Tropical Cyclones and Climate

Tropical Cyclones and Climate

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

• PaleotempestologyPaleotempestology

• ModelsModels

Effect of Climate Change on Effect of Climate Change on HurricanesHurricanes

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

Power Dissipation Based on 3 Data Sets for Power Dissipation Based on 3 Data Sets for the Western North Pacificthe Western North Pacific(smoothed with a 1-3-4-3-1 filter)

aircraft recon

Data Sources: NAVY/JTWC, Japan Meteorological Agency, UKMO/HADSST1, Jim Kossin, U. Wisconsin

Years included: 1949-2004

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

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 ProductionEnergy Production

Theoretical Upper Bound on Theoretical Upper Bound on Hurricane Maximum Wind Speed:Hurricane Maximum Wind Speed:

*2| |0

C T Tk s oV k kpot TC

oD

Air-sea enthalpy disequilibrium

Surface temperature

Outflow temperature

Ratio of exchange coefficients of enthalpy and momentum

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

PaleotempestologyPaleotempestology

barrier beach

backbarrier marshlagoon

barrier beach

backbarrier marshlagoon

a)

b)

Source: Jeff Donnelly, WHOI

upland

upland

flood tidal delta

terminal lobes

overwash fan

overwash fan

Paleotempestology

Donnelly and Woodruff (2006)

Photograph of stalagmite ATM7 showing depth of radiometric dating samples, micromilling track across approximately annually laminated couplets, and age-depth curve.

Frappier et al., Geology, 2007

Frappier et al., Geology, 2007

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.

Track:Track:

850 2501 ,track V V V V

Empirically determined constants:

0.8, 10 ,u ms

12.5v ms

Example: 200 Synthetic TracksExample: 200 Synthetic Tracks

Present Climate: Spatial Present Climate: Spatial Distribution of Genesis PointsDistribution of Genesis Points

Observed

Synthetic

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

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)

Year by Year Comparison with Best Track Year by Year Comparison with Best Track and with Knutson et al., 2007and with Knutson et al., 2007

Simulated vs. Observed Power Dissipation Trends, 1980-2006Simulated vs. Observed Power Dissipation Trends, 1980-2006

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:

Genesis DistributionsGenesis Distributions

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

Basin-Wide Percentage Change Basin-Wide Percentage Change in Storm Frequencyin Storm Frequency

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

Synthetic Events driven by GFDL Synthetic Events driven by GFDL AM2.1, Observed SSTsAM2.1, Observed SSTs

Feedback of Global Tropical Feedback of Global Tropical Cyclone Activity on the Cyclone Activity on the

Climate SystemClimate System

Strong Mixing of Upper Ocean

Direct mixing by tropical cyclones

Source: Rob Korty, CalTech

Emanuel (2001) estimated global rate of heat input as

1.4 X 1015 Watts

TC Mixing May Induce Much or Most of the Observed Poleward Heat Flux by the Oceans

Trenberth and Caron, 2001Trenberth and Caron, 2001

TC-Mixing may be Crucial for High-Latitude Warmth and Low-Latitude Moderation During Warm Climates,

such as that of the Eocene

SST: elevated mixing to 360 meters – uniformSST: elevated mixing to 360 meters – uniform

10 x CO2 in both experimentsSource: Rob Korty, CalTech

Interactive TC-Mixing Moderates Tropical Warming and Interactive TC-Mixing Moderates Tropical Warming and Amplifies High-Latitude Warming in Coupled Climate ModelsAmplifies High-Latitude Warming in Coupled Climate Models

““Slippery Sacks” Ocean Model, Patrick Slippery Sacks” Ocean Model, Patrick HaertelHaertel

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 data and paleotempestologydata and paleotempestology

• 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

• Storm-induced mixing of the upper tropical Storm-induced mixing of the upper tropical ocean may be the principal driver of the ocean may be the principal driver of the ocean’s thermohaline circulationocean’s thermohaline circulation

• Increased TC power dissipation in a warming Increased TC power dissipation in a warming climate will drive a larger poleward heat flux climate will drive a larger poleward heat flux by the oceans, tempering tropical warming by the oceans, tempering tropical warming but amplifying the warming of middle and but amplifying the warming of middle and high latitudeshigh latitudes