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Atlantic Hurricanes and Climate Change

Hurricane Katrina, Aug. 2005

GFDL model simulation of Atlantic hurricane activity

Tom Knutson

Geophysical Fluid Dynamics Lab/NOAAPrinceton, New Jersey

http://www.gfdl.noaa.gov/~tk

There is some recent evidence that overall Atlantic hurricane activity may have increased since in the 1950s and 60s in association with increasing sea surface temperatures…

Source: Kerry Emanuel, J. Climate (2007).

PDI is proportional to the time integral of the cube of the surface wind speeds accumulated across all storms over their entire life cycles.

~1950-2005

What are the implications of pronounced future warming for Atlantic Power Dissipation Index (PDI)?

Power Dissipation Index vs SST

~1950 - 2005

IPCC AR4 Climate ModelsObservations

Attribution of hurricane changes to human-induced climate change?

• Detection: is there an observed change that exceeds “internal variability”?

• Attribution: is the observed change consistent with expected anthropogenic influence? And inconsistent with alternative explanations?

• Models/theory must reconcile with observations

• Observations must be assessed for “false trends” based on evolving observational capabilities

The frequency of recorded storms (low-pass filtered) in the Atlantic basin is well-correlated with tropical Atlantic SSTs

Source: Emanuel (2006); Mann and Emanuel (2006) EOS. See also Holland and Webster (2007) Phil. Trans. R. Soc. A

But is the storm record reliable enough for this?

Trend from 1878-2006: Not significant (p=0.05, 2-sided tests, computed p-val ~0.2)Trend from 1900-2006: Is significant at p=0.05 level

Source: Vecchi and Knutson, J. Climate, in press.

+1.60 storms/century (1878-2006)+4.39 storms/century (1900-2006)

Trend from 1878-2006: Not significant (p=0.05, 2-sided tests, computed p-val ~0.2)

Trend from 1900-2006: Is significant at p=0.05 level


+1.60 storms/century (1878-2006)+4.39 storms/century (1900-2006)

TK 3

U.S. Landfalling vs basin-wide activity

Global Mean Temperature

Tropical AtlanticSea Surface Temp.

Atlantic Tropical Storm Counts (unadj.)

Adj. Atlantic Trop. Storm Counts (Vecchi/Knutson)

U.S. LandfallingTropical Storms (unadj.)

U.S. LandfallingHurricanes (unadj.)

Note: All time series are low-pass filtered (5-yr mean) and normalized to unit standard deviation (y-axis tic marks: 1 st. dev).

Source: Vecchi and Knutson, J. Climate, accepted for publication.

Storm Track Density Linear Trend (1878-2006)

Unadjusted Adjusted

TK 4

GFDL Zetac Model: A new high-resolution regional model for Atlantichurricane season simulations…

• The model runs for entire hurricane seasons.• The model generates its own sample of hurricanes during each season. • These experiments push the limits of available computing resources.

Prototype model for future global hurricane forecasting system…

Note: Model uses large-scale interior nudging to NCEP Reanalysis. Aug.-Oct. season only.

The model captures both the increase in hurricane activity since the 1980s and the year-by-year fluctuations….

Source: Updated from Knutson et al. 2007 (Bull. Amer. Meteor. Soc.)

Correlations: Model1: 0.72Model2: 0.68Ensem: 0.77



ACE:

PDI:

Cat 3-5:

Other hurricane metrics (ACE, PDI) are simulated fairly well, but major hurricanes are under-simulated in number (though still well-correlated)…

a) Observed b) Observed c) Observed

Tropical Storm Formation Tropical Storm Occurrence Hurricane Occurrence

d) Simulated e) Simulated f) Simulated

Zetac Regional Model Downscaling: geographical distribution of storms

Note: Model uses large-scale interior nudging to NCEP Reanalysis

Large-scale tropical Atlantic climate changes projected for late 21st

century by CMIP3 models (A1B scenario). Average SST change in MDR is 1.72oC with warming near 4oC in the upper troposphere.

Late 21st Century projections: increased vertical wind shear may lead to fewer Atlantic hurricanes

“storm-friendly” “storm-hostile”

Average of 18 models, Jun-Nov

Source: Vecchi and Soden, Geophys. Res. Lett., (2007)

Projected changes in Atlantic hurricane/tropical storm numbers:

Late 21st century; Zetac regional model downscaling of CMIP3 multi-model ensemble climate change signal.

Note: U.S. Landfalling hurricanes: -30%

Source: Knutson et al., 2008, submitted.

-27%

-18%

-8%

The 26.5oC “threshhold temperature” for tropical storm formation: a climate dependent threshhold…

1) Decreased frequency of tropical storms (-27%) and hurricanes (-18%).

3) Caveat: this model does not simulate hurricanes as strong as those observed.

The model provides projections of Atlantic hurricane and tropical storm frequency changes for late 21st century, downscaled fromm a multi-model ensemble climate change (IPCC A1B scenario):

Storm Intensities (Normalized by frequency)

4) A more consistent intensity increase is apparent after adjusting for decreased frequency

2) Increased frequency and intensity of the strongest hurricanes(5 12)

Source: Knutson et al., 2008, submitted.

The new model simulates increased hurricane rainfall rates in the warmer climate (late 21st century, A1B scenario) …consistent with previous studies…

Present Climate Warm Climate

Warm Climate – Present Climate

Rainfall Rates (mm/day)

Avg. Rainfall Rate Increases:50 km radius: +37%

100 km radius: +23%150 km radius: +17%400 km radius: +10%

Average Warming: 1.72oC

The control model reproduces the observed close relationship between SST and hurricane frequency (1980-2006), but this statistical relationship does not hold for future human-caused warming in the model.

Hurricane frequency actually decreases by 18% in the warm climate case… although the model does not simulate hurricanes as intense as observed.

Lesson: Caution using correlations from the present climate to make future climate projections…

Source: Knutson et al., submitted (2008).

Next Steps: Examining Model Dependence:

The regional details of future projections matter… (and these are not highly confident features of future climate projections).

Major hurricane count sensitivity varies (-8% to +~70% so far in very preliminary testing).

+70%Late 21st century

Current Climate

Changes in Atlantic hurricane activity as downscaled from GFDL CM2.1 climate model:

Hurricane Frequency: +3% Major Hurricane Frequency: +70%

Courtesy Morris Bender and Tim Marchok, NOAA/GFDL

Hurricane models project increasing hurricane intensities and rainfall rates with climate warming… …but probably not detectable at present.

Sources: Knutson and Tuleya, J. Climate, 2004 (left); Knutson and Tuleya (2008) Cambridge Univ Press (right).See also Bengtsson et al. (Tellus 2007) and Oouchi et (J. Meteor. Soc. Japan, 2006); Walsh et al. (2004) Stowasser et al. (2007).

~Late 21st century

Current climate

Hurricane Intensity Hurricane Rainfall Rates

Current climate

~Late 21st century

6-hr accumulated rainfall [cm] within ~100 km of storm center.

Sensitivity: ~4% increase in wind speed per oC SST increase

Sensitivity: ~12% increase in near-storm rainfall per oC SST increase

Alternative Downscaling Approach: Emanuel et al. (2008)

Alternative Downscaling Approach: Emanuel et al. (2008)Emanuel “Obs.” Last 25 yr: +250% (Atl.)

+47% (global)

KT04: +6%

K08: 2-3% (Atl.)

K08: -27% (? To -8%) Atl. Trop. Storms-18% (? To +7%) Atl. Hurricanes-8% (? To +70%) Major Hurricanes

K08: -25% (Atl., weak hurricanes)

(Tropical Storms)

…with some selected rough comparisons to other modeling studies

Projected 21st Century Changes in Thermodynamic Potential Intensity of Hurricanes

From Vecchi and Soden (2007, GRL)


Observations

Model/Theory

~8 m/s/oCEstimated Sensitivity of TC intensity to large-scale SST

How sensitive is hurricane intensity to greenhouse gas-induced warming?

Source for TC intensity/SST plots: Jim Kossin, personal communication, 2007.

Climate Change and Tropical Cyclones:Impediments to Scientific Understanding

1. Inhomogeneities in climate records, insufficient length of climate records related to tropical cyclones

2. Uncertainties in global climate projections (emissions, transient sensitivity)

3. Uncertainties in regional projections of climate models (wind shear, lapse rate, SST patterns, easterly waves)

4. Uncertainties in response of tropical cyclones to regional changes in climate (frequency, intensity)

5. For decadal predictions: Radiative forced response (Greenhouse gases, aerosols) vs internal climate variability (AMO)

Concluding Remarks: Where Do We Stand At This Point?

1. Based on the current state of models and ongoing data concerns, it is not appropriate at this time to make a likelihood statement attributing past changes in hurricane activity to increasing greenhouse gases or other human-caused factors.

2. Some statistical studies, particularly using data from recent decades, suggest the possibility of a human influence on hurricanes. Atlantic SST and hurricane “power dissipation” are well- correlated on low frequencies since ~1950. Studies attribute part of the Atlantic SST warming since 1950 to increasing greenhouse gases.

3. However, longer (~100-128 yr) tropical cyclone records, including consideration of data problems, give conflicting indications on whether there have been significant increases in Atlantic tropical storm and hurricane numbers. U.S. landfalling hurricanes show no increase. Basin-wide statistics are much more uncertain for these longer records than for the more recent decades.

4. Models: The recent increase in Atlantic hurricane numbers (1980-2006) is reproduced by models, and is strongly correlated to SSTs in both models and observations. However, the much larger projected greenhouse warming (late 21st century, A1B scenario) leads to decreased hurricane numbers in our model. We infer that the detailed structure of a tropical SST changes (whether it be due to internal variability, greenhouse gases, aerosols, etc.) has a major influence on how those SST changes will influence hurricanes.

5. Model dependence: While the average model projection calls for a slight decrease (-8%) in major hurricane counts by the late 21st century, at least one model (GFDL CM2.1) projects a large increase (+70%) in major hurricane counts. This indicates that further studies are needed to narrow the uncertainty in important details of future regional SSTs which may impact Atlantic hurricane activity.

6. Intensity: models simulate increased average hurricane wind speeds and rainfall rates in response to a general greenhouse gas-induced warming of tropical SSTs. Rough range of sensitivity: 1 to 8%/ deg C for wind speed; ~12%/deg C for near-storm rainfall. Significant increases of intensity have not yet been detected in observations (but stay tuned…). Attribution?

Regional model low-level winds (388 m); Red: >40 m/s (~“hurricane” strength)

GFDL Regional Model Simulation of the 2005 Atlantic Hurricane Season

Reference: T. Knutson et al., Bulletin of the American Meteorological Society, October 2007.

Developments in high-resolution hurricane modeling studies for greenhouse warming:

• Broccoli and Manabe (1990): “high resolution” was ~200 km grid model. Decreased tropical storm frequency, but physics dependence of result.

• Bengtsson et al. (1996): ~100 km grid model. Decreased tropical storm frequency. Still little credible intensity information.

• Knutson et al. (1998-2004): GFDL hurricane forecast model (18 km, later 9 km grid). Case study approach: stronger hurricanes with higher rainfall rates, even with ocean coupling. No frequency information, and wind shear effects remain a concern.

• Oouchi et al. (2006): Japan’s Earth Simulator allows 20 km grid global simulation. Frequency decreases globally but increases in Atlantic. Limited sampling of decadal variability (20 yr SST averages) raises questions about robustness of Atlantic result.

• Bengtsson et al. (2007): Range of global model resolutions from ~150 km to ~30km. Intensity increase seen at higher resolution only. A 20th century simulation (a unique experiment!) shows no significant tropical cyclone changes.

• Knutson et al. (2008): 18-km nested Atlantic model with interior nudging reproduces both the recent rise in Atlantic hurricane activity (1980-2006) and interannual (El Nino) variations. Cat 4-5 intensities are not simulated. New climate change experiment (based on multi-member ensemble of climate models) simulates decreased Atlantic hurricane numbers in the 21st century.

What type of SST “anthropogenic signal” should we look for in the tropical Atlantic?

Anthropogenic forcing: Linear trend-like, but the caveat of no indirect aerosol forcing yet.

Sources: Vecchi and Knutson, J. Climate, accepted for publication. See also Knutson et al. J. Climate, 2006

Natural + Anthropogenic forcing: Volcanic/solar/anthropogenic forcing gives a closer fit to observations … Residual (blue – black) = AMO??

Model: Radiative forcing and ocean circulation changes both provide a plausible explanation for the fluctuations in 20th

century Northern Hemisphere mean temperature (detrended)

Model: Some tentative evidence in support of a significant role for ocean circulation changes: surface and sub-surface fluctuations are out of phase in observations and modeled AMO-like change.

Sources: Zhang, Delworth and Held (2006)Zhang (2007) GRL papers.

Internal Climate Variability vs Radiative Forcing: Detrended NH Mean Temperatures

Source: Chris Landsea, NOAA/NHC

A measure of annual U.S. landfalling hurricane activity shows no clear long-term trend since 1900…

Summary Statement on Tropical Cyclones and Climate Change

WMO International Workshop on Tropical Cyclones VI

Highlights (excerpts) of Consensus Statements as of December 2006:

• No firm conclusion yet on whether there is a detectable anthropogenic signal in hurricane activity.

• Detection of trends is made more difficult by changes in hurricane observation methods over time and by strong multi-decadal variability in hurricane activity.

• Some increase in hurricane peak wind speed (and hurricane rainfall) is likely if the climate continues to warm. Projected magnitude: 3-5% increase in wind speed per degree Celsius sea surface temperature increase.

• Vulnerability to hurricane storm-surge flooding will increase if the projected rise in sea level due to global warming occurs.

Source: WMO (World Meteorological Organization)

Influence of pronounced greenhouse warming on distribution of hurricane tracks:


Source: Knutson et al. 2007 (in review)

r=0.77

r=0.73

r=0.57

r=0.87

r=0.69

r=0.41

r=0.73

r=0.16

Model performance: various Atlantic hurricane measures

Summary and Conclusions

1. Atlantic SST and hurricane “power dissipation” are well-correlated on low frequencies since ~1950. Since studies attribute part of the Atlantic SST warming since 1950 to increasing greenhouse gases, this suggests the possibility of a human influence on hurricanes.

2. Longer (~100-128 yr) tropical cyclone records, including consideration of data problems, give conflicting indications on whether there have been significant increases in Atlantic tropical storm and hurricane numbers. U.S. landfalling hurricanes show no increase.

3. In a new Atlantic nested regional hurricane model SSTs are strongly correlated with hurricane numbers (1980-2006), as in observations. However, a much larger simulated GHG-induced SST warming (late 21st century, A1B scenario) leads to decreased frequency of hurricanes in our model. This strongly suggests that the detailed structure of a climate warming (whether it be due to natural or anthropogenic causes) has a major influence on how a given warming will influence hurricanes.

4. Models simulate increased average hurricane wind speeds and rainfall rates in response to greenhouse gas-induced warming.

5. Model dependence of results is being examined using individual climate models in addition to the multi-model ensemble mean. Simulation results so far for major hurricanes: -8% (using multi- model ensemble climate change) vs. ~+70% (using GFDL CM2.1 model climate change). Further study is ongoing using other climate models and downscaling models.

6. Based on the current state of models and ongoing data concerns, it is not appropriate at this time to make a likelihood statement attributing past changes in hurricane activity to increasing greenhouse gases or other human-caused factors.

Changes in Atlantic hurricane activity as downscaled from GFDL CM2.1 modelMODEL DEPENDENCE:

-20%

+70%

Late 21st century

Current Climate

Model Dependence: Important Results To Date:

The regional details of future projections matter… (and these are not highly confident features of future climate projections).

Major hurricane count sensitivity varies (-8% to +~70% so far in very preliminary testing).


Zetac Regional Model Downscaling: Mean Precipitation and Tropical Storm Genesis

Observed (Aug.-Oct.)

Simulated (Aug.-Oct.)

mm/day

mm/day

Unadjusted tropical storm counts show a significant increasing trend…

But is the storm record reliable enough for this?


Reconstructing past tropical cyclone counts

• Satellite-era (1965-2006) storm tracks assumed perfect.• Apply satellite-era storm tracks to documented ship tracks

(ICOADS).• Storm detected if 2 ship observations within radius of tropical storm

force winds (17 m/s). First detection must occur equatorward of 40N. Monte Carlo simulation, varying storm radii within reasonable bounds.

• All land assumed to be “perfect detector” of tropical storms (equatorward of 40N)—planned to further test…

• Assume all relevant ship tracks are in data base—plan to further test with additional tracks. (First will look for evidence of storms in “new” ship data.)

Source: Vecchi and Knutson (2007) J. Climate, accepted.

Source: G. Vecchi, personal communtication

Adjusted Annual Count of Hurricanes (Cat 1-5)

Trend from 1878-2006: Not significant (p=0.05, 2-sided tests, computed p-val ~0.2)Trend from 1900-2006: Is significant at p=0.05 level


SST is still correlated with our adjusted series, though not quite as closely…

mm/day

Sample model precipitation snapshot

Warm-core structure:

Observed (Frank, 1977) Simulated hurricane composite

Deg C

Simulated Hurricane Structure – 18 km grid regional model


Showing the individual ensemble members (n=2)….

Source: Knutson et al. 2007 (BAMS)

North Atlantic Basin (August-October)Hurricane Frequency

Correlations vs. Obs: Model1: 0.76Model2: 0.76



But the model does not correlate as well with observed US landfall statistics as with basin-wide statistics...especially US hurricane landfalls

The model also reproduces the observed reduction of Atlantic hurricane activity during El Niño events fairly well…

Source: Knutson et al. (2007) Bull. Amer. Meteor. Soc.




ACE:

PDI:

Cat 3-5:

Other hurricane metrics (ACE, PDI) are simulated fairly well, but major hurricanes are under-simulated in number (though still well-correlated)…

The Zetac regional model does only a fair job at capturing the observed dependence of maximum Atlantic TC intensity on SST…(the simulated dependence is too weak)…

Large-scale tropical Atlantic climate changes projected for late 21st

century by CMIP3 models (A1B scenario). Average SST change in MDR is 1.72oC with warming near 4oC in the upper troposphere.

…and the model’s poor wind-pressure relation at lower central pressures leads to maximum near- surface winds of ~47 m/s, considerably lower than observed…

Note: Excessive simulated “core season” activity during last two seasons (2006, 2007). Why?

a) Histogram of minimum central pressure

b) Normalized histogram of minimum central pressure

The control model reproduces the observed close relationship between SST and PDI (1980-2006), but this strong relationship does not hold for future human-caused warming in the model.

PDI actually decreases by 24% in the warm climate case… although the model doesn’t simulate the hurricanes as intense as observed

Lesson: Caution using correlations from the present climate to make future climate projections…

A hint of an eastward shift in hurricane occurrence…

Some biases in hurricane occurrence may distort climate change projection

Influence of pronounced greenhouse warming on distribution of hurricane occurrence:

CMIP3 Multi-model ensemble down-scaling results: Warm climate vs. Control

U.S. landfallinghurricanes decrease by 30%, compared with an 18% decrease basin-wide.

Increased occurrence rate for strongest hurricanes is more pronounced using GFDL CM2.1 than for the multi-model ensemble …e.g. +70% for major hurricane counts…

Decreased tropical storm frequency in warm climate using either model

Late 21st century Atlantic hurricane simulations: (CMIP3 Multi-model ensemble vs. GFDL CM2.1)

Key: Black: ControlRed: Multi-modelBlue: GFDL CM2.1

Caveat: Regional model under-predicts major hurricane counts by 50% in the present climate simulation, but correlates at 0.7.

MODEL DEPENDENCE:


Source: Chris Landsea, NHC/NOAA. See EOS (2007) for final versions.

Pre-satellite era-77% strike land

…but some storms may have been missedand not recorded in the database.

Landsea: No significant trend from AMO warm phase to warm phase, or cold phase to cold phase.Holland: Assumption of constant landfalling fraction not justified. Multi-decadal variability of fraction??

Source: Adapted from Landsea (2007) EOS. Note: includes 5 of 13 additional early 20th century storms recently identified by Landsea et al.

Sources: Richard Smith, UNC, personal communication (left); Gabe Vecchi, GFDL, personal communication (right)

Application of Vecchi/Knutson approach to hurricanes

TK 8

Statistical significance testing

• Method 1: Linear least-squares regression on annual storm count series. Adjust degrees of freedom for two-sided t-test based on lag- 1 autocorrelation.

• Method 2: Same as Method 1, but for the ranks rather than the original series. Addresses issue of skewness in storm count annual data.

• Method 3: Bootstrap resampling (with replacement) of series sug- segments of length L. Compute linear trends of resampled data sets as a control comparison. L values in range of 2-8 tested. (Recommended value of 2-3 based on Wilks text.)

• The three methods give roughly similar results here, although Method 3 appears a bit more conservative (at least for some L values in range of 2-8).

One possible reason for the difference between US landfalling and basin-wide trends: storm occurrence may have decreased preferentially in the western part of the basin…


A comparison of several climate change metrics:

Global Mean Temperature

Tropical Atlantic Sea Surface Temperature

Atlantic Tropical Storm Counts (unadj.)

Atlantic Trop. Storm Counts (Vecchi/Knut. Adj.)

U.S. Landfalling Tropical Storms (unadj.)

U.S. Landfalling Hurricanes (unadj.)

Note: All time series are low-pass filtered (5-yr mean) and normalized to unit standard deviation (y-axis tic marks: 1 st. dev).


1) Decreased frequency of tropical storms (-27%) and hurricanes (-18%).

2) Increased frequency and intensity of the strongest hurricanes(5 12) or +140%

3) Caveat: this model does not simulate hurricanes as strong as those observed.

The model provides projections of Atlantic hurricane tropical storm frequency changes for late 21st century A1B warming scenario:

Application of the Vecchi/Knutson approach to Atlantic Hurricane Counts

Source: G. Vecchi, personal communtication

Research needs for this problem:

• Improved observations: particularly reanalysis efforts to produce “climate quality data” on hurricanes for detection/attribution studies (e.g., PDI error? Adjusted storm counts?)

• Improved hurricane modeling: Higher resolution models, retrospective studies of 20th century hurricane variability that can simulate or infer past hurricane activity

• Observational and modeling studies of Atlantic multidecadal variability (e.g., the Atlantic Multidecadal Oscillation—AMO, and “forced climate change” (past and future) affecting SST, shear, etc. in the basin

• Paleoclimate studies, both for the AMO and for pre-historic hurricane activity.

• Convergence of statistical models (correlations) and dynamical models (hurricane simulation)

• More confident projections of future Atlantic large-scale environment (e.g., reexamination of Vecchi/Soden with future models, improved aerosols, etc.)

Sea surface temperatures have increased in the region where Atlantic hurricanes form and intensify, and they are projected to

increase much more during the 21st century…

Future Atlantic Hurricane Activity?

• Models indicate increased hurricane intensities with warmer ocean temperatures.

• Increased vertical wind shear may reduce hurricane activity in the Atlantic.

• How do we assess which of these effects will “win out”?

Projected 21st Century Changes in Thermodynamic Potential Intensity of Hurricanes

From Vecchi and Soden (2007, GRL)

“storm-friendly”“storm-hostile”


(Alternative) Conclusions

• Observed data, including consideration of data problems, give conflicting indications on whether there have been significant increases in Atlantic tropical storm and hurricane numbers.

• High resolution models consistently project increasing hurricane intensities and rainfall rates for the late 21st

century, but whether there will be more or fewer hurricanes remains uncertain.

• A new modeling approach reproduces many important aspects of Atlantic hurricane activity observed since 1980, and thus shows promise as a tool for both understanding past variations and for making more reliable projections of future hurricane activity.

• Other slides (not used in presentation for now)

Correlations vs. Obs: Model Ensemble: 0.73


Atlantic Major Hurricane counts (basin-wide) since the mid-1940s: no long-term trend

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