Transcript Mid-latitude and tropical storms simulated changes in

Mid-latitude and Tropical Storms
Simulated Changes in
Atmospheric Drivers of Extreme
Sea Levels
Ruth McDonald
Hadley Centre, Met Office
Understanding Sea-level Rise and Variability
WCRP Workshop, Paris, June 2006
[email protected]
formerly Ruth Carnell
Outline
Introduction
Model predictions of future changes in the
frequency and intensity of mid-latitude storms
Model predictions of future changes in the
frequency of tropical storms
Why are cyclones important for sea-level?
 Storm surges
 1953 flooding around North Sea coast
 Hurricane Katrina New Orleans 2005
© Crown copyright 2005
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Introduction to tropical and extra-tropical cyclones
Tropical Cyclone
Extra-tropical cyclone
No
temperature
contrast at
surface
Baroclinic
Energy from
horizontal
temperature
contrast
Energy from
latent heat
colours = SST
blue=15°C,
blue/green = 20°C,
green =25°C
solid lines = wind
speed
Warm core
Cold core
Strongest
winds near
surface
Strongest winds
near tropopause
Scale of 100s km
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Scale of 1000s km
Contours= pressure
surfaces
Figure from Merrill (1993), downloaded from Hurricanes FAQ by Chris Landsea at http://www.aoml.noaa.gov/hrd/tcfaq/A7.html
Storm descriptions based on Hurricanes FAQ Section A7 (by Stan Goldenberg)
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Impacts of global warming on storms
 It is often assumed that global warming will cause an increase in the
frequency and intensity of extra-tropical and tropical cyclones
 Not necessarily the case
 Mid-latitude storms
 Changes occur as a result of competing effects due to changes in
atmospheric temperature and moisture
 Decrease in low level temperature contrast between pole and equator
 Less energy for storms
 Increases in upper level temperature gradient
 More energy for storms
 Local temperature contrasts are also important
 More moisture
 More latent heating. More intense storms
 Fewer storms are required to give the same energy flux between equator
and poles
 Tropical storms
 Linked to enhanced sea surface temperatures and increased moisture
 Changes to large-scale circulation, wind shear
 Changes to modes of variability like NAO and ENSO also important
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Analysis of storms in climate models
 Apply an objective technique to locate and in some studies track
storms
 Local centres of geopotential height, mean sea level
pressure or relative vorticity
 Last for at least 2 days and be non-stationary (if
tracking)
 Tropical cyclones also test for warm cores
 Use parameter diagnostics
 Band pass filter storm track to look at synoptic variability
 Eady parameter to look at baroclinicity
 Seasonal genesis parameter for tropical storms
 Lots of techniques, makes it hard to compare results
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Examples of model cyclone tracks
Mid-Latitude Storms for a single
winter
Tropical Storms for 15 years
Tracks look sensible, despite low resolution
and poor simulation of individual cyclones
Organised into tracks: Atlantic,
Pacific, Mediterranean
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Few events
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Mid-latitude Storms
Simulation of mid-latitude storms by models:Cyclone density
ERA
AMIP 13 Model Mean
Difference
Too far
north
Too
many
number of cyclone events per 145,000 km2
accumulated over nine 120 day winter
periods for the Northern Hemisphere
Lambert et al. (2002) Climate Dynamics
© Crown copyright 2005
Too few
Summary
• All models have systematic errors in the
simulation of cyclone tracks
• Cyclones tend to be too weak
• Errors often larger than climate change signal
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Future changes in mid-latitude storms:Cyclone and track density
HadAM3P A2
ECHAM5
ECHAM5-OM A1B
Bengtsson et al.
(2006) J Climate, in
press, thanks to K
Hodges
Tracks per
month per
106km2
ECHAM4/
OPYC3 IS92a
Summary: Little consensus of local changes in
IS92a
frequencyHadCM2
of storms
amongst climate models
Leckebusch et al.
(2006)
Climate Research,
in press, thanks to G
Leckebusch
Track density
North Atlantic
NE Pacific
JMA
SH
Carnell and Senior
(1998)
Climate Dynamics
Cyclones per
season per
106km2
© Crown copyright 2005
Geng and
Sugi (2003)
J Climate
Cyclones
per
4.5°x4.5°
per season
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Future changes in the frequency of winter midlatitude storms
Reference
Model
Carnell and
Senior 1998
HadCM2
N48
JMA T106
Experiments
NH Change
SH Change
IS95a
Fewer
3x30y
Geng and Sugi
20y
Fewer
Fewer
2003
OBS
Poleward and
2050s
eastward
Fyfe 2003
CCCma
3xIS92a
Sub-Antarctic 30%
500y Ctrl
fewer
Summary: There are fewer mid-latitude storms in winter in both
Lambert
2004
CGCM1
Fewer
hemispheres
in the future 1%
simulationsFewer
CGCM2 T32 1850-2100
Watterson 2006 CSIRO
30y
Fewer
Fewer
Mk2 R21,
A2
Mk3 T63
Lambert and
IPCC 4AR
20y
Fewer
Fewer
Fyfe 2006
GCMs
No shift
No shift
Bengtsson et al. ECHAM5
2006
OM T63
© Crown copyright 2005
3x30y
A1B
No change
Poleward shift
No change
Poleward shift
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Future changes in the frequency of intense
Northern Hemisphere winter mid-latitude storms
Reference
Model
Experiment Intensity
measure
Carnell and
Senior 1998
Geng and
Sugi 2003
HadCM2 IS95a
N48
3x30y
JMA T106 20y
OBS, 2050s
Central MSL
pressure
Central MSL
pressure gradient
Change in
frequency of
intense cyclones
More intense
More intense
Lambert
CGCM1
1%
Central MSL
More intense
2004
CGCM2
1850-2100
pressure
Summary:T32
There is some evidence of an increase in the frequency of
the deepest storms in the future simulations
Watterson
CSIRO
30y
Various dynamical Little change in
2006
Mk2 R21, A2
measures &
dynamical intensity,
Mk3 T63
precipitation
more precipitation
Lambert and
Fyfe
2006
Bengtsson
et al. 2006
IPCC
4AR
GCMs
ECHAM5
OM T63
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20y
Central MSL
pressure
More intense
3x30y
A1B
Central Relative
vorticity
Fewer weak
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Summary of other studies on changes in midlatitude storms
Ref
Model
Exp
Changes
Lionello et al. 2002
ECHAM4
30y
2xCO2
Fewer tracks in Mediterranean region
Fyfe 2003
CCCma
IS92a x3
1850-2100
Sub-Antarctic fewer cyclones
Lozano et al 2004
ECHAM4 AGCM
T106
2xCO2 30y
Coastal storms
Fewer but more intense in region
covering Ireland and Scotland
Leckebusch and
Ulbrich 2004
HadCM3
A1, B2
30y
More North Atlantic cyclone tracks,
and more intense
Yin 2005
IPCC 4AR GCMs
A1B
20y
BPF EKE
Poleward and upward shift
intensification
Inatsu and Kimoto
2005
CCSR/NIES/FRC
GC AGCM
20y x7
2xCO2
timeslice
EKE shows W Pacific tracks stronger
and W Hemisphere track weaker
Fischer-Bruns et al.
2006
ECHAM4HOPEG T30
A2, B2
Max wind speed events, poleward
shift, more N Atl, SO, fewer Pacific
ocean
Leckebusch et al.
2006
5 GCMs
A1 and
IS92a
30y
Fewer in N Atlantic, Oct-Mar More
intense
© Crown copyright 2005
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Tropical Storms
Simulation of tropical storms by models:cyclone genesis density
T106 JMA
N144 HadAM3
Observations 10y
Model CLIM1 10y
McDonald et al. (2005) Climate
Dynamics
Cyclone genesis per
0.83°x1.25° x per 17y
Too few in NE
Pacific
Yoshimura and Sugi (2006) SOLA
Models simulate cyclones
in S Atlantic
Summary: Simulation of TC genesis is realistic but there are some errors
© Crown copyright 2005
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Future changes in the frequency of tropical storms
N144 HadAM3
20km MRI/JMA
McDonald et al. (2005) Climate Dynamics Cyclone
genesis per 0.83°x1.25° x per 17y
T106 JMA
Oouchi et al. (2006) J Met Soc Japan
NW Pacific
© Crown copyright 2005
Sugi et al. J Met Soc Japan
N Atlantic
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Future changes in the frequency of tropical storms
Ratio (%) of number of storms in global warming experiment to number in control
experiment
model
reference
Global
Ocean basin
NA
WNP ENP
T106 JMA 10y
Sugi et al. 2002
66
161
34
33
109
43
69
T42 NCAR CCM2 10y
Tsutsui 2002
102
86
111
91
116
124
99
N144 HadAM3 15y
McDonald et al. 2005
94
75
70
180
142
110
82
T106
CCSR/NIES/FRCGC
Hasegawa and Emori
2005
T106 JMA 10y
Yoshimura & Sugi 05
fewer
T63 ECHAM5-OM
Bengtsson et al. 2006
94
20km MRI/JMA
Oouchi et al. 2006
70
66
48
72
57
NI
SI
SWP
96
134
62
Red = significantly more tropical storms in the future simulation
Blue = significantly fewer tropical storms in the future simulation
Summary: fewer tropical cyclones globally in the future simulations, sign of
regional changes varies between model and basin
© Crown copyright 2005
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Summary
 IPCC TAR: simulated future changes in storms are
inconclusive
 There are still large uncertainties in the future changes
of mid-latitude and tropical storms
 Mid-latitude storms
 Models are tending to agree on there being fewer Northern and
Southern Hemisphere extra-tropical storms in winter
 Models do not agree on regional changes in frequency of storms
 Some models show poleward shifts in the storm tracks
 There is some agreement on increased frequency of extra-tropical
storms with central pressure below 970 hPa
 Tropical storms
 Models are tending to show reduced frequency of tropical storms, but
the sign of the changes varies with region
© Crown copyright 2005
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References
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