Three Lectures on Tropical Cyclones

1. Three Lectures on Tropical Cyclones Kerry Emanuel Massachusetts Institute of Technology Spring School on Fluid Mechanics of Environmental Hazards

2. Lecture 3:Using Physics to Assess Tropical Cyclone Risk in a Changing Climate

3. Tropical Cyclones Do Respond to Climate Change!

4. Atlantic Sea Surface Temperatures and Storm Max Power Dissipation (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

5. 10-year Running Average of Aug-Oct NH Surface T and MDR SST

7. 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.

8. 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.

9. Effect of Increased Potential Intensity on Hurricane Katrina

10. Paleotempestology

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

12. Donnelly and Woodruff (2006)

13. 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

14. Frappier et al., Geology, 2007

15. Assessing Tropical Cyclone Risk: Historical Statistics Are Inadequate

16. U.S. Hurricane Damage, 1900-2004,Adjusted for Inflation, Wealth, and Population

17. Top 10 Northeast Storms Since 1851

18. Issues with Direct Use of Global Climate Models: • Today’s global models are too coarse to simulate high intensity events • Not practical to run models for long enough to generate high quality regional statistics • Embedding regional models is feasible but expensive

19. Our Approach: Step 1: Randomly seed ocean basins with weak (12 m/s) warm-core vortices Step 2: Determine tracks of candidate storms using a simple model that moves storms with mean background wind Step 3: Run a deterministic coupled tropical cyclone intensity model along each synthetic track, discarding all storms that fail to achieve winds of at least 17 m/s (random seeding method) Step 4: Assess risk using statistics of surviving events

20. Synthetic Track Generation,Using Synthetic Wind Time Series • Postulate that TCs move with vertically averaged environmental flow plus a “beta drift” correction (Beta and Advection Model, or “BAMS”) • Approximate “vertically averaged” by weighted mean of 850 and 250 hPa flow

21. Synthetic wind time series Monthly mean, variances and co-variances from NCEP re-analysis data Synthetic time series constrained to have the correct mean, variance, co-variances and an power series

22. 250 hPa zonal wind modeled as Fourier series in time with random phase: where T is a time scale corresponding to the period of the lowest frequency wave in the series, N is the total number of waves retained, and is, for each n, a random number between 0 and 1.

23. The time series of other flow components: or where each Fi has a different random phase, and A satisfies where COV is the symmetric matrix containing the variances and covariances of the flow components.

24. Example:

25. Track: Empirically determined constants:

26. Run coupled deterministic model (CHIPS, Emanuel et al., 2004) along each track Use monthly mean potential intensity, ocean mixed layer depth, and sub-mixed layer thermal stratification Use shear from synthetic wind time series Initial intensity specified as Tracks terminated when v < Tropical Cyclone Intensity

27. 6-hour zonal displacements in region bounded by 10o and 30o N latitude, and 80o and 30o W longitude, using only post-1970 hurricane data

28. Example: 50 Synthetic Tracks

29. 200 Random Western North Pacific Events

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

31. Return Periods

36. Random Seeding Method: Calibration Absolute genesis frequency calibrated to North Atlantic during the period 1980-2005

37. Genesis rates Western North Pacific Southern Hemisphere Eastern North Pacific North Indian Ocean Atlantic Calibrated to Atlantic

38. Seasonal Cycles Western North Pacific

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

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

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

42. Global Percentage of Cat 4 & Cat 5 Storms

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

44. Compare two simulations each from 7 IPCC models: 1.Last 20 years of 20th century simulations2. Years 2180-2200 of IPCC Scenario A1b (CO2 stabilized at 720 ppm)

45. Basin-Wide Percentage Change in Power Dissipation Different Climate Models

46. Basin-Wide Percentage Change in Storm Frequency Different Climate Models

47. 7 Model Consensus Change in Storm Frequency Reds: Increases Blues: Decreases