Atlantic Hurricanes and Climate Change

1
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
2
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?

3
The earths surface temperature has been
increasing. but what about hurricanes?
Sources NASA/GISS.
4
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
PDI is proportional to the time integral of the
cube of the surface wind speeds accumulated
across all storms over their entire life cycles.
Source Kerry Emanuel, MIT, personal
communication 2007.
5
But a measure of annual U.S. landfalling
hurricane activity shows no clear long-term trend
since 1900

Source Chris Landsea, NOAA/NHC
6
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)
7
(No Transcript)
8
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 lag-1
  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 sug-segments 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.

9
but some storms may have been missed and not
recorded in the database.
Pre-satellite era- 77 strike land
Source Chris Landsea, NHC/NOAA
Source Chris Landsea, NOAA/NHC
10
Landsea No significant trend from AMO warm
phase to warm phase, or cold phase to cold phase.
11
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
  baseplan to further test with additional tracks.
  (First will look for evidence of storms in new
  ship data.)

12
Trend from 1878-2006 Not significant (p0.05,
2-sided tests) Trend from 1900-2006 Significant

13
(No Transcript)
14
(No Transcript)
15
The Atlantic Multidecadal Oscillation (AMO) and
Tropical Atlantic (Aug-Oct) SSTs
GFDL Model All Forcings (n8)

GFDL Model Natural Forcings Only (n4)
GFDL Model Anthropogenic Forcings (n4)
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).
16
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).
17
(No Transcript)
18
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
19
NW Pacific Basin Intensity vs. SST
Minimum surface pressure (mb)
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.
20
(No Transcript)
21
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 12 increase in near-storm
rainfall per oC SST increase
Sensitivity 4 increase in wind speed 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).
22
Late 21st Century projections increased
vertical wind shear may lead to fewer Atlantic
hurricanes
7 6 5 4 3 2 1
0 1 2 3 4 5 6
7
Percent change in shear per degree Celsius global
warming
Source Vecchi and Soden (Geophysical Research
Letters 2007)
23
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?

24
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.

25
Sample hurricane from the Zetac 18-km grid model
Surface winds (m/s) and rainfall (mm/day)
Atmospheric warm core and wind
speeds
26
(No Transcript)
27
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
Note Model uses large-scale interior nudging
to NCEP Reanalysis
Source Knutson et al. 2007 (in review)
28
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
Note Model uses large-scale interior nudging
to NCEP Reanalysis
Source Knutson et al. 2007 (BAMS, in press)
29
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
Note Model uses large-scale interior nudging
to NCEP Reanalysis
Source Knutson et al. 2007 (BAMS, in press)
30
Correlations Model1 0.36 Model2 0.51
Ensem 0.57
Correlations Model1 0.30 Model2 0.32
Ensem 0.41
31
Correlations Model1 0.72 Model2 0.66
Ensem 0.76
Correlations Model1 0.64 Model2 0.54
Ensem 0.70
Correlations Model1 0.70 Model2 0.60
Ensem 0.72
Correlations Model1 -0.01 Model2 0.20
Ensem 0.13
32
The Zetac model reproduces the observed reduction
of N. Atlantic activity during El Nino events
fairly well
33
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.