Transcript Atlantic Hurricanes and Climate Change

Atlantic Hurricanes and Climate Change
Tom Knutson
Geophysical Fluid Dynamics Lab/NOAA
Princeton, New Jersey U.S.A.
http://www.gfdl.noaa.gov/~tk
NOTE: SOME SLIDES REMOVED FOR WEB VERSION
Hurricane Katrina, Aug. 2005
GFDL 18km-grid simulation of Atlantic hurricane activity
Focus questions for this talk
• Are there significant trends in Atlantic
tropical cyclone activity?
• How well can we simulate past Atlantic
hurricane activity?
• What are the prospects for future Atlantic
hurricane activity?
The earth’s surface
temperature has been
increasing….
…but what about
hurricanes?
Sources: NASA/GISS.
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, MIT, personal communication 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.
But a measure of annual U.S. landfalling hurricane
activity shows no clear long-term trend since 1900…
Source: Chris Landsea, NOAA/NHC
The frequency of recorded storms (low-pass filtered) in the
Atlantic basin is well-correlated with tropical Atlantic SSTs
Source: Emanuel (2006); Mann and Emanuel (2006)
Statistical significance testing
• Method 1: Linear least-squares regression on annual storm count
series. Adjust degrees of freedom for two-sided t-test based on lag1 autocorrelation.
• Method 2: Same as Method 1, but for the ranks rather than the
original series. Addresses issue of skewness in storm count annual
data.
• Method 3: Bootstrap resampling (with replacement) of series sugsegments of length L. Compute linear trends of resampled data sets
as a control comparison. L values in range of 2-8 tested.
(Recommended value of 2-3 based on Wilks text.)
• The three methods give roughly similar results here.
…but some storms may have been missed
and not recorded in the database.
Pre-satellite era77% strike land
Source: Chris Landsea, NOAA/NHC
Source: Chris Landsea, NHC/NOAA
Landsea: No significant trend from AMO warm phase to warm phase, or cold phase to cold phase.
Reconstructing past tropical cyclone counts
• Satellite-era (1965-2006) storm tracks assumed perfect.
• Apply satellite-era storm tracks to documented ship
tracks (ICOADS).
• Storm detected if ship within radius of tropical storm
force winds (17 m/s). First detection must occur
equatorward of 40N. Monte Carlo simulation, varying
storm radii within reasonable bounds.
• All land assumed to be “perfect detector” of tropical
storms (equatorward of 40N)—planned to further test…
• Assume all relevant ship tracks are in data base—plan to
further test with additional tracks. (First will look for
evidence of storms in “new” ship data.)
Trend from 1878-2006: Not significant (p=0.05, 2-sided tests)
Trend from 1900-2006: Significant
“
“
The Atlantic Multidecadal Oscillation (AMO) and Tropical Atlantic (Aug-Oct) SSTs
GFDL Model: All Forcings (n=8)
GFDL Model: Natural Forcings Only (n=4)
GFDL Model: Anthropogenic Forcings (n=4)
Note: No indirect aerosol forcing is included in any of these runs.
Sources: C. Landsea, NHC/NOAA update of Goldenberg et al. 2000; Knutson and Tuleya (2006; accepted for publication, Cambridge Univ. Press).
A comparison of several climate change metrics:
Global Mean Temperature
Tropical Atlantic Sea Surface Temperature
Atlantic Tropical Storm Counts (unadj.)
Atlantic Trop. Storm Counts (Vecchi/Knut. Adj.)
U.S. Landfalling Tropical Storms (unadj.)
U.S. Landfalling Hurricanes (unadj.)
Note: All time series are low-pass filtered (5-yr mean) and normalized to unit standard deviation (y-axis tic marks: 1 st. dev).
Sea surface temperatures have increased in the region where
Atlantic hurricanes form and intensify, and they are projected to
increase much more during the 21st century…
Minimum surface pressure (mb)
NW Pacific Basin: Intensity vs. SST
The most
intense
storms
occur at
high SSTs
Sea surface temperature (deg C)
Source: Baik and Paek, J. Meteor. Soc. Japan (1998). Used with permission.
Hurricane models project increasing hurricane
intensities and rainfall rates with climate warming …
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, 2007; accepted for publication, Cambridge Univ Press (right).
See also Bengtsson et al. (Tellus 2007) and Oouchi et (J. Meteor. Soc. Japan, 2006).
Late 21st Century projections: increased vertical wind shear may
lead to fewer Atlantic hurricanes
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Percent change in shear per degree Celsius global warming
Source: Vecchi and Soden (Geophysical Research Letters 2007)
21st Century Atlantic Hurricane
Activity?
• Models indicate increased hurricane intensities
with warmer ocean temperatures.
• Climate models project that greenhouse
warming may be accompanied by increased
vertical wind shear in some regions of the
Atlantic, which should act to reduce storm
frequency and intensification in those regions.
• How do we assess which of these competing
effects will “win out”?
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.
Sample hurricane from the Zetac 18-km grid model
Surface winds (m/s) and rainfall (mm/day)
Atmospheric “warm core” and wind speeds
The model captures both the increase in hurricane activity
since the 1980s and the year-by-year fluctuations….
North Atlantic Basin (August-October)
Hurricane Frequency
Correlations vs. Obs: Model1: 0.76
Model2: 0.76
Source: Knutson et al. 2007 (in review)
Note: Model uses large-scale interior nudging to
The model captures both the increase in hurricane
activity since the 1980s and the year-by-year
fluctuations….
North Atlantic Basin (August-October)
Hurricane Frequency
Correlation: 0.86
Source: Knutson et al. 2007 (BAMS, in press)
Note: Model uses large-scale interior nudging to
NCEP Reanalysis
The model captures both the increase in tropical storm
activity since the 1980s and the year-by-year
fluctuations….
North Atlantic Basin (August-October)
Tropical Storm Frequency
Correlation: 0.74
Source: Knutson et al. 2007 (BAMS, in press)
Note: Model uses large-scale interior nudging to
NCEP Reanalysis
Correlations: Model1: 0.36
Model2: 0.51
Ensem: 0.57
Correlations: Model1: 0.30
Model2: 0.32
Ensem: 0.41
Correlations: Model1: 0.72
Model2: 0.66
Ensem: 0.76
Correlations: Model1: 0.70
Model2: 0.60
Ensem: 0.72
Correlations: Model1: 0.64
Model2: 0.54
Ensem: 0.70
Correlations: Model1: -0.01
Model2: 0.20
Ensem: 0.13
The Zetac model reproduces the observed reduction of N. Atlantic activity
during El Nino events fairly well…
Conclusions
• Observed data give conflicting indications on whether humans
might have caused significant increases in Atlantic tropical storm
and hurricane numbers.
• Data quality issues reduce confidence in current assessments
and statistical analyses of past Atlantic (and global) hurricane
activity. Statistical techniques will be important both for addressing
data quality issues and for assessing significance of trends and
other features in the data.
• Models project increased intensities of the most intense
hurricanes and increased hurricane rainfall rates for the late 21st
century.
• A new regional dynamical downscaling model reproduces the
interannual and decadal scale variability of Atlantic hurricane activity
during 1980-2006, and shows promise as a tool for exploring the
causes of these changes and the possible future changes in Atlantic
hurricane activity associated with 21st century climate warming.