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Tropical Cyclones and Climate Kerry Emanuel Massachusetts Institute of Technology Issues • Effect of climate change on tropical cyclone activity • Role of tropical cyclones in the climate system Approaches • The historical record • Physics • Paleotempestology • Models Effect of Climate Change on Hurricanes Global TC Frequency, 1970-2006 Data Sources: NOAA/TPC and NAVY/JTWC Better Intensity Metric: The Power Dissipation Index PDI V dt 0 3 max 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 (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 Atlantic Storm Maximum Power Dissipation Power Dissipation Index (PDI) (Smoothed with a 1-3-4-3-1 filter) Data Source: NOAA/TPC Years included: 1870-2006 Atlantic Sea Surface Temperatures and Storm Max Power Dissipation Data Sources: NOAA/TPC, UKMO/HADSST1 Years included: 1870-2006 Scaled Temperature Power Dissipation Index (PDI) (Smoothed with a 1-3-4-3-1 filter) Tropical Atlantic SST(blue), Global Mean Surface Temperature (red), Aerosol Forcing (aqua) Global mean surface temperature Tropical Atlantic sea surface temperature Sulfate aerosol radiative forcing Mann, M. E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. EOS, 87, 233-244. Best Fit Linear Combination of Global Warming and Aerosol Forcing (red) versus Tropical Atlantic SST (blue) Tropical Atlantic sea surface temperature Global Surface T + Aerosol Forcing Mann, M. E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. EOS, 87, 233-244. Physics Energy Production Theoretical Upper Bound on Hurricane Maximum Wind Speed: Surface temperature C T T * 2 k s o |V pot | k k T 0 C o D Ratio of exchange coefficients of enthalpy and momentum Outflow Air-sea enthalpy temperature disequilibrium Observed Tropical Atlantic Potential Intensity Emanuel, K., J. Climate, 2007 Data Sources: NCAR/NCEP re-analysis with pre-1979 bias correction, UKMO/HADSST1 Paleotempestology Paleotempestology barrier beach upland overwash fan backbarrier marsh a) lagoon barrier beach upland overwash fan b) backbarrier marsh lagoon terminal lobes flood tidal delta Source: Jeff Donnelly, WHOI Donnelly and Woodruff (2006) Photograph of stalagmite ATM7 showing depth of radiometric dating samples, micromilling track across approximately annually laminated couplets, and agedepth curve. Frappier et al., Geology, 2007 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: Vtrack V850 1 V250 V , Empirically determined constants: 0.8, 1 u 0 ms , v 2.5 ms 1 Example: 200 Synthetic Tracks Present Climate: Spatial Distribution of Genesis Points Observed Synthetic Calibration • Absolute genesis frequency calibrated to North Atlantic during the period 1980-2005 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, 1980-2006 Now Use Daily Output from IPCC Models to Derive Wind Statistics, Thermodynamic State Needed by Synthetic Track Technique Compare two simulations each from 7 IPCC models: 1. Last 20 years of 20th century simulations 2. Years 2180-2200 of IPCC Scenario A1b (CO2 stabilized at 720 ppm) 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 Emanuel (2001) estimated global rate of heat input as 1.4 X 1015 Watts Source: Rob Korty, CalTech 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 Interactive TC-Mixing Moderates Tropical Warming and Amplifies High-Latitude Warming in Coupled Climate Models DSST: elevated mixing to 360 meters – uniform Source: Rob Korty, CalTech 10 x CO2 in both experiments “Slippery Sacks” Ocean Model, Patrick Haertel Summary: • Tropical 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 intensity • New 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 warming • Storm-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 latitudes