1. Global Warming and Hurricanes
Thomas R. Knutson
NOAA/Geophysical Fluid Dynamics Lab
Princeton, New Jersey
Acknowledgements: Dr. Chris Landsea (NOAA/NHC)
Dr. Kerry Emanuel (MIT)

2. Global Warming and Hurricanes: Historical Perspective and Future Outlook
Global warming and its potential impact on the built environment (overview)
Historical observations: sea surface temperatures and hurricanes
Future simulations of tropical sea surface temperatures and hurricane intensity
Future research directions

3. Global Warming and Hurricanes: Historical Perspective and Future Outlook
Global warming and its potential impact on the built environment (overview)
Historical observations: sea surface temperatures and hurricanes
Future simulations of tropical sea surface temperatures and hurricane intensity
Future research directions

5. Surface Temperature Anomalies during most recent Inactive and Active Atlantic Major Hurricane Eras
Inactive Era (July Dec.1989) Active Era (Jan – Aug. 2005)
Data Source: HadCRUT2v (Climatic Research Unit, U.K.)

7. Global Mean Surface Temperature Simulations: 1870-2000
a) CM2.1 All Forcings
Global Mean Surface Temperature Simulations:
Krakatau eruption
b) CM2.1 Natural Forcings
c) CM2.1 Anthropogenic Forcings

8. Paleoclimate evidence…
Source: Petit et al., Nature, 1999.
CO2 and temperature are highly correlated on glacial/interglacial time scales…
Estimates of radiative forcing during last ice age: ~ -6.5 W/m2 (Hansen 1993) are comparable in magnitude to future positive forcing over the next years under some IPCC scenarios…

9. Potential Impacts of Climate Change on the Built Environment
Heat and thermal stress
Infrastructure sensitive to thermal stress; heat mortality; wildfire risk
Floods and drought
Flood risk; agricultural impacts; wildfire risk
Sea level rise
Exacerbate coastal flooding, including from storm surge
Tropical cyclones
Increased wind damage, flooding from rains, and storm surge

10. # of models that predict increase in precipitation by 2100
in A1B scenario, out of 20 models used by IPCC/AR4
Red = wetter
Blue = drier
Source: Isaac Held, GFDL/NOAA

11. Potential increasing risk of great floods under future climate change…
Modeled return period of 100-yr flood under 4xCO2 climate change
Source: Milly et al. 2002, Nature, v. 415, p Based on one model (GFDL R30).

13. Global Warming and Hurricanes: Historical Perspective and Future Outlook
Global warming and its potential impact on the built environment (overview)
Historical observations: sea surface temperatures and hurricanes
Future simulations of tropical sea surface temperatures and hurricane intensity
Future research directions

14. Trends in Tropical Atlantic “Main Development Region” SSTs…

15. Source: Chris Landsea (NOAA/National Hurricane Center)

18. Why have Tropical Atlantic (MDR) SSTs warmed?
All Forcings (n=8)
Why have Tropical Atlantic (MDR) SSTs warmed?
GFDL CM2 coupled model historical simulations ( ); Aug-Oct season
Natural Forcings Only (n=4)
Anthropogenic Forcings Only (n=4)
5-yr running means

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. 2005
N=28
Source: Chris Landsea, NOAA/National Hurricane Center. TK Added 2005 bar (unofficial). Note that 1933 had 21 named storms.

21. 2005
N=7
Source: Chris Landsea, NOAA/National Hurricane Center. TK Added 2005 bar (unofficial).

22. Original PDI from Emanuel (2005)
Revised PDI from Landsea ( updated)
Emanuel (2005)

23. Emanuel’s Multi-basin Tropical Cyclone Power Dissipation Index (PDI) has increased substantially over past 50 years, along with tropical SSTs
Source: Kerry Emanuel, MIT, SST anomaly (deg C) with arbitrary vertical offset. PDI scaled by constant.

24. Storm-Maximum Power Dissipation Index – Atlantic Basin
Source: K. Emanuel, MIT, 2006

25. Atlantic Basin: SSTs vs number of tropical cyclones
Source: Kerry Emanuel, MIT

26. Question: Are the historical data adequate for this conclusion?
Webster et al.: The percentage of hurricanes which reach Category 4-5 has increased in all basins, comparing two recent 15-year periods…
Question: Are the historical data adequate for this conclusion?
Source: Adapted from Webster et al., Science, Sept

27. Knaff and Sampson’s reanalysis of NW Pacific max intensities produces a reduced Cat 4-5 trend, relative to “best track”
Source: Knaff and Sampson, AMS Hurricanes Conference Proceedings, 2006

28. Sriver and Huber’s PDI from reanalysis, although weaker, is well-correlated after 1978 with Emanuel’s PDI from “best track” data (Atlantic + NW Pacific)
Source: Sriver and Huber, Geophysical Research Letters, in press.

29. Source: Chris Landsea, NOAA/NHC

30. Source: Chris Landsea (NOAA/National Hurricane Center)

31. Georgia-New England Major Hurricanes
New England
Major Hurricanes
Source: Chris Landsea (NOAA/National Hurricane Center)

32. Source: Chris Landsea, NOAA/NHC

33. Conditions Associated With the Active 2004 Atlantic Hurricane Season
Source: Chris Landsea (NOAA/National Hurricane Center)

34. A tale of three seasons…
1995…very active… but few strong U.S. landfalls
2004…active… four Florida hurricanes
Category
2005… very active… four major U.S. Gulf Coast hurricanes

35. A tale of three seasons…
1995…very active… but few strong U.S. landfalls
2004…active… four Florida hurricanes
2005… very active… four major U.S. Gulf Coast hurricanes
Plots: September 500mb geopotential heights

36. Global Warming and Hurricanes: Historical Perspective and Future Outlook
Global warming and its potential impact on the built environment (overview)
Historical observations: sea surface temperatures and hurricanes
Future simulations of tropical sea surface temperatures and hurricane intensity
Future research directions

37. Hurricane–region SSTs in the 21st Century

39. GFDL Simulations: Hurricanes are more intense for warmer climate conditions …(~4% per deg C)
Note: Min. central pressures are averages over day 5 of integrations.
Source: Knutson and Tuleya, J. Climate, 2004.

40. GFDL Simulations: Hurricanes have significantly more near-storm rainfall for warmer climate conditions …(~12% per deg C)
Average rainfall in a 32,700 km2 region of highest 6-hour accumulation (equivalent to 100km radius region).

41. So What’s the Problem?

42. Reality check: comparing models with observations…
GFDL Model wind speed intensity, V (and hurricane theory) vs SST: ~4-5% per oC
Emanuel (2005) for Atl, NW Pac, NE Pac: V3 increases 50% for 0.5oC, so V: ~30% per oC
Emanuel (2006) for Atlantic only:
Century-scale data: V increases ~10% per oC; Data since 1980 only: ~20% per oC
Factor of 2 to 6 discrepancy in sensitivity…
Implications for future projections??

43. Resolving the Discrepancy?
Possibilities:
Past trend of intensity over-estimated? (i.e.: the data is wrong)
Hurricane model/theory not sensitive enough to SST change? (i.e., the models are wrong)
Other factors besides SST which can affect potential intensity are playing a role? (i.e., our simple analysis is wrong)

44. Global Warming and Hurricanes: Historical Perspective and Future Outlook
Global warming and its potential impact on the built environment (overview)
Historical observations: sea surface temperatures and hurricanes
Future simulations of tropical sea surface temperatures and hurricane intensity
Future research directions

45. Where are we going from here?
Simulations of Atlantic hurricane frequency using nested regional models…

46. GFDL Zetac Nonhydrostatic Regional Model: 18km Tropical N
GFDL Zetac Nonhydrostatic Regional Model: 18km Tropical N. Atlantic Simulation
Simulated hurricanes

47. Observed vs Simulated Atlantic Tropical Storm Tracks
1982: Inactive year
1995: Active year
Observed (Aug-Oct)
Observed (Aug-Oct)
Simulation 2 (48-hr nudging)
Simulation 2 (48-hr nudging)
Category
GFDL Zetac regional model, 18km resolution, large-scale nudged toward observed large scale

48. Note: Uses large-scale interior nudging

49. Note: Winds derived from pressure.

50. Tropical Storm Formation Tropical Storm Occurrence Hurricane Occurrence
Observed Observed Observed
Simulated Simulated Simulated
Period of Simulation: August-October seasons,
Model: GFDL Zetac Regional Model with large-scale interior nudging to NCEP Reanalysis

51. Summary of Main Points: Global Warming and Hurricanes
Tropical SSTs (including tropical North Atlantic):
Substantial warming (~0.6oC) occurred in 20th century, roughly tracking global mean temperature
Substantially greater 21st century warming (~2oC) is anticipated due to anthropogenic forcing (greenhouse gas emissions, etc.)
Historical hurricane observations give conflicting information on past trends:
Several Atlantic hurricane activity measures are dominated by multi-decadal “cycles” or noise (e.g., landfalling PDI)—not trends. Some basin-wide indices show unprecedented levels in recent years, correlated with rising SSTs. Data quality issues remain unresolved at this time.
Hurricane intensity sensitivity implied by some studies greatly exceeds that of current model simulation and theory, a discrepancy that remains unresolved at this time.
GFDL hurricane model future simulations:
Maximum intensities increase (roughly ½ category or 7% -- per 100 yr)
Near-hurricane precipitation increases (roughly 20% per 100yr)
Ongoing work at GFDL: high-resolution seasonal Atlantic simulations
Future frequency changes? highly uncertain
Futurte regionally specific effects? highly uncertain