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

6. Hurricane–region SSTs in the 21 st Century

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

8. NW Pacific Basin: Intensity vs. SST Source: Baik and Paek, J. Meteor. Soc. Japan (1998). Used with permission. Minimum surface pressure (mb) Sea surface temperature (deg C) The most intense storms occur at high SSTs

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 Source: Knutson, Tuleya, and Kurihara, Science, v. 279, 1998. Note: Earlier (18km inner-nest) version of the model was used for these simulations

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 km 2 region of highest 6-hour accumulation (equivalent to 100km radius region).

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

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

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

17. Source: Chris Landsea, NOAA/NHC

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

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

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.http://wind.mit.edu/~emanuel/anthro2.htm

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

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 o C Emanuel (2005) for Atl, NW Pac, NE Pac: V 3 increases 50% for 0.5 o C, so V: ~30% per o C Emanuel (2006) for Atlantic only: Century-scale data: V increases ~10% per o C; Data since 1980 only: ~20% per o C Factor of 2 to 6 discrepancy in sensitivity… Implications for future projections??

25. Resolving the Discrepancy? 1.Past trend of intensity over-estimated? (i.e.: the data is wrong) 2.Hurricane model/theory not sensitive enough to SST change? (i.e., the models are wrong) 3.Other factors besides SST which can affect potential intensity are playing a role? (i.e., our simple analysis is wrong) Possibilities:

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 (~0.6 o C) occurred in 20 th century, roughly tracking global mean temperature  Substantially greater 21 st century warming (~2 o C) 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