Tropical Cyclones and Climate Kerry Emanuel Massachusetts Institute of Technology

Issues Effect of climate change on tropical cyclone activityEffect of climate change on tropical cyclone activity Role of tropical cyclones in the climate systemRole of tropical cyclones in the climate system

Approaches The historical recordThe historical record PhysicsPhysics PaleotempestologyPaleotempestology ModelsModels

Effect of Climate Change on Hurricanes

Global TC Frequency, Data Sources: NOAA/TPC and NAVY/JTWC

Better 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

Power Dissipation Based on 3 Data Sets for the Western North Pacific Power Dissipation Based on 3 Data Sets for the Western North Pacific (smoothed with a filter) aircraft recon Data Sources: NAVY/JTWC, Japan Meteorological Agency, UKMO/HADSST1, Jim Kossin, U. Wisconsin Years included:

Atlantic Storm Maximum Power Dissipation (Smoothed with a filter) Power Dissipation Index (PDI) Years included: Data Source: NOAA/TPC

Atlantic Sea Surface Temperatures and Storm Max Power Dissipation (Smoothed with a filter) Scaled Temperature Power Dissipation Index (PDI) Years included: Data Sources: NOAA/TPC, UKMO/HADSST1

Tropical Atlantic SST(blue), Global Mean Surface Temperature (red), Aerosol Forcing (aqua) Mann, M. E., and K. A. Emanuel, Atlantic hurricane trends linked to climate change. EOS, 87, Global mean surface temperature Tropical Atlantic sea surface temperature Sulfate aerosol radiative forcing

Best Fit Linear Combination of Global Warming and Aerosol Forcing (red) versus Tropical Atlantic SST (blue) Mann, M. E., and K. A. Emanuel, Atlantic hurricane trends linked to climate change. EOS, 87, Tropical Atlantic sea surface temperature Global Surface T + Aerosol Forcing

Physics

Energy Production

Theoretical Upper Bound on Hurricane Maximum Wind Speed: Air-sea enthalpy disequilibrium Surface temperature Outflow temperature Ratio of exchange coefficients of enthalpy and momentum

Observed Tropical Atlantic Potential Intensity Data Sources: NCAR/NCEP re-analysis with pre-1979 bias correction, UKMO/HADSST1 Emanuel, K., J. Climate, 2007

Paleotempestology

barrier beach backbarrier marsh lagoon barrier beach backbarrier marsh lagoon a) b) Source: Jeff Donnelly, WHOI upland flood tidal delta terminal lobes 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

Projecting into the Future: Downscaling from Global Climate Models

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

Our 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: Empirically determined constants:

Example: 200 Synthetic Tracks

Present Climate: Spatial Distribution of Genesis Points Observed Synthetic

Calibration Absolute genesis frequency calibrated to North Atlantic during the period Absolute genesis frequency calibrated to North Atlantic during the period

Genesis rates

Seasonal Cycles Atlantic

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

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

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

Simulated vs. Observed Power Dissipation Trends,

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

Last 20 years of 20 th century simulations 2. Years of IPCC Scenario A1b (CO 2 stabilized at 720 ppm) 1. Last 20 years of 20 th century simulations 2. Years of IPCC Scenario A1b (CO 2 stabilized at 720 ppm) Compare two simulations each from 7 IPCC models:

Genesis Distributions

Basin-Wide Percentage Change in Power Dissipation

Basin-Wide Percentage Change in Storm Frequency

7 Model Consensus Change in Storm Frequency

Synthetic Events driven by GFDL AM2.1, Observed SSTs

Feedback of Global Tropical Cyclone Activity on the Climate 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 Watts

TC Mixing May Induce Much or Most of the Observed Poleward Heat Flux by the Oceans Trenberth 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 – uniform 10 x CO 2 in both experiments Source: Rob Korty, CalTech Interactive TC-Mixing Moderates Tropical Warming and Amplifies High-Latitude Warming in Coupled Climate Models

“Slippery Sacks” Ocean Model, Patrick Haertel

Summary: Tropical cyclones are sensitive to the climate state, as revealed by historical data and paleotempestologyTropical cyclones are sensitive to the climate state, as revealed by historical data and paleotempestology Observations together with detailed modeling suggest that TC power dissipation increases by ~65% for a 10% increase in potential intensityObservations together with detailed modeling suggest that TC power dissipation increases by ~65% for a 10% increase in potential intensity

New technique for downscaling climate models shows promise for predicting response of global tropical cyclone activity to climate changeNew technique for downscaling climate models shows promise for predicting response of global tropical cyclone activity to climate change Climate models may have systematic errors that compromise estimates of tropical cyclone response to global warmingClimate models may have systematic errors that compromise estimates of tropical cyclone response to global warming

Storm-induced mixing of the upper tropical ocean may be the principal driver of the ocean’s thermohaline circulationStorm-induced mixing of the upper tropical ocean may be the principal driver of the ocean’s thermohaline circulation Increased TC power dissipation in a warming climate will drive a larger poleward heat flux by the oceans, tempering tropical warming but amplifying the warming of middle and high latitudesIncreased TC power dissipation in a warming climate will drive a larger poleward heat flux by the oceans, tempering tropical warming but amplifying the warming of middle and high latitudes