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)

A Look at Tropical Atlantic SSTs…

Main Development Region

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

Hurricane–region SSTs in the 21st Century

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

NW Pacific Basin: Intensity vs. SST

Source: Baik and Paek, J. Meteor. Soc. Japan (1998). Used with permission.

Min

imum

sur

face

pre

ssur

e (m

b)

Sea surface temperature (deg C)

The most intense storms occur at high SSTs

Potential Intensity theories simulate an increase in the intensity of hurricanes for higher sea surface temperatures

Source: Kerry Emanuel, MIT.

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

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

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.

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).

Tropical Cyclone-generated sea surface cooling

Tropical cyclone

Cool wake

Model: GFDL Coupled Hurricane-Ocean model

SST (deg C)

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

Emanuel (2005)

Original PDI from Emanuel

(2005)

Revised PDI from Landsea

(2005 - updated)

Source: Chris Landsea, NOAA/NHC

Source: K. Emanuel, MIT, 2006

Storm-Maximum Power Dissipation Index – Atlantic Basin

Source: Kerry Emanuel, MIT

Atlantic Basin: SSTs vs number of tropical cyclones

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.

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NorthAtlantic

East Pacifc WestPacific

SouthwestPacific

NorthIndian

SouthIndian

1975-1989

1990-2004

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.

Figure 4. The annual counts and associated trends with category 4&5 TCs occurring in the western North Pacific Basin for the years 1966-2004 are shown. Notice the change in the trends as a result of the reanalysis. Again, blue and red bars and lines are associated with the best track and the reanalysis using the MSLP data, respectively.

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”

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1960 1970 1980 1990 2000

Figure 1

ERA Global PDERA Global PDIERA at+wp PDIEmanuel at+wp PDI

Stan

dard

Dev

iatio

ns

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.

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??

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:

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

Simulated hurricanes

Note: Uses large-scale interior nudging

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