? 3.How is extratropical storm climate variability linked to hemispheric temperature variations? 4.How did wind storm impact on storm surges and ocean waves develop in the past decades?

99%iles of annual geostrophic wind speeds for a series of station triangles in the North Sea regions and in the Baltic Sea region. Alexandersson et al., 2002

Time series of pressure-based storminess indices derived from pressure readings in Lund (blue) and Stockholm (red). From top to bottom: Annual number of pressure observations below 980 hPa (N p980 ), annual number of absolute pressure differences exceeding 16 hPa/12 h (N Dp/Dt ), Intra-annual 95-percentile and 99-percentile of the pressure differences (P 95 and P 99 ) in units of hPa. From Bärring and von Storch, 2005 (GRL) Stockholm Lund

How has the NE Atlantic storm climate developed in the last few decades and last few centuries? Assessment of systematic changes of storminess needs long series; certainly more than 50 years. Intensification related to change in NAO; no links to anthropogenic climate change established in long-term indicators of storminess.

1.How to determine decadal and longer variations in the storm climate? 2.How has the NE Atlantic storm climate developed in the last few decades and last few centuries? 3.How is the extratropical storm climate variability linked to hemispheric temperature variations? 4.How did wind storm-impact on storm surges and ocean waves develop in the past decades?

“Forced” simulation, Simulation with climate model exposed to estimated volcanic, solar and GHG forcing. Model is Atmosphere-Ocean GCM ECHO-G with Atmospheric Model ECHAM4 (T30) (~3.75°x 3.75°  ~300 km x 300 km) and Ocean Model HOPE-G (T43) (~2.8°x 2.8°  ~200 km x 200 km) Number of strong wind events per season (wind at 10 m;  8 Bft, gales; every 12 hours) were counted.

Warming and Storms in the N Atlantic (11-yr running means) NH Temp & NA storm count: No obvious correlation in simulated historical times Fischer-Bruns et al., 2005

How is the extratropical storm climate variability linked to hemispheric temperature variations? During pre-industrial times, no obvious link between extra-tropical storminess and hemispheric mean temperatures exist.

1.How to determine decadal and longer variations in the storm climate? 2.How has the NE Atlantic storm climate developed in the last few decades and last few centuries? 3.How is the extratropical storm climate variability linked to hemispheric temperature variations? 4.How did wind storm-impact on storm surges and ocean waves develop in the past decades?

SLP & Wind NCEP Re-analyse, Downscaling mit REMO-SN Simulation with barotropic (2d) hydrodynamic model TRIM and wave model

Weisse et al., J. Climate, 2005 Change of # Bft 8/year t ≤ T t ≥ T Stormcount

Trends of annual percentiles of surge heights Weisse & Plüß, %iles 90%iles

Weisse, Gaslikova, pers. comm. Ocean wave height reconstruction

How did wind storm-impact on storm surges and ocean waves develop in the past decades? In the recent decades, , an increase of marine storminess was recorded – simultaneously with an intensification of the NAO (North Atlantic Oscillation); Since 1995 this trend has reversed in most marine areas. The limited evidence available for longer times ( years) indicates no systematic increase of marine storminess in the NE Atlantic region. No link between ongoing anthropogenic warming (the existence of which has been demonstrated) and NE Atlantic storminess has been established.

1.How to determine decadal and longer variations in the storm climate? Use homogenous proxies for storminess. 2.How has the NE Atlantic storm climate developed in the last few decades and last few centuries? An intensification in ; thereafter activity has ceased somewhat. No significant changes since How is the extratropical storm climate variability linked to hemispheric temperature variations? Model simulation indicate that there is no obvious link during historical times, but that parallel signals a probable in the course of emerging anthropogenic climate change. 4.How did wind storm-impact on storm surges and ocean waves develop in the past decades? Storm surges as well as ocean wave extremes develop mostly parallel to wind conditions; no significant increases in the past, except for an ongoing increase in the Southern North Sea.

Outlook Scenarios available for North Sea futures (changes of the order of 10% at end of 21st century; no detectability given for the present time) Localization-step for scenarios Similar problems with other storms – tropical „typhoons“ and extra-tropical „polar lows“: > homogeneity of data > insufficient time series lengths New efforts underway at GKSS

Damages and storms Recent meeting of scientists and re-insurances (Munich Re; Hohenkammer, May 2006) Consensus statement: „1. Climate change is real, and has a significant human component related to greenhouse gases. 2.Direct economic losses of global disasters have increased in recent decades with particularly large increases since the 1980s. 8.Analyses of long-term records of disaster losses indicate that societal change and economic development are the principal factors responsible for the documented increasing losses to date. 9.The vulnerability of communities to natural disasters is determined by their economic development and other social characteristics. 10.There is evidence that changing patterns of extreme events are drivers for recent increases in global losses. 13.In the near future the quantitative link (attribution) of trends in storm and flood losses to climate changes related to GHG emissions is unlikely to be answered unequivocally.“