Tropical Cyclone Intensities: Recent observational studies and simulated response to CO2-induced warming

1. Tropical Cyclone Intensities: Recent observational studies and simulated response to CO2-induced warming Thomas R. Knutson NOAA/Geophysical Fluid Dynamics Lab Princeton, New Jersey Acknowledgements: Dr. Chris Landsea (NOAA/NHC) Dr. Kerry Emanuel (MIT)

2. A Look at Tropical Atlantic SSTs… Main Development Region

5. All Forcings (n=8) Why have Tropical Atlantic (MDR) SSTs warmed? GFDL CM2 coupled model historical simulations (1860-2000); Aug-Oct season Natural Forcings Only (n=4) Anthropogenic Forcings Only (n=4) 5-yr running means

6. Hurricane–region SSTs in the 21st Century

7. What is the response of hurricane intensities to increasing tropical SSTs?

8. 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.

9. Potential Intensity theories simulate an increase in the intensity of hurricanes for higher sea surface temperatures Source: Kerry Emanuel, MIT.

10. Future Hurricane Intensities: Simulations using a high-resolution hurricane model • 9 km grid spacing near storm-- partially resolves eye of hurricane • Similar versions of this model are used operationally for hurricane prediction at NOAA’s National Centers for Environmental Prediction

11. GFDL hurricane model: simulated max. surface wind speeds in the NW Pacific Observed (1971-92) Control (51 cases) m/sec Note: Earlier (18km inner-nest) version of the model was used for these simulations Source: Knutson, Tuleya, and Kurihara, Science, v. 279, 1998.

12. 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.

13. 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).

14. Tropical Cyclone-generated sea surface cooling Cool wake Tropical cyclone SST (deg C) Model: GFDL Coupled Hurricane-Ocean model

15. What do the historical tropical cyclone data show in terms of long-term trends?

16. Original PDI from Emanuel (2005) Revised PDI from Landsea (2005 - updated) Emanuel (2005)

17. Source: Chris Landsea, NOAA/NHC

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

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

20. 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, http://wind.mit.edu/~emanuel/anthro2.htm. SST anomaly (deg C) with arbitrary vertical offset. PDI scaled by constant.

21. 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. 2005.

22. Knaff and Sampson’s reanalysis of 1966-87 NW Pacific max intensities produces a reduced Cat 4-5 trend, relative to “best track” Source: Knaff and Sampson, AMS Hurricanes Conference Proceedings, 2006

23. 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.

24. Comparison of 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??

25. 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)

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

27. Note: Uses large-scale interior nudging

28. 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.) • Intensity simulations with a high-resolution hurricane prediction model: • Maximum intensities increase (roughly 4% -- per deg Celsius SST increase) • Near-hurricane precipitation increases (roughly 12% per deg Celsius) • 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. • Ongoing work at GFDL: high-resolution seasonal Atlantic simulations • Future frequency changes? highly uncertain • Future regionally specific effects? highly uncertain