STORMS IN NORTHERN EUROPE – CHANGES, PERSPECTIVES, AND IMPACTS HANS VON STORCH Institute of Coastal Research, (Helmholtz Zentrum Geesthacht), and KlimaCampus Hamburg, Germany TOKIO MARINE'S 2011 SUMMIT ON GLOBAL WARMING AND CLIMATE CHANGE, APRIL 29, 2011, ATLANTA USA

Counting storms in weather maps – steady increase of NE Atlantic storms since the 1930s ….

Popular argument When it is getting warmer, then we have more water vapor in the atmosphere, and mid-latitude storms become more intense („more fuel“). However at mid latitudes, intense storms are baroclinic, and thus driven by temperature differences – explaining, why we have more storms in the cold season that in the warm season. Unchanging extratropical storm conditions are not inconsistent with a rise in temperature

Page 4 Causes of inhomogenities: Changes in – Instruments – Sampling frequencies – Measuring units – Environments (e.g. trees, buildings) – Station relocations (Dotted lines) Homogeneity of local observations (Wind speed measurement; SYNOP Measuring net of German Weather service; Coastal stations at the German Bight; Observation period: ) Lindenberg et al., 2011

Page 5 Representativity of near surface wind speed measurements

Page 6 The major problem for applying statistical analysis in climate change studies is related - to the assumptions of a stationary sampling process, and - to the assumption of representativity. The local data change their statistics because of changing observational practices and conditions (inhomogeneity); also their representativity for a larger area is often compromised. Usage of weather analyses, incl. re-analyses and proxies such as damages are generally not suitable.

a)Use of proxies derived from air pressure readings. b)Simulation by empirical or dynamical downscaling of large scale information. For assessing recent and ongoing changes of mid- latitude storm conditions, principally two approaches are possible:

A proxy of storminess: Intraseasonal statistics of daily geostrophic winds (pressure gradients) Air pressure readings are mostly homogenous The scaled pressure gradient is called “geostrophic wind”. The tropospheric wind is to first order approximation proportional to atmospheric pressure gradient We use annual/seasonal percentiles (e.g., 95 or 99%iles ) of geostrophic wind derived from triangles of pressure readings as proxies for annual /seasonal storminess. Schmidt and von Storch, 1993

Krueger and von Storch, 2011 Testing the robustness of the link seasonal percentiles of geostrophic wind speed and of surface wind speeds in the virtual reality of a 50 years regional simulation Triangles used Correlations between time series of geostrophic wind speed percentiles and wind speed percentiles

Geostropic wind stats N Europe 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

Dynamical downscaling to obtain high- resolution (10-50 km grid; 1 hourly) description of weather stream- with spectral nudging constraint - use of NCEP or ERA re-analysis allows reconstruction of regional weather in past decades ( ) - when global scenarios are used, regional scenarios with better description of space/time detail can be derived. Meteorological data are fed into dynamical models of weather-sensitive systems, such as ocean waves, catchment hydrology, long-range pollution etc. Dynamical (process based model) downscaling cascade for constructing variable regional marine weather statistics

Weisse, pers. comm. Extreme value analysis of wind speed at platform K13 (southern North Sea) January 1980-January 1997

Page 13 Stormcount Weisse et al., J. Climate, 2005 Change of # Bft 8/year t ≤ T t ≥ T

Skill in representing wave conditions Red: buoy, yellow: radar, blue: wave model run with REMO winds wave direction significant wave height [days] Gerd Gayer, pers. comm., 2001

The CoastDat data set: Long (50 years) and high-resolution reconstructions of recent offshore and coastal conditions mainly in terms of wind, storms, waves, surges and currents and other variables in N Europe Scenarios (100 years) of possible consistent futures of coastal and offshore conditions extension – ecological variables, SE Asia Clients: Governmental: various coastal agencies dealing with coastal defense and coastal traffic Companies: assessments of risks (ship and offshore building and operations) and opportunities (wind energy) General public / media: explanations of causes of change; perspectives and options of change

Wave Energy Flux [kW/m] Currents Power [W/m 2 ] Some applications of - Ship design - Navigational safety - Offshore wind - Interpretation of measurements - Oils spill risk and chronic oil pollution - Ocean energy - Scenarios of storm surge conditions - Scenarios of future wave conditions

Range of changes in seasonal maximum wind (10 m) in Northern Germany in a series of dynamically downscaled scenarios (HadCM, MPI; A2, B2)

© Dundee Satellite Receiving Station ~300 km Scandinavia Spitz- bergen Polar lows Mesoscale (< 1000 km) sized maritime storms ● intense/ strong winds, severe weather ● occur polewards of the Polar Fronts in both hemispheres during winter ● typically induced by disturbances in the air flow ● typically driven by convective processes ● Here: only Northern North Atlantic

Past annual frequencies of polar lows PLS: Polar Low Season (July-June) Zahn and von Storch, 2008

Downscaling scenarios Downscaling scenarios and simulations C20 ( ) A2, B1, A1B1 ( ) … run with ECHAM5/MPIOM1 … downscaled with CLM, employing spectral nudging.

Projected changes in polar low frequency and vertical atmospheric stability A2 C20 A1B B1 Zahn and von Storch, 2010 Differences of the area and time-averaged ice- free SST and T 500hPa over the maritime northern North Atlantic as proxy for frequency of favorable polar low conditions (CMIP3/IPCC AR4)

Past changes according to dynamical downscaling of NCEP/NCAR re- analysis  Strong inter annual variability  Frequency remains on a similar level – no systematic trend Polar lows become less frequent in the Northern North Atlantic in the coming 100 years according to  regional modeling (dynamical downscaling) - control and scenario simulations with one GCM, and by  analysis of vertical stability in a large set of (CMIP3) global climate simulations; all simulations show an increase in projected stability. The genesis regions shift northward. Past and future occurrence of North Atlantic Polar Lows

Page 23 Conclusion: Usage of proxies 1.Monitoring extra-tropical storminess may be based on air pressure proxies. 2.This allows assessments for 100 and more years. 3.Decades long upward and downwards trends have been detected in recent years in the Northern European and other regions. 4.These trends are not sustained and have show recent reversals in all considered regions. 5.Recent trends are not beyond the range of natural variations, as given by the historical past, but are more of intermittent character. Regional temperatures rose significantly at the same time.

Page 24 Conclusion: Usage of dynamical downscaling 1.Dynamical downscaling for describing synoptic and mesoscale variability is doable. 2.Analysis of 60 year simulations point to strong year-to-year variability, to less decade-to-decade variability and no noteworthy trend in the region of the North Atlantic/European Sector – both in terms of baroclinic storms and polar lows. 3.Similar study done for North Atlantic Polar Lows, E Asian typhoons and medicanes. (work in progress)

Mediterranean hurricanes are strong mesoscale cyclones with some resemblance with tropical cyclones and polar lows (warm core, cloud-free eye, winds up to hurricane speed) that develop occasionally over the Mediterranean Sea. Medicanes 15 Jan 1995 Leone Cavicchia, pers. comm Dynamically downscaled

Take-Home Message 1.Baroclinic storms and polar lows represent key climatic risks in Northern Europe. 2.In the past, no noteworthy systematic changes have been detected, when homogeneous, sufficiently long times series of evidence has been considered. 3.For the futures, scenarios describe the possibility for a weak intensification of strong wind cases (baroclinic storms) in winter, and a reduction and northward shift of polar low activity. 4.Scenarios are consistent with ongoing change. 5.The methodology, based on dynamical downscaling and the usage of proxies for storminess are well developed. They may be employed for analyzing conditions in other parts of the world.

Reserve

How do size and surface conditions influence the description of storm activity?

Single-station based proxies correlations between annual 99th percentiles of ground level wind speed and annual number of pressure observations below 980 hPa (median 0.198, 0.05-quantile -0.09)

Local pressure stats since 1800 Time series of pressure-based storminess indices derived from pressure readings in Lund and Stockholm. 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: see also BACC Stockholm Lund

Special application – assessing the lull wind conditions, leading to insufficient energy harvest by wind mills in Germany, winters Weisse, pers. Comm.

Downscaling vs. “obs” C=0,72 NCEP-based downscaling (black) and observations (red) of MetNo (Noer, pers comm) Monthly comparison of NCEP- downscaling (in black) with analysed observed data (in red; Blechschmidt, 2008)

Bracegirdle, T. J. and S. L. Gray, 2008 Density of polar low genesis Genesis in NCEP downscaling RCM simulation