Tropical Cyclones and Climate:Tropical Cyclones and Climate:Some New FindingsSome New Findings

Kerry EmanuelKerry EmanuelProgram in Atmospheres, Oceans, and Program in Atmospheres, Oceans, and

ClimateClimateMITMIT

Some IssuesSome Issues

• What processes control rates of genesis of What processes control rates of genesis of tropical cyclones?tropical cyclones?

• What processes control the actual and What processes control the actual and potential intensity of TCs? potential intensity of TCs?

• Do TCs have important feedbacks on climate?Do TCs have important feedbacks on climate?

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)

Scal

ed T

empe

ratu

re

Pow

er D

issi

patio

n In

dex

(PD

I)

Years included: 1870-2006

Data Sources: NOAA/TPC, UKMO/HADSST1

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

2 *| | 0s b

C T Tk s oV T s spot C TD o

Air-sea enthalpy disequilibrium

Surface temperature

Outflow temperature

Ratio of exchange coefficients of enthalpy and momentum

s0* = saturation entropy of sea surface

sb = actual entropy of subcloud layer

Condition of convective neutrality:

sb = s* of free troposphere

Also, s* of free troposphere is approximately spatially uniform (WTG approximation)

*2 *| | 0s

C T Tk s oV T s spot C TD o

approximately constant

What matters, apparently, is the SST (sWhat matters, apparently, is the SST (s00*) relative *) relative to the tropospheric temperature (s*)to the tropospheric temperature (s*)

Can SST Vary Independently of s*?Can SST Vary Independently of s*?

* *0

* *0

* *0

| | ( )

| |

| |

k a s rad ocean

rad ocean

k a s

rad ocean

k a s

C T s s F h F

F h Fs s

C T

and

F h Fs s

C T

10

10

10

V

V

V

Ocean Surface Energy Balance:

Answer: On time scales long enough for the ocean Answer: On time scales long enough for the ocean mixed layer to be in thermal equilibrium (>~ 1 year)mixed layer to be in thermal equilibrium (>~ 1 year)

• Potential intensity is determined by local radiative balance, local convergence of ocean heat flux, local surface wind speed, and local outflow temperature only

• Remote influences limited to remote effects on surface wind surface radiation ocean heat flux and, in marginal zones, on outflow temperature

• SST cannot vary independently of free atmospheric temperature

Interpretation of Recent Trends in Interpretation of Recent Trends in Potential IntensityPotential Intensity

Based on NCAR/NCEP Reanalysis

Importance of Trends in Outflow TemperatureImportance of Trends in Outflow Temperature

Outflow Temperature, September, 1995

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

60oS

30oS

0o

30oN

60oN

200 210 220 230 240 250 260 270 280

Very High Resolution Modeling Very High Resolution Modeling of the Response of Tropical of the Response of Tropical Cyclones to Climate ChangeCyclones to Climate Change

The Problem:The Problem:

• Global models are far too coarse to simulate high Global models are far too coarse to simulate high intensity tropical cyclonesintensity tropical cyclones

• Embedding regional models within global models Embedding regional models within global models introduces problems stemming from incompatibility introduces problems stemming from incompatibility of models, and even regional models are usually too of models, and even regional models are usually too coarsecoarse

Histograms of Tropical Cyclone Intensity as Simulated by a Global Model with 50 km grid point spacing. (Courtesy Isaac Held, GFDL)

Category 3

Probability Density of TC Damage, U.S. East Coast

Damage Multiplied by Probability Density of TC Damage, U.S. East Coast

To the extent that they simulate tropical cyclones at all, global models simulate storms that are largely irrelevant to society and to the climate system itself, given that ocean stirring effects are heavily weighted towards the most intense storms

What are the true resolution What are the true resolution requirements for simulating requirements for simulating

tropical cyclones?tropical cyclones?

Numerical convergence in an axisymmetric, nonhydrostatic model (Rotunno and Emanuel, 1987)

Another Major Problem with Using Another Major Problem with Using Global and/or Regional Models to Global and/or Regional Models to

Simulate Tropical Cyclones:Simulate Tropical Cyclones:

Model TCs are not coupled Model TCs are not coupled to the oceanto the ocean

Comparing Fixed to Interactive SST:Comparing Fixed to Interactive SST:

Our Solution:Our Solution:

Drive a simple but very high resolution, coupled ocean-atmosphere TC model using boundary conditions supplied by the global model or reanalysis data set

CHIPS: A Time-dependent, axisymmetric CHIPS: A Time-dependent, axisymmetric model phrased in R spacemodel phrased in R space

• Hydrostatic and gradient balance above PBL• Moist adiabatic lapse rates on M surfaces

above PBL• Boundary layer quasi-equilibrium• Deformation-based radial diffusion

21

2M rV fr

21

2fR M

Detailed view of Entropy and Angular MomentumDetailed view of Entropy and Angular Momentum

Ocean Component: ((Schade, L.R., 1997: A physical interpreatation of SST-feedback. Preprints of the 22nd Conf. on Hurr. Trop. Meteor., Amer. Meteor.

Soc., Boston, pgs. 439-440.)• Mixing by bulk-Richardson number closure• Mixed-layer current driven by hurricane model surface

wind

Ocean columns integrated only Along predicted storm track.Predicted storm center SST anomaly used for input to ALLatmospheric points.

Comparison with same atmospheric model coupled Comparison with same atmospheric model coupled to 3-D ocean model; idealized runs:to 3-D ocean model; idealized runs:

Full model (black), string model (red)Full model (black), string model (red)

Hindcast of KatrinaHindcast of Katrina

Comparison to Skill of Other ModelsComparison to Skill of Other Models

Application to Assessing Tropical Application to Assessing Tropical Cyclone Risk in a Changing ClimateCyclone Risk in a Changing Climate

Approach: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, plus a correction for beta drift

• Step 3: Run the CHIPS 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.

Details: Emanuel et al., BAMS, 2008

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

6-hour zonal displacements in region bounded by 106-hour zonal displacements in region bounded by 10oo and 30 and 30oo N latitude, and 80N latitude, and 80oo and 30 and 30oo W longitude, using only post-1970 W longitude, using only post-1970

hurricane datahurricane data

CalibrationCalibration

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

Genesis ratesGenesis rates

AtlanticAtlantic

Eastern North Pacific

Western North Pacific

North Indian Ocean

Southern Hemisphere

Calibrated to AtlanticCalibrated to Atlantic

Seasonal CyclesSeasonal Cycles

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

3000 Tracks within 100 km of Miami3000 Tracks within 100 km of Miami

95% confidence bounds

Return PeriodsReturn Periods

Sample Storm Wind SwathSample Storm Wind Swath

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

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

Couple Hurricane Model to Storm Surge Model (ADCIRC)Couple Hurricane Model to Storm Surge Model (ADCIRC)Results for the Battery, New York CityResults for the Battery, New York City

Importance of Potential Intensity in Importance of Potential Intensity in Controlling Tropical Cyclone ActivityControlling Tropical Cyclone Activity

Decomposition of Frequency TrendsDecomposition of Frequency Trends

Decomposition of PDI TrendsDecomposition of PDI Trends

Now Use Daily Output from IPCC Now Use Daily Output from IPCC Models to Derive Wind Statistics, Models to Derive Wind Statistics, Thermodynamic State Needed by Thermodynamic State Needed by

Synthetic Track TechniqueSynthetic Track Technique

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

2.2. Years 2180-2200 of IPCC Scenario Years 2180-2200 of IPCC Scenario A1b (COA1b (CO22 stabilized at 720 ppm) stabilized at 720 ppm)

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

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

Northeast U.S. Damage FunctionNortheast U.S. Damage Function

Application to the Climate of the Application to the Climate of the PliocenePliocene

with Alexey Federov and Chris Brierley, Yale

50

Pacific ocean SSTs

51

; Indo-Pacific

; Atlantic

Mg/Ca – solid boxes Alkenone – open

Pacific ocean SSTs

Brierley, Fedorov, Liu, Herbert, Lawrence, LaRiviere 2009, Science

4Ma

52

Pacific ocean SSTs

4Ma

; Indo-Pacific

; Atlantic

Mg/Ca – solid boxes Alkenone – open

Brierley, Fedorov, Liu, Herbert, Lawrence, LaRiviere 2009, Science

53

Tropical warm pool (observations)

Hypothetical warm pool in the early Pliocene (~4Ma)

oC oC

SummarySummary

• Potential intensity is an important (but not the only!) control on tropical cyclone activity

• On time scales long enough for the ocean mixed layer to be in thermal equilibrium, potential intensity is controlled largely by surface radiation, surface wind speed, ocean heat fluxes, and outflow temperature

• Simple but high resolution coupled TC model can be used to ‘downscale” TC activity from global climate data sets

• Studies based on this downscaling suggest large sensitivity of TCs to climate state, and possibly important role for TC-induced ocean mixing in regulating climate