1. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Extra-tropical cyclones in a warmer climate. Will they be more intense? Professor Lennart Bengtsson ESSC Many thanks to Kevin Hodges, Noel Keenlyside and MPI modeling team

2. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Extra-tropical cyclones Societal damages due to mid-latitude cyclones are generally related to high winds and flooding. According to Munich Re (2002) damages due to wind storms since 1950 were 324 G$ and insured losses 106 G$ Damages due to tropical cyclones dominate but extreme winter storms in Europe may cause annual damages of several GEuro. The question whether cyclones may intensify in a future climate is consequently an issue of primary importance for society. There is evidence form both theory and model experiments that this may happen for tropical storms but will it also occur for extra-tropical storms? The concern is further enhanced by the ongoing increased exposure to extreme weather independent of climate change.

3. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Extra-tropical cyclones in a warmer climate. Will they be more intense? We have addressed the following scientific objectives How well does the ECHAM5 T213 model represents the dynamics and energetics of intense extra-tropical cyclones? How is maximum wind speed and precipitation related to the life cycle of the cyclone? What is the importance of resolution? What changes might occur in a warmer climate?

4. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Background Are there any physical reasons that extra-tropical cyclones might become more intense in a warmer climate? Do we have any evidence that extra-tropical cyclones have become more intense? Are present GCM able to represent intense extra- tropical cyclones? What are the evidence from climate change experiments?

5. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Do we have any evidence that extra-tropical cyclones have become more intense? Extreme storms are rare and require long and reliable observational records. Indications are that several decades of homogeneous data are needed. There are still general problems to detect extreme storms in sufficient details as observational records are insufficient, although the situation today is significantly better than in the past. For this reason trend calculations must be critically assessed.

6. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Do we have any evidence that extra-tropical cyclones have become more intense? Several interesting studies have been published most are limited to Northern and Western Europe. WASA group (1998) Alexandersson et al. (2000) Weisse et al, (2005) Here are some findings from Weisse et al.(ibid) A general increase in extreme cyclones (10m wind) from 1958 until 1990, therefter a weakening. The pattern follows variations in the large scale atmospheric circulation (e.g. NAO) There is no robust trend indicating an increase of extreme winds

7. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson, Longer term records using geostrophic winds indicate that extreme winds in Northwestern Europe were as intense in the end of the 19th century as in the end of the last century IPCC, 2007

8. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Do we have any physical reasons why extra-tropical cyclones might become more intense in a warmer climate? As extra-tropical storms depends on available potential energy (proportional to temperature variance in the low and middle troposphere). Changes here may effect the number and intensity of the storms.? Release of latent heat is also important so more water vapor in the atmosphere may be important. Probably Tropical storms move into the extra-tropics and may also contribute.Yes Upper level cyclogenesis related to sharp gradients in potential vorticity may also contribute to low level intensification.?

9. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson 5 year global temperature change in the last two decades

10. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Change in temperature at different levels (21C - 20C). Black 1000 hPa, red 850 hPa, green 600 hPa, blue 500 hPa, yellow 400 hPa, cyan 300hPa, magenta 200hPa. Left DJF, right JJA. ECHAM 5, A1B, DT in 110 years

11. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Globally integrated water vapor 1979-2005 From Held and Soden, 2006 DT +0.45K De s +3% Full line GFDL model Dashed line measurements

12. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Clausius - Clapeyron expression A typical value of alfa in the lower troposphere is 0.07/K or 7% increase in the saturation water vapor for each 1K in temperature

13. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Semenov and Bengtsson, 2002 Secular trends in daily precipitation characteristics Clim.Dyn. 123-140 +30%

14. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson What happens to the hydrological cycle in a warmer climate? See e.g. Held and Soden, 2006, J. of Clim. Observations and models show that water vapor follows temperature according to Clausius- Clapeyron expression. The increase in precipitation is much slower. This increases the residence time of water in the atmosphere. This reduces the large scale vertical mass flux This slows down the large scale circulation

15. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Top, IPCC models: massflux, water vapor and precipitation Below, GFDl model P = Mq ( From Held and Soden (2006))

16. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson What is new and typical of this study? We explore a global climate simulation using a higher horizontal resolution than previously used in similar studies. We use the ECHAM5 model at T213 resolution (ca 60km) and investigate 32 years from the 20th century, 1959-1990 (20C) and 32 years from the 21st century, 2069-2100 (21C). SST data are taken from a T63 coupled model. We are making use of the A1B scenario We explore transient storm track in a Lagrangian sense using data for every 6 hr.

17. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Selection of storm tracks Level 850 hPa Lifetime ≥ 48 hours Intensity in vorticity ≥10 -5 s -1 Movements ≥1000km

18. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Storm track density and storm track genesis at 20C DJF (T213) Track density Genesis

19. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Identification of extreme extra-tropical events We identify cyclones by searching for maximum of 850 hPa vorticity using data for every 6 hrs. We search for the maximum wind within a radius of 5° of the vorticity centre. Wind speed is determined at 925 hPa We calculate total precipitation in a circular area within 5° of the centre. ( ca 10 6 km 2 ) We also use surface pressure minima and surface pressure tendencies(deepening rates)

20. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Example of an extreme extra-tropical cyclone and its evolution

21. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Structure and evolution of extra-tropical cyclones We explore here the 100 most intense cyclones in terms of maximum wind speed.

22. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Composite vortex (100 most intense) at maximum intensification left pressure and wind, right pressure and precipitation 20C, DJF. Movement of cyclone is to the right. mm/hour

23. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Composite vortex (100 most intense) at maximum precipitation left pressure and wind, right pressure and precipitation. 15 hrs later 20C, DJF Movement of cyclone is to the right

24. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Composite vortex (100 most intense) at maximum intensity left pressure and wind, right pressure and precipitation. 15 hrs later 20C, DJF Movement of cyclone is to the right

25. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson ECHAM 5 T213 Cyclones in different stages of development Max. deepening Max. precipitation Max. intensity

26. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms) (DJF) at 20C. Time units are in 6 hours centered at the time of minimum pressure. Evolution of central pressure, vorticity, wind speed and precipitation 30 hours

27. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Vertical tilt of the composite cyclone 36 hours before and after maximum intensity

28. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Comparison with ERA-40 The number of cyclones and their relative intensity is virtual identical to model results ERA-40 underestimates precipitation ( initialization problems) ERA-40 underestimate wind speed maximum and vorticity ( mainly resolution reasons, interimanalysis agrees better with the model results)

29. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Number of cyclones at T63 and T213 as a function of vorticity at T42 resolution

30. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson ERA-40 Cyclones in different stages of development

31. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF) at 20C. (ECHAM5/T213) (left) and ERA 40(right).Time units are in 6 hours centered at the time of minimum pressure. Evolution of central pressure, vorticity, wind speed and precipitation

32. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Number of storm tracks for a given maximum wind speed. ERA-40 for three different periods and ECHAM5. The higher maximum wind speeds in ECHAM5 are likely due to higher resolution

33. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Vertical tilt of the composite cyclone 36 hours before and after maximum intensity, ECHAM5 (top), ERA-40 (below)

34. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Cases of extreme10 m wind (red) and at 925 hPa for ERA-40 (black) and ECHAM5(blue)

35. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Number of extreme storms in the Atlantic and the Pacific from ERA-40 and ECHAM5 Reanalysis data are for 1979-2001. ECHAM5 has been normalized to 22 years Area/Model ( total nr. of storms ca. 16000) NH 25N-90N AtlanticPacific ERA-40 >45 m/s 865229 ECHAM5 >45 m/s 393197188 ECHAM5 >50 m/s 875232

36. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Comparison between ERA 40 and ECMWF Interim reanalysis (composite of 50 cyclones)

37. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Effect of resolution We compare T213 with T63 Using the same resolution (T42) the number and relative distribution of cyclones are the same) At full resolution intensities are underestimated

38. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Number of cyclones at &63 and T213 as a function of vorticity at T42 resolution

39. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Intense cyclones at T63 and T213 resolution mm/1hr

40. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Wind speeds at 925 hPa (ca 400m above the surface) at the 99.9 percentile, ECHAM5 model at T213 resolution(60 km) For the period 1960-1990

41. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Wind speeds at 925 hPa (ca 400m above the surface) at the 99.9 percentile, ECHAM5 model at T63 resolution(250 km) For the period 1960-1990

42. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Extremes of winds at Ekofisk (North Sea) at 925 hPa 90, 95, 99 and 99,5 percentiles

43. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Extra-tropical cyclones in a warmer climate ( C21: 2070-2100. A1B) Main investigation is for NH winter (DJF) Structure and distribution of cyclones virtually identical No significant increase in wind speed maximum, vorticity or minimum pressure Significant increase in precipitation (i) global precipitation + 6% (ii) accumulated precipitation along storm tracks +11% (iii) maximum precipitation > 30%

44. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Storm tracks (DJF) over 32 years with maximum wind speed of 50m/s. Left 1959-1990 (20C), right 2069-2100 (C21), Scenario A1B. Model ECHAM5 (T213) 20C21C

45. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Storm tracks (DJF) over 32 years with maximum wind speed > 45m/s. Left 1959- 1990 (20C), right 2069-2100 (C21), Scenario A1B. Model ECHAM5 (T213). Colored points indicate centre of cyclone at the time of maximum wind speed within 5 degrees from the centre.

46. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF) at 21C. Time units are in 6 hours centered at the time of minimum pressure. Evolution of central pressure, vorticity, wind speed and precipitation

47. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Lifecycle composites of the 100 most intense storms in T42 ξ 850 for the NH, DJF for T213 ECHAM5 in 21C and 20C (transparent colour). Parameters shown are MSLP (hPa) ( in black), ξ 850 (10 -5 s -1 ), (in red) 925hPa winds (m s -1 ) (in green) and area averaged total precipitation (mm hr -1 ) ( in blue). Time step is 6 hours.

48. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Wind in a grid point space DJF

49. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Wind speeds at 925 hPa (ca 400m above the surface) at the 99.9 percentile, ECHAM5 model at T213 resolution(60 km) For the period 1960-1990

50. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Wind speeds at 925 hPa (ca 400m above the surface) at the 99.9 percentile, ECHAM5 model at T213 resolution(60 km) For the period 2070-2100, scenario A1B

51. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Change in wind speed maximum at the 99 percentiles. Calculated from all gridpoints every 6 hours, DJF

52. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Change in wind speed maximum at the 99.9 percentiles. Calculated from all gridpoints every 6 hours, DJF

53. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Precipitation in a grid point space 6 hourly DJF

54. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Hourly precipitation intensity at 20C (DJF), 99 percentile

55. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Percentage change in hourly precipitation intensity between 21C and 20C (DJF), 99 percentile

56. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Hourly precipitation intensity at 20C (DJF), 99.9 percentile

57. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Percentage change in hourly precipitation intensity between 21C and 20C (DJF), 99.9 percentile

58. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Other seasons

59. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Minimum surface pressure at NH and SH. 20C (full) and 21C (dashed)

60. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Maximum wind speed at NH and SH. 20C (full) and 21C (dashed)

61. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Areas investigated NH:- (0, 360), (25.0, 90.0)N Atl.:-(60W, 0), (25.0, 70.0)N AtlEuro:-(60W, 40E), (25.0, 70.0)N Pac.:-(120E, 120W), (25.0, 70.0)N Arctic:-(0, 360), (70.0, 90.0)N NEuro:-(10W, 40E), (47.5, 70.0)N SEuro:-(10W, 40E), 30.0, 47.5)N

62. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Number of extreme cyclones at 20C and 21 C at different seasons. Winds are at 925 hPa. Winds>45 m/s at 925 hPa corresponds broadly to > 12 Bf at 10m above the surface. Red color indicate where there are more events at 21C. Storm tracks generated between 20 and 30N are excluded. >45m/s NHAtl.Atl/EuPac.ArcticNEurSEur DJF20C546277 25615141 DJF21C51725125425516221 MAM20C20810510696920 MAM21C175767795310 JJA20C 1613 3 000 JJA21C 1710 7 000 SON20C2721411441227120 SON21C249125126122371

63. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Number of extreme cyclones at 20C and 21 C at different seasons. Winds are at 925 hPa. Winds>35 m/s at 925 hPa corresponds broadly to > 10 Bf at 10m above the surface. Red color indicate where there are more events at 21C. >35m/sAtl/EuropArcticNEuroSEuro DJF20C149825730470 DJF21C136929927052 MAM20C8401159829 MAM21C8081417729 JJA20C35026150 JJA21C33634130 SON20C97914516822 SON21C94913619721

64. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Extreme winds, 99.5 percentiles both hemispheres DJF and JJA

65. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Precipitation all seasons both hemispheres, 20C (full), 21C (dashed)

66. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Percentage increase i 6hr prec from 20C to 21C NH extra- tropics North Atlantic North Europe South Europe MeanDJF8.04.015.4-19.9 MeanJJA8.01.9-3.5-47.1 99th percentiles DJF15.813.919.4-6.0 99th percentiles JJA17.911.16.6-42.3 Max.DJF29.430.220.312.6 Max.JJA37.129.932.44.1

67. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Conclusions 1. There is an overall reduction in the number of extra-tropical storms. This covers virtually all areas and all seasons. For cyclones reaching a maximum wind speed of 25m/s or higher at 925 hPa or 8 Bf in our scaling, the total reduction is around 5%. The same proportional reduction occurs if we consider cyclones with wind speeds above 45m/s or 12 Bf. 2. The largest reduction in the most intense cyclones (>12Bf) occurs during DJF and MAM. During JJA there is an increase in 21C. This increase in intensity is related to more powerful tropical cyclones that enter mid latitude regions. This mainly occurs in the Pacific Ocean. 3. Using surface pressure below a given limit as a proxy for wind speed is misleading. The minimum surface pressure of the most intense cyclones is actually lower in 21C but maximum wind speed and vorticity is slightly lower than at 20C

68. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Conclusions 4. There is a slight poleward movements of the cyclones having the effect the number of cyclones in Northern Europe and Arctic is practically unchanged. For the same reason the number of cyclones in the Mediterranean region is proportionally more reduced. 5. There is an increase in the number of intense cyclones in the Arctic (ca 10%) but no clear tendency over Northern Europe. In order to get a representative number this is based on storms >35m/s. 6. The distribution of cyclones as a function of maximum wind speed is similar to ERA-40 but wind speeds are systematically stronger in ECHAM5 7. There is slight regional intensification (stronger wind speeds at the higher percentiles) over part of eastern Atlantic and western Europe as obtained from the set of grid point data. We suggest that this may be related to the strengthening of the SST gradient between 40 and 50N south of Greenland 8. We see no indication of any effect from the higher level of latent heat at 21C. Generally release of latent hear has little effect on extra-tropical cyclones because the way precipitation is organized around frontal surfaces, the rapidity of the dynamical processes which is on the same time scale as that of geostrophic adjustment.

69. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Can we have sufficient confidence in one particular climate model? The overall response to warmer climate is a marked increase in the column water vapour. This increase follows the Clausius-Clapeyron relation is common with other GCMs. Similarly, the much slower increase in total precipitation is also common with other GCMs ( Held and Soden, 2006). However also in agreement with other studies, extreme precipitation does increase more rapidly. This is physically credible as convective weather systems are expected to more effectively draw on the higher level of water vapour. We believe we can have confidence in the change in maximum wind speed. To predict maximum wind speed in a local environment exposed to coastal and sharp orographic obstacles is not feasible with a model of the kind used here, but the model should be capable of providing reliable predictors for such predictions. We see no reasons that such predictors will change in a warmer climate and consequently even local extremes are not likely to change either. Our confidence in the model is based on its ability to practically perfectly reproduce the high frequency change of surface pressure in the composite cyclone. Furthermore, we have no indications of any systematic increase in available potential energy in the warmer climate nor in a higher level of baroclinicity. We expect that other state of the art GCMs will behave similarly.

70. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Is the integration sufficiently long? 32 years is likely to be too short for regional assessment of extreme conditions in extra-tropical cyclones ( Weisse et al., 2005). However, based on the assessment of three different 30-year integrations with the same model at lower resolution ( Bengtsson et al., 2006) suggest that overall hemispheric statistics should be robust.

71. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Is the resolution and the physical parameterization adequate? Here we are on less safe ground and it could well be the case that future ultra-high resolution using, say, non- hydrostatic equations will give a different result. However, extra-tropical cyclones are well described even by the quasi-geostrophic equations, so we do not expect that the larger cyclones will be very much different. Additional short- and medium-range prediction with similar models are accurate and forecast errors are more related to errors in the initial state. In areas, where extreme statistics is available, the results agree with observations as well as with that of limited area models.

72. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson. How confident can we be that the feedback from the release of latent heat is insignificant in generating stronger winds? The fact that extra-tropical cyclones are most intense during the cold season and in situations with strong frontal gradients and a well developed jet stream with strong upper air winds strongly suggest that these conditions, that also follow from theoretical considerations, are the main drivers of extreme extra-tropical cyclones. In difference to tropical cyclones where the release of latent heat is quasi-symmetrically organized around the cyclone the extra-tropical cyclones are different in this respect. Moreover, the evolution of an extra-tropical cyclone is characterized by a rapid transient process with a fast built up of frontal precipitation and an equally fast collapse of organized precipitation as the cyclone occludes and rapidly weakens. In order to better explore the influence of latent heat release we ordered the 100 most intense cyclones based upon precipitation intensity instead of wind speed maximum ( not shown). These cyclones have weaker maximum winds but the interesting result was that there was no increase in extreme winds but rather a slight decrease at 21C. However, there are reports of intense small-scale extra-tropical cyclones and polar lows which may have features more in common with tropical cyclones, where an enhancement by latent heat cannot be excluded. We intend to investigate this in a future study.

73. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Summary Accumulated precipitation around extra tropical cyclones increase by some 11%. Extreme precipitation ( accumulated over 6 hours) increases by more than 30% in some areas in the storm track region by more than 50%. Extreme precipitation in a warmer climate will clearly fall outside the range of present climate. Extreme winds are likely to fall within the range of the present climate.

74. 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson END