Atlantic Hurricanes and Climate Change

Hurricane Katrina, Aug. 2005

GFDL model simulation of Atlantic hurricane activity

Tom Knutson

NOAA / Geophysical Fluid Dynamics LabPrinceton, New Jersey

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

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Outline:

• Observations(Historical Trends?)

• Model Projections(with climate warming)

• Conclusions

Climate Change Attribution

• are observed changes consistent with

expected responses to forcings

inconsistent with alternative explanations

Observations

All forcing

Solar+volcanic

Source: IPCC 4th Assessment Report. Used with permission.

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.

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Increasing data uncertainty

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Source: Vecchi et al. Science (2008)

Projection 1: Absolute SST

• ~300% projected increase in Power Dissipation

• Indirect attribution: CO2 SST Hurricanes

Projection 2: Relative SST

• Projected change: sign uncertain, +/- 80%

• No Attribution

• Supported by dynamical models

.

Two future projections of Atlantic tropical cyclone power dissipation

The frequency of tropical storms (low-pass filtered) in the Atlantic basin since 1870 has some correlation 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?

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Tropical storm occurrence has apparently decreased in the Gulf of Mexico and Caribbean…Increases are mostly located in the open Atlantic and off the U.S. East Coast (in original, unadjusted data)…

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

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Ship tracks have changed in density and location over time

Source: Vecchi and Knutson , J. Climate, 2008.

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Atlantic tropical storms (< 2 day duration) show a strong rising trend, but storms of >2 day duration--adjusted for missing storms--do not show a trend.

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Sources: Vecchi and Knutson (2008) Landsea et al. (2009) Vecchi and Knutson (in preparation)

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Adjustments to storm counts based on ship/storm track locations and density

Source: Elsner et al., Nature, 2008.

There is some statistical evidence that the strongest hurricanes are getting stronger. This signal is most pronounced in the Atlantic. However, the satellite-based data for the global analysis are only available for 1981-2006.

Quantile regression computes linear trends for particular parts of the distribution. The largest increases of intensity are found in the upper quantiles (upper extremes) of the distribution.

10Global Tropical Cyclone

Intensity Trends

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Projections of Future Changes in Climate

Best estimate for low scenario (B1) is 1.8°C (likely range is 1.1°C to 2.9°C), and for high scenario (A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C).

Broadly consistent with span quoted for SRES in TAR, but not directly comparable

Source: IPCC 4th Assessment Report. Used with permission.

Projected Atlantic region climate changes: 18-Model CMIP3 ensemble

Higher shear

Higher potential intensity

Zetac Regional Model reproduces the interannual variability and trend of Atlantic hurricane counts (1980-2006)

18-km grid model nudged toward large-scale (wave 0-2) NCEP Reanalyses

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Source: Knutson et al., 2007, Bull. Amer. Meteor. Soc.

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

Source: Knutson et al., 2008, Nature Geoscience.

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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 from 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, Nature Geoscience.

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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

Source: Knutson et al., 2008, Nature Geoscience.

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Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes, Science, January 2010.

Morris A. Bender, Thomas R. Knutson, Joseph J. Sirutis, Robert E. Tuleya, Gabriel A. Vecchi, Stephen T. Garner and Isaac M. Held

Geophysical Fluid Dynamics Laboratory/NOAA

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18km grid Zetac regional climate model

9 km GFDL hurricane model

observed

Simulated distributions of maximum wind speeds(Atlantic tropical storms and hurricanes, 1980-2006)

Down-scaled GFDL hurricane prediction model produced much more realistic distribution of maximum wind speeds compared to Zetac.

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20 30 40 50 60 70 80Maximum Wind Speed (m/s)

No

rmalized

occu

rrences

20 In a warmer climate (late 21st century A1B scenario) the hurricane model simulates an expanded distribution of Atlantic hurricane intensities.

The strongest hurricanes increase in number for the downscaled ensemble-mean climate warming…

…and increase for 3 of 4 individual climate models tested.

Control

Source: Bender et al., Science, 2010.

Late 21st Century Climate Warming Projection-- Average of 18 CMIP3 Models

(27 Simulated Hurricane Seasons) Source: Bender et al., Science, 2010

Control Climate (Odd Years Only)

GFDL-CM2.1

MRI-CGCM

MPI-ECHAM5

UKMO-HADCM3

Degrees Longitude East

Deg

rees Latitu

de

NWS VERSION (GFDL)

Tracks of Storms that Reached Category 4 or 5 Intensity

Degrees Longitude East Degrees Longitude East

Deg

rees Latitu

de

Late 21st Century Warmed Climate Projection based on 4 Individual CMIP3 Climate Models

SUMMARY OF PROJECTED CHANGE

• Colored bars show changes for the18 model CMIP3 ensemble (27 seasons); dots show range of changes across 4 individual CMIP models (13 seasons).

Cat 4+5 frequency: 81% increase, or 10% per decade

Source: Bender et al., Science, 2010.

Estimated net impact of these changes on damage potential:

+28%

Emergence Time Scale: If the observed Cat 4+5 data since 1944 represents the noise (e.g. through bootstrap resampling), how long would it take for a trend of ~10% per decade in Cat 4+5 frequency to emerge from noise? Answer: ~60 yr (by then 95% of cases are positive)

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Source: Bender et al., Science, 2010.

Instead, assume residuals from a 4th order polynomial: 55 yr

Instead, resample chunks of length 3-7 yr: 65-70 yr

Conclusions: What can data and climate models tell us about global warming and Atlantic hurricanes?

i) Models, together with observations and theory, provide a compelling case that human emissions of greenhouse gases have caused much of the long-term global warming over the past 50 yr and 140 yr.

ii) Models project substantial further warming over the 21st century, including in the tropical Atlantic.

iii) Sea level rise is expected to exacerbate storm surge impacts even assuming storms themselves do not change.

iv) Models project reductions or little change in Atlantic tropical storm or hurricane numbers, or in aggregate hurricane activity (e.g., annual Power Dissipation Index). (Increased shear outweighs warmer SSTs.)

v) Models suggest that the (fewer) surviving storms may at times reach favorable areas with both warmer SSTs and low shear, leading to a greater number of very intense category 4 and 5 hurricanes than at present (possibly a doubling in annual frequency by 2100)

vi) However, we cannot yet conclude that humans have already caused a detectable change in Atlantic hurricane activity. Note that humans may have already caused changes that are either below the 'detection threshold' or are not yet properly modeled (e.g., aerosol effects).

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Decreasing

Confidence

Extra slides

• In case some questions come up during Q&A.

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Tropical Cyclones Frequency Projections (Late 21st century) - Summary

Blue = decrease

Red = increase

Tropical Cyclone Intensity Projections

Blue = decrease

Red = increase

Tropical Cyclone Frequency Projections: Higher Intensity Storms

Blue = decrease

Red = increase

Tropical Cyclone Precipitation Rate Projections

Blue = decrease

Red = increase

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)

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Projected Changes in Regional Hurricane Activity

GFDL 50-km HIRAM, using four projections of late 21st Century SSTs.

Red/yellow = increaseBlue/green = decrease

• Regional increases/decreases much larger than global-mean.

• Pattern depends on details of SST change.

Source: Zhao, Held, Lin and Vecchi (J. Climate, in press)

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Unit: Number per year

18-model CMIP3 Ensemble GFDL CM2.1

HadCM3 ECHAM5

Tropical Storms

All Hurricanes (Cat 1-5)

Major Hurricanes (Cat 3-5)

Cat 4-5 Hurricanes (>131 mph)

Most Intense Hurricanes (>145 mph)

GFDL CM2.1

MRI

18-Model Ensemble

MPI

HadCM3

Ensemble: +220%. Range: - 60% to +180%

Tropical storms and hurricanes consistently decrease in number in the warmer climate, but…

Hurricane Intensity

Class

…the rarer most intense simulated hurricanes occur up to 3 times as often in the warmer climate, and increased for 3 of 4 individual models

1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 BASE YEAR

Ensemble: +75%. Range: - 53% to +110%

Ensemble: -18%. Range: - 60% to +40%

Ensemble: -33%. Range: - 60% to -7.5%

Ensemble: -27%. Range: - 48% to - 3%

Changes in Atlantic Hurricane Counts by Intensity Class: Late 21st century A1B Projection

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Source: Bender et al., Science, 2010.

HIRAM 50 km grid model TC correlations

for several basins

North Atlantic

East Pacific

West Pacific

red: observationsblue: HiRAM ensemble meanshading: model uncertainty

corr=0.83

corr=0.62

corr=0.52

Hurricane counts for each basinare normalized by atime-independent multiplicative factor

Correlation for the SouthPacific is ~0.3 and insignificantfor the Indian Ocean

Source: Zhao, Held, Lin, and Vecchi (J. Climate, in press)

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Tom Knutson, Co-Chair Geophysical Fluid Dynamics Laboratory/NOAA,

Princeton, USA

John McBride, Co-Chair Center for Australian Weather and Climate

Research, Melbourne, Australia

Johnny Chan University of Hong Kong, Hong Kong, China

Kerry Emanuel Massachusetts Institute of Technology, Cambridge, USA

Greg Holland National Center for Atmospheric Research, Boulder, USA

Chris Landsea National Hurricane Center/NOAA, Miami, USA

Isaac Held Geophysical Fluid Dynamics Laboratory/NOAA, USA

Jim Kossin National Climatic Data Center/NOAA, Madison, USA

A.K. Srivastava India Meteorological Department, Pune, India

Masato Sugi Research Institute for Global Change/JAMSTEC, Yokohama, Japan

Author Team:

SUMMARY ASSESSMENT:

Detection and Attribution:

It remains uncertain whether past changes in any tropical cyclone activity (frequency, intensity, rainfall, etc.) exceed the variability expected through natural causes, after accounting for changes over time in observing capabilities.

SUMMARY ASSESSMENT:

Tropical Cyclone Projections: Frequency

It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged due to greenhouse warming. We have very low confidence in projected changes in individual basins. Current models project changes ranging from -6 to -34% globally, and up to ± 50% or more in individual basins by the late 21st century.

“Likely”: >67% probability of occurrence, assessed using expert judgment

SUMMARY ASSESSMENT:

Tropical Cyclone Projections: Intensity

Some increase in mean tropical cyclone maximum wind speed is likely (+2 to +11% globally) with projected 21st century warming, although increases may not occur in all tropical regions. The frequency of the most intense (rare/high-impact) storms will more likely than not increase by a substantially larger percentage in some basins.

“More likely than not”: >50% probability of occurrence, assessed using expert judgment

SUMMARY ASSESSMENT:

Tropical Cyclone Projections: Rainfall

Tropical cyclone rainfall rates are likely to increase. The projected magnitude is on the order of +20% within 100 km of the tropical cyclone center.

SUMMARY ASSESSMENT:

Tropical Cyclone Projections: Genesis, Tracks, Duration, and Surge Flooding

We have low confidence in projected changes in genesis location, tracks, duration, or areas of impact. Existing model projections do not show dramatic large-scale changes in these features. The vulnerability of coastal regions to storm surge flooding is expected to increase with future sea level rise and coastal development, although this vulnerability will also depend on future storm characteristics.

On longer time scales, the rising trend in Atlantic tropical storms is due mostly to very short lived storms (< 2 day duration)

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Source: Landsea et al., J. Climate, in press.

Source: Elsner et al., Nature, 2008.

There statistical evidence that the strongest hurricanes are getting stronger is most convincing for the Atlantic (1981-2006).

The North and South Indian Ocean data also suggest increased intensity, but data are more uncertain for those regions (e.g., satellite view angle changes).

Atlantic

Western North Pacific Eastern North Pacific

South Indian

South Pacific North Indian

The intensity change signal is quite weak for the Pacific basins.

Tropical Cyclone Intensity Trends in Various Basins

Eastern North Pacific(corrected)

b

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