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Is Global Warming Affecting Hurricanes?. Kerry Emanuel Massachusetts Institute of Technology. Program. Overview of hurricane risk Evidence connecting hurricane activity to tropical sea surface temperature The evidence for anthropogenic forcing of tropical ocean temperature The future.

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is global warming affecting hurricanes

Is Global Warming Affecting Hurricanes?

Kerry Emanuel

Massachusetts Institute of Technology

program
Program
  • Overview of hurricane risk
  • Evidence connecting hurricane activity to tropical sea surface temperature
  • The evidence for anthropogenic forcing of tropical ocean temperature
  • The future
hurricane risk
Hurricane Risk
  • Tropical cyclones account for the bulk of natural catastrophe U.S. insurance losses
  • Losses vary roughly as the cube of the maximum wind speed
  • Katrina caused > 1300 deaths and > $130 billion in damage
summary of u s hurricane damage statistics
Summary of U.S. Hurricane Damage Statistics:
  • >50% of all normalized damage caused by top 8 events, all category 3, 4 and 5
  • >90% of all damage caused by storms of category 3 and greater
  • Category 3,4 and 5 events are only 13% of total landfalling events; only 30 since 1870
  • Landfalling storm statistics are grossly inadequate for assessing hurricane risk
intensity metric the power dissipation index
Intensity Metric:The Power Dissipation Index

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

slide13
Power Dissipation Based on 3 Data Sets for the Western North Pacific(smoothed with a 1-3-4-3-1 filter)

Years included: 1949-2004

aircraft recon

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

slide14

North Atlantic PDI and Sea Surface Temperatures

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

Years included: 1970-2006

Scaled Temperature

Power Dissipation Index (PDI)

Data Sources: NOAA/TPC, UKMO/HADSST1

slide15

Atlantic Sea Surface Temperatures and Storm Max PDI

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

Years included: 1870-2006

Power Dissipation Index (PDI)

Scaled Temperature

Data Sources: NOAA/TPC, UKMO/HADSST1

what environmental factors control hurricane power dissipation
What Environmental Factors Control Hurricane Power Dissipation?
  • Potential Intensity
  • Wind Shear
  • Low level environmental vorticity (“spin”)
slide19

Distribution of Entropy in Hurricane Inez, 1966

Source: Hawkins and Imbembo, 1976

maximum theoretical wind speed v pot
Maximum Theoretical Wind Speed, Vpot

Net outgoing radiation

Sea Surface Temperature

Incoming solar radiation

Ocean mixed layer entrainment

Temperature at top of storm

Surface Trade Wind speed

potential intensity and sst can be changed by
Potential intensity and SST can be changed by:
  • Changing solar and infrared radiation
  • Changing ocean mixed layer entrainment
  • Changing mean surface wind speed

Also, Potential Intensity (but NOT SST) can be changed by changing the storm top temperature

slide24

Contributions to North Atlantic Potential Intensity

(Log of each contribution, minus long-term mean)

contributions to north atlantic hurricane power dissipation
Contributions to North Atlantic Hurricane Power Dissipation:

(Log of each contribution, minus long-term mean)

slide27

Atlantic Sea Surface Temperatures andSurface Temperature

Aug-Oct Sea Surface Temperatures (at key latitudes)

Aug-Oct HADCRU NH Surface Temperature

slide29

Begin with Global Mean

Surface Temperature

greenhouse gases and associated radiative forcing have been increasing

Carbon Dioxide

Methane

Nitrous Oxide

Halocarbons

Total LLGHG

Greenhouse Gases and Associated Radiative Forcing have been Increasing

3

2.5

2

Radiative forcing (Watts/square meter)

1.5

1

0.5

0

1750

1800

1850

1900

1950

2000

Year

slide32

Pelée

Agung

El Chichón

Pinatubo

slide33

Northern hemisphere surface temperature (and late summer-early fall tropical Atlantic sea surface temperature) represents a linear combination of global warming and aerosol cooling

Hypothesis about Why the Northern Hemisphere Differs from the Globe:

Mann and Emanuel 2006

tropical atlantic sst blue global mean surface temperature red aerosol forcing aqua
Tropical Atlantic SST(blue), Global Mean Surface Temperature (red), Aerosol Forcing (aqua)

Global Mean Surface T

MDR SST

Aerosol forcing

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

slide35
Best Fit Linear Combination of Global Warming and Aerosol Forcing (red) versus Tropical Atlantic SST (blue)

MDR SST

Global mean T

+ aerosol forcing

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

using physics to improve hurricane risk assessment
Using Physics to Improve Hurricane Risk Assessment
  • Generate very large number of synthetic storm tracks consistent with the general circulation of the atmosphere in a given climate
  • Run a coupled ocean-atmosphere model of hurricane intensity along each track to generate wind fields
slide41

Use Daily Output from Climate Models to Derive Wind Statistics, Thermodynamic State Needed by Synthetic Track Technique(but hold genesis PDF constant!)

slide42

Compare two simulations from IPCC set: 1. Last 20 years of 20th century simulations2. Years 2180-2200 of IPCC Scenario A1b (CO2 stabilized at 720 ppm)

slide43

Results Using 2000 Atlantic and 2000 North Pacific Tracks from 5 Models:

Percent Increase in Basin Power Dissipation

slide44

Results Using 2000 Atlantic and 2000 North Pacific Tracks from 5 Models:

Percent Increase in Landfall Power Dissipation

summary
Summary
  • Atlantic TC frequency, intensity and duration are co-varying with tropical Atlantic SST
  • Changes in tropical cyclone power are driven by changing potential intensity, wind shear, and “spin” of the low-level winds
slide46
Changes in tropical North Atlantic sea temperature mirror changes in northern hemispheric temperature and are probably driven by a combination of cooling by volcanoes and air pollution, and warming by greenhouse gases
  • Long-term risk assessments must account for climate change
scientific basis of the natural cycles story
Scientific Basis of the “Natural Cycles” Story

The Atlantic Multi-Decadal Oscillation (AMO)

slide49

The AMO is a Pattern of Sea

Surface Temperature

High-latitude North Atlantic

“Main development region”

S. B. Goldenberg et al., 2001. Science, 293, 474-479

slide50

Variation with time of amplitude of third rotated EOF of the non-ENSO residual 1856-1991 de-trended SST data

slide53

A

De-trended Aug-Oct Northern Hemisphere Surface Temperature

(Hadley Centre Global Surface Temperature Data)

Variation with Time of the Strength of the AMO

(Goldenberg et al. 2001)

boston
Boston

HURDAT: 28 events Method 2: 3000 events

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