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
Surface Temperature Anomalies during most recent Inactive
and Active Atlantic Major Hurricane Eras
Inactive Era (July 1966 - Dec.1989)
Active Era (Jan. 1995 – Aug. 2005)
Data Source: HadCRUT2v (Climatic Research Unit, U.K.)
a) CM2.1 All Forcings
Global Mean Surface
Temperature Simulations:
1870-2000
b) CM2.1 Natural Forcings
Krakatau eruption
c) CM2.1 Anthropogenic Forcings
Paleoclimate evidence…
Source: Petit et al., Nature, 1999.


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…
Potential Impacts of Climate Change
on the Built Environment

Heat and thermal stress


Floods and drought


Flood risk; agricultural impacts; wildfire risk
Sea level rise


Infrastructure sensitive to thermal stress; heat mortality;
wildfire risk
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
Red = wetter
Blue = drier
Source: Isaac Held, GFDL/NOAA
Potential increasing risk of great floods under future climate change…
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).
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)
All Forcings (n=8)
Why have Tropical Atlantic
(MDR) SSTs warmed?
GFDL CM2 coupled model
historical simulations (18602000); Aug-Oct season
Natural Forcings Only (n=4)
5-yr running means
Anthropogenic Forcings Only (n=4)
Minimum surface pressure (mb)
NW Pacific Basin: Intensity vs. SST
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.
2005
N=28
Source: Chris Landsea, NOAA/National Hurricane Center. TK Added 2005 bar (unofficial). Note that 1933 had 21 named storms.
2005
N=7
Source: Chris Landsea, NOAA/National Hurricane Center. TK Added 2005 bar (unofficial).
Original PDI
from Emanuel
(2005)
Revised PDI
from Landsea
(2005 - updated)
Emanuel (2005)
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.
Storm-Maximum Power Dissipation Index – Atlantic Basin
Source: K. Emanuel, MIT, 2006
Atlantic Basin: SSTs vs number of tropical cyclones
Source: Kerry Emanuel, MIT
Webster et al.: The percentage of hurricanes which reach Category 4-5
has increased in all basins, comparing two recent 15-year periods…
45
40
35
30
1975-1989
1990-2004
25
20
15
10
5
0
North
Atlantic
East Pacifc
West
Pacific
Southwest
Pacific
North
Indian
South
Indian
Question: Are the historical data adequate for this conclusion?
Source: Adapted from Webster et al., Science, Sept. 2005.
Knaff and Sampson’s reanalysis of 1966-87 NW Pacific max
intensities produces a reduced Cat 4-5 trend, relative to “best track”
Figure 4. The annual counts and
sociated
as
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
k and
trac
the reanalysis using
e th
MSLP data, respectively.
Source: Knaff and Sampson, AMS Hurricanes Conference Proceedings, 2006
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)
Figure 1
5
ERA Global PD
ERA Global PDI
ERA at+wp PDI
Emanuel at+wp PDI
4
3
Standard Deviations
2
1
0
-1
-2
-3
1960
1970
1980
1990
Source: Sriver and Huber, Geophysical Research Letters, in press.
2000
Source: Chris Landsea, NOAA/NHC
Source: Chris Landsea (NOAA/National Hurricane Center)
Georgia-New England Major Hurricanes
1851-2004
Source: Chris Landsea (NOAA/National Hurricane Center)
Source: Chris Landsea, NOAA/NHC
Conditions Associated With the
Active 2004 Atlantic Hurricane Season
Source: Chris Landsea (NOAA/National Hurricane Center)
A tale of three
seasons…
1995…very active…
but few strong U.S.
landfalls
2004…active…
four Florida
hurricanes
Category
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?
Possibilities:
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)
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
Observed (Aug-Oct)
Simulation 2 (48-hr nudging)
1995: Active year
Observed (Aug-Oct)
Simulation 2 (48-hr nudging)
Category
GFDL Zetac regional model, 18km resolution, large-scale nudged toward observed large scale
Note: Uses
large-scale
interior
nudging
Note: Winds derived from pressure.
Tropical Storm Formation
Tropical Storm Occurrence
Hurricane Occurrence
Observed
Observed
Observed
Simulated
Simulated
Simulated
Period of Simulation: August-October seasons, 1980-2005
Model: GFDL Zetac Regional Model with large-scale interior nudging to NCEP Reanalysis
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