Atlantic Hurricanes and Climate Change

Transcription

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Atlantic Hurricanes and Climate Change
Tom Knutson
Geophysical Fluid Dynamics Lab/NOAA
Princeton, New Jersey
http://www.gfdl.noaa.gov/~tk
Hurricane Katrina, Aug. 2005
GFDL model simulation of Atlantic hurricane activity
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Collaborators
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Joe Sirutis
Isaac Held
Gabe Vecchi
Bob Tuleya
Morris Bender
Steve Garner
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Overview
• Observations of Hurricanes and Climate Warming:
– Some worrisome time series…
– Some alternative perspectives…
• Dynamical Modeling Perspective:
– Downscaling past Atlantic hurricane activity…Does
regional downscaling work?
– Projections of changes in hurricane activity under climate
warming scenarios
– A further downscaling step to higher resolution examine
the more intense hurricanes
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There is some recent evidence that overall Atlantic hurricane
activity may have increased since in the 1950s and 60s in
association with increasing sea surface temperatures…
Source: Kerry Emanuel, J. Climate (2007).
PDI is proportional to the time
integral of the cube of the surface
wind speeds accumulated across all
storms over their entire life cycles.
The frequency of recorded storms (low-pass filtered) in the
Atlantic basin is well-correlated with tropical Atlantic SSTs
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But is the
storm record
reliable
enough for
this?
Source: Emanuel (2006); Mann and Emanuel (2006) EOS. See also Holland and Webster (2007) Phil. Trans. R. Soc. A
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Recent TC Intensity Trends
There is some statistical
evidence that the strongest
hurricanes are getting stronger.
This signal is most pronounced in
the Atlantic. However, the
satellite-based data for the global
analysis are only available for
1981-2006.
Quantile regression
computes linear trends for
particular parts of the
distribution. The largest
increases of intensity are
found in the upper quantiles
(upper extremes) of the
distribution.
Source: Elsner et al., Nature, 2008.
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Regional Structure of Tropical Cyclone Intensity Trends
Western North Pacific
b
Eastern
Eastern North
North Pacific
Pacific
(corrected)
There statistical evidence that the
strongest hurricanes are getting
stronger is most convincing for
the Atlantic (1981-2006).
South Indian
South Pacific
Source: Elsner et al., Nature, 2008.
Atlantic
North Indian
The North and South Indian
Ocean data also suggest
increased intensity, but data
homogeneity concerns are still
being debated for those regions.
The intensity change signal is
quite weak for the Pacific
basin.
Two future projections of Atlantic tropical cyclone power dissipation
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Projection 1:
Tropical Atlantic SST
Projection 2:
Tropical Atlantic SST
Relative to Tropical
Mean SST
Source: Vecchi et al. Science (2008)
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+1.60 storms/century (1878-2006)
+4.39 storms/century (1900-2006)
Trend from 1878-2006: Not significant (p=0.05, 2-sided tests, computed p-val ~0.2)
Trend from 1900-2006: Is significant at p=0.05 level
Source: Vecchi and Knutson, J. Climate, 2008.
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U.S. Landfalling vs
basin-wide tropical
storm activity
Global Mean
Temperature
Tropical Atlantic
Sea Surface Temp.
Atlantic Tropical
Storm Counts (unadj.)
Adj. Atlantic Trop. Storm
Counts (Vecchi/Knutson)
U.S. Landfalling
Tropical Storms (unadj.)
U.S. Landfalling
Hurricanes (unadj.)
Storm Track Density Linear Trend (1878-2006)
Unadjusted
Note: All time series are low-pass filtered (5-yr mean) and
normalized to unit standard deviation (y-axis tic marks: 1 st.
dev).
Source: Vecchi and Knutson, J. Climate, 2008.
Adjusted
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Zetac Model Characteristics
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1/6 degree (18 km) grid; 45 vertical levels
Non-hydrostatic dynamical core
Resolved convection
Lin Microphysics
Mellor-Yamada boundary layer formulation
Atlantic Basin domain (105W-10E; 10S-45N)
Boundary forcing: Observed SSTs + NCEP 4x daily Reanalyses
Large-scale (waves 0-2) interior spectral nudging of all variables toward
Reanalysis with a timescale of 36 hours.
Interactive land model (with spun-up initial condition based on reanalysis
forcing
Time step: 30 sec
CPU requirements: ~300 CPU hr / simulated day (Altix – 90 CPUs) or
750,000 CPU hours for 27 three-month seasons.
Typically Aug 1 – Oct 31 simulations (+ 3-day spin-up)
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GFDL Zetac Model: A new high-resolution regional model for Atlantic
hurricane season simulations…
• The model runs for entire hurricane seasons.
• The model generates its own sample of hurricanes during each season.
• These experiments push the limits of available computing resources.
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Sample hurricane from the Zetac 18-km grid model
Surface winds (m/s) and rainfall (mm/day)
Source: Knutson et al., Bull. Amer. Meteor. Soc, 2007.
Atmospheric “warm core” and wind speeds
The model captures both the increase in hurricane
activity since the 1980s and the year-by-year
fluctuations….
Note: Model uses large-scale interior nudging to
NCEP Reanalysis. Aug.-Oct. season only.
Source: Knutson et al. 2008 (Nature Geoscience)
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Other hurricane metrics (ACE, PDI) are simulated fairly well, but major
hurricanes are under-simulated in number (though still well-correlated)…
ACE:
Correlations: Model1: 0.72
Model2: 0.68
Ensem: 0.77
PDI:
Model2: 0.62
Ensem: 0.73
Cat 3-5:
Model2: 0.51
Ensem: 0.69
Source: Knutson et al., Bull. Amer. Meteor. Soc, 2007, updated.
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Zetac Regional Model Downscaling: geographical distribution of storms
Tropical Storm Formation
a)
Tropical Storm Occurrence
Hurricane Occurrence
Observed
b) Observed
c)
Observed
d) Simulated
e) Simulated
f) Simulated
Note: Model uses large-scale interior nudging to NCEP Reanalysis
Source: Knutson et al., Bull. Amer. Meteor. Soc, 2007.
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A Climate Change Experiment
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Re-run all 27 seasons with the Zetac regional
model, but modify the NCEP Reanalysis forcing by
a 3-D climate change perturbation field:
– Multi-model ensemble climate change. IPCC A1B
scenarion. 14 CMIP3 climate models (similar to
Vecchi and Soden, 2007).
– Compare modified climate runs with original 26 year
control runs.
– Interannual/decadal variability and weather are
unaltered from the control run.
– Multi-model ensemble climate change approach
reduces problems with corruption of climate change
signal by internal multi-decadal variability in the
models.
Source: Knutson et al., Nature Geoscience, 2008.
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Large-scale tropical
Atlantic climate changes
projected for late 21st
century by CMIP3
models (A1B scenario).
Average SST change in
MDR is 1.72oC with
warming near 4oC in the
upper troposphere.
Source: Knutson et al., Nature Geoscience, 2008.
Late 21st Century projections: increased vertical
wind shear may lead to fewer Atlantic hurricanes
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Average of 18 models, Jun-Nov
“storm-friendly”
Source: Vecchi and Soden, Geophys. Res. Lett., (2007)
“storm-hostile”
The model provides projections of Atlantic hurricane and tropical
storm frequency changes for late 21st century, downscaled from a
multi-model ensemble climate change (IPCC A1B scenario):
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1) Decreased frequency of
tropical storms (-27%) and
hurricanes (-18%).
Storm Intensities (Normalized by frequency)
2) Increased frequency and
intensity of the strongest
hurricanes
(5 à 12)
4) A more consistent
intensity increase is
apparent after adjusting
for decreased frequency
3) Caveat: this model does
not simulate hurricanes as
strong as those observed.
Source: Knutson et al., 2008, Nature Geoscience.
The new model simulates increased hurricane rainfall rates in
the warmer climate (late 21st century, A1B scenario)
…consistent with previous studies…
Present Climate
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Warm Climate
Rainfall Rates
(mm/day)
Warm Climate – Present Climate
Avg. Rainfall Rate Increases:
50 km radius: +37%
100 km radius: +23%
150 km radius: +17%
400 km radius: +10%
Average Warming: 1.72oC
Source: Knutson et al., 2008, Nature Geoscience.
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Climate Model Dependence?
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Rather than an average climate change
perturbation derived from multiple models,
consider the change from individual CMIP3
models separately.
– For each model, use linear trend analysis to
extract the A1B scenario 21st century climate
change perturbation field.
– Models include GFDL CM2.1, MPI, MRI,
HadCM3 (so far).
– Selection of models attempts to capture
extremes of the distribution of model
responses
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Climate Model Dependence: Zetac model downscaling (Warm minus Control)
Control: 11.1 /yr
Hurricanes:
Range: +2% to -58%
Ensemble: -26%
Control: 6.5 /yr
Major Hurricanes:
Range: +57% to -43%
Ensemble: -14%
Control:1.4 /yr
Source: Knutson, Sirutis, et al., unpublished – do not quote or cite
Increasing Storm Intensity
Tropical Storms:
Range: -9% to -52%
Ensemble: -20%
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Intensities of the strongest storms?
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Since the 18-km grid zetac model fails to
simulate wind speeds greater than ~47 m/s, a
further downscaling step is necessary.
Use GFDL hurricane prediction model to resimulate all individual storms in the zetac runs
(control and warm climates).
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Hurricane models project increasing hurricane
intensities and rainfall rates with climate
warming…
…but probably not detectable at
present.
Hurricane Intensity
Hurricane Rainfall Rates
Current
climate
Current climate
~Late 21st century
~Late 21st century
6-hr accumulated rainfall [cm] within ~100 km of storm center.
Sensitivity: ~4% increase in wind speed
per oC SST increase
Sensitivity: ~12% increase in near-storm
rainfall per oC SST increase
Sources: Knutson and Tuleya, J. Climate, 2004 (left); Knutson and Tuleya (2008) Cambridge Univ Press (right).
See also Bengtsson et al. (Tellus 2007) and Oouchi et (J. Meteor. Soc. Japan, 2006); Walsh et al. (2004) Stowasser et al. (2007).
EXPERIMENTAL DESIGN DETAILS
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Latest version of the GFDL Hurricane Prediction system used for this
study (Operational at NCEP and FLEET since 1995).
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Every Zetac Tropical Storm was downscaled to the GFDL hurricane
prediction model for the 27 years from 1980 to 2006 (control and
warm climate).
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All forecasts were begun 3 days before maximum intensity obtained
by the Zetac model or when Zetac model first designated system as
tropical storm (if less then 3 days before maximum intensity).
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GFDL Hurricane Model then run for 5 days.
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In the Zetac Warmed climate simulations, the initial conditions and
boundary forcing conditions (from NCEP Reanalysis) were perturbed
by a late 21st century climate change anomaly from an ensemble of
climate models or 4 individual international models (GFDL CM2.1,
Hadley Center, MPI, MRI).
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Maximum surface wind obtained during 5 day simulations emphasized in
study.
An Alternative Downscaling Approach: Emanuel et al. (2008)
Source: Emanuel et al., 2008, Bull. Amer. Meteor. Soc. Note: multi-decadal variability from the global climate models may contaminate results.
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Climate Change and Tropical Cyclones:
Impediments to Scientific Understanding
1. Inhomogeneities in climate records, insufficient length
of climate records related to tropical cyclones
2. Uncertainties in global climate projections (emissions,
transient sensitivity)
3. Uncertainties in regional projections of climate models
(wind shear, lapse rate, SST patterns, easterly waves)
4. Uncertainties in response of tropical cyclones to
regional changes in climate (frequency, intensity)
5. For decadal predictions: Radiative forced response
(Greenhouse gases, aerosols) vs internal climate
variability (AMO)
Main Conclusions
i)
It is premature to conclude that human
activity--and particularly greenhouse
warming--has already had a discernible
impact on Atlantic hurricane activity.
ii)
Latest modeling results suggest that
greenhouse warming may actually decrease
the number of hurricanes in the Atlantic, but
that the occurrence of rarer, more intense
hurricanes may increase by a substantial
fraction.
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Detailed Summary of the Main Points…
Main Conclusions:
i)
It is premature to conclude that human activity--and particularly greenhouse warming--has
already had a discernible impact on Atlantic hurricane activity.
ii)
Latest modeling results suggest that greenhouse warming may actually decrease the
number of hurricanes in the Atlantic, but that the occurrence of rarer, more intense
hurricanes may increase by a substantial fraction.
Background details:
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Some statistical studies suggest the possibility that humans have already influenced
hurricane climate. Atlantic hurricane “power dissipation” appears well-correlated with warming
Atlantic SSTs since ~1950. Studies attribute part of the Atlantic SST warming since 1950 to
increasing greenhouse gases. But Atlantic hurricane activity since 1950 is also correlated to other
SST measures that don’t increase very much with greenhouse warming. The intensities of the
strongest storms have increased in the Atlantic since 1981; there is some weaker evidence for an
increase in other basins.
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Longer (~128 yr) Atlantic tropical cyclone records, including adjustments for estimated “missing
storms”, do not indicate a significant increasing trend in tropical storm numbers. U.S.
landfalling hurricanes show no increase. Basin-wide statistics are much more uncertain for
these longer records than for the more recent decades.
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Downscaling hurricane frequency: The recent increase in Atlantic hurricane numbers (1980-2006)
is well-reproduced by models, and is strongly correlated to SSTs in both models and
observations. However, the much larger projected greenhouse warming (late 21st century, A1B
scenario) leads to decreased hurricane numbers in our downscaling models.
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Atlantic Hurricanes and Climate Change

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