Retrospective analysis of NE Atlantic weather especially storms

1
Regional storm climate and related marine hazards
in the NE Atlantic
Hans von Storch and Ralf Weisse Institute for
Coastal Research, GKSS, Germany
Climate Variability and Extremes During the Past
100 Years, Gwatt (near Thun), Switzerland, 24-26
July 2006
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• How to determine decadal and longer variations in
  the storm climate?
• How has the NE Atlantic storm climate developed
  in the last few decades and last few centuries?
• 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?

3

• How to determine decadal and longer variations in
  the storm climate?
• How has the NE Atlantic storm climate developed
  in the last few decades and last few centuries?
• 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?

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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.

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10-yearly sum of events with winds stronger than
7 Bft in Hamburg
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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 99iles)
• 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

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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.

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• How to determine decadal and longer variations in
  the storm climate?
• How has the NE Atlantic storm climate developed
  in the last few decades and last few centuries?
• 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?

9
99iles 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
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Stockholm Lund
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
(Np980), annual number of absolute pressure
differences exceeding 16 hPa/12 h
(NDp/Dt), Intra-annual 95-percentile and
99-percentile of the pressure differences (P95
and P99) in units of hPa. From Bärring and von
Storch, 2005 (GRL)
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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 1960-1990 related to change in
  NAO no links to anthropogenic climate change
  established in long-term indicators of storminess.

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• How to determine decadal and longer variations in
  the storm climate?
• How has the NE Atlantic storm climate developed
  in the last few decades and last few centuries?
• 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?

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Forced simulation, 1000-2000

• Simulation with climate model exposed to
  estimated volcanic, solar and GHG forcing.
• Model is Atmosphere-Ocean GCM ECHO-G with
  Atmospheric Model ECHAM4 (T30) (3.75x 3.75 ?
  300 km x 300 km) and Ocean Model HOPE-G (T43)
  (2.8x 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.

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Warming and Storms in the N Atlantic
N Atlantic


Fischer-Bruns et al., 2005
(11-yr running means)
NH Temp NA storm count No obvious correlation
in simulated historical times
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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.

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• How to determine decadal and longer variations in
  the storm climate?
• How has the NE Atlantic storm climate developed
  in the last few decades and last few centuries?
• 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?

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NCEP Re-analyse, 1958-2004
Downscaling mit REMO-SN
Simulation with barotropic (2d) hydrodynamic
model TRIM and wave model
SLP Wind
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Stormcount 1958-2001
Change of Bft 8/year
t T
t T
Weisse et al., J. Climate, 2005
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Trends of annual percentiles of surge heights
1958-2002
50iles
Weisse Plüß, 2005
1958-2002
90iles
20
Ocean wave height reconstruction
Weisse, Gaslikova, pers. comm.
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How did wind storm-impact on storm surges and
ocean waves develop in the past decades?

• In the recent decades, 1960-1990, 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
  (100-200 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.

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• How to determine decadal and longer variations in
  the storm climate? Use homogenous proxies for
  storminess.
• How has the NE Atlantic storm climate developed
  in the last few decades and last few centuries?
  An intensification in 1960-1990 thereafter
  activity has ceased somewhat. No significant
  changes since 1800.
• 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.
• 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.

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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 lowsgt
  homogeneity of datagt insufficient time series
  lengthsNew efforts underway at GKSS

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Damages and storms

• Recent meeting of scientists and re-insurances
  (Munich Re Hohenkammer, May 2006)
• Consensus statement1. Climate change is real,
  and has a significant human component related to
  greenhouse gases.2. Direct economic losses of
  global disasters have increased inrecent 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.