1. 25 October 2006MPI for Meteorology Hamburg Hurricane type vortices in ECHAM5. How will they change in a warmer climate? Lennart Bengtsson with Kevin Hodges, ESSC Monika Esch and co-workers at MPI

2. 25 October 2006MPI for Meteorology Hamburg Tropical cyclones in a future climate what could be expected? Higher SST and higher atmospheric moisture would generally favor more intense storms ( e.g. Emanuel 1988, 1999) This is supported by modeling results by Knutson and Tuleya (2004) driving an limited area model with CMIP2+ boundary data ( 9 different models). Increasing vertical wind-shear and reduced relative humidity would counteract this tendency. Such influences occur in the tropical N. Atlantic during El Nino. How will the number of storms change? What are the general conditions controlling the number of tropical storms? What are the critical conditions in modeling tropical storms? Are results from large scale models with limited resolution credible?

3. 25 October 2006MPI for Meteorology Hamburg After Emanuel

4. 25 October 2006MPI for Meteorology Hamburg Modeling approach Simulation of tropical cyclones per se in a global GCM Using limited area models at high resolution Identifying climate predictors in a GCM (SST, vorticity, static stability, relative humidity, vertical wind-shear)

5. 25 October 2006MPI for Meteorology Hamburg Simulation of tropical cyclones per se in a global GCM Disadvantage: Difficulties to resolve the intense features of a tropical storm Examples of studies: Bengtsson et al. 1995, 1996, Tellus Sugi et al. 2002, JMS, Japan McDonald et al. 2005, Clim.Dyn. Chauvin et al. 2006, Clim Dyn. Oouchi et al. 2006, JMS, Japan Yoshimori et al. 2006, JMS, Japan

6. 25 October 2006MPI for Meteorology Hamburg Simulation of hurricane type vortices with a GCM Typical criteria: An identifiable vortex A minimum in the surface pressure Surface wind speed above a given value A warm core vortex ( reduced circulation with height)

7. 25 October 2006MPI for Meteorology Hamburg Effect of 2xCO 2 From Bengtsson et al., 1996 (Tellus) ( number of cyclones /basin)

8. 25 October 2006MPI for Meteorology Hamburg Using limited area models at high resolution Disadvantages: Generation of storms, Large scale influences difficult to handle Examples of studies: Knutson et al. 1998, Science Knutson and Tuleya, 1999, Clim. Dyn. Knutson and Tuleya, 2004, J Clim.

9. 25 October 2006MPI for Meteorology Hamburg Impact of CO 2 -induced warming on simulated hurricane intensity Knutson and Tuleya (2004, J of Climate) They used a high resolution limited area model driven by the SST and moisture of 9 CGCM from the CMIP 2+ project. CMIP2 uses 1%yr -1 increase over an 80-year period implying an increase by a factor of 2.2. Model calculations are undertaken in NW Pacific-, NE Pacific- and Atlantic basin Four different convective schemes are tested (no significant differences) RESULTS: Max. surface wind speed increases by 6% Min. central pressure by 14% Max. precipitation by 24% Hurricane increase by a factor of 1/2 in the Simpson-Saffir scale

10. 25 October 2006MPI for Meteorology Hamburg Intensification of hurricanes at 2xCO 2 Knutson and Tuleya (2004)

11. 25 October 2006MPI for Meteorology Hamburg Identifying climate predictors in a GCM Disadvantage: Lack of proper understanding, ad hoc selection of predictors, overestimation of the effect of SST Examples of studies Gray, 1979 Met. over the Tropical oceans, RMetSoc Royer et al. 1998, Clim. Change Chauvin et al. 2006, Clim. Dyn.

12. 25 October 2006MPI for Meteorology Hamburg Objectives of the present study Hurricane type vortices (HTV) in ECHAM5. How do the HTVs respond to anthropogenic climate change and how does this depend on resolution? What changes occur in intensity, life time and power dissipation index? What possible mechanisms control the change in HTV? What are the dominant factors?

13. 25 October 2006MPI for Meteorology Hamburg Tracking methods and vortex identification Tracks are followed from its generation (6x10 -6 s -1 ) until it disappears as an extra-tropical cyclones north of 60N We calculate the total life-time of the HTVs We are able to identify the transition from a tropical to an extra-tropical vortex Alternatively we use the wind speed at 925 hPa as a selection criteria for intense storms We have also calculated the potential dissipation index, PDI. See Emanuel, Nature, 2005

14. 25 October 2006MPI for Meteorology Hamburg Hurricane Katrina August 2005 ECMWF operational analyses, 850 hPa vorticity

15. 25 October 2006MPI for Meteorology Hamburg Katrina vorticity at different levels

16. 25 October 2006MPI for Meteorology Hamburg Katrina 850 hPa vorticity at different resolution

17. 25 October 2006MPI for Meteorology Hamburg Katrina Wind speed max. and surface pressure min.

18. 25 October 2006MPI for Meteorology Hamburg Selection of HTV indicators We use criteria for minimum vorticity at 850 hPa (V), minimum vertical vorticity gradient (G) between 850 and 250 hPa, and number of time steps of 6 hrs (T) when these conditions are fulfilled. (V, G, T) = (6, 6, 4) vorticity at 850 hPa = 6x10 -5 s -1 vorticity 850- 250 hPa = 6x10 -5 s -1 conditions fulfilled at least 24 hrs (6, 6, 4) is defined as an HTV

19. 25 October 2006MPI for Meteorology Hamburg Selection of criteria for selecting hurricane type vortices (HTV) All Tropical Storms Hurricanes, Typhoons, Cyclones >33ms -1 (6, 6, 4) (10, 6, 4) (12, 6, 4) 20037533714839 20047236795241 20058038836248

20. 25 October 2006MPI for Meteorology Hamburg Objectives of the present study Hurricane type vortices (HTV) in ECHAM5 We have used scenario A1B and studied the periods 1861- 1890, 1961-1990 and 2071-2100 We have explored the coupled T63 run (3 runs) for all periods and T213 time - slice 1961-1990 and 2071-2100 T319 time - slice 1971-1990, (2081-2100 to be done) We have also used AMIP2 runs (20 years) with T63 and T159 as a validation study

21. 25 October 2006MPI for Meteorology Hamburg Comparison with observations from the Tropical Warning Centers and with ERA-40 re-analyses

22. 25 October 2006MPI for Meteorology Hamburg Super Typhoon 21 (1991) in ERA-40 (left) and selected similar storm in ECHAM5 (right) Intensity (vorticity at 850 hPa)

23. 25 October 2006MPI for Meteorology Hamburg Lifetime of HTVs in days

24. 25 October 2006MPI for Meteorology Hamburg Hurricane genesis (a) observed, (b) ERA-40 and (c ) ECHAM5 T159

25. 25 October 2006MPI for Meteorology Hamburg Hurricane track density (a) observed, (b) ERA-40 and (c ) ECHAM5 T159

26. 25 October 2006MPI for Meteorology Hamburg Hurricane track density (Atlantic) (d) observed, (e) ERA-40 and (f ) ECHAM5 T159

27. 25 October 2006MPI for Meteorology Hamburg Hurricane type vortices (21 years) ECHAM, ERA-40, JRA 25 and Observed >18ms -1 >33ms -1 >50ms -1 ECHAM5 T1592017731 72 ERA401447297 1 JRA251234323 2 Observed1236724346 All (6, 6, 4)>2x10 -4 >5x10 -4 >1x10 -3 ECHAM5 T1592073184859924 ERA4017471557400 JRA25130693370

28. 25 October 2006MPI for Meteorology Hamburg ECHAM 5 max. wind speed in different regions

29. 25 October 2006MPI for Meteorology Hamburg ERA-40 max. wind speed in different regions

30. 25 October 2006MPI for Meteorology Hamburg “Observed” max. wind speed in different regions

31. 25 October 2006MPI for Meteorology Hamburg Annual variability of HTVs Observed, ERA-40 and ECHAM5- T159

32. 25 October 2006MPI for Meteorology Hamburg Interannual variability of HTVs > 33ms -1 Observed, ERA-40 and ECHAM5- T159

33. 25 October 2006MPI for Meteorology Hamburg Active Atlantic years - non active years (left) Active West-Pacific years - less active (right) From above: SST, 850 hPa Vel. Pot., wind shear 250-850 pPa

34. 25 October 2006MPI for Meteorology Hamburg Courtesy J. O’Brien

35. 25 October 2006MPI for Meteorology Hamburg The same ERA-40 response Active Atlantic (left), Active West Pacific (right)

36. 25 October 2006MPI for Meteorology Hamburg Are there observational evidence that hurricanes are becoming more intense? Why is it so difficult? Longer term records are needed due to internal variability There have been large changes in the observing systems making it easier to detect more tropical cyclones in later years. Recent papers have used PDI ( time integral of max. wind cube) which is overly sensitive to observational accuracy Model studies (e.g. Knutson and Tuleya, 2004) indicate small changes in intensity as of now which are hardly detectable

37. 25 October 2006MPI for Meteorology Hamburg Are there observational evidence that hurricanes are becoming more intense? YES Webster et al. (2005), Science, Emanuel (2005), Nature, Sriver and Huber (2006), GRL NO Chan (2006), Science, Klotzbach ( 2006), GRL, Landsea et al. (2006), Science

38. 25 October 2006MPI for Meteorology Hamburg There are recent claims that there is an increase in hurricane intensity ( e.g. Emanuel (2005), Webster et al. (2005) Are these findings credible? They are generally not supported by operational meteorologists According to Knutson and Tuleya (2004) any changes are probably undetectable “for decades to come” Results from this study and some additional work may presumably reduce the likelihood of detection further There are structural problems in the detection of trends Changes in observing systems Difficulties to separate a genuine change in storms from societal causes behind the huge increase in damages and damage cost

39. 25 October 2006MPI for Meteorology Hamburg What may happen in a warmer climate? We have used the AIB scenario And the coupled MPI model at T63 resolution used in the IPCC 4th assessment Higher resolution experiments use the transient SST from T63 (time - window) We study C20 (1961- 1990) C21 ( 2071-2100)

40. 25 October 2006MPI for Meteorology Hamburg What is A1B? Middle of the line scenario Carbon emission peaking in the 2050s (16 Gt/year) CO 2 reaching 450 ppm. in 2030 CO 2 reaching 700 ppm. in 2100 SO 2 peaking in 2020 then coming done to 20% thereof in 2100

41. 25 October 2006MPI for Meteorology Hamburg SST difference (C 21-C 20)

42. 25 October 2006MPI for Meteorology Hamburg HTVs at T63 resolution C19 (black), C20 (red) and C21(blue)

43. 25 October 2006MPI for Meteorology Hamburg Number of HTVs (T63) for C 20 and C21 for wind speed and vorticity T63>18ms -1 >33ms -1 >50ms -1 20C (1961-1990)346.10.1 21C (2071-2100)266.40.1 T63All (6, 6, 4)>2x10 -4 s -1 >5x10 -4 s -1 >1x10 -3 s -1 20C (1961-1990) 355.500 21C (2071-2100) 275.600

44. 25 October 2006MPI for Meteorology Hamburg Hurricane genesis T63 From top C21-C20, C21 and C20

45. 25 October 2006MPI for Meteorology Hamburg Hurricane track density T63 From top C21-C20, C21 and C20

46. 25 October 2006MPI for Meteorology Hamburg Changes of HTVs in four hurricane regions (T63)

47. 25 October 2006MPI for Meteorology Hamburg Number of HTVs/year (T213) for C 20 and C21 for wind speed and vorticity T213All (6, 6, 4)>2x10 -4 s -1 >5x10 -4 s -1 >1x10 -3 s -1 20C (1961-1990)10497406.0 21C (2071-2100)9490499.8 T213>18ms -1 >33ms -1 >50ms -1 20C (1961-1990)100333.7 21C (2071-2100)92364.9

48. 25 October 2006MPI for Meteorology Hamburg Hurricane genesis T213 From top C21-C20, C21 and C20

49. 25 October 2006MPI for Meteorology Hamburg Hurricane track density, T213 From top C21-C20, C21 and C20

50. 25 October 2006MPI for Meteorology Hamburg Changes of HTVs in four hurricane regions (T213)

51. 25 October 2006MPI for Meteorology Hamburg Change in HTV lifetime (T213)

52. 25 October 2006MPI for Meteorology Hamburg Change in min. surface pressure (T213)

53. 25 October 2006MPI for Meteorology Hamburg Accumulated precipitation ( in mm and for an area with a radius of 5 degrees) along the track of the HTV for C20 and C21(T213) HTVs reaching >33ms -1 C21 C20 Total increase 30%

54. 25 October 2006MPI for Meteorology Hamburg Climate change and the water cycle Atmosphere appears to conserve relative humidity. This means that atmospheric water vapor follows Clausius- Clapeyron relation. (Soden and Held, 2006, J. Clim.) We see an increase of 27% in atmospheric water vapor at C21 compared to C20 Precipitation must be balanced by evaporation. Evaporation is driven by the surface energy balance which increases slower than atmospheric water vapor. In fact it can even diminish at a high aerosol concentration (ECHAM4). Precipitation increases by 6% both globally and in the tropics This means that the residence time of water in the atmosphere increases from 8.7 to 10.3 days or by 16%.

55. 25 October 2006MPI for Meteorology Hamburg Result from present study: Reduced number of hurricanes There is s reduction in the number of tropical cyclones in agreement with most previous studies. We suggest this is due to a weakening of the tropical circulation. This can best be seen as a slowing down of the hydrological cycle by some 16% The radiative cooling of the tropics increases due to more water vapor in the upper troposphere and dynamical cooling due to increased static stability. This processes can compensate warming from release of latent heat (6%) with a less active tropical circulation.

56. 25 October 2006MPI for Meteorology Hamburg Result from present study: More intense hurricanes Given favorable atmospheric condition we suggest the ideas put forward by Emanuel and Holland comparing a hurricane with a Carnot cycle can be applied. This will provide an energy input broadly proportional to the specific humidity at a higher temperature (following SST) The intensification of the tropical cyclones depends on sufficient model resolution to accurately describe the convergence of momentum which generate the very high wind speeds at the core of the vortex. For HTVs reaching 33ms -1 PDI (power dissipation index) increase by 16%

57. 25 October 2006MPI for Meteorology Hamburg What is the effect of even higher resolution?

58. 25 October 2006MPI for Meteorology Hamburg HTVs at C20 at four different resolutions Number/year as a function of vorticity

59. 25 October 2006MPI for Meteorology Hamburg END