Hurricanes and Global Warming

Hurricanes and Global Warming

Kerry EmanuelMassachusetts Institute of Technology

IssuesIssues

• Effect of climate change on tropical cyclone Effect of climate change on tropical cyclone activityactivity

• Role of tropical cyclones in the climate Role of tropical cyclones in the climate systemsystem

ApproachesApproaches

• The historical recordThe historical record

• PhysicsPhysics

• ModelsModels

The Historical RecordThe Historical Record

Global TC Frequency, 1970-2006Global TC Frequency, 1970-2006

Data Sources: NOAA/TPC and NAVY/JTWC

Better Intensity Metric:Better Intensity Metric:

The Power Dissipation IndexThe Power Dissipation Index

0

3maxPDI V dt

A measure of the total frictional dissipation of kinetic A measure of the total frictional dissipation of kinetic energy in the hurricane boundary layer over the energy in the hurricane boundary layer over the

lifetime of the stormlifetime of the storm

Atlantic Storm Maximum Power DissipationAtlantic Storm Maximum Power Dissipation(Smoothed with a 1-3-4-3-1 filter)

Po

wer

Dis

sip

atio

n In

dex

(P

DI)

Years included: 1870-2006

Data Source: NOAA/TPC

Atlantic Sea Surface Temperatures and Atlantic Sea Surface Temperatures and Storm Max Power DissipationStorm Max Power Dissipation

(Smoothed with a 1-3-4-3-1 filter)

Sca

led

Tem

per

atu

re

Po

wer

Dis

sip

atio

n In

dex

(P

DI)

Years included: 1870-2006

Data Sources: NOAA/TPC, UKMO/HADSST1

10-year Running Average of Aug-Oct NH Surface T and 10-year Running Average of Aug-Oct NH Surface T and MDR SSTMDR SST

Tropical Atlantic SST(blue), Global Mean Surface Tropical Atlantic SST(blue), Global Mean Surface Temperature (red), Temperature (red),

Aerosol Forcing (aqua)Aerosol Forcing (aqua)

Mann, M. E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. EOS, 87, 233-244.

Global mean surface temperature

Tropical Atlantic sea surface temperature

Sulfate aerosol radiative forcing

Best Fit Linear Combination of Global Warming Best Fit Linear Combination of Global Warming and Aerosol Forcing (red) versus Tropical Atlantic and Aerosol Forcing (red) versus Tropical Atlantic

SST (blue)SST (blue)

Mann, M. E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. EOS, 87, 233-244.

Tropical Atlantic sea surface temperature

Global Surface T + Aerosol Forcing

PhysicsPhysics

Energy Production: The HurricaneEnergy Production: The Hurricaneas a Carnot Heat Engineas a Carnot Heat Engine

0o 60oE 120oE 180oW 120oW 60oW

60oS

30oS

0o

30oN

60oN

0 10 20 30 40 50 60 70 80

Annual Maximum Potential Intensity (m/s)Annual Maximum Potential Intensity (m/s)

Observed Tropical Atlantic Potential IntensityObserved Tropical Atlantic Potential Intensity

Data Sources: NCAR/NCEP re-analysis with pre-1979 bias correction, UKMO/HADSST1

Emanuel, K., J. Climate, 2007

Effect of Increased Potential Effect of Increased Potential Intensity on Hurricane KatrinaIntensity on Hurricane Katrina

Projecting into the Future: Projecting into the Future: Downscaling from Global Downscaling from Global

Climate ModelsClimate Models

Today’s global climate Today’s global climate models are far too coarse to models are far too coarse to simulate tropical cyclonessimulate tropical cyclones

Our ApproachOur Approach• Step 1: Seed each ocean basin with a very large

number of weak, randomly located cyclones

• Step 2: Cyclones are assumed to move with the large scale atmospheric flow in which they are embedded

• Step 3: Run a coupled, ocean-atmosphere computer model for each cyclone, and note how many achieve at least tropical storm strength

• Step 4: Using the small fraction of surviving events, determine storm statistics.

200 Synthetic U.S. Landfalling tracks (color 200 Synthetic U.S. Landfalling tracks (color coded by Saffir-Simpson Scale)coded by Saffir-Simpson Scale)

CalibrationCalibration

• Absolute genesis frequency calibrated Absolute genesis frequency calibrated to North Atlantic during the period to North Atlantic during the period 1980-20051980-2005

Genesis ratesGenesis rates

AtlanticAtlantic

Eastern North Pacific

Western North Pacific

North Indian Ocean

Southern Hemisphere

Calibrated to AtlanticCalibrated to Atlantic

Seasonal CyclesSeasonal Cycles

AtlanticAtlantic

Cumulative Distribution of Storm Lifetime Cumulative Distribution of Storm Lifetime Peak Wind Speed, with Sample of 2946Peak Wind Speed, with Sample of 2946

Synthetic TracksSynthetic Tracks

Captures effects of regional climate Captures effects of regional climate phenomena (e.g. ENSO, AMM)phenomena (e.g. ENSO, AMM)

Global Percentage of Cat 4 & Cat 5 Global Percentage of Cat 4 & Cat 5 StormsStorms

Now Use Daily Output from IPCC Now Use Daily Output from IPCC Models to Derive Wind Models to Derive Wind

Statistics, Thermodynamic State Statistics, Thermodynamic State Needed by Synthetic Track Needed by Synthetic Track

TechniqueTechnique

1. Last 20 years of 20Last 20 years of 20thth century century simulationssimulations

2.2. Years 2180-2200 of IPCC Years 2180-2200 of IPCC Scenario A1b (COScenario A1b (CO22 stabilized at stabilized at

720 ppm)720 ppm)

Compare two simulations each Compare two simulations each from 7 IPCC models:from 7 IPCC models:

Basin-Wide Percentage Change Basin-Wide Percentage Change in Power Dissipationin Power Dissipation

7 Model Consensus Change in 7 Model Consensus Change in Storm FrequencyStorm Frequency

U.S. Coastal Damage PotentialU.S. Coastal Damage Potential

Summary:Summary:

• Tropical cyclones are sensitive to the Tropical cyclones are sensitive to the climate state, as revealed by historical climate state, as revealed by historical (and geological) data(and geological) data

• Observations together with detailed Observations together with detailed modeling suggest that TC power modeling suggest that TC power dissipation increases by ~65% for a dissipation increases by ~65% for a 10% increase in potential intensity10% increase in potential intensity

• New technique for downscaling climate New technique for downscaling climate models shows promise for predicting models shows promise for predicting response of global tropical cyclone response of global tropical cyclone activity to climate changeactivity to climate change

• Climate models may have systematic Climate models may have systematic errors that compromise estimates of errors that compromise estimates of tropical cyclone response to global tropical cyclone response to global warmingwarming