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Tropical Cyclone Intensities: Recent observational studies and simulated response to CO2-induced warming Thomas R. Knutson NOAA/Geophysical Fluid Dynamics.

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Presentation on theme: "Tropical Cyclone Intensities: Recent observational studies and simulated response to CO2-induced warming Thomas R. Knutson NOAA/Geophysical Fluid Dynamics."— Presentation transcript:

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, 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 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


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