30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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.

30 April 2008 MPI, Hamburg 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?

30 April 2008 MPI, Hamburg 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?

30 April 2008 MPI, Hamburg 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.

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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. Yes 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.?

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson 5 year global temperature change in the last two decades

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Globally integrated water vapor From Held and Soden, 2006 DT +0.45K De s +3% Full line GFDL model Dashed line measurements

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Semenov and Bengtsson, 2002 Secular trends in daily precipitation characteristics Clim.Dyn %

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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))

30 April 2008 MPI, Hamburg 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, (20C) and 32 years from the 21st century, (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.

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Storm track density and storm track genesis at 20C DJF (T213)

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Storm tracks (DJF) over 32 years with maximum wind speed of 50m/s. Left (20C), right (C21), Scenario A1B. Model ECHAM5 (T213) 20C21C

30 April 2008 MPI, Hamburg 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.

30 April 2008 MPI, Hamburg 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 use maximum wind of 25m/s, 35m/s and 45 m/s. This corresponds broadly to 8Bf, 10Bf and 12 Bf at 10 m, respectively We also use surface pressure minima and surface pressure tendencies(deepening rates)

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Temperature( hPa) at maximum deepening rate

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Vortex vertical structure Wind field at maximum intensification

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Vortex vertical structure Wind field at maximum intensity

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Temperature( hPa) at minimum pressure

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Vertical tilt of the composite cyclone 36 hours before and after maximum intensity

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Changes in eddy available potential energy (PE) and kinetic energy (KE). Units in MJ/m 2 and 6hours

30 April 2008 MPI, Hamburg 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 agreees better with the model results)

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Number of cyclones at T63 and T213 as a function of vorticity at T42 resolution

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF) at 20C. From ERA-040. Time units are in 6 hours centered at the time of minimum pressure. Evolution of central pressure, vorticity, wind speed and precipitation

30 April 2008 MPI, Hamburg 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 April 2008 MPI, Hamburg 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 to be due to the higher resolution

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Cases of extreme10 m wind (red) and at 925 hPa for ERA-40 (black) and ECHAM5(blue)

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Number of cyclones at &63 and T213 as a function of vorticity at T42 resolution

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Intense cyclones at T63 and T213 resolution mm/1hr

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Extremes of winds at Ekofisk (North Sea) at 925 hPa 90, 95, 99 and 99,5 percentiles

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Extra-tropical cyclones in a warmer climate ( C21: 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 +11% (iii) maximum precipitation > 30%

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Storm tracks (DJF) over 32 years with maximum wind speed of 50m/s. Left (20C), right (C21), Scenario A1B. Model ECHAM5 (T213) 20C21C

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Storm tracks (DJF) over 32 years with maximum wind speed > 45m/s. Left (20C), right (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.

30 April 2008 MPI, Hamburg 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 ECHAM5 has been normalized to 22 years Area/Model ( total nr. of storms ca ) NH 25N-90N AtlanticPacific ERA-40 >45 m/s ECHAM5 >45 m/s ECHAM5 >50 m/s 5232

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF). Time units are in 6 hours centered at the time of minimum pressure. Full lines 20C, dashed lines 21C Surface pressure in hPa

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF), showing maximum wind speed. Time units are in 6 hours centered at the time of minimum pressure. Full lines 20C, dashed lines 21C Maximum wind speed within 5 degrees from the centre of the cyclone Maximum wind speed within 5 degrees from the centre

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF). Time units are in 6 hours centered at the time of minimum pressure. Full lines 20C, dashed lines 21C Precipitation intensity in mm/hr averaged over a circular geodetic area with 5 degree radius following the storm.

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Wind in a grid point space DJF

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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 , scenario A1B

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Change in wind speed maximum at the 99 percentiles. Calculated from all gridpoints every 6 hours, DJF

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Precipitation in a grid point space 6 hourly DJF

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Hourly precipitation intensity at 20C (DJF), 99 percentile

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Percentage change in hourly precipitation intensity between 21C and 20C (DJF), 99 percentile

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Hourly precipitation intensity at 20C (DJF), 99.9 percentile

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Percentage change in hourly precipitation intensity between 21C and 20C (DJF), 99.9 percentile

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Other seasons

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Number of events as a function of maximum wind speed

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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. Figures within brackets exclude storm tracks which are generated between 20 and 30N. >45m/s NHAtl.Atl/EuPac.ArcticNEurSEur DJF20C DJF21C MAM20C MAM21C JJA20C 56(15)16(12) 16 39(3) 000 JJA21C 77(14)17(10) 17 59(4) 000 SON20C SON21C

30 April 2008 MPI, Hamburg 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 DJF20C DJF21C MAM20C MAM21C JJA20C JJA21C SON20C SON21C

30 April 2008 MPI, Hamburg 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

30 April 2008 MPI, Hamburg 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 storm 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.

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson Conclusions 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.

30 April 2008 MPI, Hamburg ETC in a warmer climate? Lennart Bengtsson END