Transcript Retrospective analysis of NE Atlantic weather

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
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
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)
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.
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, 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)
(~3.75°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.
(11-yr running means)
Fischer-Bruns et al., 2005
Warming
and Storms in the N Atlantic
N Atlantic
NH Temp & NA storm count:
No obvious correlation in simulated historical times
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?
NCEP Re-analyse, 1958-2004
Downscaling mit REMO-SN
Simulation with barotropic
(2d) hydrodynamic model
TRIM and wave model
SLP & Wind
t≤T
Change of # Bft 8/year
t≥T
Weisse et al., J. Climate, 2005
Stormcount 1958-2001
Trends of annual
percentiles of surge
heights
1958-2002
50%iles
Weisse & Plüß, 2005
1958-2002
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, 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 (100200 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 1960-1990; thereafter activity has ceased
somewhat. No significant changes since 1800.
3. 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.“