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)

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

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

Inactive Era (July 1966 - Dec.1989) Active Era (Jan. 1995 – Aug. 2005)

Surface Temperature Anomalies during most recent Inactive and Active Atlantic Major Hurricane Eras

Data Source: HadCRUT2v (Climatic Research Unit, U.K.)

a) CM2.1 All Forcings

b) CM2.1 Natural Forcings c) CM2.1 Anthropogenic Forcings

Global Mean Surface Temperature Simulations: 1870-2000

Krakatau eruption

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 100-150 years under some IPCC scenarios…

Source: Petit et al., Nature, 1999.

Paleoclimate evidence…

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

# of models that predict increase in precipitation by 2100 in A1B scenario, out of 20 models used by IPCC/AR4

Source: Isaac Held, GFDL/NOAA

Red = wetter

Blue = drier

Modeled return period of 100-yr flood under 4xCO2 climate change

Source: Milly et al. 2002, Nature, v. 415, p. 514-517. Based on one model (GFDL R30).

Potential increasing risk of great floods under future climate change…

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

Trends in Tropical Atlantic “Main Development Region” SSTs…

Main Development Region

Source: Chris Landsea (NOAA/National Hurricane Center)

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

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

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

2005

N=28

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

2005

N=7

Emanuel (2005)

Original PDI from Emanuel

(2005)

Revised PDI from Landsea

(2005 - updated)

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.

Source: K. Emanuel, MIT, 2006

Storm-Maximum Power Dissipation Index – Atlantic Basin

Source: Kerry Emanuel, MIT

Atlantic Basin: SSTs vs number of tropical cyclones

0

5

10

15

20

25

30

35

40

45

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”

-3

-2

-1

0

1

2

3

4

5

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.

Source: Chris Landsea, NOAA/NHC

Source: Chris Landsea (NOAA/National Hurricane Center)

Georgia-New England Major Hurricanes1851-2004

Source: Chris Landsea (NOAA/National Hurricane Center)

Source: Chris Landsea, NOAA/NHC

Conditions Associated With theActive 2004 Atlantic Hurricane Season

Source: Chris Landsea (NOAA/National Hurricane Center)

Category

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

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

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

Hurricane–region SSTs in the 21st Century

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).

So What’s the Problem?

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??

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:

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

Where are we going from here?

Simulations of Atlantic hurricane frequency using nested regional models…

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

Simulated hurricanes

Observed vs Simulated Atlantic Tropical Storm Tracks

1982: Inactive year 1995: Active year

GFDL Zetac regional model, 18km resolution, large-scale nudged toward observed large scale

Observed (Aug-Oct) Observed (Aug-Oct)

Simulation 2 (48-hr nudging)Simulation 2 (48-hr nudging) Category

Note: Uses large-scale interior nudging

Note: Winds derived from pressure.

Tropical Storm Formation Tropical Storm Occurrence Hurricane Occurrence

Observed Observed Observed

Model: GFDL Zetac Regional Model with large-scale interior nudging to NCEP Reanalysis

Period of Simulation: August-October seasons, 1980-2005

Simulated Simulated Simulated

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