1. Atlantic Hurricanes and Climate Change1
Atlantic Hurricanes and Climate Change
Hurricane Katrina, Aug. 2005
Tom Knutson
NOAA / Geophysical Fluid Dynamics Lab
Princeton, New Jersey
Outline:
Observations
(Historical Trends?)
Model Projections
(with climate warming)
Conclusions
GFDL model simulation of Atlantic hurricane activity 1

2. Climate Change Attribution
Observations
are observed changes consistent with
expected responses to forcings
inconsistent with alternative explanations
All forcing
Solar+volcanic
Attribution of climate change to causes involves READ
Climate models are important tools for attributing and understanding climate change. Understanding observed changes is based on our best understanding of climate physics, as contained in simple to complex climate models. For the 4rth assessment report, we had a new and very comprehensive archive of 20th century simulations available. This has greatly helped.
This figure gives an example.
You see observed global and annual mean temperature in black over the 20th century compared to that simulated by a wide range of these models. On the top, in red, are individual model simulations and their overall mean shown fat, that are driven by external influences including increases in greenhouse gases, in aerosols, in changes in solar radiation and by volcanic eruptions. The observations rarely leave the range of model simulations. The trends and individual events like cooling in response to volcanic eruptions (POINT) are well reproduced. The fuzzy range gives an idea of uncertainty with variability in the climate system.
Source: IPCC 4th Assessment Report. Used with permission.

3. 3
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…
Increasing data uncertainty
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, J. Climate (2007). 3

4. Two future projections of Atlantic tropical cyclone power dissipation4
Two future projections of Atlantic tropical cyclone power dissipation
Projection 1: Absolute SST
~300% projected increase in
Power Dissipation
Indirect attribution:
CO2  SST  Hurricanes
Projection 2: Relative SST
Projected change:
sign uncertain, +/- 80%
No Attribution
Supported by dynamical
models .
Source: Vecchi et al. Science (2008)

5. The frequency of tropical storms (low-pass filtered) in the Atlantic basin since 1870 has some correlation with tropical Atlantic SSTs
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 5

6. Tropical storm occurrence has apparently decreased in the Gulf of Mexico and Caribbean…Increases are mostly located in the open Atlantic and off the U.S. East Coast (in original, unadjusted data)…
Source: Vecchi and Knutson, J. Climate, accepted for publication.

7. Ship tracks have changed in density and location over time
Source: Vecchi and Knutson , J. Climate, 2008.

8. Atlantic tropical storms (< 2 day duration) show a strong rising trend, but storms of >2 day duration--adjusted for missing storms--do not show a trend.

9. Adjustments to storm counts based on ship/storm track locations and density
Sources:
Vecchi and Knutson (2008)
Landsea et al. (2009)
Vecchi and Knutson (in preparation)

10. Global Tropical Cyclone Intensity Trends10
Global Tropical Cyclone 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
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.

11. Projections of Future Changes in Climate
Best estimate for low scenario (B1) is 1.8°C (likely range is 1.1°C to 2.9°C), and for high scenario (A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C).
Broadly consistent with span quoted for SRES in TAR, but not directly comparable
Source: IPCC 4th Assessment Report. Used with permission.

12. Projected Atlantic region climate changes: 18-Model CMIP3 ensemble
Higher shear
Projected Atlantic region climate changes: 18-Model CMIP3 ensemble
Higher potential intensity

13. 18-km grid model nudged toward large-scale (wave 0-2) NCEP Reanalyses
Zetac Regional Model reproduces the interannual variability and trend of Atlantic hurricane counts ( )
18-km grid model nudged toward large-scale (wave 0-2) NCEP Reanalyses 13
Source: Knutson et al., 2007, Bull. Amer. Meteor. Soc.

14. 14
The 26.5oC “threshhold temperature” for tropical storm formation: a climate dependent threshhold…
Source: Knutson et al., 2008, Nature Geoscience.

15. 15
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):
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  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.

16. 16
The new model simulates increased hurricane rainfall rates in the warmer climate (late 21st century, A1B scenario) …consistent with previous studies…
Present Climate
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.

17. Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes, Science, January Morris A. Bender, Thomas R. Knutson, Joseph J. Sirutis, Robert E. Tuleya, Gabriel A. Vecchi, Stephen T. Garner and Isaac M. Held Geophysical Fluid Dynamics Laboratory/NOAA

18. 18

19. (Atlantic tropical storms and hurricanes, 1980-2006)
Down-scaled GFDL hurricane prediction model produced much more realistic distribution of maximum wind speeds compared to Zetac. 35
Simulated distributions of maximum wind speeds
(Atlantic tropical storms and hurricanes, )
Normalized occurrences
18km grid Zetac regional climate model
9 km GFDL hurricane model
observed
Maximum Wind Speed (m/s)

20. In a warmer climate (late 21st century A1B scenario) the hurricane model simulates an expanded distribution of Atlantic hurricane intensities. 20
Control
The strongest hurricanes increase in number for the downscaled ensemble-mean climate warming…
…and increase for 3 of 4 individual climate models tested.
Source: Bender et al., Science,

21. Late 21st Century Climate Warming Projection-- Average of 18 CMIP3 Models
(27 Simulated Hurricane Seasons) Source: Bender et al., Science, 2010

22. Tracks of Storms that Reached Category 4 or 5 Intensity
Degrees Latitude
NWS VERSION (GFDL)
Late 21st Century Warmed Climate Projection based on 4 Individual CMIP3 Climate Models
Control Climate
(Odd Years Only)
Degrees Longitude East
Degrees Latitude
MPI-ECHAM5
GFDL-CM2.1
MRI-CGCM
UKMO-HADCM3
Degrees Longitude East
Degrees Longitude East

23. SUMMARY OF PROJECTED CHANGE
Cat 4+5 frequency: 81% increase, or 10% per decade
Estimated net impact of these changes on damage potential: +28%
Colored bars show changes for the18 model CMIP3 ensemble (27 seasons); dots show range of changes across 4 individual CMIP models (13 seasons).
Source: Bender et al., Science, 2010.

24. Instead, assume residuals from a 4th order polynomial: 55 yr
Emergence Time Scale: If the observed Cat 4+5 data since 1944 represents the noise (e.g. through bootstrap resampling), how long would it take for a trend of ~10% per decade in Cat 4+5 frequency to emerge from noise? Answer: ~60 yr (by then 95% of cases are positive)
Instead, assume residuals from a 4th order polynomial: 55 yr
Instead, resample chunks of length 3-7 yr: yr
Source: Bender et al., Science, 2010.

25. 25
Conclusions: What can data and climate models tell us about global warming and Atlantic hurricanes?
Models, together with observations and theory, provide a compelling case that human emissions of greenhouse gases have caused much of the long-term global warming over the past 50 yr and 140 yr.
Models project substantial further warming over the 21st century, including in the tropical Atlantic.
Sea level rise is expected to exacerbate storm surge impacts even assuming storms themselves do not change.
Models project reductions or little change in Atlantic tropical storm or hurricane numbers, or in aggregate hurricane activity (e.g., annual Power Dissipation Index). (Increased shear outweighs warmer SSTs.)
Models suggest that the (fewer) surviving storms may at times reach favorable areas with both warmer SSTs and low shear, leading to a greater number of very intense category 4 and 5 hurricanes than at present (possibly a doubling in annual frequency by 2100)
However, we cannot yet conclude that humans have already caused a detectable change in Atlantic hurricane activity. Note that humans may have already caused changes that are either below the 'detection threshold' or are not yet properly modeled (e.g., aerosol effects).
Decreasing Confidence

26. Extra slides
In case some questions come up during Q&A.

27. Tropical Cyclones Frequency Projections (Late 21st century) - Summary
Blue = decrease
Red = increase

28. Tropical Cyclone Intensity Projections
Blue = decrease
Red = increase

29. Tropical Cyclone Frequency Projections: Higher Intensity Storms
Blue = decrease
Red = increase

30. Tropical Cyclone Precipitation Rate Projections
Blue = decrease
Red = increase

31. 31
Late 21st Century projections: increased vertical wind shear may lead to fewer Atlantic hurricanes
Average of 18 models, Jun-Nov
“storm-friendly”
“storm-hostile”
Source: Vecchi and Soden, Geophys. Res. Lett., (2007)

32. Projected Changes in Regional Hurricane Activity
GFDL 50-km HIRAM, using four projections of late 21st Century SSTs.
18-model CMIP3 Ensemble
GFDL CM2.1
HadCM3
ECHAM5
Red/yellow = increase
Blue/green = decrease
Unit: Number per year
Regional increases/decreases much larger than global-mean.
Pattern depends on details of SST change.
Source: Zhao, Held, Lin and Vecchi (J. Climate, in press)

33. Hurricane Intensity Class
Changes in Atlantic Hurricane Counts by Intensity Class: Late 21st century A1B Projection 33
Tropical Storms
Ensemble: -27%. Range: - 48% to - 3%
GFDL CM2.1
MRI
18-Model Ensemble
MPI
HadCM3
All Hurricanes (Cat 1-5)
Tropical storms and hurricanes consistently decrease in number in the warmer climate, but…
Hurricane Intensity Class
Ensemble: -33%. Range: - 60% to -7.5%
Major Hurricanes (Cat 3-5)
Ensemble: -18%. Range: - 60% to +40%
Cat 4-5 Hurricanes (>131 mph)
Ensemble: +75%. Range: - 53% to +110%
…the rarer most intense simulated hurricanes occur up to 3 times as often in the warmer climate, and increased for 3 of 4 individual models
Most Intense Hurricanes (>145 mph)
Ensemble: +220%. Range: - 60% to +180%
BASE YEAR
Source: Bender et al., Science, 2010.

34. HIRAM 50 km grid model TC correlations for several basins
North Atlantic
HIRAM 50 km grid model TC correlations for several basins
corr=0.83
red: observations
blue: HiRAM ensemble mean
shading: model uncertainty
East Pacific
corr=0.62
Hurricane counts for each basin
are normalized by a
time-independent
multiplicative factor
West Pacific
corr=0.52
Correlation for the South
Pacific is ~0.3 and insignificant
for the Indian Ocean
Source: Zhao, Held, Lin, and Vecchi (J. Climate, in press)

35. Author Team:
Tom Knutson, Co-Chair Geophysical Fluid Dynamics Laboratory/NOAA,
Princeton, USA
John McBride, Co-Chair Center for Australian Weather and Climate
Research, Melbourne, Australia
Johnny Chan University of Hong Kong, Hong Kong, China
Kerry Emanuel Massachusetts Institute of Technology, Cambridge, USA
Greg Holland National Center for Atmospheric Research, Boulder, USA
Chris Landsea National Hurricane Center/NOAA, Miami, USA
Isaac Held Geophysical Fluid Dynamics Laboratory/NOAA, USA
Jim Kossin National Climatic Data Center/NOAA, Madison, USA
A.K. Srivastava India Meteorological Department, Pune, India
Masato Sugi Research Institute for Global Change/JAMSTEC, Yokohama, Japan

36. Detection and Attribution:
SUMMARY ASSESSMENT:
Detection and Attribution:
It remains uncertain whether past changes in any tropical cyclone activity (frequency, intensity, rainfall, etc.) exceed the variability expected through natural causes, after accounting for changes over time in observing capabilities.

37. Tropical Cyclone Projections: Frequency
SUMMARY ASSESSMENT:
Tropical Cyclone Projections: Frequency
It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged due to greenhouse warming. We have very low confidence in projected changes in individual basins. Current models project changes ranging from -6 to -34% globally, and up to ± 50% or more in individual basins by the late 21st century.
“Likely”: >67% probability of occurrence, assessed using expert judgment

38. Tropical Cyclone Projections: Intensity
SUMMARY ASSESSMENT:
Tropical Cyclone Projections: Intensity
Some increase in mean tropical cyclone maximum wind speed is likely (+2 to +11% globally) with projected 21st century warming, although increases may not occur in all tropical regions. The frequency of the most intense (rare/high-impact) storms will more likely than not increase by a substantially larger percentage in some basins.
“More likely than not”: >50% probability of occurrence,
assessed using expert judgment

39. Tropical Cyclone Projections: Rainfall
SUMMARY ASSESSMENT:
Tropical Cyclone Projections: Rainfall
Tropical cyclone rainfall rates are likely to increase. The projected magnitude is on the order of +20% within 100 km of the tropical cyclone center.

40. SUMMARY ASSESSMENT:
Tropical Cyclone Projections: Genesis, Tracks, Duration, and Surge Flooding
We have low confidence in projected changes in genesis location, tracks, duration, or areas of impact. Existing model projections do not show dramatic large-scale changes in these features. The vulnerability of coastal regions to storm surge flooding is expected to increase with future sea level rise and coastal development, although this vulnerability will also depend on future storm characteristics.

41. On longer time scales, the rising trend in Atlantic tropical storms is due mostly to very short lived storms (< 2 day duration)
Source: Landsea et al., J. Climate, in press.

43. Tropical Cyclone Intensity Trends in Various Basins
Western North Pacific b
Eastern North Pacific
Eastern North Pacific
(corrected)
There statistical evidence that the strongest hurricanes are getting stronger is most convincing for the Atlantic ( ).
South Indian
Atlantic
The North and South Indian Ocean data also suggest increased intensity, but data are more uncertain for those regions (e.g., satellite view angle changes).
South Pacific
North Indian
The intensity change signal is quite weak for the Pacific basins.
Source: Elsner et al., Nature, 2008.